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Network Working Group                                     B. Carpenter
+
Network Working Group                                       S. Blake
−
Request for Comments: 2775                                          IBM
+
Request for Comments: 2475            Torrent Networking Technologies
−
Category: Informational                                   February 2000
+
Category: Informational                                     D. Black
 +
                                                  EMC Corporation
 +
                                                        M. Carlson
 +
                                                  Sun Microsystems
 +
                                                        E. Davies
 +
                                                        Nortel UK
 +
                                                          Z. Wang
 +
                                    Bell Labs Lucent Technologies
 +
                                                          W. Weiss
 +
                                              Lucent Technologies
 +
                                                    December 1998
  
−
                      Internet Transparency
+
          An Architecture for Differentiated Services
  
−
Status of this Memo
+
'''Status of this Memo'''
  
 
This memo provides information for the Internet community.  It does
 
This memo provides information for the Internet community.  It does
Line 11: Line 21:
 
memo is unlimited.
 
memo is unlimited.
  
−
Copyright Notice
+
'''Copyright Notice'''
  
−
Copyright (C) The Internet Society (2000).  All Rights Reserved.
+
Copyright (C) The Internet Society (1998).  All Rights Reserved.
  
−
Abstract
+
'''Abstract'''
  
−
This document describes the current state of the Internet from the
+
This document defines an architecture for implementing scalable
−
architectural viewpoint, concentrating on issues of end-to-end
+
service differentiation in the Internet.  This architecture achieves
−
connectivity and transparency. It concludes with a summary of some
+
scalability by aggregating traffic classification state which is
−
major architectural alternatives facing the Internet network layer.
+
conveyed by means of IP-layer packet marking using the DS field
 +
[DSFIELD].  Packets are classified and marked to receive a particular
 +
per-hop forwarding behavior on nodes along their path.  Sophisticated
 +
classification, marking, policing, and shaping operations need only
 +
be implemented at network boundaries or hosts. Network resources are
 +
allocated to traffic streams by service provisioning policies which
 +
govern how traffic is marked and conditioned upon entry to a
 +
differentiated services-capable network, and how that traffic is
 +
forwarded within that network.  A wide variety of services can be
 +
implemented on top of these building blocks.
  
−
This document was used as input to the IAB workshop on the future of
+
    2.3.4  Location of Traffic Conditioners and MF Classifiers ... 17
−
the network layer held in July 1999. For this reason, it does not
+
 
−
claim to be complete and definitive, and it refrains from making
+
4. Interoperability with Non-Differentiated Services-Compliant
−
recommendations.
 
  
 
== Introduction ==
 
== Introduction ==
  
−
   "There's a freedom about the Internet: As long as we accept the
+
=== Overview ===
−
    rules of sending packets around, we can send packets containing
+
 
−
    anything to anywhere." [Berners-Lee]
+
This document defines an architecture for implementing scalable
 +
service differentiation in the Internet.  A "Service" defines some
 +
significant characteristics of packet transmission in one direction
 +
across a set of one or more paths within a network.  These
 +
 
 +
characteristics may be specified in quantitative or statistical terms
 +
of throughput, delay, jitter, and/or loss, or may otherwise be
 +
specified in terms of some relative priority of access to network
 +
resources.  Service differentiation is desired to accommodate
 +
heterogeneous application requirements and user expectations, and to
 +
permit differentiated pricing of Internet service.
 +
 
 +
This architecture is composed of a number of functional elements
 +
implemented in network nodes, including a small set of per-hop
 +
forwarding behaviors, packet classification functions, and traffic
 +
conditioning functions including metering, marking, shaping, and
 +
policing.  This architecture achieves scalability by implementing
 +
complex classification and conditioning functions only at network
 +
boundary nodes, and by applying per-hop behaviors to aggregates of
 +
traffic which have been appropriately marked using the DS field in
 +
the IPv4 or IPv6 headers [DSFIELD].  Per-hop behaviors are defined to
 +
permit a reasonably granular means of allocating buffer and bandwidth
 +
resources at each node among competing traffic streams.  Per-
 +
application flow or per-customer forwarding state need not be
 +
maintained within the core of the network.  A distinction is
 +
maintained between:
 +
 
 +
o  the service provided to a traffic aggregate,
 +
 
 +
o  the conditioning functions and per-hop behaviors used to realize
 +
  services,
 +
 
 +
o  the DS field value (DS codepoint) used to mark packets to select a
 +
  per-hop behavior, and
 +
 
 +
o  the particular node implementation mechanisms which realize a
 +
  per-hop behavior.
 +
 
 +
Service provisioning and traffic conditioning policies are
 +
sufficiently decoupled from the forwarding behaviors within the
 +
network interior to permit implementation of a wide variety of
 +
service behaviors, with room for future expansion.
 +
 
 +
This architecture only provides service differentiation in one
 +
direction of traffic flow and is therefore asymmetric.  Development
 +
of a complementary symmetric architecture is a topic of current
 +
research but is outside the scope of this document; see for example
 +
[EXPLICIT].
 +
 
 +
Sect. 1.2 is a glossary of terms used within this document.  Sec. 1.3
 +
lists requirements addressed by this architecture, and Sec. 1.4
 +
provides a brief comparison to other approaches for service
 +
differentiation.  Sec. 2 discusses the components of the architecture
 +
 
 +
in detail.  Sec. 3 proposes guidelines for per-hop behavior
 +
specifications.  Sec. 4 discusses interoperability issues with nodes
 +
and networks which do not implement differentiated services as
 +
defined in this document and in [DSFIELD].  Sec. 5 discusses issues
 +
with multicast service delivery.  Sec. 6 addresses security and
 +
tunnel considerations.
 +
 
 +
=== Terminology ===
 +
 
 +
This section gives a general conceptual overview of the terms used in
 +
this document.  Some of these terms are more precisely defined in
 +
later sections of this document.
 +
 
 +
Behavior Aggregate (BA)  a DS behavior aggregate.
 +
 
 +
BA classifier            a classifier that selects packets based
 +
                          only on the contents of the DS field.
 +
 
 +
Boundary link            a link connecting the edge nodes of two
 +
                          domains.
 +
 
 +
Classifier                an entity which selects packets based on
 +
                          the content of packet headers according to
 +
                          defined rules.
 +
 
 +
DS behavior aggregate    a collection of packets with the same DS
 +
                          codepoint crossing a link in a particular
 +
                          direction.
 +
 
 +
DS boundary node          a DS node that connects one DS domain to a
 +
                          node either in another DS domain or in a
 +
                          domain that is not DS-capable.
 +
 
 +
DS-capable                capable of implementing differentiated
 +
                          services as described in this architecture;
 +
                          usually used in reference to a domain
 +
                          consisting of DS-compliant nodes.
 +
 
 +
DS codepoint              a specific value of the DSCP portion of the
 +
                          DS field, used to select a PHB.
 +
 
 +
DS-compliant              enabled to support differentiated services
 +
                          functions and behaviors as defined in
 +
                          [DSFIELD], this document, and other
 +
                          differentiated services documents; usually
 +
                          used in reference to a node or device.
 +
 
 +
DS domain                a DS-capable domain; a contiguous set of
 +
                          nodes which operate with a common set of
 +
                          service provisioning policies and PHB
 +
                          definitions.
 +
 
 +
DS egress node            a DS boundary node in its role in handling
 +
                          traffic as it leaves a DS domain.
 +
 
 +
DS ingress node          a DS boundary node in its role in handling
 +
                          traffic as it enters a DS domain.
 +
 
 +
DS interior node          a DS node that is not a DS boundary node.
 +
 
 +
DS field                  the IPv4 header TOS octet or the IPv6
 +
                          Traffic Class octet when interpreted in
 +
                          conformance with the definition given in
 +
                          [DSFIELD].  The bits of the DSCP field
 +
                          encode the DS codepoint, while the
 +
                          remaining bits are currently unused.
 +
 
 +
DS node                  a DS-compliant node.
 +
 
 +
DS region                a set of contiguous DS domains which can
 +
                          offer differentiated services over paths
 +
                          across those DS domains.
 +
 
 +
Downstream DS domain      the DS domain downstream of traffic flow on
 +
                          a boundary link.
 +
 
 +
Dropper                  a device that performs dropping.
 +
 
 +
Dropping                  the process of discarding packets based on
 +
                          specified rules; policing.
 +
 
 +
Legacy node              a node which implements IPv4 Precedence as
 +
                          defined in [RFC791,RFC1812] but which is
 +
                          otherwise not DS-compliant.
 +
 
 +
Marker                    a device that performs marking.
 +
 
 +
Marking                  the process of setting the DS codepoint in
 +
                          a packet based on defined rules; pre-
 +
                          marking, re-marking.
 +
 
 +
Mechanism                a specific algorithm or operation (e.g.,
 +
                          queueing discipline) that is implemented in
 +
                          a node to realize a set of one or more per-
 +
                          hop behaviors.
 +
 
 +
Meter                    a device that performs metering.
 +
 
 +
Metering                  the process of measuring the temporal
 +
                          properties (e.g., rate) of a traffic stream
 +
                          selected by a classifier.  The
 +
                          instantaneous state of this process may be
 +
                          used to affect the operation of a marker,
 +
                          shaper, or dropper, and/or may be used for
 +
                          accounting and measurement purposes.
 +
 
 +
Microflow                a single instance of an application-to-
 +
                          application flow of packets which is
 +
                          identified by source address, source port,
 +
                          destination address, destination port and
 +
                          protocol id.
 +
 
 +
MF Classifier            a multi-field (MF) classifier which selects
 +
                          packets based on the content of some
 +
                          arbitrary number of header fields;
 +
                          typically some combination of source
 +
                          address, destination address, DS field,
 +
                          protocol ID, source port and destination
 +
                          port.
 +
 
 +
Per-Hop-Behavior (PHB)   the externally observable forwarding
 +
                          behavior applied at a DS-compliant node to
 +
                          a DS behavior aggregate.
 +
 
 +
PHB group                a set of one or more PHBs that can only be
 +
                          meaningfully specified and implemented
 +
                          simultaneously, due to a common constraint
 +
                          applying to all PHBs in the set such as a
 +
                          queue servicing or queue management policy.
 +
                          A PHB group provides a service building
 +
                          block that allows a set of related
 +
                          forwarding behaviors to be specified
 +
                          together (e.g., four dropping priorities).
 +
                          A single PHB is a special case of a PHB
 +
                          group.
 +
 
 +
Policing                  the process of discarding packets (by a
 +
                          dropper) within a traffic stream in
 +
                          accordance with the state of a
 +
                          corresponding meter enforcing a traffic
 +
                          profile.
 +
 
 +
Pre-mark                  to set the DS codepoint of a packet prior
 +
                          to entry into a downstream DS domain.
 +
 
 +
Provider DS domain        the DS-capable provider of services to a
 +
                          source domain.
 +
 
 +
Re-mark                  to change the DS codepoint of a packet,
 +
                          usually performed by a marker in accordance
 +
                          with a TCA.
 +
 
 +
Service                  the overall treatment of a defined subset
 +
                          of a customer's traffic within a DS domain
 +
                          or end-to-end.
 +
 
 +
Service Level Agreement  a service contract between a customer and a
 +
(SLA)                    service provider that specifies the
 +
                          forwarding service a customer should
 +
                          receive.  A customer may be a user
 +
                          organization (source domain) or another DS
 +
                          domain (upstream domain).  A SLA may
 +
                          include traffic conditioning rules which
 +
                          constitute a TCA in whole or in part.
 +
 
 +
Service Provisioning      a policy which defines how traffic
 +
Policy                    conditioners are configured on DS boundary
 +
                          nodes and how traffic streams are mapped to
 +
                          DS behavior aggregates to achieve a range
 +
                          of services.
 +
 
 +
Shaper                    a device that performs shaping.
 +
 
 +
Shaping                  the process of delaying packets within a
 +
                          traffic stream to cause it to conform to
 +
                          some defined traffic profile.
 +
 
 +
Source domain            a domain which contains the node(s)
 +
                          originating the traffic receiving a
 +
                          particular service.
 +
 
 +
Traffic conditioner      an entity which performs traffic
 +
                          conditioning functions and which may
 +
                          contain meters, markers, droppers, and
 +
                          shapers. Traffic conditioners are typically
 +
                          deployed in DS boundary nodes only.  A
 +
                          traffic conditioner may re-mark a traffic
 +
                          stream or may discard or shape packets to
 +
                          alter the temporal characteristics of the
 +
                          stream and bring it into compliance with a
 +
                          traffic profile.
 +
 
 +
Traffic conditioning      control functions performed to enforce
 +
                          rules specified in a TCA, including
 +
                          metering, marking, shaping, and policing.
 +
 
 +
Traffic Conditioning      an agreement specifying classifier rules
 +
Agreement (TCA)          and any corresponding traffic profiles and
 +
                          metering, marking, discarding and/or
 +
                          shaping rules which are to apply to the
 +
                          traffic streams selected by the classifier.
 +
                          A TCA encompasses all of the traffic
 +
                          conditioning rules explicitly specified
 +
                          within a SLA along with all of the rules
 +
                          implicit from the relevant service
 +
                          requirements and/or from a DS domain's
 +
                          service provisioning policy.
 +
 
 +
Traffic profile          a description of the temporal properties
 +
                          of a traffic stream such as rate and burst
 +
                          size.
 +
 
 +
Traffic stream            an administratively significant set of one
 +
                          or more microflows which traverse a path
 +
                          segment.  A traffic stream may consist of
 +
                          the set of active microflows which are
 +
                          selected by a particular classifier.
 +
 
 +
Upstream DS domain        the DS domain upstream of traffic flow on a
 +
                          boundary link.
 +
 
 +
=== Requirements ===
 +
 
 +
The history of the Internet has been one of continuous growth in the
 +
number of hosts, the number and variety of applications, and the
 +
capacity of the network infrastructure, and this growth is expected
 +
to continue for the foreseeable future.  A scalable architecture for
 +
service differentiation must be able to accommodate this continued
 +
growth.
 +
 
 +
The following requirements were identified and are addressed in this
 +
architecture:
 +
 
 +
o  should accommodate a wide variety of services and provisioning
 +
  policies, extending end-to-end or within a particular (set of)
 +
  network(s),
 +
 
 +
o  should allow decoupling of the service from the particular
 +
  application in use,
 +
 
 +
o  should work with existing applications without the need for
 +
  application programming interface changes or host software
 +
  modifications (assuming suitable deployment of classifiers,
 +
  markers, and other traffic conditioning functions),
 +
 
 +
o  should decouple traffic conditioning and service provisioning
 +
  functions from forwarding behaviors implemented within the core
 +
  network nodes,
 +
 
 +
o  should not depend on hop-by-hop application signaling,
 +
 
 +
o  should require only a small set of forwarding behaviors whose
 +
  implementation complexity does not dominate the cost of a network
 +
  device, and which will not introduce bottlenecks for future high-
 +
  speed system implementations,
 +
 
 +
o  should avoid per-microflow or per-customer state within core
 +
  network nodes,
 +
 
 +
o  should utilize only aggregated classification state within the
 +
  network core,
 +
 
 +
o  should permit simple packet classification implementations in core
 +
  network nodes (BA classifier),
 +
 
 +
o  should permit reasonable interoperability with non-DS-compliant
 +
  network nodes,
 +
 
 +
o  should accommodate incremental deployment.
 +
 
 +
=== Comparisons with Other Approaches ===
 +
 
 +
The differentiated services architecture specified in this document
 +
can be contrasted with other existing models of service
 +
differentiation.  We classify these alternative models into the
 +
following categories: relative priority marking, service marking,
 +
label switching, Integrated Services/RSVP, and static per-hop
 +
classification.
 +
 
 +
Examples of the relative priority marking model include IPv4
 +
Precedence marking as defined in [[RFC791]], 802.5 Token Ring priority
 +
[TR], and the default interpretation of 802.1p traffic classes
 +
[802.1p].  In this model the application, host, or proxy node selects
 +
a relative priority or "precedence" for a packet (e.g., delay or
 +
discard priority), and the network nodes along the transit path apply
 +
the appropriate priority forwarding behavior corresponding to the
 +
priority value within the packet's header.  Our architecture can be
 +
considered as a refinement to this model, since we more clearly
 +
 
 +
specify the role and importance of boundary nodes and traffic
 +
conditioners, and since our per-hop behavior model permits more
 +
general forwarding behaviors than relative delay or discard priority.
 +
 
 +
An example of a service marking model is IPv4 TOS as defined in
 +
[[RFC1349]].  In this example each packet is marked with a request for
 +
a "type of service", which may include "minimize delay", "maximize
 +
throughput", "maximize reliability", or "minimize cost".  Network
 +
nodes may select routing paths or forwarding behaviors which are
 +
suitably engineered to satisfy the service request.  This model is
 +
subtly different from our architecture.  Note that we do not describe
 +
the use of the DS field as an input to route selection.  The TOS
 +
markings defined in [[RFC1349]] are very generic and do not span the
 +
range of possible service semantics.  Furthermore, the service
 +
request is associated with each individual packet, whereas some
 +
service semantics may depend on the aggregate forwarding behavior of
 +
a sequence of packets.  The service marking model does not easily
 +
accommodate growth in the number and range of future services (since
 +
the codepoint space is small) and involves configuration of the
 +
"TOS->forwarding behavior" association in each core network node.
 +
Standardizing service markings implies standardizing service
 +
offerings, which is outside the scope of the IETF.  Note that
 +
provisions are made in the allocation of the DS codepoint space to
 +
allow for locally significant codepoints which may be used by a
 +
provider to support service marking semantics [DSFIELD].
 +
 
 +
Examples of the label switching (or virtual circuit) model include
 +
Frame Relay, ATM, and MPLS [FRELAY, ATM].  In this model path
 +
forwarding state and traffic management or QoS state is established
 +
for traffic streams on each hop along a network path.  Traffic
 +
aggregates of varying granularity are associated with a label
 +
switched path at an ingress node, and packets/cells within each label
 +
switched path are marked with a forwarding label that is used to
 +
lookup the next-hop node, the per-hop forwarding behavior, and the
 +
replacement label at each hop.  This model permits finer granularity
 +
resource allocation to traffic streams, since label values are not
 +
globally significant but are only significant on a single link;
 +
therefore resources can be reserved for the aggregate of packets/
 +
cells received on a link with a particular label, and the label
 +
switching semantics govern the next-hop selection, allowing a traffic
 +
stream to follow a specially engineered path through the network.
 +
This improved granularity comes at the cost of additional management
 +
and configuration requirements to establish and maintain the label
 +
switched paths.  In addition, the amount of forwarding state
 +
maintained at each node scales in proportion to the number of edge
 +
nodes of the network in the best case (assuming multipoint-to-point
 +
 
 +
label switched paths), and it scales in proportion with the square of
 +
the number of edge nodes in the worst case, when edge-edge label
 +
switched paths with provisioned resources are employed.
 +
 
 +
The Integrated Services/RSVP model relies upon traditional datagram
 +
forwarding in the default case, but allows sources and receivers to
 +
exchange signaling messages which establish additional packet
 +
classification and forwarding state on each node along the path
 +
between them [RFC1633, RSVP].  In the absence of state aggregation,
 +
the amount of state on each node scales in proportion to the number
 +
of concurrent reservations, which can be potentially large on high-
 +
speed links.  This model also requires application support for the
 +
RSVP signaling protocol.  Differentiated services mechanisms can be
 +
utilized to aggregate Integrated Services/RSVP state in the core of
 +
the network [Bernet].
 +
 
 +
A variant of the Integrated Services/RSVP model eliminates the
 +
requirement for hop-by-hop signaling by utilizing only "static"
 +
classification and forwarding policies which are implemented in each
 +
node along a network path.  These policies are updated on
 +
administrative timescales and not in response to the instantaneous
 +
mix of microflows active in the network.  The state requirements for
 +
this variant are potentially worse than those encountered when RSVP
 +
is used, especially in backbone nodes, since the number of static
 +
policies that might be applicable at a node over time may be larger
 +
than the number of active sender-receiver sessions that might have
 +
installed reservation state on a node.  Although the support of large
 +
numbers of classifier rules and forwarding policies may be
 +
computationally feasible, the management burden associated with
 +
installing and maintaining these rules on each node within a backbone
 +
network which might be traversed by a traffic stream is substantial.
 +
 
 +
Although we contrast our architecture with these alternative models
 +
of service differentiation, it should be noted that links and nodes
 +
employing these techniques may be utilized to extend differentiated
 +
services behaviors and semantics across a layer-2 switched
 +
infrastructure (e.g., 802.1p LANs, Frame Relay/ATM backbones)
 +
interconnecting DS nodes, and in the case of MPLS may be used as an
 +
alternative intra-domain implementation technology.  The constraints
 +
imposed by the use of a specific link-layer technology in particular
 +
regions of a DS domain (or in a network providing access to DS
 +
domains) may imply the differentiation of traffic on a coarser grain
 +
basis.  Depending on the mapping of PHBs to different link-layer
 +
services and the way in which packets are scheduled over a restricted
 +
set of priority classes (or virtual circuits of different category
 +
and capacity), all or a subset of the PHBs in use may be supportable
 +
(or may be indistinguishable).
 +
 
 +
== Differentiated Services Architectural Model ==
 +
 
 +
The differentiated services architecture is based on a simple model
 +
where traffic entering a network is classified and possibly
 +
conditioned at the boundaries of the network, and assigned to
 +
different behavior aggregates.  Each behavior aggregate is identified
 +
by a single DS codepoint.  Within the core of the network, packets
 +
are forwarded according to the per-hop behavior associated with the
 +
DS codepoint.  In this section, we discuss the key components within
 +
a differentiated services region, traffic classification and
 +
conditioning functions, and how differentiated services are achieved
 +
through the combination of traffic conditioning and PHB-based
 +
forwarding.
 +
 
 +
=== Differentiated Services Domain ===
 +
 
 +
A DS domain is a contiguous set of DS nodes which operate with a
 +
common service provisioning policy and set of PHB groups implemented
 +
on each node.  A DS domain has a well-defined boundary consisting of
 +
DS boundary nodes which classify and possibly condition ingress
 +
traffic to ensure that packets which transit the domain are
 +
appropriately marked to select a PHB from one of the PHB groups
 +
supported within the domain. Nodes within the DS domain select the
 +
forwarding behavior for packets based on their DS codepoint, mapping
 +
that value to one of the supported PHBs using either the recommended
 +
codepoint->PHB mapping or a locally customized mapping [DSFIELD].
 +
Inclusion of non-DS-compliant nodes within a DS domain may result in
 +
unpredictable performance and may impede the ability to satisfy
 +
service level agreements (SLAs).
 +
 
 +
A DS domain normally consists of one or more networks under the same
 +
administration; for example, an organization's intranet or an ISP.
 +
The administration of the domain is responsible for ensuring that
 +
adequate resources are provisioned and/or reserved to support the
 +
SLAs offered by the domain.
 +
 
 +
==== DS Boundary Nodes and Interior Nodes ====
 +
 
 +
A DS domain consists of DS boundary nodes and DS interior nodes.  DS
 +
boundary nodes interconnect the DS domain to other DS or non-DS-
 +
capable domains, whilst DS interior nodes only connect to other DS
 +
interior or boundary nodes within the same DS domain.
 +
 
 +
Both DS boundary nodes and interior nodes must be able to apply the
 +
appropriate PHB to packets based on the DS codepoint; otherwise
 +
unpredictable behavior may result.  In addition, DS boundary nodes
 +
may be required to perform traffic conditioning functions as defined
 +
by a traffic conditioning agreement (TCA) between their DS domain and
  
−
The Internet is experiencing growing pains which are often referred
+
the peering domain which they connect to (see Sec. 2.3.3).
−
to as "the end-to-end problem". This document attempts to analyse
 
−
those growing pains by reviewing the current state of the network
 
−
layer, especially its progressive loss of transparency. For the
 
−
purposes of this document, "transparency" refers to the original
 
−
Internet concept of a single universal logical addressing scheme, and
 
−
the mechanisms by which packets may flow from source to destination
 
−
essentially unaltered.
 
  
−
The causes of this loss of transparency are partly artefacts of
+
Interior nodes may be able to perform limited traffic conditioning
−
parsimonious allocation of the limited address space available to
+
functions such as DS codepoint re-marking.  Interior nodes which
−
IPv4, and partly the result of broader issues resulting from the
+
implement more complex classification and traffic conditioning
−
widespread use of TCP/IP technology by businesses and consumers. For
+
functions are analogous to DS boundary nodes (see Sec. 2.3.4.4).
−
example, network address translation is an artefact, but Intranets
 
−
are not.
 
  
−
Thus the way forward must recognise the fundamental changes in the
+
A host in a network containing a DS domain may act as a DS boundary
−
usage of TCP/IP that are driving current Internet growth. In one
+
node for traffic from applications running on that host; we therefore
−
scenario, a complete migration to IPv6 potentially allows the
+
say that the host is within the DS domain. If a host does not act as
−
restoration of global address transparency, but without removing
+
a boundary node, then the DS node topologically closest to that host
−
firewalls and proxies from the picture. At the other extreme, a total
+
acts as the DS boundary node for that host's traffic.
−
failure of IPv6 leads to complete fragmentation of the network layer,
 
−
with global connectivity depending on endless patchwork.
 
  
 +
==== DS Ingress Node and Egress Node ====
  
−
This document does not discuss the routing implications of address
+
DS boundary nodes act both as a DS ingress node and as a DS egress
−
space, nor the implications of quality of service management on
+
node for different directions of traffic.  Traffic enters a DS domain
−
router state, although both these matters interact with transparency
+
at a DS ingress node and leaves a DS domain at a DS egress node.  A
−
to some extent. It also does not substantively discuss namespace
+
DS ingress node is responsible for ensuring that the traffic entering
−
issues.
+
the DS domain conforms to any TCA between it and the other domain to
 +
which the ingress node is connected.  A DS egress node may perform
 +
traffic conditioning functions on traffic forwarded to a directly
 +
connected peering domain, depending on the details of the TCA between
 +
the two domains. Note that a DS boundary node may act as a DS
 +
interior node for some set of interfaces.
  
−
== Aspects of end-to-end connectivity ==
+
=== Differentiated Services Region ===
  
−
The phrase "end to end", often abbreviated as "e2e", is widely used
+
A differentiated services region (DS Region) is a set of one or more
−
in architectural discussions of the Internet. For the purposes of
+
contiguous DS domains.  DS regions are capable of supporting
−
this paper, we first present three distinct aspects of end-to-
+
differentiated services along paths which span the domains within the
−
endness.
+
region.
  
−
2.1 The end-to-end argument
+
The DS domains in a DS region may support different PHB groups
 +
internally and different codepoint->PHB mappings.  However, to permit
 +
services which span across the domains, the peering DS domains must
 +
each establish a peering SLA which defines (either explicitly or
 +
implicitly) a TCA which specifies how transit traffic from one DS
 +
domain to another is conditioned at the boundary between the two DS
 +
domains.
  
−
This is an argument first described in [Saltzer] and reviewed in [RFC
+
It is possible that several DS domains within a DS region may adopt a
−
1958], from which an extended quotation follows:
+
common service provisioning policy and may support a common set of
 +
PHB groups and codepoint mappings, thus eliminating the need for
 +
traffic conditioning between those DS domains.
  
−
  "The basic argument is that, as a first principle, certain
+
=== Traffic Classification and Conditioning ===
−
  required end-to-end functions can only be performed correctly by
 
−
  the end-systems themselves. A specific case is that any network,
 
−
  however carefully designed, will be subject to failures of
 
−
  transmission at some statistically determined rate. The best way
 
−
  to cope with this is to accept it, and give responsibility for the
 
−
  integrity of communication to the end systems. Another specific
 
−
  case is end-to-end security.
 
  
−
  "To quote from [Saltzer], 'The function in question can completely
+
Differentiated services are extended across a DS domain boundary by
−
  and correctly be implemented only with the knowledge and help of
+
establishing a SLA between an upstream network and a downstream DS
−
  the application standing at the endpoints of the communication
+
domain.  The SLA may specify packet classification and re-marking
−
  system.  Therefore, providing that questioned function as a
+
rules and may also specify traffic profiles and actions to traffic
−
  feature of the communication system itself is not possible.
+
streams which are in- or out-of-profile (see Sec. 2.3.2).  The TCA
−
  (Sometimes an incomplete version of the function provided by the
+
between the domains is derived (explicitly or implicitly) from this
−
  communication system may be useful as a performance enhancement.)'
+
SLA.
  
−
  "This principle has important consequences if we require
+
The packet classification policy identifies the subset of traffic
−
  applications to survive partial network failures. An end-to-end
+
which may receive a differentiated service by being conditioned and/
−
  protocol design should not rely on the maintenance of state (i.e.
+
or mapped to one or more behavior aggregates (by DS codepoint re-
−
  information about the state of the end-to-end communication)
+
marking) within the DS domain.
−
  inside the network. Such state should be maintained only in the
 
−
  endpoints, in such a way that the state can only be destroyed when
 
−
  the endpoint itself breaks (known as fate-sharing). An immediate
 
−
  consequence of this is that datagrams are better than classical
 
−
  virtual circuits.  The network's job is to transmit datagrams as
 
−
  efficiently and flexibly as possible.  Everything else should be
 
−
  done at the fringes."
 
  
 +
Traffic conditioning performs metering, shaping, policing and/or re-
 +
marking to ensure that the traffic entering the DS domain conforms to
 +
the rules specified in the TCA, in accordance with the domain's
 +
service provisioning policy.  The extent of traffic conditioning
 +
required is dependent on the specifics of the service offering, and
 +
may range from simple codepoint re-marking to complex policing and
 +
shaping operations.  The details of traffic conditioning policies
 +
which are negotiated between networks is outside the scope of this
 +
document.
  
−
Thus this first aspect of end-to-endness limits what the network is
+
==== Classifiers ====
−
expected to do, and clarifies what the end-system is expected to do.
 
−
The end-to-end argument underlies the rest of this document.
 
  
−
2.2 End-to-end performance
+
Packet classifiers select packets in a traffic stream based on the
 +
content of some portion of the packet header. We define two types of
 +
classifiers.  The BA (Behavior Aggregate) Classifier classifies
 +
packets based on the DS codepoint only.  The MF (Multi-Field)
 +
classifier selects packets based on the value of a combination of one
 +
or more header fields, such as source address, destination address,
 +
DS field, protocol ID, source port and destination port numbers, and
 +
other information such as incoming interface.
  
−
Another aspect, in which the behaviour of the network and that of the
+
Classifiers are used to "steer" packets matching some specified rule
−
end-systems interact in a complex way, is performance, in a
+
to an element of a traffic conditioner for further processing.
−
generalised sense. This is not a primary focus of the present
+
Classifiers must be configured by some management procedure in
−
document, but it is mentioned briefly since it is often referred to
+
accordance with the appropriate TCA.
−
when discussing end-to-end issues.
 
  
−
Much work has been done over many years to improve and optimise the
+
The classifier should authenticate the information which it uses to
−
performance of TCP. Interestingly, this has led to comparatively
+
classify the packet (see Sec. 6).
−
minor changes to TCP itself; [STD 7] is still valid apart from minor
 
−
additions [[[RFC1323|RFC 1323]], [[RFC2581|RFC 2581]], [[RFC2018|RFC 2018]]]. However a great deal of
 
−
knowledge about good practice in TCP implementations has built up,
 
−
and the queuing and discard mechanisms in routers have been fine-
 
−
tuned to improve system performance in congested conditions.
 
  
−
Unfortunately all this experience in TCP performance does not help
+
Note that in the event of upstream packet fragmentation, MF
−
with transport protocols that do not exhibit TCP-like response to
+
classifiers which examine the contents of transport-layer header
−
congestion [[[RFC2309|RFC 2309]]]. Also, the requirement for specified quality of
+
fields may incorrectly classify packet fragments subsequent to the
−
service for different applications and/or customers has led to much
+
first. A possible solution to this problem is to maintain
−
new development, especially the Integrated Services [[[RFC1633|RFC 1633]], RFC
 
−
2210] and Differentiated Services [[[RFC2475|RFC 2475]]] models. At the same time
 
−
new transport-related protocols have appeared [[[RFC1889|RFC 1889]], [[RFC2326|RFC 2326]]] or
 
−
are in discussion in the IETF. It should also be noted that since the
 
−
speed of light is not set by an IETF standard, our current notions of
 
−
end-to-end performance will be largely irrelevant to interplanetary
 
−
networking.
 
  
−
Thus, despite the fact that performance and congestion issues for TCP
+
fragmentation state; however, this is not a general solution due to
−
are now quite well understood, the arrival of QOS mechanisms and of
+
the possibility of upstream fragment re-ordering or divergent routing
−
new transport protocols raise new questions about end-to-end
+
paths.  The policy to apply to packet fragments is outside the scope
−
performance, but these are not further discussed here.
+
of this document.
  
−
2.3 End-to-end address transparency
+
==== Traffic Profiles ====
  
−
When the catenet concept (a network of networks) was first described
+
A traffic profile specifies the temporal properties of a traffic
−
by Cerf in 1978 [IEN 48] following an earlier suggestion by Pouzin in
+
stream selected by a classifier.  It provides rules for determining
−
1974 [CATENET], a clear assumption was that a single logical address
+
whether a particular packet is in-profile or out-of-profile. For
−
space would cover the whole catenet (or Internet as we now know it).
+
example, a profile based on a token bucket may look like:
−
This applied not only to the early TCP/IP Internet, but also to the
 
−
Xerox PUP design, the OSI connectionless network design, XNS, and
 
−
numerous other proprietary network architectures.
 
  
 +
  codepoint=X, use token-bucket r, b
  
−
This concept had two clear consequences - packets could flow
+
The above profile indicates that all packets marked with DS codepoint
−
essentially unaltered throughout the network, and their source and
+
X should be measured against a token bucket meter with rate r and
−
destination addresses could be used as unique labels for the end
+
burst size b.  In this example out-of-profile packets are those
−
systems.
+
packets in the traffic stream which arrive when insufficient tokens
 +
are available in the bucket.  The concept of in- and out-of-profile
 +
can be extended to more than two levels, e.g., multiple levels of
 +
conformance with a profile may be defined and enforced.
  
−
The first of these consequences is not absolute.  In practice changes
+
Different conditioning actions may be applied to the in-profile
−
can be made to packets in transit. Some of these are reversible at
+
packets and out-of-profile packets, or different accounting actions
−
the destination (such as fragmentation and compression). Others may
+
may be triggered.  In-profile packets may be allowed to enter the DS
−
be irreversible (such as changing type of service bits or
+
domain without further conditioning; or, alternatively, their DS
−
decrementing a hop limit), but do not seriously obstruct the end-to-
+
codepoint may be changed.  The latter happens when the DS codepoint
−
end principle of Section 2.1. However, any change made to a packet in
+
is set to a non-Default value for the first time [DSFIELD], or when
−
transit that requires per-flow state information to be kept at an
+
the packets enter a DS domain that uses a different PHB group or
−
intermediate point would violate the fate-sharing aspect of the end-
+
codepoint->PHB mapping policy for this traffic stream. Out-of-
−
to-end principle.
+
profile packets may be queued until they are in-profile (shaped),
 +
discarded (policed), marked with a new codepoint (re-marked), or
 +
forwarded unchanged while triggering some accounting procedure.
 +
Out-of-profile packets may be mapped to one or more behavior
 +
aggregates that are "inferior" in some dimension of forwarding
 +
performance to the BA into which in-profile packets are mapped.
  
−
The second consequence, using addresses as unique labels, was in a
+
Note that a traffic profile is an optional component of a TCA and its
−
sense a side-effect of the catenet concept. However, it was a side-
+
use is dependent on the specifics of the service offering and the
−
effect that came to be highly significant. The uniqueness and
+
domain's service provisioning policy.
−
durability of addresses have been exploited in many ways, in
 
−
particular by incorporating them in transport identifiers.  Thus they
 
−
have been built into transport checksums, cryptographic signatures,
 
−
Web documents, and proprietary software licence servers. [[[RFC2101|RFC 2101]]]
 
−
explores this topic in some detail. Its main conclusion is that IPv4
 
−
addresses can no longer be assumed to be either globally unique or
 
−
invariant, and any protocol or applications design that assumes these
 
−
properties will fail unpredictably. Work in the IAB and the NAT
 
−
working group [NAT-ARCH] has analysed the impact of one specific
 
−
cause of non-uniqueness and non-invariance, i.e., network address
 
−
translators. Again the conclusion is that many applications will
 
−
fail, unless they are specifically adapted to avoid the assumption of
 
−
address transparency. One form of adaptation is the insertion of some
 
−
form of application level gateway, and another form is for the NAT to
 
−
modify payloads on the fly, but in either case the adaptation is
 
−
application-specific.
 
  
−
Non-transparency of addresses is part of a more general phenomenon.
+
==== Traffic Conditioners ====
−
We have to recognise that the Internet has lost end-to-end
 
−
transparency, and this requires further analysis.
 
  
−
== Multiple causes of loss of transparency ==
+
A traffic conditioner may contain the following elements: meter,
 +
marker, shaper, and dropper.  A traffic stream is selected by a
 +
classifier, which steers the packets to a logical instance of a
 +
traffic conditioner.  A meter is used (where appropriate) to measure
 +
the traffic stream against a traffic profile.  The state of the meter
  
−
This section describes various recent inventions that have led to the
+
with respect to a particular packet (e.g., whether it is in- or out-
−
loss of end-to-end transparency in the Internet.
+
of-profile) may be used to affect a marking, dropping, or shaping
 +
action.
  
 +
When packets exit the traffic conditioner of a DS boundary node the
 +
DS codepoint of each packet must be set to an appropriate value.
  
−
3.1 The Intranet model
+
Fig. 1 shows the block diagram of a classifier and traffic
 +
conditioner.  Note that a traffic conditioner may not necessarily
 +
contain all four elements.  For example, in the case where no traffic
 +
profile is in effect, packets may only pass through a classifier and
 +
a marker.
  
−
Underlying a number of the specific developments mentioned below is
+
                            +-------+
−
the concept of an "Intranet", loosely defined as a private corporate
+
                            |      |-------------------+
−
network using TCP/IP technology, and connected to the Internet at
+
                    +----->| Meter |                  |
−
large in a carefully controlled manner. The Intranet is presumed to
+
                    |      |      |--+                |
−
be used by corporate employees for business purposes, and to
+
                    |      +-------+  |                |
−
interconnect hosts that carry sensitive or confidential information.
+
                    |                V                V
−
It is also held to a higher standard of operational availability than
+
              +------------+      +--------+      +---------+
−
the Internet at large. Its usage can be monitored and controlled, and
+
              |            |      |        |      | Shaper/ |
−
its resources can be better planned and tuned than those of the
+
packets =====>| Classifier |=====>| Marker |=====>| Dropper |=====>
−
public network. These arguments of security and resource management
+
              |            |      |        |      |        |
−
have ensured the dominance of the Intranet model in most corporations
+
              +------------+      +--------+      +---------+
−
and campuses.
 
  
−
The emergence of the Intranet model has had a profound effect on the
+
Fig. 1: Logical View of a Packet Classifier and Traffic Conditioner
−
notion of application transparency. Many corporate network managers
 
−
feel it is for them alone to determine which applications can
 
−
traverse the Internet/Intranet boundary. In this world view, address
 
−
transparency may seem to be an unimportant consideration.
 
  
−
3.2 Dynamic address allocation
+
===== Meters =====
  
−
3.2.1 SLIP and PPP
+
Traffic meters measure the temporal properties of the stream of
 +
packets selected by a classifier against a traffic profile specified
 +
in a TCA. A meter passes state information to other conditioning
 +
functions to trigger a particular action for each packet which is
 +
either in- or out-of-profile (to some extent).
  
−
It is to be noted that with the advent of vast numbers of dial-up
+
===== Markers =====
−
Internet users, whose addresses are allocated at dial-up time, and
 
−
whose traffic may be tunneled back to their home ISP, the actual IP
 
−
addresses of such users are purely transient. During their period of
 
−
validity they can be relied on end-to-end, but they must be forgotten
 
−
at the end of every session. In particular they can have no permanent
 
−
association with the domain name of the host borrowing them.
 
  
−
3.2.2 DHCP
+
Packet markers set the DS field of a packet to a particular
 +
codepoint, adding the marked packet to a particular DS behavior
 +
aggregate. The marker may be configured to mark all packets which
 +
are steered to it to a single codepoint, or may be configured to mark
 +
a packet to one of a set of codepoints used to select a PHB in a PHB
 +
group, according to the state of a meter.  When the marker changes
 +
the codepoint in a packet it is said to have "re-marked" the packet.
  
−
Similarly, LAN-based users of the Internet today frequently use DHCP
+
===== Shapers =====
−
to acquire a new address at system restart, so here again the actual
 
−
value of the address is potentially transient and must not be stored
 
−
between sessions.
 
  
−
3.3 Firewalls
+
Shapers delay some or all of the packets in a traffic stream in order
 +
to bring the stream into compliance with a traffic profile.  A shaper
 +
usually has a finite-size buffer, and packets may be discarded if
 +
there is not sufficient buffer space to hold the delayed packets.
  
−
3.3.1 Basic firewalls
+
===== Droppers =====
  
−
Intranet managers have a major concern about security: unauthorised
+
Droppers discard some or all of the packets in a traffic stream in
−
traffic must be kept out of the Intranet at all costs. This concern
+
order to bring the stream into compliance with a traffic profile.
−
led directly to the firewall concept (a system that intercepts all
+
This process is know as "policing" the stream.  Note that a dropper
−
traffic between the Internet and the Intranet, and only lets through
+
can be implemented as a special case of a shaper by setting the
 +
shaper buffer size to zero (or a few) packets.
  
 +
==== Location of Traffic Conditioners and MF Classifiers ====
  
−
selected traffic, usually belonging to a very limited set of
+
Traffic conditioners are usually located within DS ingress and egress
−
applications). Firewalls, by their nature, fundamentally limit
+
boundary nodes, but may also be located in nodes within the interior
−
transparency.
+
of a DS domain, or within a non-DS-capable domain.
  
−
3.3.2 SOCKS
+
===== Within the Source Domain =====
  
−
A footnote to the effect of firewalls is the SOCKS mechanism [RFC
+
We define the source domain as the domain containing the node(s)
−
1928] by which untrusted applications such as telnet and ftp can
+
which originate the traffic receiving a particular service.  Traffic
−
punch through a firewall.  SOCKS requires a shim library in the
+
sources and intermediate nodes within a source domain may perform
−
Intranet client, and a server in the firewall which is essentially an
+
traffic classification and conditioning functions. The traffic
−
application level relay. As a result, the remote server does not see
+
originating from the source domain across a boundary may be marked by
−
the real client; it believes that the firewall is the client.
+
the traffic sources directly or by intermediate nodes before leaving
 +
the source domain.  This is referred to as initial marking or "pre-
 +
marking".
  
−
3.4 Private addresses
+
Consider the example of a company that has the policy that its CEO's
 +
packets should have higher priority.  The CEO's host may mark the DS
 +
field of all outgoing packets with a DS codepoint that indicates
 +
"higher priority".  Alternatively, the first-hop router directly
 +
connected to the CEO's host may classify the traffic and mark the
 +
CEO's packets with the correct DS codepoint.  Such high priority
 +
traffic may also be conditioned near the source so that there is a
 +
limit on the amount of high priority traffic forwarded from a
 +
particular source.
  
−
When the threat of IPv4 address exhaustion first arose, and in some
+
There are some advantages to marking packets close to the traffic
−
cases user sites were known to be "pirating" addresses for private
+
source.  First, a traffic source can more easily take an
−
use, a set of official private addresses were hurriedly allocated
+
application's preferences into account when deciding which packets
−
[[[RFC1597|RFC 1597]]] and later more carefully defined [[[BCP5|BCP 5]]].  The legitimate
+
should receive better forwarding treatment. Also, classification of
−
existence of such an address allocation proved to very appealing, so
 
−
Intranets with large numbers of non-global addresses came into
 
−
existence. Unfortunately, such addresses by their nature cannot be
 
−
used for communication across the public Internet; without special
 
−
measures, hosts using private addresses are cut off from the world.
 
  
−
Note that private address space is sometimes asserted to be a
+
packets is much simpler before the traffic has been aggregated with
−
security feature, based on the notion that outside knowledge of
+
packets from other sources, since the number of classification rules
−
internal addresses might help intruders. This is a false argument,
+
which need to be applied within a single node is reduced.
−
since it is trivial to hide addresses by suitable access control
 
−
lists, even if they are globally unique - indeed that is a basic
 
−
feature of a filtering router, the simplest form of firewall. A
 
−
system with a hidden address is just as private as a system with a
 
−
private address.  There is of course no possible point in hiding the
 
−
addresses of servers to which outside access is required.
 
  
−
It is also worth noting that the IPv6 equivalent of private
+
Since packet marking may be distributed across multiple nodes, the
−
addresses, i.e. site-local addresses, have similar characteristics to
+
source DS domain is responsible for ensuring that the aggregated
−
[[BCP5|BCP 5]] addresses, but their use will not be forced by a lack of
+
traffic towards its provider DS domain conforms to the appropriate
−
globally unique IPv6 addresses.
+
TCA. Additional allocation mechanisms such as bandwidth brokers or
 +
RSVP may be used to dynamically allocate resources for a particular
 +
DS behavior aggregate within the provider's network [2BIT, Bernet].
 +
The boundary node of the source domain should also monitor
 +
conformance to the TCA, and may police, shape, or re-mark packets as
 +
necessary.
  
−
3.5 Network address translators
+
===== At the Boundary of a DS Domain =====
  
−
Network address translators (NATs) are an almost inevitable
+
Traffic streams may be classified, marked, and otherwise conditioned
−
consequence of the existence of Intranets using private addresses yet
+
on either end of a boundary link (the DS egress node of the upstream
−
needing to communicate with the Internet at large. Their
+
domain or the DS ingress node of the downstream domain). The SLA
−
architectural implications are discussed at length in [NAT-ARCH], the
+
between the domains should specify which domain has responsibility
−
fundamental point being that address translation on the fly destroys
+
for mapping traffic streams to DS behavior aggregates and
−
end-to-end address transparency and breaks any middleware or
+
conditioning those aggregates in conformance with the appropriate
 +
TCA.  However, a DS ingress node must assume that the incoming
 +
traffic may not conform to the TCA and must be prepared to enforce
 +
the TCA in accordance with local policy.
  
 +
When packets are pre-marked and conditioned in the upstream domain,
 +
potentially fewer classification and traffic conditioning rules need
 +
to be supported in the downstream DS domain.  In this circumstance
 +
the downstream DS domain may only need to re-mark or police the
 +
incoming behavior aggregates to enforce the TCA.  However, more
 +
sophisticated services which are path- or source-dependent may
 +
require MF classification in the downstream DS domain's ingress
 +
nodes.
  
−
applications that depend on it. Numerous protocols, for example
+
If a DS ingress node is connected to an upstream non-DS-capable
−
H.323, carry IP addresses at application level and fail to traverse a
+
domain, the DS ingress node must be able to perform all necessary
−
simple NAT box correctly. If the full range of Internet applications
+
traffic conditioning functions on the incoming traffic.
−
is to be used, NATs have to be coupled with application level
 
−
gateways (ALGs) or proxies. Furthermore, the ALG or proxy must be
 
−
updated whenever a new address-dependent application comes along.  In
 
−
practice, NAT functionality is built into many firewall products, and
 
−
all useful NATs have associated ALGs, so it is difficult to
 
−
disentangle their various impacts.
 
  
−
3.6 Application level gateways, relays, proxies, and caches
+
===== In non-DS-Capable Domains =====
  
−
It is reasonable to position application level gateways, relays,
+
Traffic sources or intermediate nodes in a non-DS-capable domain may
−
proxies, and caches at certain critical topological points,
+
employ traffic conditioners to pre-mark traffic before it reaches the
−
especially the Intranet/Internet boundary.  For example, if an
+
ingress of a downstream DS domain.  In this way the local policies
−
Intranet does not use SMTP as its mail protocol, an SMTP gateway is
+
for classification and marking may be concealed.
−
needed. Even in the normal case, an SMTP relay is common, and can
 
−
perform useful mail routing functions, spam filtering, etc. (It may
 
−
be observed that spam filtering is in some ways a firewall function,
 
−
but the store-and-forward nature of electronic mail and the
 
−
availability of MX records allow it to be a distinct and separate
 
−
function.)
 
  
−
Similarly, for a protocol such as HTTP with a well-defined voluntary
+
===== In Interior DS Nodes =====
−
proxy mechanism, application proxies also serving as caches are very
 
−
useful. Although these devices interfere with transparency, they do
 
−
so in a precise way, correctly terminating network, transport and
 
−
application protocols on both sides. They can however exhibit some
 
−
shortfalls in ease of configuration and failover.
 
  
−
However, there appear to be cases of "involuntary" applications level
+
Although the basic architecture assumes that complex classification
−
devices such as proxies that grab and modify HTTP traffic without
+
and traffic conditioning functions are located only in a network's
−
using the appropriate mechanisms, sometimes known as "transparent
+
ingress and egress boundary nodes, deployment of these functions in
−
caches", or mail relays that purport to remove undesirable words.
+
the interior of the network is not precluded.  For example, more
−
These devices are by definition not transparent, and may have totally
+
restrictive access policies may be enforced on a transoceanic link,
−
unforeseeable side effects.  (A possible conclusion is that even for
+
requiring MF classification and traffic conditioning functionality in
−
non-store-and-forward protocols, a generic diversion mechanism
+
the upstream node on the link.  This approach may have scaling
−
analogous to the MX record would be of benefit. The SRV record [RFC
+
limits, due to the potentially large number of classification and
−
2052] is a step in this direction.)
+
conditioning rules that might need to be maintained.
  
−
3.7 Voluntary isolation and peer networks
+
=== Per-Hop Behaviors ===
  
−
There are communities that think of themselves as being so different
+
A per-hop behavior (PHB) is a description of the externally
−
that they require isolation via an explicit proxy, and even by using
+
observable forwarding behavior of a DS node applied to a particular
−
proprietary protocols and addressing schemes within their network. An
+
DS behavior aggregate.  "Forwarding behavior" is a general concept in
−
example is the WAP Forum which targets very small phone-like devices
+
this context.  For example, in the event that only one behavior
−
with some capabilities for Internet connectivity. However, it's not
+
aggregate occupies a link, the observable forwarding behavior (i.e.,
 +
loss, delay, jitter) will often depend only on the relative loading
 +
of the link (i.e., in the event that the behavior assumes a work-
 +
conserving scheduling discipline).  Useful behavioral distinctions
 +
are mainly observed when multiple behavior aggregates compete for
 +
buffer and bandwidth resources on a node. The PHB is the means by
 +
which a node allocates resources to behavior aggregates, and it is on
 +
top of this basic hop-by-hop resource allocation mechanism that
 +
useful differentiated services may be constructed.
  
 +
The most simple example of a PHB is one which guarantees a minimal
 +
bandwidth allocation of X% of a link (over some reasonable time
 +
interval) to a behavior aggregate.  This PHB can be fairly easily
 +
measured under a variety of competing traffic conditions.  A slightly
 +
more complex PHB would guarantee a minimal bandwidth allocation of X%
 +
of a link, with proportional fair sharing of any excess link
 +
capacity.  In general, the observable behavior of a PHB may depend on
 +
certain constraints on the traffic characteristics of the associated
 +
behavior aggregate, or the characteristics of other behavior
 +
aggregates.
  
−
the Internet they're connecting directly to. They have to go through
+
PHBs may be specified in terms of their resource (e.g., buffer,
−
a proxy. This could potentially mean that millions of devices will
+
bandwidth) priority relative to other PHBs, or in terms of their
−
never know the benefits of end-to-end connectivity to the Internet.
+
relative observable traffic characteristics (e.g., delay, loss).
 +
These PHBs may be used as building blocks to allocate resources and
 +
should be specified as a group (PHB group) for consistency. PHB
 +
groups will usually share a common constraint applying to each PHB
 +
within the group, such as a packet scheduling or buffer management
 +
policy.  The relationship between PHBs in a group may be in terms of
 +
absolute or relative priority (e.g., discard priority by means of
  
−
A similar effect arises when applications such as telephony span both
+
deterministic or stochastic thresholds), but this is not required
−
an IP network and a peer network layer using a different technology.
+
(e.g., N equal link shares).  A single PHB defined in isolation is a
−
Although the application may work end-to-end, there is no possibility
+
special case of a PHB group.
−
of end-to-end packet transmission.
 
  
−
3.8 Split DNS
+
PHBs are implemented in nodes by means of some buffer management and
 +
packet scheduling mechanisms.  PHBs are defined in terms of behavior
 +
characteristics relevant to service provisioning policies, and not in
 +
terms of particular implementation mechanisms.  In general, a variety
 +
of implementation mechanisms may be suitable for implementing a
 +
particular PHB group.  Furthermore, it is likely that more than one
 +
PHB group may be implemented on a node and utilized within a domain.
 +
PHB groups should be defined such that the proper resource allocation
 +
between groups can be inferred, and integrated mechanisms can be
 +
implemented which can simultaneously support two or more groups.  A
 +
PHB group definition should indicate possible conflicts with
 +
previously documented PHB groups which might prevent simultaneous
 +
operation.
  
−
Another consequence of the Intranet/Internet split is "split DNS" or
+
As described in [DSFIELD], a PHB is selected at a node by a mapping
−
"two faced DNS", where a corporate network serves up partly or
+
of the DS codepoint in a received packet.  Standardized PHBs have a
−
completely different DNS inside and outside its firewall. There are
+
recommended codepoint.  However, the total space of codepoints is
−
many possible variants on this; the basic point is that the
+
larger than the space available for recommended codepoints for
−
correspondence between a given FQDN (fully qualified domain name) and
+
standardized PHBs, and [DSFIELD] leaves provisions for locally
−
a given IPv4 address is no longer universal and stable over long
+
configurable mappings.  A codepoint->PHB mapping table may contain
−
periods.
+
both 1->1 and N->1 mappings. All codepoints must be mapped to some
 +
PHB; in the absence of some local policy, codepoints which are not
 +
mapped to a standardized PHB in accordance with that PHB's
 +
specification should be mapped to the Default PHB.
  
−
3.9 Various load-sharing tricks
+
=== Network Resource Allocation ===
  
−
IPv4 was not designed to support anycast [[[RFC1546|RFC 1546]]], so there is no
+
The implementation, configuration, operation and administration of
−
natural approach to load-sharing when one server cannot do the job.
+
the supported PHB groups in the nodes of a DS Domain should
−
Various tricks have been used to resolve this (multicast to find a
+
effectively partition the resources of those nodes and the inter-node
−
free server, the DNS returns different addresses for the same FQDN in
+
links between behavior aggregates, in accordance with the domain's
−
a round-robin, a router actually routes packets sent to the same
+
service provisioning policy. Traffic conditioners can further
−
address automatically to different servers, etc.). While these tricks
+
control the usage of these resources through enforcement of TCAs and
−
are not particularly harmful in the overall picture, they can be
+
possibly through operational feedback from the nodes and traffic
−
implemented in such a way as to interfere with name or address
+
conditioners in the domain.  Although a range of services can be
−
transparency.
+
deployed in the absence of complex traffic conditioning functions
 +
(e.g., using only static marking policies), functions such as
 +
policing, shaping, and dynamic re-marking enable the deployment of
 +
services providing quantitative performance metrics.
  
−
== Summary of current status and impact ==
+
The configuration of and interaction between traffic conditioners and
 +
interior nodes should be managed by the administrative control of the
 +
domain and may require operational control through protocols and a
 +
control entity.  There is a wide range of possible control models.
  
−
It is impossible to estimate with any numerical reliability how
+
The precise nature and implementation of the interaction between
−
widely the above inventions have been deployed. Since many of them
+
these components is outside the scope of this architecture.  However,
−
preserve the illusion of transparency while actually interfering with
+
scalability requires that the control of the domain does not require
−
it, they are extremely difficult to measure.
+
micro-management of the network resources.  The most scalable control
 +
model would operate nodes in open-loop in the operational timeframe,
 +
and would only require administrative-timescale management as SLAs
 +
are varied.  This simple model may be unsuitable in some
 +
circumstances, and some automated but slowly varying operational
 +
control (minutes rather than seconds) may be desirable to balance the
 +
utilization of the network against the recent load profile.
  
−
However it is certain that all the mechanisms just described are in
+
== Per-Hop Behavior Specification Guidelines ==
−
very widespread use; they are not a marginal phenomenon. In corporate
 
−
networks, many of them are the norm. Some of them (firewalls, relays,
 
−
proxies and caches) clearly have intrinsic value given the Intranet
 
−
concept. The others are largely artefacts and pragmatic responses to
 
−
various pressures in the operational Internet, and they must be
 
−
costing the industry very dearly in constant administration and
 
−
complex fault diagnosis.
 
  
 +
Basic requirements for per-hop behavior standardization are given in
 +
[DSFIELD].  This section elaborates on that text by describing
 +
additional guidelines for PHB (group) specifications.  This is
 +
intended to help foster implementation consistency.  Before a PHB
 +
group is proposed for standardization it should satisfy these
 +
guidelines, as appropriate, to preserve the integrity of this
 +
architecture.
  
−
In particular, the decline of transparency is having a severe effect
+
G.1:  A PHB standard must specify a recommended DS codepoint selected
−
on deployment of end-to-end IP security. The Internet security model
+
from the codepoint space reserved for standard mappings [DSFIELD].
−
[SECMECH] calls for security at several levels (roughly, network,
+
Recommended codepoints will be assigned by the IANA.  A PHB proposal
−
applications, and object levels). The current network level security
+
may recommend a temporary codepoint from the EXP/LU space to
−
model [[[RFC2401|RFC 2401]]] was constructed prior to the recognition that end-
+
facilitate inter-domain experimentation.  Determination of a packet's
−
to-end address transparency was under severe threat.  Although
+
PHB must not require inspection of additional packet header fields
−
alternative proposals have begun to emerge [HIP] the current reality
+
beyond the DS field.
−
is that IPSEC cannot be deployed end-to-end in the general case.
 
−
Tunnel-mode IPSEC can be deployed between corporate gateways or
 
−
firewalls.  Transport-mode IPSEC can be deployed within a corporate
 
−
network in some cases, but it cannot span from Intranet to Internet
 
−
and back to another Intranet if there is any chance of a NAT along
 
−
the way.
 
  
−
Indeed, NAT breaks other security mechanisms as well, such as DNSSEC
+
G.2:  The specification of each newly proposed PHB group should
−
and Kerberos, since they rely on address values.
+
include an overview of the behavior and the purpose of the behavior
 +
being proposed.  The overview should include a problem or problems
 +
statement for which the PHB group is targeted.  The overview should
 +
include the basic concepts behind the PHB group.  These concepts
 +
should include, but are not restricted to, queueing behavior, discard
 +
behavior, and output link selection behavior.  Lastly, the overview
 +
should specify the method by which the PHB group solves the problem
 +
or problems specified in the problem statement.
  
−
The loss of transparency brought about by private addresses and NATs
+
G.3:  A PHB group specification should indicate the number of
−
affects many applications protocols to a greater or lesser extent.
+
individual PHBs specified.  In the event that multiple PHBs are
−
This is explored in detail in [NAT-PROT]. A more subtle effect is
+
specified, the interactions between these PHBs and constraints that
−
that the prevalence of dynamic addresses (from DHCP, SLIP and PPP)
+
must be respected globally by all the PHBs within the group should be
−
has fed upon the trend towards client/server computing.  Today it is
+
clearly specified.  As an example, the specification must indicate
−
largely true that servers have fixed addresses, clients have dynamic
+
whether the probability of packet reordering within a microflow is
−
addresses, and servers can in no way assume that a client's IP
+
increased if different packets in that microflow are marked for
−
address identifies the client. On the other hand, clients rely on
+
different PHBs within the group.
−
servers having stable addresses since a DNS lookup is the only
 
−
generally deployed mechanism by which a client can find a server and
 
−
solicit service.  In this environment, there is little scope for true
 
−
peer-to-peer applications protocols, and no easy solution for mobile
 
−
servers. Indeed, the very limited demand for Mobile IP might be
 
−
partly attributed to the market dominance of client/server
 
−
applications in which the client's address is of transient
 
−
significance. We also see a trend towards single points of failure
 
−
such as media gateways, again resulting from the difficulty of
 
−
implementing peer-to-peer solutions directly between clients with no
 
−
fixed address.
 
  
−
The notion that servers can use precious globally unique addresses
+
G.4:  When proper functioning of a PHB group is dependent on
−
from a small pool, because there will always be fewer servers than
+
constraints such as a provisioning restriction, then the PHB
−
clients, may become anachronistic when most electrical devices become
+
definition should describe the behavior when these constraints are
−
network-manageable and thus become servers for a management protocol.
+
violated. Further, if actions such as packet discard or re-marking
−
Similarly, if every PC becomes a telephone (or the converse), capable
+
are required when these constraints are violated, then these actions
−
of receiving unsolicited incoming calls, the lack of stable IP
+
should be specifically stipulated.
−
addresses for PCs will be an issue. Another impending paradigm shift
 
−
is when domestic and small-office subscribers move from dial-up to
 
−
always-on Internet connectivity, at which point transient address
 
−
assignment from a pool becomes much less appropriate.
 
  
 +
G.5:  A PHB group may be specified for local use within a domain in
 +
order to provide some domain-specific functionality or domain-
 +
specific services.  In this event, the PHB specification is useful
 +
for providing vendors with a consistent definition of the PHB group.
 +
However, any PHB group which is defined for local use should not be
 +
considered for standardization, but may be published as an
 +
Informational RFC.  In contrast, a PHB group which is intended for
 +
general use will follow a stricter standardization process.
 +
Therefore all PHB proposals should specifically state whether they
 +
are to be considered for general or local use.
  
−
Many of the inventions described in the previous section lead to the
+
It is recognized that PHB groups can be designed with the intent of
−
datagram traffic between two hosts being directly or indirectly
+
providing host-to-host, WAN edge-to-WAN edge, and/or domain edge-to-
−
mediated by at least one other host. For example a client may depend
+
domain edge services. Use of the term "end-to-end" in a PHB
−
on a DHCP server, a server may depend on a NAT, and any host may
+
definition should be interpreted to mean "host-to-host" for
−
depend on a firewall. This violates the fate-sharing principle of
+
consistency.
−
[Saltzer] and introduces single points of failure. Worse, most of
 
−
these points of failure require configuration data, yet another
 
−
source of operational risk. The original notion that datagrams would
 
−
find their way around failures, especially around failed routers, has
 
−
been lost; indeed the overloading of border routers with additional
 
−
functions has turned them into critical rather than redundant
 
−
components, even for multihomed sites.
 
  
−
The loss of address transparency has other negative effects.  For
+
Other PHB groups may be defined and deployed locally within domains,
−
example, large scale servers may use heuristics or even formal
+
for experimental or operational purposes. There is no requirement
−
policies that assign different priorities to service for different
+
that these PHB groups must be publicly documented, but they should
−
clients, based on their addresses. As addresses lose their global
+
utilize DS codepoints from one of the EXP/LU pools as defined in
−
meaning, this mechanism will fail. Similarly, any anti-spam or anti-
+
[DSFIELD].
−
spoofing techniques that rely on reverse DNS lookup of address values
 
−
can be confused by translated addresses. (Uncoordinated renumbering
 
−
can have similar disadvantages.)
 
  
−
The above issues are not academic. They add up to complexity in
+
G.6:  It may be possible or appropriate for a packet marked for a PHB
−
applications design, complexity in network configuration, complexity
+
within a PHB group to be re-marked to select another PHB within the
−
in security mechanisms, and complexity in network management.
+
group; either within a domain or across a domain boundary. Typically
−
Specifically, they make fault diagnosis much harder, and by
+
there are three reasons for such PHB modification:
−
introducing more single points of failure, they make faults more
 
−
likely to occur.
 
  
−
== Possible future directions ==
+
a. The codepoints associated with the PHB group are collectively
 +
  intended to carry state about the network,
 +
b. Conditions exist which require PHB promotion or demotion of a
 +
  packet (this assumes that PHBs within the group can be ranked in
 +
  some order),
 +
c. The boundary between two domains is not covered by a SLA.  In this
 +
  case the codepoint/PHB to select when crossing the boundary link
 +
  will be determined by the local policy of the upstream domain.
  
−
5.1 Successful migration to IPv6
+
A PHB specification should clearly state the circumstances under
 +
which packets marked for a PHB within a PHB group may, or should be
 +
modified (e.g., promoted or demoted) to another PHB within the group.
 +
If it is undesirable for a packet's PHB to be modified, the
  
−
In this scenario, IPv6 becomes fully implemented on all hosts and
+
specification should clearly state the consequent risks when the PHB
−
routers, including the adaptation of middleware, applications, and
+
is modified.  A possible risk to changing a packet's PHB, either
−
management systems. Since the address space then becomes big enough
+
within or outside a PHB group, is a higher probability of packet re-
−
for all conceivable needs, address transparency can be restored.
+
ordering within a microflow. PHBs within a group may carry some
−
Transport-mode IPSEC can in principle deploy, given adequate security
+
host-to-host, WAN edge-to-WAN edge, and/or domain edge-to-domain edge
−
policy tools and a key infrastructure.  However, it is widely
+
semantics which may be difficult to duplicate if packets are re-
−
believed that the Intranet/firewall model will certainly persist.
+
marked to select another PHB from the group (or otherwise).
  
−
Note that it is a basic assumption of IPv6 that no artificial
+
For certain PHB groups, it may be appropriate to reflect a state
−
constraints will be placed on the supply of addresses, given that
+
change in the node by re-marking packets to specify another PHB from
−
there are so many of them. Current practices by which some ISPs
+
within the group.  If a PHB group is designed to reflect the state of
−
strongly limit the number of IPv4 addresses per client will have no
+
a network, the PHB definition must adequately describe the
−
reason to exist for IPv6. (However, addresses will still be assigned
+
relationship between the PHBs and the states they reflect. Further,
−
prudently, according to guidelines designed to favour hierarchical
+
if these PHBs limit the forwarding actions a node can perform in some
−
routing.)
+
way, these constraints may be specified as actions the node should,
 +
or must perform.
  
 +
G.7:  A PHB group specification should include a section defining the
 +
implications of tunneling on the utility of the PHB group.  This
 +
section should specify the implications for the utility of the PHB
 +
group of a newly created outer header when the original DS field of
 +
the inner header is encapsulated in a tunnel.  This section should
 +
also discuss what possible changes should be applied to the inner
 +
header at the egress of the tunnel, when both the codepoints from the
 +
inner header and the outer header are accessible (see Sec. 6.2).
  
−
Clearly this is in any case a very long term scenario, since it
+
G.8:  The process of specifying PHB groups is likely to be
−
assumes that IPv4 has declined to the point where IPv6 is required
+
incremental in nature.  When new PHB groups are proposed, their known
−
for universal connectivity. Thus, a viable version of Scenario 5.3 is
+
interactions with previously specified PHB groups should be
−
a prerequisite for Scenario 5.1.
+
documented.  When a new PHB group is created, it can be entirely new
 +
in scope or it can be an extension to an existing PHB group.  If the
 +
PHB group is entirely independent of some or all of the existing PHB
 +
specifications, a section should be included in the PHB specification
 +
which details how the new PHB group can co-exist with those PHB
 +
groups already standardized. For example, this section might
 +
indicate the possibility of packet re-ordering within a microflow for
 +
packets marked by codepoints associated with two separate PHB groups.
 +
If concurrent operation of two (or more) different PHB groups in the
 +
same node is impossible or detrimental this should be stated. If the
 +
concurrent operation of two (or more) different PHB groups requires
 +
some specific behaviors by the node when packets marked for PHBs from
 +
these different PHB groups are being processed by the node at the
 +
same time, these behaviors should be stated.
  
−
5.2 Complete failure of IPv6
+
Care should be taken to avoid circularity in the definitions of PHB
 +
groups.
  
−
In this scenario, IPv6 fails to reach any significant level of
+
If the proposed PHB group is an extension to an existing PHB group, a
−
operational deployment, IPv4 addressing is the only available
+
section should be included in the PHB group specification which
−
mechanism, and address transparency cannot be restored. IPSEC cannot
+
details how this extension interoperates with the behavior being
−
be deployed globally in its current form. In the very long term, the
+
extended. Further, if the extension alters or more narrowly defines
−
pool of globally unique IPv4 addresses will be nearly totally
+
the existing behavior in some way, this should also be clearly
−
allocated, and new addresses will generally not be available for any
+
indicated.
−
purpose.
 
  
−
It is unclear exactly what is likely to happen if the Internet
+
G.9:  Each PHB specification should include a section specifying
−
continues to rely exclusively on IPv4, because in that eventuality a
+
minimal conformance requirements for implementations of the PHB
−
variety of schemes are likely to promulgated, with a view toward
+
group.  This conformance section is intended to provide a means for
−
providing an acceptable evolutionary path for the network. However,
+
specifying the details of a behavior while allowing for
−
we can examine two of the more simplistic sub-scenarios which are
+
implementation variation to the extent permitted by the PHB
−
possible, while realising that the future would be unlikely to match
+
specification. This conformance section can take the form of rules,
−
either one exactly:
+
tables, pseudo-code, or tests.
  
−
5.2.1 Re-allocating the IPv4 address space
+
G.10:  A PHB specification should include a section detailing the
 +
security implications of the behavior. This section should include a
 +
discussion of the re-marking of the inner header's codepoint at the
 +
egress of a tunnel and its effect on the desired forwarding behavior.
  
−
Suppose that a mechanism is created to continuously recover and re-
+
Further, this section should also discuss how the proposed PHB group
−
allocate IPv4 addresses. A single global address space is maintained,
+
could be used in denial-of-service attacks, reduction of service
−
with all sites progressively moving to an Intranet private address
+
contract attacks, and service contract violation attacks. Lastly,
−
model, with global addresses being assigned temporarily from a pool
+
this section should discuss possible means for detecting such attacks
−
of several billion.
+
as they are relevant to the proposed behavior.
  
−
5.2.1.1 A sub-sub-scenario of this is generalised use of NAT and NAPT
+
G.11:  A PHB specification should include a section detailing
−
        (NAT with port number translation). This has the disadvantage
+
configuration and management issues which may affect the operation of
−
        that the host is unaware of the unique address being used for
+
the PHB and which may impact candidate services that might utilize
−
        its traffic, being aware only of its ambiguous private
+
the PHB.
−
        address, with all the problems that this generates. See
 
−
        [NAT-ARCH].
 
  
−
5.2.1.2 Another sub-sub-scenario is the use of realm-specific IP
+
G.12:  It is strongly recommended that an appendix be provided with
−
        addressing implemented at the host rather than by a NAT box.
+
each PHB specification that considers the implications of the
−
        See [RSIP]. In this case the host is aware of its unique
+
proposed behavior on current and potential services. These services
−
        address, allowing for substantial restoration of the end-to-
+
could include but are not restricted to be user-specific, device-
−
        end usefulness of addresses, e.g. for TCP or cryptographic
+
specific, domain-specific or end-to-end services.  It is also
−
        checksums.
+
strongly recommended that the appendix include a section describing
 +
how the services are verified by users, devices, and/or domains.
  
 +
G.13:  It is recommended that an appendix be provided with each PHB
 +
specification that is targeted for local use within a domain,
 +
providing guidance for PHB selection for packets which are forwarded
 +
into a peer domain which does not support the PHB group.
  
−
5.2.1.3 A final sub-sub-scenario is the "map and encapsulate" model
+
G.14:  It is recommended that an appendix be provided with each PHB
−
        in which address translation is replaced by systematic
+
specification which considers the impact of the proposed PHB group on
−
        encapsulation of all packets for wide-area transport.  This
+
existing higher-layer protocols.  Under some circumstances PHBs may
−
        model has never been fully developed, although it is
+
allow for possible changes to higher-layer protocols which may
−
        completely compatible with end-to-end addressing.
+
increase or decrease the utility of the proposed PHB group.
  
−
5.2.2 Exhaustion
+
G.15:  It is recommended that an appendix be provided with each PHB
 +
specification which recommends mappings to link-layer QoS mechanisms
 +
to support the intended behavior of the PHB across a shared-medium or
 +
switched link-layer.  The determination of the most appropriate
 +
mapping between a PHB and a link-layer QoS mechanism is dependent on
 +
many factors and is outside the scope of this document; however, the
 +
specification should attempt to offer some guidance.
  
−
Suppose that no mechanism is created to recover addresses (except
+
== Interoperability with Non-Differentiated Services-Compliant Nodes ==
−
perhaps black or open market trading). Sites with large address
 
−
blocks keep them.  All the scenarios of 5.2.1 appear but are
 
−
insufficient.  Eventually the global address space is no longer
 
−
adequate.  This is a nightmare scenario - NATs appear within the
 
−
"global" address space, for example at ISP-ISP boundaries. It is
 
−
unclear how a global routing system and a global DNS system can be
 
−
maintained; the Internet is permanently fragmented.
 
  
−
5.3 Partial deployment of IPv6
+
We define a non-differentiated services-compliant node (non-DS-
 +
compliant node) as any node which does not interpret the DS field as
 +
specified in [DSFIELD] and/or does not implement some or all of the
 +
standardized PHBs (or those in use within a particular DS domain).
 +
This may be due to the capabilities or configuration of the node.  We
 +
define a legacy node as a special case of a non-DS-compliant node
 +
which implements IPv4 Precedence classification and forwarding as
 +
defined in [RFC791, RFC1812], but which is otherwise not DS-
 +
compliant.  The precedence values in the IPv4 TOS octet are
 +
compatible by intention with the Class Selector Codepoints defined in
 +
[DSFIELD], and the precedence forwarding behaviors defined in
 +
[RFC791, RFC1812] comply with the Class Selector PHB Requirements
 +
also defined in [DSFIELD]. A key distinction between a legacy node
 +
and a DS-compliant node is that the legacy node may or may not
 +
interpret bits 3-6 of the TOS octet as defined in [[RFC1349]] (the
 +
"DTRC" bits); in practice it will not interpret these bit as
 +
specified in [DSFIELD].  We assume that the use of the TOS markings
 +
defined in [[RFC1349]] is deprecated.  Nodes which are non-DS-compliant
 +
and which are not legacy nodes may exhibit unpredictable forwarding
 +
behaviors for packets with non-zero DS codepoints.
  
−
In this scenario, IPv6 is completely implemented but only deploys in
+
Differentiated services depend on the resource allocation mechanisms
−
certain segments of the network (e.g. in certain countries, or in
+
provided by per-hop behavior implementations in nodes. The quality
−
certain areas of application such as mobile hand-held devices).
+
or statistical assurance level of a service may break down in the
−
Instead of being transitional in nature, some of the IPv6 transition
+
event that traffic transits a non-DS-compliant node, or a non-DS-
−
techniques become permanent features of the landscape. Sometimes
+
capable domain.
−
addresses are 32 bits, sometimes they are 128 bits. DNS lookups may
 
−
return either or both. In the 32 bit world, the scenarios 5.2.1 and
 
−
5.2.2 may occur. IPSEC can only deploy partially.
 
  
−
== Conclusion ==
+
We will examine two separate cases.  The first case concerns the use
 +
of non-DS-compliant nodes within a DS domain.  Note that PHB
 +
forwarding is primarily useful for allocating scarce node and link
 +
resources in a controlled manner.  On high-speed, lightly loaded
 +
links, the worst-case packet delay, jitter, and loss may be
  
−
None of the above scenarios is clean, simple and straightforward.
+
negligible, and the use of a non-DS-compliant node on the upstream
−
Although the pure IPv6 scenario is the cleanest and simplest, it is
+
end of such a link may not result in service degradation. In more
−
not straightforward to reach it. The various scenarios without use of
+
realistic circumstances, the lack of PHB forwarding in a node may
−
IPv6 are all messy and ultimately seem to lead to dead ends of one
+
make it impossible to offer low-delay, low-loss, or provisioned
−
kind or another. Partial deployment of IPv6, which is a required step
+
bandwidth services across paths which traverse the node.  However,
−
on the road to full deployment, is also messy but avoids the dead
+
use of a legacy node may be an acceptable alternative, assuming that
−
ends.
+
the DS domain restricts itself to using only the Class Selector
 +
Codepoints defined in [DSFIELD], and assuming that the particular
 +
precedence implementation in the legacy node provides forwarding
 +
behaviors which are compatible with the services offered along paths
 +
which traverse that node.  Note that it is important to restrict the
 +
codepoints in use to the Class Selector Codepoints, since the legacy
 +
node may or may not interpret bits 3-5 in accordance with [[RFC1349]],
 +
thereby resulting in unpredictable forwarding results.
  
−
== Security Considerations ==
+
The second case concerns the behavior of services which traverse
 +
non-DS-capable domains.  We assume for the sake of argument that a
 +
non-DS-capable domain does not deploy traffic conditioning functions
 +
on domain boundary nodes; therefore, even in the event that the
 +
domain consists of legacy or DS-compliant interior nodes, the lack of
 +
traffic enforcement at the boundaries will limit the ability to
 +
consistently deliver some types of services across the domain.  A DS
 +
domain and a non-DS-capable domain may negotiate an agreement which
 +
governs how egress traffic from the DS-domain should be marked before
 +
entry into the non-DS-capable domain.  This agreement might be
 +
monitored for compliance by traffic sampling instead of by rigorous
 +
traffic conditioning.  Alternatively, where there is knowledge that
 +
the non-DS-capable domain consists of legacy nodes, the upstream DS
 +
domain may opportunistically re-mark differentiated services traffic
 +
to one or more of the Class Selector Codepoints.  Where there is no
 +
knowledge of the traffic management capabilities of the downstream
 +
domain, and no agreement in place, a DS domain egress node may choose
 +
to re-mark DS codepoints to zero, under the assumption that the non-
 +
DS-capable domain will treat the traffic uniformly with best-effort
 +
service.
  
−
The loss of transparency is both a bug and a feature from the
+
In the event that a non-DS-capable domain peers with a DS domain,
−
security viewpoint. To the extent that it prevents the end-to-end
+
traffic flowing from the non-DS-capable domain should be conditioned
−
deployment of IPSEC, it damages security and creates vulnerabilities.
+
at the DS ingress node of the DS domain according to the appropriate
−
For example, if a standard NAT box is in the path, then the best that
+
SLA or policy.
−
can be done is to decrypt and re-encrypt IP traffic in the NAT.  The
 
−
traffic will therefore be momentarily in clear text and potentially
 
−
vulnerable. Furthermore, the NAT will possess many keys and will be a
 
−
prime target of attack. In environments where this is unacceptable,
 
  
 +
== Multicast Considerations ==
  
−
encryption must be applied above the network layer instead, of course
+
Use of differentiated services by multicast traffic introduces a
−
with no usage whatever of IP addresses in data that is
+
number of issues for service provisioning.  First, multicast packets
−
cryptographically protected. See section 4 for further discussion.
+
which enter a DS domain at an ingress node may simultaneously take
 +
multiple paths through some segments of the domain due to multicast
 +
packet replication. In this way they consume more network resources
  
−
In certain scenarios, i.e. 5.1 (full IPv6) and 5.2.1.2 (RSIP), end-
+
than unicast packets.  Where multicast group membership is dynamic,
−
to-end IPSEC would become possible, especially using the "distributed
+
it is difficult to predict in advance the amount of network resources
−
firewalls" model advocated in [Bellovin].
+
that may be consumed by multicast traffic originating from an
 +
upstream network for a particular group. A consequence of this
 +
uncertainty is that it may be difficult to provide quantitative
 +
service guarantees to multicast senders. Further, it may be
 +
necessary to reserve codepoints and PHBs for exclusive use by unicast
 +
traffic, to provide resource isolation from multicast traffic.
  
−
The loss of transparency at the Intranet/Internet boundary may be
+
The second issue is the selection of the DS codepoint for a multicast
−
considered a security feature, since it provides a well defined point
+
packet arriving at a DS ingress node.  Because that packet may exit
−
at which to apply restrictions. This form of security is subject to
+
the DS domain at multiple DS egress nodes which peer with multiple
−
the "crunchy outside, soft inside" risk, whereby any successful
+
downstream domains, the DS codepoint used should not result in the
−
penetration of the boundary exposes the entire Intranet to trivial
+
request for a service from a downstream DS domain which is in
−
attack. The lack of end-to-end security applied within the Intranet
+
violation of a peering SLA.  When establishing classifier and traffic
−
also ignores insider threats.
+
conditioner state at an DS ingress node for an aggregate of traffic
 +
receiving a differentiated service which spans across the egress
 +
boundary of the domain, the identity of the adjacent downstream
 +
transit domain and the specifics of the corresponding peering SLA can
 +
be factored into the configuration decision (subject to routing
 +
policy and the stability of the routing infrastructure).  In this way
 +
peering SLAs with downstream DS domains can be partially enforced at
 +
the ingress of the upstream domain, reducing the classification and
 +
traffic conditioning burden at the egress node of the upstream
 +
domain.  This is not so easily performed in the case of multicast
 +
traffic, due to the possibility of dynamic group membership.  The
 +
result is that the service guarantees for unicast traffic may be
 +
impacted. One means of addressing this problem is to establish a
 +
separate peering SLA for multicast traffic, and to either utilize a
 +
particular set of codepoints for multicast packets, or to implement
 +
the necessary classification and traffic conditioning mechanisms in
 +
the DS egress nodes to provide preferential isolation for unicast
 +
traffic in conformance with the peering SLA with the downstream
 +
domain.
  
−
It should be noted that security applied above the transport level,
+
== Security and Tunneling Considerations ==
−
such as SSL, SSH, PGP or S/MIME, is not affected by network layer
 
−
transparency issues.
 
  
−
Acknowledgements
+
This section addresses security issues raised by the introduction of
 +
differentiated services, primarily the potential for denial-of-
 +
service attacks, and the related potential for theft of service by
 +
unauthorized traffic (Sec. 6.1).  In addition, the operation of
 +
differentiated services in the presence of IPsec and its interaction
 +
with IPsec are also discussed (Sec. 6.2), as well as auditing
 +
requirements (Sec. 6.3).  This section considers issues introduced by
 +
the use of both IPsec and non-IPsec tunnels.
  
−
This document and the related issues have been discussed extensively
+
=== Theft and Denial of Service ===
−
by the IAB. Special thanks to Steve Deering for a detailed review and
 
−
to Noel Chiappa. Useful comments or ideas were also received from Rob
 
−
Austein, John Bartas, Jim Bound, Scott Bradner, Vint Cerf, Spencer
 
−
Dawkins, Anoop Ghanwani, Erik Guttmann, Eric A. Hall, Graham Klyne,
 
−
Dan Kohn, Gabriel Montenegro, Thomas Narten, Erik Nordmark, Vern
 
−
Paxson, Michael Quinlan, Eric Rosen, Daniel Senie, Henning
 
−
Schulzrinne, Bill Sommerfeld, and George Tsirtsis.
 
  
−
References
+
The primary goal of differentiated services is to allow different
 +
levels of service to be provided for traffic streams on a common
 +
network infrastructure.  A variety of resource management techniques
 +
may be used to achieve this, but the end result will be that some
 +
packets receive different (e.g., better) service than others.  The
 +
mapping of network traffic to the specific behaviors that result in
 +
different (e.g., better or worse) service is indicated primarily by
 +
the DS field, and hence an adversary may be able to obtain better
 +
service by modifying the DS field to codepoints indicating behaviors
 +
used for enhanced services or by injecting packets with the DS field
 +
set to such codepoints.  Taken to its limits, this theft of service
 +
becomes a denial-of-service attack when the modified or injected
 +
traffic depletes the resources available to forward it and other
 +
traffic streams.  The defense against such theft- and denial-of-
 +
service attacks consists of the combination of traffic conditioning
 +
at DS boundary nodes along with security and integrity of the network
 +
infrastructure within a DS domain.
  
−
[Bellovin]    Distributed Firewalls, S. Bellovin, available at
+
As described in Sec. 2, DS ingress nodes must condition all traffic
−
              http://www.research.att.com/~smb/papers/distfw.pdf or
+
entering a DS domain to ensure that it has acceptable DS codepoints.
−
              http://www.research.att.com/~smb/papers/distfw.ps (work
+
This means that the codepoints must conform to the applicable TCA(s)
−
              in progress).
+
and the domain's service provisioning policy.  Hence, the ingress
 +
nodes are the primary line of defense against theft- and denial-of-
 +
service attacks based on modified DS codepoints (e.g., codepoints to
 +
which the traffic is not entitled), as success of any such attack
 +
constitutes a violation of the applicable TCA(s) and/or service
 +
provisioning policy. An important instance of an ingress node is
 +
that any traffic-originating node in a DS domain is the ingress node
 +
for that traffic, and must ensure that all originated traffic carries
 +
acceptable DS codepoints.
  
−
[Berners-Lee] Weaving the Web, T. Berners-Lee, M. Fischetti,
+
Both a domain's service provisioning policy and TCAs may require the
−
              HarperCollins, 1999, p 208.
+
ingress nodes to change the DS codepoint on some entering packets
 +
(e.g., an ingress router may set the DS codepoint of a customer's
 +
traffic in accordance with the appropriate SLA).  Ingress nodes must
 +
condition all other inbound traffic to ensure that the DS codepoints
 +
are acceptable; packets found to have unacceptable codepoints must
 +
either be discarded or must have their DS codepoints modified to
 +
acceptable values before being forwarded.  For example, an ingress
 +
node receiving traffic from a domain with which no enhanced service
 +
agreement exists may reset the DS codepoint to the Default PHB
 +
codepoint [DSFIELD].  Traffic authentication may be required to
 +
validate the use of some DS codepoints (e.g., those corresponding to
 +
enhanced services), and such authentication may be performed by
 +
technical means (e.g., IPsec) and/or non-technical means (e.g., the
 +
inbound link is known to be connected to exactly one customer site).
  
−
[Saltzer]    End-To-End Arguments in System Design, J.H. Saltzer,
+
An inter-domain agreement may reduce or eliminate the need for
−
              D.P.Reed, D.D.Clark, ACM TOCS, Vol 2, Number 4,
+
ingress node traffic conditioning by making the upstream domain
−
              November 1984, pp 277-288.
+
partly or completely responsible for ensuring that traffic has DS
 +
codepoints acceptable to the downstream domain. In this case, the
 +
ingress node may still perform redundant traffic conditioning checks
 +
to reduce the dependence on the upstream domain (e.g., such checks
 +
can prevent theft-of-service attacks from propagating across the
 +
domain boundary). If such a check fails because the upstream domain
 +
is not fulfilling its responsibilities, that failure is an auditable
 +
event; the generated audit log entry should include the date/time the
 +
packet was received, the source and destination IP addresses, and the
 +
DS codepoint that caused the failure.  In practice, the limited gains
 +
from such checks need to be weighed against their potential
 +
performance impact in determining what, if any, checks to perform
 +
under these circumstances.
  
−
[IEN 48]      Cerf, V., "The Catenet Model for Internetworking,"
+
Interior nodes in a DS domain may rely on the DS field to associate
−
              Information Processing Techniques Office, Defense
+
differentiated services traffic with the behaviors used to implement
−
              Advanced Research Projects Agency, IEN 48, July 1978.
+
enhanced services.  Any node doing so depends on the correct
 +
operation of the DS domain to prevent the arrival of traffic with
 +
unacceptable DS codepoints.  Robustness concerns dictate that the
 +
arrival of packets with unacceptable DS codepoints must not cause the
 +
failure (e.g., crash) of network nodes.  Interior nodes are not
 +
responsible for enforcing the service provisioning policy (or
 +
individual SLAs) and hence are not required to check DS codepoints
 +
before using them.  Interior nodes may perform some traffic
 +
conditioning checks on DS codepoints (e.g., check for DS codepoints
 +
that are never used for traffic on a specific link) to improve
 +
security and robustness (e.g., resistance to theft-of-service attacks
 +
based on DS codepoint modifications).  Any detected failure of such a
 +
check is an auditable event and the generated audit log entry should
 +
include the date/time the packet was received, the source and
 +
destination IP addresses, and the DS codepoint that caused the
 +
failure.  In practice, the limited gains from such checks need to be
 +
weighed against their potential performance impact in determining
 +
what, if any, checks to perform at interior nodes.
  
 +
Any link that cannot be adequately secured against modification of DS
 +
codepoints or traffic injection by adversaries should be treated as a
 +
boundary link (and hence any arriving traffic on that link is treated
 +
as if it were entering the domain at an ingress node).  Local
 +
security policy provides the definition of "adequately secured," and
 +
such a definition may include a determination that the risks and
 +
consequences of DS codepoint modification and/or traffic injection do
 +
not justify any additional security measures for a link.  Link
 +
security can be enhanced via physical access controls and/or software
 +
means such as tunnels that ensure packet integrity.
  
−
[CATENET]    Pouzin, L., "A Proposal for Interconnecting Packet
+
=== IPsec and Tunneling Interactions ===
−
              Switching Networks," Proceedings of EUROCOMP, Brunel
 
−
              University, May 1974, pp. 1023-36.
 
  
−
[STD 7]       Postel, J., "Transmission Control Protocol", STD 7, RFC
+
The IPsec protocol, as defined in [ESP, AH], does not include the IP
−
              793, September 1981.
+
header's DS field in any of its cryptographic calculations (in the
 +
case of tunnel mode, it is the outer IP header's DS field that is not
 +
included).  Hence modification of the DS field by a network node has
 +
no effect on IPsec's end-to-end security, because it cannot cause any
 +
IPsec integrity check to fail.  As a consequence, IPsec does not
 +
provide any defense against an adversary's modification of the DS
 +
field (i.e., a man-in-the-middle attack), as the adversary's
 +
modification will also have no effect on IPsec's end-to-end security.
 +
In some environments, the ability to modify the DS field without
 +
affecting IPsec integrity checks may constitute a covert channel; if
 +
it is necessary to eliminate such a channel or reduce its bandwidth,
 +
the DS domains should be configured so that the required processing
 +
(e.g., set all DS fields on sensitive traffic to a single value) can
 +
be performed at DS egress nodes where traffic exits higher security
 +
domains.
  
−
[[[RFC1546|RFC 1546]]]    Partridge, C., Mendez, T. and W. Milliken, "Host
+
IPsec's tunnel mode provides security for the encapsulated IP
−
              Anycasting Service", [[RFC1546|RFC 1546]], November 1993.
+
header's DS field.  A tunnel mode IPsec packet contains two IP
 +
headers: an outer header supplied by the tunnel ingress node and an
 +
encapsulated inner header supplied by the original source of the
 +
packet.  When an IPsec tunnel is hosted (in whole or in part) on a
 +
differentiated services network, the intermediate network nodes
 +
operate on the DS field in the outer header.  At the tunnel egress
 +
node, IPsec processing includes stripping the outer header and
 +
forwarding the packet (if required) using the inner header.     If
 +
the inner IP header has not been processed by a DS ingress node for
 +
the tunnel egress node's DS domain, the tunnel egress node is the DS
 +
ingress node for traffic exiting the tunnel, and hence must carry out
 +
the corresponding traffic conditioning responsibilities (see Sec.
 +
6.1).  If the IPsec processing includes a sufficiently strong
 +
cryptographic integrity check of the encapsulated packet (where
 +
sufficiency is determined by local security policy), the tunnel
 +
egress node can safely assume that the DS field in the inner header
 +
has the same value as it had at the tunnel ingress node.  This allows
 +
a tunnel egress node in the same DS domain as the tunnel ingress
 +
node, to safely treat a packet passing such an integrity check as if
 +
it had arrived from another node within the same DS domain, omitting
 +
the DS ingress node traffic conditioning that would otherwise be
 +
required.  An important consequence is that otherwise insecure links
 +
internal to a DS domain can be secured by a sufficiently strong IPsec
 +
tunnel.
  
−
[[[RFC1597|RFC 1597]]]    Rekhter, Y., Moskowitz, B., Karrenberg, D. and G. de
+
This analysis and its implications apply to any tunneling protocol
−
              Groot, "Address Allocation for Private Internets", RFC
+
that performs integrity checks, but the level of assurance of the
−
              1597, March 1994.
+
inner header's DS field depends on the strength of the integrity
  
−
[[[RFC1633|RFC 1633]]]    Braden, R., Clark, D. and S. Shenker, "Integrated
+
check performed by the tunneling protocol. In the absence of
−
              Services in the Internet Architecture: an Overview",
+
sufficient assurance for a tunnel that may transit nodes outside the
−
              [[RFC1633|RFC 1633]], June 1994.
+
current DS domain (or is otherwise vulnerable), the encapsulated
 +
packet must be treated as if it had arrived at a DS ingress node from
 +
outside the domain.
  
−
[[[RFC1889|RFC 1889]]]    Schulzrinne, H., Casner, S., Frederick, R. and V.
+
The IPsec protocol currently requires that the inner header's DS
−
              Jacobson, "RTP: A Transport Protocol for Real-Time
+
field not be changed by IPsec decapsulation processing at a tunnel
−
              Applications", [[RFC1889|RFC 1889]], January 1996.
+
egress node. This ensures that an adversary's modifications to the
 +
DS field cannot be used to launch theft- or denial-of-service attacks
 +
across an IPsec tunnel endpoint, as any such modifications will be
 +
discarded at the tunnel endpoint.  This document makes no change to
 +
that IPsec requirement.
  
−
[[[BCP5|BCP 5]]]      Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
+
If the IPsec specifications are modified in the future to permit a
−
              G. and E. Lear, "Address Allocation for Private
+
tunnel egress node to modify the DS field in an inner IP header based
−
              Internets", [[BCP5|BCP 5]], [[RFC1918|RFC 1918]], February 1996.
+
on the DS field value in the outer header (e.g., copying part or all
 +
of the outer DS field to the inner DS field), then additional
 +
considerations would apply. For a tunnel contained entirely within a
 +
single DS domain and for which the links are adequately secured
 +
against modifications of the outer DS field, the only limits on inner
 +
DS field modifications would be those imposed by the domain's service
 +
provisioning policy. Otherwise, the tunnel egress node performing
 +
such modifications would be acting as a DS ingress node for traffic
 +
exiting the tunnel and must carry out the traffic conditioning
 +
responsibilities of an ingress node, including defense against theft-
 +
and denial-of-service attacks (See Sec. 6.1). If the tunnel enters
 +
the DS domain at a node different from the tunnel egress node, the
 +
tunnel egress node may depend on the upstream DS ingress node having
 +
ensured that the outer DS field values are acceptable.  Even in this
 +
case, there are some checks that can only be performed by the tunnel
 +
egress node (e.g., a consistency check between the inner and outer DS
 +
codepoints for an encrypted tunnel).  Any detected failure of such a
 +
check is an auditable event and the generated audit log entry should
 +
include the date/time the packet was received, the source and
 +
destination IP addresses, and the DS codepoint that was unacceptable.
  
−
[[[RFC1928|RFC 1928]]]    Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D.
+
An IPsec tunnel can be viewed in at least two different ways from an
−
              and L. Jones, "SOCKS Protocol Version 5", [[RFC1928|RFC 1928]],
+
architectural perspective. If the tunnel is viewed as a logical
−
              March 1996.
+
single hop "virtual wire", the actions of intermediate nodes in
 +
forwarding the tunneled traffic should not be visible beyond the ends
 +
of the tunnel and hence the DS field should not be modified as part
 +
of decapsulation processing. In contrast, if the tunnel is viewed as
 +
a multi-hop participant in forwarding traffic, then modification of
 +
the DS field as part of tunnel decapsulation processing may be
 +
desirable. A specific example of the latter situation occurs when a
 +
tunnel terminates at an interior node of a DS domain at which the
 +
domain administrator does not wish to deploy traffic conditioning
  
−
[[[RFC1958|RFC 1958]]]    Carpenter, B., "Architectural Principles of the
+
logic (e.g., to simplify traffic management). This could be
−
              Internet", [[RFC1958|RFC 1958]], June 1996.
+
supported by using the DS codepoint in the outer IP header (which was
 +
subject to traffic conditioning at the DS ingress node) to reset the
 +
DS codepoint in the inner IP header, effectively moving DS ingress
 +
traffic conditioning responsibilities from the IPsec tunnel egress
 +
node to the appropriate upstream DS ingress node (which must already
 +
perform that function for unencapsulated traffic).
  
−
[[[RFC2018|RFC 2018]]]    Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
+
=== Auditing ===
−
              Selective Acknowledgement Options", [[RFC2018|RFC 2018]], October
 
−
              1996.
 
  
−
[[[RFC2052|RFC 2052]]]    Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
+
Not all systems that support differentiated services will implement
−
              the location of services (DNS SRV)", [[RFC2052|RFC 2052]], October
+
auditing.  However, if differentiated services support is
−
              1996.
+
incorporated into a system that supports auditing, then the
 +
differentiated services implementation should also support auditing.
 +
If such support is present the implementation must allow a system
 +
administrator to enable or disable auditing for differentiated
 +
services as a whole, and may allow such auditing to be enabled or
 +
disabled in part.
  
−
[[[RFC2101|RFC 2101]]]    Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4
+
For the most part, the granularity of auditing is a local matter.
−
              Address Behaviour Today", [[RFC2101|RFC 2101]], February 1997.
+
However, several auditable events are identified in this document and
 +
for each of these events a minimum set of information that should be
 +
included in an audit log is defined.  Additional information (e.g.,
 +
packets related to the one that triggered the auditable event) may
 +
also be included in the audit log for each of these events, and
 +
additional events, not explicitly called out in this specification,
 +
also may result in audit log entries.  There is no requirement for
 +
the receiver to transmit any message to the purported sender in
 +
response to the detection of an auditable event, because of the
 +
potential to induce denial of service via such action.
  
−
[[[RFC2210|RFC 2210]]]    Wroclawski, J., "The Use of RSVP with IETF Integrated
+
== Acknowledgements ==
−
              Services", [[RFC2210|RFC 2210]], September 1997.
 
  
 +
This document has benefitted from earlier drafts by Steven Blake,
 +
David Clark, Ed Ellesson, Paul Ferguson, Juha Heinanen, Van Jacobson,
 +
Kalevi Kilkki, Kathleen Nichols, Walter Weiss, John Wroclawski, and
 +
Lixia Zhang.
  
−
[[[RFC2309|RFC 2309]]]    Braden, B., Clark, D., Crowcroft, J., Davie, B.,
+
The authors would like to acknowledge the following individuals for
−
              Deering, S., Estrin, D., Floyd, S., Jacobson, V.,
+
their helpful comments and suggestions: Kathleen Nichols, Brian
−
              Minshall, G., Partridge, C., Peterson, L.,
+
Carpenter, Konstantinos Dovrolis, Shivkumar Kalyana, Wu-chang Feng,
−
              Ramakrishnan, K., Shenker, S., Wroclawski, J. and L.
+
Marty Borden, Yoram Bernet, Ronald Bonica, James Binder, Borje
−
              Zhang, "Recommendations on Queue Management and
+
Ohlman, Alessio Casati, Scott Brim, Curtis Villamizar, Hamid Ould-
−
              Congestion Avoidance in the Internet", [[RFC2309|RFC 2309]], April
+
Brahi, Andrew Smith, John Renwick, Werner Almesberger, Alan O'Neill,
−
              1998.
+
James Fu, and Bob Braden.
  
−
[[[RFC2326|RFC 2326]]]    Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time
+
== References ==
−
              Streaming Protocol (RTSP)", [[RFC2326|RFC 2326]], April 1998.
 
  
−
[[[RFC2401|RFC 2401]]]    Kent, S. and R. Atkinson, "Security Architecture for
+
[802.1p]    ISO/IEC Final CD 15802-3 Information technology - Tele-
−
              the Internet Protocol", [[RFC2401|RFC 2401]], November 1998.
+
            communications and information exchange between systems -
 +
            Local and metropolitan area networks - Common
 +
            specifications - Part 3: Media Access Control (MAC)
 +
            bridges, (current draft available as IEEE P802.1D/D15).
  
−
[[[RFC2475|RFC 2475]]]    Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
+
[AH]       Kent, S. and R. Atkinson, "IP Authentication Header", RFC
−
              and W. Weiss, "An Architecture for Differentiated
+
            2402, November 1998.
−
              Service", [[RFC2475|RFC 2475]], December 1998.
 
  
−
[[[RFC2581|RFC 2581]]]    Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
+
[ATM]       ATM Traffic Management Specification Version 4.0 <af-tm-
−
              Control", [[RFC2581|RFC 2581]], April 1999.
+
            0056.000>, ATM Forum, April 1996.
  
−
[NAT-ARCH]    Hain, T., "Architectural Implications of NAT", Work in
+
[Bernet]    Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang, K.
−
              Progress.
+
            Nichols, and M. Speer, "A Framework for Use of RSVP with
 +
            Diff-serv Networks", Work in Progress.
  
−
[NAT-PROT]   Holdrege, M. and P. Srisuresh, "Protocol Complications
+
[DSFIELD]   Nichols, K., Blake, S., Baker, F. and D. Black,
−
              with the IP Network Address Translator (NAT)", Work in
+
            "Definition of the Differentiated Services Field (DS
−
              Progress.
+
            Field) in the IPv4 and IPv6 Headers", [[RFC2474|RFC 2474]], December
 +
            1998.
  
−
[SECMECH]     Bellovin, S., "Security Mechanisms for the Internet",
+
[EXPLICIT] D. Clark and W. Fang, "Explicit Allocation of Best Effort
−
              Work in Progress.
+
            Packet Delivery Service", IEEE/ACM Trans. on Networking,
 +
            vol. 6, no. 4, August 1998, pp. 362-373.
  
−
[RSIP]       Lo, J., Borella, M. and D. Grabelsky, "Realm Specific
+
[ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security
−
              IP: A Framework", Work in Progress.
+
            Payload (ESP)", [[RFC2406|RFC 2406]], November 1998.
  
−
[HIP]         Moskowitz, R., "The Host Identity Payload", Work in
+
[FRELAY]   ANSI T1S1, "DSSI Core Aspects of Frame Rely", March 1990.
−
              Progress.
 
  
 +
[[RFC791]]    Postel, J., Editor, "Internet Protocol", [[STD5|STD 5]], [[RFC791|RFC 791]],
 +
            September 1981.
  
−
Author's Address
+
[[RFC1349]]  Almquist, P., "Type of Service in the Internet Protocol
 +
            Suite", [[RFC1349|RFC 1349]], July 1992.
  
−
Brian E. Carpenter
+
[[RFC1633]]  Braden, R., Clark, D. and S. Shenker, "Integrated
−
IBM
+
            Services in the Internet Architecture: An Overview", RFC
−
c/o iCAIR
+
            1633, July 1994.
−
Suite 150
 
−
1890 Maple Avenue
 
−
Evanston, IL 60201
 
−
USA
 
  
−
EMail: brian@icair.org
+
[[RFC1812]]  Baker, F., Editor, "Requirements for IP Version 4
 +
            Routers", [[RFC1812|RFC 1812]], June 1995.
  
 +
[RSVP]      Braden, B., Zhang, L., Berson S., Herzog, S. and S.
 +
            Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
 +
            Functional Specification", [[RFC2205|RFC 2205]], September 1997.
 +
 +
[2BIT]      K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit
 +
            Differentiated Services Architecture for the Internet",
 +
            ftp://ftp.ee.lbl.gov/papers/dsarch.pdf, November 1997.
 +
 +
[TR]        ISO/IEC 8802-5 Information technology -
 +
            Telecommunications and information exchange between
 +
            systems - Local and metropolitan area networks - Common
 +
            specifications - Part 5: Token Ring Access Method and
 +
            Physical Layer Specifications, (also ANSI/IEEE Std 802.5-
 +
            1995), 1995.
 +
 +
Authors' Addresses
 +
 +
Steven Blake
 +
Torrent Networking Technologies
 +
3000 Aerial Center, Suite 140
 +
Morrisville, NC  27560
 +
 +
Phone:  +1-919-468-8466 x232
 +
 +
 +
David L. Black
 +
EMC Corporation
 +
35 Parkwood Drive
 +
Hopkinton, MA  01748
 +
 +
Phone:  +1-508-435-1000 x76140
 +
 +
 +
Mark A. Carlson
 +
Sun Microsystems, Inc.
 +
2990 Center Green Court South
 +
Boulder, CO  80301
 +
 +
Phone:  +1-303-448-0048 x115
 +
 +
 +
Elwyn Davies
 +
Nortel UK
 +
London Road
 +
Harlow, Essex  CM17 9NA, UK
 +
 +
Phone:  +44-1279-405498
 +
 +
 +
Zheng Wang
 +
Bell Labs Lucent Technologies
 +
101 Crawfords Corner Road
 +
Holmdel, NJ  07733
 +
 +
 +
 +
Walter Weiss
 +
Lucent Technologies
 +
300 Baker Avenue, Suite 100
 +
Concord, MA  01742-2168
 +
 +
  
 
Full Copyright Statement
 
Full Copyright Statement
  
−
Copyright (C) The Internet Society (2000).  All Rights Reserved.
+
Copyright (C) The Internet Society (1998).  All Rights Reserved.
  
 
This document and translations of it may be copied and furnished to
 
This document and translations of it may be copied and furnished to
Line 768: Line 1,625:
 
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
  
−
Acknowledgement
+
[[Category:Informational]]
−
 
 
−
Funding for the RFC Editor function is currently provided by the
 
−
Internet Society.
 
−
 
 
−
[[Category:Informational|2775]]
 
−
[[Category:Network Working Group|2775]]
 

Latest revision as of 22:18, 19 October 2020

Network Working Group S. Blake Request for Comments: 2475 Torrent Networking Technologies Category: Informational D. Black 

                                                  EMC Corporation
                                                       M. Carlson
                                                 Sun Microsystems
                                                        E. Davies
                                                        Nortel UK
                                                          Z. Wang
                                    Bell Labs Lucent Technologies
                                                         W. Weiss
                                              Lucent Technologies
                                                    December 1998

          An Architecture for Differentiated Services

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This document defines an architecture for implementing scalable service differentiation in the Internet. This architecture achieves scalability by aggregating traffic classification state which is conveyed by means of IP-layer packet marking using the DS field [DSFIELD]. Packets are classified and marked to receive a particular per-hop forwarding behavior on nodes along their path. Sophisticated classification, marking, policing, and shaping operations need only be implemented at network boundaries or hosts. Network resources are allocated to traffic streams by service provisioning policies which govern how traffic is marked and conditioned upon entry to a differentiated services-capable network, and how that traffic is forwarded within that network. A wide variety of services can be implemented on top of these building blocks. 

   2.3.4  Location of Traffic Conditioners and MF Classifiers ... 17

4. Interoperability with Non-Differentiated Services-Compliant

Introduction

Overview

This document defines an architecture for implementing scalable service differentiation in the Internet. A "Service" defines some significant characteristics of packet transmission in one direction across a set of one or more paths within a network. These

characteristics may be specified in quantitative or statistical terms of throughput, delay, jitter, and/or loss, or may otherwise be specified in terms of some relative priority of access to network resources. Service differentiation is desired to accommodate heterogeneous application requirements and user expectations, and to permit differentiated pricing of Internet service.

This architecture is composed of a number of functional elements implemented in network nodes, including a small set of per-hop forwarding behaviors, packet classification functions, and traffic conditioning functions including metering, marking, shaping, and policing. This architecture achieves scalability by implementing complex classification and conditioning functions only at network boundary nodes, and by applying per-hop behaviors to aggregates of traffic which have been appropriately marked using the DS field in the IPv4 or IPv6 headers [DSFIELD]. Per-hop behaviors are defined to permit a reasonably granular means of allocating buffer and bandwidth resources at each node among competing traffic streams. Per- application flow or per-customer forwarding state need not be maintained within the core of the network. A distinction is maintained between:

o the service provided to a traffic aggregate,

o the conditioning functions and per-hop behaviors used to realize 

  services,

o the DS field value (DS codepoint) used to mark packets to select a 

  per-hop behavior, and

o the particular node implementation mechanisms which realize a 

  per-hop behavior.

Service provisioning and traffic conditioning policies are sufficiently decoupled from the forwarding behaviors within the network interior to permit implementation of a wide variety of service behaviors, with room for future expansion.

This architecture only provides service differentiation in one direction of traffic flow and is therefore asymmetric. Development of a complementary symmetric architecture is a topic of current research but is outside the scope of this document; see for example [EXPLICIT].

Sect. 1.2 is a glossary of terms used within this document. Sec. 1.3 lists requirements addressed by this architecture, and Sec. 1.4 provides a brief comparison to other approaches for service differentiation. Sec. 2 discusses the components of the architecture

in detail. Sec. 3 proposes guidelines for per-hop behavior specifications. Sec. 4 discusses interoperability issues with nodes and networks which do not implement differentiated services as defined in this document and in [DSFIELD]. Sec. 5 discusses issues with multicast service delivery. Sec. 6 addresses security and tunnel considerations.

Terminology

This section gives a general conceptual overview of the terms used in this document. Some of these terms are more precisely defined in later sections of this document.

Behavior Aggregate (BA) a DS behavior aggregate.

BA classifier a classifier that selects packets based 

                         only on the contents of the DS field.

Boundary link a link connecting the edge nodes of two 

                         domains.

Classifier an entity which selects packets based on 

                         the content of packet headers according to
                         defined rules.

DS behavior aggregate a collection of packets with the same DS 

                         codepoint crossing a link in a particular
                         direction.

DS boundary node a DS node that connects one DS domain to a 

                         node either in another DS domain or in a
                         domain that is not DS-capable.

DS-capable capable of implementing differentiated 

                         services as described in this architecture;
                         usually used in reference to a domain
                         consisting of DS-compliant nodes.

DS codepoint a specific value of the DSCP portion of the 

                         DS field, used to select a PHB.

DS-compliant enabled to support differentiated services 

                         functions and behaviors as defined in
                         [DSFIELD], this document, and other
                         differentiated services documents; usually
                         used in reference to a node or device.

DS domain a DS-capable domain; a contiguous set of 

                         nodes which operate with a common set of
                         service provisioning policies and PHB
                         definitions.

DS egress node a DS boundary node in its role in handling 

                         traffic as it leaves a DS domain.

DS ingress node a DS boundary node in its role in handling 

                         traffic as it enters a DS domain.

DS interior node a DS node that is not a DS boundary node.

DS field the IPv4 header TOS octet or the IPv6 

                         Traffic Class octet when interpreted in
                         conformance with the definition given in
                         [DSFIELD].  The bits of the DSCP field
                         encode the DS codepoint, while the
                         remaining bits are currently unused.

DS node a DS-compliant node.

DS region a set of contiguous DS domains which can 

                         offer differentiated services over paths
                         across those DS domains.

Downstream DS domain the DS domain downstream of traffic flow on 

                         a boundary link.

Dropper a device that performs dropping.

Dropping the process of discarding packets based on 

                         specified rules; policing.

Legacy node a node which implements IPv4 Precedence as 

                         defined in [RFC791,RFC1812] but which is
                         otherwise not DS-compliant.

Marker a device that performs marking.

Marking the process of setting the DS codepoint in 

                         a packet based on defined rules; pre-
                         marking, re-marking.

Mechanism a specific algorithm or operation (e.g., 

                         queueing discipline) that is implemented in
                         a node to realize a set of one or more per-
                         hop behaviors.

Meter a device that performs metering.

Metering the process of measuring the temporal 

                         properties (e.g., rate) of a traffic stream
                         selected by a classifier.  The
                         instantaneous state of this process may be
                         used to affect the operation of a marker,
                         shaper, or dropper, and/or may be used for
                         accounting and measurement purposes.

Microflow a single instance of an application-to- 

                         application flow of packets which is
                         identified by source address, source port,
                         destination address, destination port and
                         protocol id.

MF Classifier a multi-field (MF) classifier which selects 

                         packets based on the content of some
                         arbitrary number of header fields;
                         typically some combination of source
                         address, destination address, DS field,
                         protocol ID, source port and destination
                         port.

Per-Hop-Behavior (PHB) the externally observable forwarding 

                         behavior applied at a DS-compliant node to
                         a DS behavior aggregate.

PHB group a set of one or more PHBs that can only be 

                         meaningfully specified and implemented
                         simultaneously, due to a common constraint
                         applying to all PHBs in the set such as a
                         queue servicing or queue management policy.
                         A PHB group provides a service building
                         block that allows a set of related
                         forwarding behaviors to be specified
                         together (e.g., four dropping priorities).
                         A single PHB is a special case of a PHB
                         group.

Policing the process of discarding packets (by a 

                         dropper) within a traffic stream in
                         accordance with the state of a
                         corresponding meter enforcing a traffic
                         profile.

Pre-mark to set the DS codepoint of a packet prior 

                         to entry into a downstream DS domain.

Provider DS domain the DS-capable provider of services to a 

                         source domain.

Re-mark to change the DS codepoint of a packet, 

                         usually performed by a marker in accordance
                         with a TCA.

Service the overall treatment of a defined subset 

                         of a customer's traffic within a DS domain
                         or end-to-end.

Service Level Agreement a service contract between a customer and a (SLA) service provider that specifies the 

                         forwarding service a customer should
                         receive.  A customer may be a user
                         organization (source domain) or another DS
                         domain (upstream domain).  A SLA may
                         include traffic conditioning rules which
                         constitute a TCA in whole or in part.

Service Provisioning a policy which defines how traffic Policy conditioners are configured on DS boundary 

                         nodes and how traffic streams are mapped to
                         DS behavior aggregates to achieve a range
                         of services.

Shaper a device that performs shaping.

Shaping the process of delaying packets within a 

                         traffic stream to cause it to conform to
                         some defined traffic profile.

Source domain a domain which contains the node(s) 

                         originating the traffic receiving a
                         particular service.

Traffic conditioner an entity which performs traffic 

                         conditioning functions and which may
                         contain meters, markers, droppers, and
                         shapers. Traffic conditioners are typically
                         deployed in DS boundary nodes only.  A
                         traffic conditioner may re-mark a traffic
                         stream or may discard or shape packets to
                         alter the temporal characteristics of the
                         stream and bring it into compliance with a
                         traffic profile.

Traffic conditioning control functions performed to enforce 

                         rules specified in a TCA, including
                         metering, marking, shaping, and policing.

Traffic Conditioning an agreement specifying classifier rules Agreement (TCA) and any corresponding traffic profiles and 

                         metering, marking, discarding and/or
                         shaping rules which are to apply to the
                         traffic streams selected by the classifier.
                         A TCA encompasses all of the traffic
                         conditioning rules explicitly specified
                         within a SLA along with all of the rules
                         implicit from the relevant service
                         requirements and/or from a DS domain's
                         service provisioning policy.

Traffic profile a description of the temporal properties 

                         of a traffic stream such as rate and burst
                         size.

Traffic stream an administratively significant set of one 

                         or more microflows which traverse a path
                         segment.  A traffic stream may consist of
                         the set of active microflows which are
                         selected by a particular classifier.

Upstream DS domain the DS domain upstream of traffic flow on a 

                         boundary link.

Requirements

The history of the Internet has been one of continuous growth in the number of hosts, the number and variety of applications, and the capacity of the network infrastructure, and this growth is expected to continue for the foreseeable future. A scalable architecture for service differentiation must be able to accommodate this continued growth.

The following requirements were identified and are addressed in this architecture:

o should accommodate a wide variety of services and provisioning 

  policies, extending end-to-end or within a particular (set of)
  network(s),

o should allow decoupling of the service from the particular 

  application in use,

o should work with existing applications without the need for 

  application programming interface changes or host software
  modifications (assuming suitable deployment of classifiers,
  markers, and other traffic conditioning functions),

o should decouple traffic conditioning and service provisioning 

  functions from forwarding behaviors implemented within the core
  network nodes,

o should not depend on hop-by-hop application signaling,

o should require only a small set of forwarding behaviors whose 

  implementation complexity does not dominate the cost of a network
  device, and which will not introduce bottlenecks for future high-
  speed system implementations,

o should avoid per-microflow or per-customer state within core 

  network nodes,

o should utilize only aggregated classification state within the 

  network core,

o should permit simple packet classification implementations in core 

  network nodes (BA classifier),

o should permit reasonable interoperability with non-DS-compliant 

  network nodes,

o should accommodate incremental deployment.

Comparisons with Other Approaches

The differentiated services architecture specified in this document can be contrasted with other existing models of service differentiation. We classify these alternative models into the following categories: relative priority marking, service marking, label switching, Integrated Services/RSVP, and static per-hop classification.

Examples of the relative priority marking model include IPv4 Precedence marking as defined in RFC791, 802.5 Token Ring priority [TR], and the default interpretation of 802.1p traffic classes [802.1p]. In this model the application, host, or proxy node selects a relative priority or "precedence" for a packet (e.g., delay or discard priority), and the network nodes along the transit path apply the appropriate priority forwarding behavior corresponding to the priority value within the packet's header. Our architecture can be considered as a refinement to this model, since we more clearly

specify the role and importance of boundary nodes and traffic conditioners, and since our per-hop behavior model permits more general forwarding behaviors than relative delay or discard priority.

An example of a service marking model is IPv4 TOS as defined in RFC1349. In this example each packet is marked with a request for a "type of service", which may include "minimize delay", "maximize throughput", "maximize reliability", or "minimize cost". Network nodes may select routing paths or forwarding behaviors which are suitably engineered to satisfy the service request. This model is subtly different from our architecture. Note that we do not describe the use of the DS field as an input to route selection. The TOS markings defined in RFC1349 are very generic and do not span the range of possible service semantics. Furthermore, the service request is associated with each individual packet, whereas some service semantics may depend on the aggregate forwarding behavior of a sequence of packets. The service marking model does not easily accommodate growth in the number and range of future services (since the codepoint space is small) and involves configuration of the "TOS->forwarding behavior" association in each core network node. Standardizing service markings implies standardizing service offerings, which is outside the scope of the IETF. Note that provisions are made in the allocation of the DS codepoint space to allow for locally significant codepoints which may be used by a provider to support service marking semantics [DSFIELD].

Examples of the label switching (or virtual circuit) model include Frame Relay, ATM, and MPLS [FRELAY, ATM]. In this model path forwarding state and traffic management or QoS state is established for traffic streams on each hop along a network path. Traffic aggregates of varying granularity are associated with a label switched path at an ingress node, and packets/cells within each label switched path are marked with a forwarding label that is used to lookup the next-hop node, the per-hop forwarding behavior, and the replacement label at each hop. This model permits finer granularity resource allocation to traffic streams, since label values are not globally significant but are only significant on a single link; therefore resources can be reserved for the aggregate of packets/ cells received on a link with a particular label, and the label switching semantics govern the next-hop selection, allowing a traffic stream to follow a specially engineered path through the network. This improved granularity comes at the cost of additional management and configuration requirements to establish and maintain the label switched paths. In addition, the amount of forwarding state maintained at each node scales in proportion to the number of edge nodes of the network in the best case (assuming multipoint-to-point

label switched paths), and it scales in proportion with the square of the number of edge nodes in the worst case, when edge-edge label switched paths with provisioned resources are employed.

The Integrated Services/RSVP model relies upon traditional datagram forwarding in the default case, but allows sources and receivers to exchange signaling messages which establish additional packet classification and forwarding state on each node along the path between them [RFC1633, RSVP]. In the absence of state aggregation, the amount of state on each node scales in proportion to the number of concurrent reservations, which can be potentially large on high- speed links. This model also requires application support for the RSVP signaling protocol. Differentiated services mechanisms can be utilized to aggregate Integrated Services/RSVP state in the core of the network [Bernet].

A variant of the Integrated Services/RSVP model eliminates the requirement for hop-by-hop signaling by utilizing only "static" classification and forwarding policies which are implemented in each node along a network path. These policies are updated on administrative timescales and not in response to the instantaneous mix of microflows active in the network. The state requirements for this variant are potentially worse than those encountered when RSVP is used, especially in backbone nodes, since the number of static policies that might be applicable at a node over time may be larger than the number of active sender-receiver sessions that might have installed reservation state on a node. Although the support of large numbers of classifier rules and forwarding policies may be computationally feasible, the management burden associated with installing and maintaining these rules on each node within a backbone network which might be traversed by a traffic stream is substantial.

Although we contrast our architecture with these alternative models of service differentiation, it should be noted that links and nodes employing these techniques may be utilized to extend differentiated services behaviors and semantics across a layer-2 switched infrastructure (e.g., 802.1p LANs, Frame Relay/ATM backbones) interconnecting DS nodes, and in the case of MPLS may be used as an alternative intra-domain implementation technology. The constraints imposed by the use of a specific link-layer technology in particular regions of a DS domain (or in a network providing access to DS domains) may imply the differentiation of traffic on a coarser grain basis. Depending on the mapping of PHBs to different link-layer services and the way in which packets are scheduled over a restricted set of priority classes (or virtual circuits of different category and capacity), all or a subset of the PHBs in use may be supportable (or may be indistinguishable).

Differentiated Services Architectural Model

The differentiated services architecture is based on a simple model where traffic entering a network is classified and possibly conditioned at the boundaries of the network, and assigned to different behavior aggregates. Each behavior aggregate is identified by a single DS codepoint. Within the core of the network, packets are forwarded according to the per-hop behavior associated with the DS codepoint. In this section, we discuss the key components within a differentiated services region, traffic classification and conditioning functions, and how differentiated services are achieved through the combination of traffic conditioning and PHB-based forwarding.

Differentiated Services Domain

A DS domain is a contiguous set of DS nodes which operate with a common service provisioning policy and set of PHB groups implemented on each node. A DS domain has a well-defined boundary consisting of DS boundary nodes which classify and possibly condition ingress traffic to ensure that packets which transit the domain are appropriately marked to select a PHB from one of the PHB groups supported within the domain. Nodes within the DS domain select the forwarding behavior for packets based on their DS codepoint, mapping that value to one of the supported PHBs using either the recommended codepoint->PHB mapping or a locally customized mapping [DSFIELD]. Inclusion of non-DS-compliant nodes within a DS domain may result in unpredictable performance and may impede the ability to satisfy service level agreements (SLAs).

A DS domain normally consists of one or more networks under the same administration; for example, an organization's intranet or an ISP. The administration of the domain is responsible for ensuring that adequate resources are provisioned and/or reserved to support the SLAs offered by the domain.

DS Boundary Nodes and Interior Nodes

A DS domain consists of DS boundary nodes and DS interior nodes. DS boundary nodes interconnect the DS domain to other DS or non-DS- capable domains, whilst DS interior nodes only connect to other DS interior or boundary nodes within the same DS domain.

Both DS boundary nodes and interior nodes must be able to apply the appropriate PHB to packets based on the DS codepoint; otherwise unpredictable behavior may result. In addition, DS boundary nodes may be required to perform traffic conditioning functions as defined by a traffic conditioning agreement (TCA) between their DS domain and

the peering domain which they connect to (see Sec. 2.3.3).

Interior nodes may be able to perform limited traffic conditioning functions such as DS codepoint re-marking. Interior nodes which implement more complex classification and traffic conditioning functions are analogous to DS boundary nodes (see Sec. 2.3.4.4).

A host in a network containing a DS domain may act as a DS boundary node for traffic from applications running on that host; we therefore say that the host is within the DS domain. If a host does not act as a boundary node, then the DS node topologically closest to that host acts as the DS boundary node for that host's traffic.

DS Ingress Node and Egress Node

DS boundary nodes act both as a DS ingress node and as a DS egress node for different directions of traffic. Traffic enters a DS domain at a DS ingress node and leaves a DS domain at a DS egress node. A DS ingress node is responsible for ensuring that the traffic entering the DS domain conforms to any TCA between it and the other domain to which the ingress node is connected. A DS egress node may perform traffic conditioning functions on traffic forwarded to a directly connected peering domain, depending on the details of the TCA between the two domains. Note that a DS boundary node may act as a DS interior node for some set of interfaces.

Differentiated Services Region

A differentiated services region (DS Region) is a set of one or more contiguous DS domains. DS regions are capable of supporting differentiated services along paths which span the domains within the region.

The DS domains in a DS region may support different PHB groups internally and different codepoint->PHB mappings. However, to permit services which span across the domains, the peering DS domains must each establish a peering SLA which defines (either explicitly or implicitly) a TCA which specifies how transit traffic from one DS domain to another is conditioned at the boundary between the two DS domains.

It is possible that several DS domains within a DS region may adopt a common service provisioning policy and may support a common set of PHB groups and codepoint mappings, thus eliminating the need for traffic conditioning between those DS domains.

Traffic Classification and Conditioning

Differentiated services are extended across a DS domain boundary by establishing a SLA between an upstream network and a downstream DS domain. The SLA may specify packet classification and re-marking rules and may also specify traffic profiles and actions to traffic streams which are in- or out-of-profile (see Sec. 2.3.2). The TCA between the domains is derived (explicitly or implicitly) from this SLA.

The packet classification policy identifies the subset of traffic which may receive a differentiated service by being conditioned and/ or mapped to one or more behavior aggregates (by DS codepoint re- marking) within the DS domain.

Traffic conditioning performs metering, shaping, policing and/or re- marking to ensure that the traffic entering the DS domain conforms to the rules specified in the TCA, in accordance with the domain's service provisioning policy. The extent of traffic conditioning required is dependent on the specifics of the service offering, and may range from simple codepoint re-marking to complex policing and shaping operations. The details of traffic conditioning policies which are negotiated between networks is outside the scope of this document.

Classifiers

Packet classifiers select packets in a traffic stream based on the content of some portion of the packet header. We define two types of classifiers. The BA (Behavior Aggregate) Classifier classifies packets based on the DS codepoint only. The MF (Multi-Field) classifier selects packets based on the value of a combination of one or more header fields, such as source address, destination address, DS field, protocol ID, source port and destination port numbers, and other information such as incoming interface.

Classifiers are used to "steer" packets matching some specified rule to an element of a traffic conditioner for further processing. Classifiers must be configured by some management procedure in accordance with the appropriate TCA.

The classifier should authenticate the information which it uses to classify the packet (see Sec. 6).

Note that in the event of upstream packet fragmentation, MF classifiers which examine the contents of transport-layer header fields may incorrectly classify packet fragments subsequent to the first. A possible solution to this problem is to maintain

fragmentation state; however, this is not a general solution due to the possibility of upstream fragment re-ordering or divergent routing paths. The policy to apply to packet fragments is outside the scope of this document.

Traffic Profiles

A traffic profile specifies the temporal properties of a traffic stream selected by a classifier. It provides rules for determining whether a particular packet is in-profile or out-of-profile. For example, a profile based on a token bucket may look like: 

 codepoint=X, use token-bucket r, b

The above profile indicates that all packets marked with DS codepoint X should be measured against a token bucket meter with rate r and burst size b. In this example out-of-profile packets are those packets in the traffic stream which arrive when insufficient tokens are available in the bucket. The concept of in- and out-of-profile can be extended to more than two levels, e.g., multiple levels of conformance with a profile may be defined and enforced.

Different conditioning actions may be applied to the in-profile packets and out-of-profile packets, or different accounting actions may be triggered. In-profile packets may be allowed to enter the DS domain without further conditioning; or, alternatively, their DS codepoint may be changed. The latter happens when the DS codepoint is set to a non-Default value for the first time [DSFIELD], or when the packets enter a DS domain that uses a different PHB group or codepoint->PHB mapping policy for this traffic stream. Out-of- profile packets may be queued until they are in-profile (shaped), discarded (policed), marked with a new codepoint (re-marked), or forwarded unchanged while triggering some accounting procedure. Out-of-profile packets may be mapped to one or more behavior aggregates that are "inferior" in some dimension of forwarding performance to the BA into which in-profile packets are mapped.

Note that a traffic profile is an optional component of a TCA and its use is dependent on the specifics of the service offering and the domain's service provisioning policy.

Traffic Conditioners

A traffic conditioner may contain the following elements: meter, marker, shaper, and dropper. A traffic stream is selected by a classifier, which steers the packets to a logical instance of a traffic conditioner. A meter is used (where appropriate) to measure the traffic stream against a traffic profile. The state of the meter

with respect to a particular packet (e.g., whether it is in- or out- of-profile) may be used to affect a marking, dropping, or shaping action.

When packets exit the traffic conditioner of a DS boundary node the DS codepoint of each packet must be set to an appropriate value.

Fig. 1 shows the block diagram of a classifier and traffic conditioner. Note that a traffic conditioner may not necessarily contain all four elements. For example, in the case where no traffic profile is in effect, packets may only pass through a classifier and a marker. 

                           +-------+
                           |       |-------------------+
                    +----->| Meter |                   |
                    |      |       |--+                |
                    |      +-------+  |                |
                    |                 V                V
              +------------+      +--------+      +---------+
              |            |      |        |      | Shaper/ |
packets =====>| Classifier |=====>| Marker |=====>| Dropper |=====>
              |            |      |        |      |         |
              +------------+      +--------+      +---------+

Fig. 1: Logical View of a Packet Classifier and Traffic Conditioner

Meters

Traffic meters measure the temporal properties of the stream of packets selected by a classifier against a traffic profile specified in a TCA. A meter passes state information to other conditioning functions to trigger a particular action for each packet which is either in- or out-of-profile (to some extent).

Markers

Packet markers set the DS field of a packet to a particular codepoint, adding the marked packet to a particular DS behavior aggregate. The marker may be configured to mark all packets which are steered to it to a single codepoint, or may be configured to mark a packet to one of a set of codepoints used to select a PHB in a PHB group, according to the state of a meter. When the marker changes the codepoint in a packet it is said to have "re-marked" the packet.

Shapers

Shapers delay some or all of the packets in a traffic stream in order to bring the stream into compliance with a traffic profile. A shaper usually has a finite-size buffer, and packets may be discarded if there is not sufficient buffer space to hold the delayed packets.

Droppers

Droppers discard some or all of the packets in a traffic stream in order to bring the stream into compliance with a traffic profile. This process is know as "policing" the stream. Note that a dropper can be implemented as a special case of a shaper by setting the shaper buffer size to zero (or a few) packets.

Location of Traffic Conditioners and MF Classifiers

Traffic conditioners are usually located within DS ingress and egress boundary nodes, but may also be located in nodes within the interior of a DS domain, or within a non-DS-capable domain.

Within the Source Domain

We define the source domain as the domain containing the node(s) which originate the traffic receiving a particular service. Traffic sources and intermediate nodes within a source domain may perform traffic classification and conditioning functions. The traffic originating from the source domain across a boundary may be marked by the traffic sources directly or by intermediate nodes before leaving the source domain. This is referred to as initial marking or "pre- marking".

Consider the example of a company that has the policy that its CEO's packets should have higher priority. The CEO's host may mark the DS field of all outgoing packets with a DS codepoint that indicates "higher priority". Alternatively, the first-hop router directly connected to the CEO's host may classify the traffic and mark the CEO's packets with the correct DS codepoint. Such high priority traffic may also be conditioned near the source so that there is a limit on the amount of high priority traffic forwarded from a particular source.

There are some advantages to marking packets close to the traffic source. First, a traffic source can more easily take an application's preferences into account when deciding which packets should receive better forwarding treatment. Also, classification of

packets is much simpler before the traffic has been aggregated with packets from other sources, since the number of classification rules which need to be applied within a single node is reduced.

Since packet marking may be distributed across multiple nodes, the source DS domain is responsible for ensuring that the aggregated traffic towards its provider DS domain conforms to the appropriate TCA. Additional allocation mechanisms such as bandwidth brokers or RSVP may be used to dynamically allocate resources for a particular DS behavior aggregate within the provider's network [2BIT, Bernet]. The boundary node of the source domain should also monitor conformance to the TCA, and may police, shape, or re-mark packets as necessary.

At the Boundary of a DS Domain

Traffic streams may be classified, marked, and otherwise conditioned on either end of a boundary link (the DS egress node of the upstream domain or the DS ingress node of the downstream domain). The SLA between the domains should specify which domain has responsibility for mapping traffic streams to DS behavior aggregates and conditioning those aggregates in conformance with the appropriate TCA. However, a DS ingress node must assume that the incoming traffic may not conform to the TCA and must be prepared to enforce the TCA in accordance with local policy.

When packets are pre-marked and conditioned in the upstream domain, potentially fewer classification and traffic conditioning rules need to be supported in the downstream DS domain. In this circumstance the downstream DS domain may only need to re-mark or police the incoming behavior aggregates to enforce the TCA. However, more sophisticated services which are path- or source-dependent may require MF classification in the downstream DS domain's ingress nodes.

If a DS ingress node is connected to an upstream non-DS-capable domain, the DS ingress node must be able to perform all necessary traffic conditioning functions on the incoming traffic.

In non-DS-Capable Domains

Traffic sources or intermediate nodes in a non-DS-capable domain may employ traffic conditioners to pre-mark traffic before it reaches the ingress of a downstream DS domain. In this way the local policies for classification and marking may be concealed.

In Interior DS Nodes

Although the basic architecture assumes that complex classification and traffic conditioning functions are located only in a network's ingress and egress boundary nodes, deployment of these functions in the interior of the network is not precluded. For example, more restrictive access policies may be enforced on a transoceanic link, requiring MF classification and traffic conditioning functionality in the upstream node on the link. This approach may have scaling limits, due to the potentially large number of classification and conditioning rules that might need to be maintained.

Per-Hop Behaviors

A per-hop behavior (PHB) is a description of the externally observable forwarding behavior of a DS node applied to a particular DS behavior aggregate. "Forwarding behavior" is a general concept in this context. For example, in the event that only one behavior aggregate occupies a link, the observable forwarding behavior (i.e., loss, delay, jitter) will often depend only on the relative loading of the link (i.e., in the event that the behavior assumes a work- conserving scheduling discipline). Useful behavioral distinctions are mainly observed when multiple behavior aggregates compete for buffer and bandwidth resources on a node. The PHB is the means by which a node allocates resources to behavior aggregates, and it is on top of this basic hop-by-hop resource allocation mechanism that useful differentiated services may be constructed.

The most simple example of a PHB is one which guarantees a minimal bandwidth allocation of X% of a link (over some reasonable time interval) to a behavior aggregate. This PHB can be fairly easily measured under a variety of competing traffic conditions. A slightly more complex PHB would guarantee a minimal bandwidth allocation of X% of a link, with proportional fair sharing of any excess link capacity. In general, the observable behavior of a PHB may depend on certain constraints on the traffic characteristics of the associated behavior aggregate, or the characteristics of other behavior aggregates.

PHBs may be specified in terms of their resource (e.g., buffer, bandwidth) priority relative to other PHBs, or in terms of their relative observable traffic characteristics (e.g., delay, loss). These PHBs may be used as building blocks to allocate resources and should be specified as a group (PHB group) for consistency. PHB groups will usually share a common constraint applying to each PHB within the group, such as a packet scheduling or buffer management policy. The relationship between PHBs in a group may be in terms of absolute or relative priority (e.g., discard priority by means of

deterministic or stochastic thresholds), but this is not required (e.g., N equal link shares). A single PHB defined in isolation is a special case of a PHB group.

PHBs are implemented in nodes by means of some buffer management and packet scheduling mechanisms. PHBs are defined in terms of behavior characteristics relevant to service provisioning policies, and not in terms of particular implementation mechanisms. In general, a variety of implementation mechanisms may be suitable for implementing a particular PHB group. Furthermore, it is likely that more than one PHB group may be implemented on a node and utilized within a domain. PHB groups should be defined such that the proper resource allocation between groups can be inferred, and integrated mechanisms can be implemented which can simultaneously support two or more groups. A PHB group definition should indicate possible conflicts with previously documented PHB groups which might prevent simultaneous operation.

As described in [DSFIELD], a PHB is selected at a node by a mapping of the DS codepoint in a received packet. Standardized PHBs have a recommended codepoint. However, the total space of codepoints is larger than the space available for recommended codepoints for standardized PHBs, and [DSFIELD] leaves provisions for locally configurable mappings. A codepoint->PHB mapping table may contain both 1->1 and N->1 mappings. All codepoints must be mapped to some PHB; in the absence of some local policy, codepoints which are not mapped to a standardized PHB in accordance with that PHB's specification should be mapped to the Default PHB.

Network Resource Allocation

The implementation, configuration, operation and administration of the supported PHB groups in the nodes of a DS Domain should effectively partition the resources of those nodes and the inter-node links between behavior aggregates, in accordance with the domain's service provisioning policy. Traffic conditioners can further control the usage of these resources through enforcement of TCAs and possibly through operational feedback from the nodes and traffic conditioners in the domain. Although a range of services can be deployed in the absence of complex traffic conditioning functions (e.g., using only static marking policies), functions such as policing, shaping, and dynamic re-marking enable the deployment of services providing quantitative performance metrics.

The configuration of and interaction between traffic conditioners and interior nodes should be managed by the administrative control of the domain and may require operational control through protocols and a control entity. There is a wide range of possible control models.

The precise nature and implementation of the interaction between these components is outside the scope of this architecture. However, scalability requires that the control of the domain does not require micro-management of the network resources. The most scalable control model would operate nodes in open-loop in the operational timeframe, and would only require administrative-timescale management as SLAs are varied. This simple model may be unsuitable in some circumstances, and some automated but slowly varying operational control (minutes rather than seconds) may be desirable to balance the utilization of the network against the recent load profile.

Per-Hop Behavior Specification Guidelines

Basic requirements for per-hop behavior standardization are given in [DSFIELD]. This section elaborates on that text by describing additional guidelines for PHB (group) specifications. This is intended to help foster implementation consistency. Before a PHB group is proposed for standardization it should satisfy these guidelines, as appropriate, to preserve the integrity of this architecture.

G.1: A PHB standard must specify a recommended DS codepoint selected from the codepoint space reserved for standard mappings [DSFIELD]. Recommended codepoints will be assigned by the IANA. A PHB proposal may recommend a temporary codepoint from the EXP/LU space to facilitate inter-domain experimentation. Determination of a packet's PHB must not require inspection of additional packet header fields beyond the DS field.

G.2: The specification of each newly proposed PHB group should include an overview of the behavior and the purpose of the behavior being proposed. The overview should include a problem or problems statement for which the PHB group is targeted. The overview should include the basic concepts behind the PHB group. These concepts should include, but are not restricted to, queueing behavior, discard behavior, and output link selection behavior. Lastly, the overview should specify the method by which the PHB group solves the problem or problems specified in the problem statement.

G.3: A PHB group specification should indicate the number of individual PHBs specified. In the event that multiple PHBs are specified, the interactions between these PHBs and constraints that must be respected globally by all the PHBs within the group should be clearly specified. As an example, the specification must indicate whether the probability of packet reordering within a microflow is increased if different packets in that microflow are marked for different PHBs within the group.

G.4: When proper functioning of a PHB group is dependent on constraints such as a provisioning restriction, then the PHB definition should describe the behavior when these constraints are violated. Further, if actions such as packet discard or re-marking are required when these constraints are violated, then these actions should be specifically stipulated.

G.5: A PHB group may be specified for local use within a domain in order to provide some domain-specific functionality or domain- specific services. In this event, the PHB specification is useful for providing vendors with a consistent definition of the PHB group. However, any PHB group which is defined for local use should not be considered for standardization, but may be published as an Informational RFC. In contrast, a PHB group which is intended for general use will follow a stricter standardization process. Therefore all PHB proposals should specifically state whether they are to be considered for general or local use.

It is recognized that PHB groups can be designed with the intent of providing host-to-host, WAN edge-to-WAN edge, and/or domain edge-to- domain edge services. Use of the term "end-to-end" in a PHB definition should be interpreted to mean "host-to-host" for consistency.

Other PHB groups may be defined and deployed locally within domains, for experimental or operational purposes. There is no requirement that these PHB groups must be publicly documented, but they should utilize DS codepoints from one of the EXP/LU pools as defined in [DSFIELD].

G.6: It may be possible or appropriate for a packet marked for a PHB within a PHB group to be re-marked to select another PHB within the group; either within a domain or across a domain boundary. Typically there are three reasons for such PHB modification:

a. The codepoints associated with the PHB group are collectively 

  intended to carry state about the network,

b. Conditions exist which require PHB promotion or demotion of a 

  packet (this assumes that PHBs within the group can be ranked in
  some order),

c. The boundary between two domains is not covered by a SLA. In this 

  case the codepoint/PHB to select when crossing the boundary link
  will be determined by the local policy of the upstream domain.

A PHB specification should clearly state the circumstances under which packets marked for a PHB within a PHB group may, or should be modified (e.g., promoted or demoted) to another PHB within the group. If it is undesirable for a packet's PHB to be modified, the

specification should clearly state the consequent risks when the PHB is modified. A possible risk to changing a packet's PHB, either within or outside a PHB group, is a higher probability of packet re- ordering within a microflow. PHBs within a group may carry some host-to-host, WAN edge-to-WAN edge, and/or domain edge-to-domain edge semantics which may be difficult to duplicate if packets are re- marked to select another PHB from the group (or otherwise).

For certain PHB groups, it may be appropriate to reflect a state change in the node by re-marking packets to specify another PHB from within the group. If a PHB group is designed to reflect the state of a network, the PHB definition must adequately describe the relationship between the PHBs and the states they reflect. Further, if these PHBs limit the forwarding actions a node can perform in some way, these constraints may be specified as actions the node should, or must perform.

G.7: A PHB group specification should include a section defining the implications of tunneling on the utility of the PHB group. This section should specify the implications for the utility of the PHB group of a newly created outer header when the original DS field of the inner header is encapsulated in a tunnel. This section should also discuss what possible changes should be applied to the inner header at the egress of the tunnel, when both the codepoints from the inner header and the outer header are accessible (see Sec. 6.2).

G.8: The process of specifying PHB groups is likely to be incremental in nature. When new PHB groups are proposed, their known interactions with previously specified PHB groups should be documented. When a new PHB group is created, it can be entirely new in scope or it can be an extension to an existing PHB group. If the PHB group is entirely independent of some or all of the existing PHB specifications, a section should be included in the PHB specification which details how the new PHB group can co-exist with those PHB groups already standardized. For example, this section might indicate the possibility of packet re-ordering within a microflow for packets marked by codepoints associated with two separate PHB groups. If concurrent operation of two (or more) different PHB groups in the same node is impossible or detrimental this should be stated. If the concurrent operation of two (or more) different PHB groups requires some specific behaviors by the node when packets marked for PHBs from these different PHB groups are being processed by the node at the same time, these behaviors should be stated.

Care should be taken to avoid circularity in the definitions of PHB groups.

If the proposed PHB group is an extension to an existing PHB group, a section should be included in the PHB group specification which details how this extension interoperates with the behavior being extended. Further, if the extension alters or more narrowly defines the existing behavior in some way, this should also be clearly indicated.

G.9: Each PHB specification should include a section specifying minimal conformance requirements for implementations of the PHB group. This conformance section is intended to provide a means for specifying the details of a behavior while allowing for implementation variation to the extent permitted by the PHB specification. This conformance section can take the form of rules, tables, pseudo-code, or tests.

G.10: A PHB specification should include a section detailing the security implications of the behavior. This section should include a discussion of the re-marking of the inner header's codepoint at the egress of a tunnel and its effect on the desired forwarding behavior.

Further, this section should also discuss how the proposed PHB group could be used in denial-of-service attacks, reduction of service contract attacks, and service contract violation attacks. Lastly, this section should discuss possible means for detecting such attacks as they are relevant to the proposed behavior.

G.11: A PHB specification should include a section detailing configuration and management issues which may affect the operation of the PHB and which may impact candidate services that might utilize the PHB.

G.12: It is strongly recommended that an appendix be provided with each PHB specification that considers the implications of the proposed behavior on current and potential services. These services could include but are not restricted to be user-specific, device- specific, domain-specific or end-to-end services. It is also strongly recommended that the appendix include a section describing how the services are verified by users, devices, and/or domains.

G.13: It is recommended that an appendix be provided with each PHB specification that is targeted for local use within a domain, providing guidance for PHB selection for packets which are forwarded into a peer domain which does not support the PHB group.

G.14: It is recommended that an appendix be provided with each PHB specification which considers the impact of the proposed PHB group on existing higher-layer protocols. Under some circumstances PHBs may allow for possible changes to higher-layer protocols which may increase or decrease the utility of the proposed PHB group.

G.15: It is recommended that an appendix be provided with each PHB specification which recommends mappings to link-layer QoS mechanisms to support the intended behavior of the PHB across a shared-medium or switched link-layer. The determination of the most appropriate mapping between a PHB and a link-layer QoS mechanism is dependent on many factors and is outside the scope of this document; however, the specification should attempt to offer some guidance.

Interoperability with Non-Differentiated Services-Compliant Nodes

We define a non-differentiated services-compliant node (non-DS- compliant node) as any node which does not interpret the DS field as specified in [DSFIELD] and/or does not implement some or all of the standardized PHBs (or those in use within a particular DS domain). This may be due to the capabilities or configuration of the node. We define a legacy node as a special case of a non-DS-compliant node which implements IPv4 Precedence classification and forwarding as defined in [RFC791, RFC1812], but which is otherwise not DS- compliant. The precedence values in the IPv4 TOS octet are compatible by intention with the Class Selector Codepoints defined in [DSFIELD], and the precedence forwarding behaviors defined in [RFC791, RFC1812] comply with the Class Selector PHB Requirements also defined in [DSFIELD]. A key distinction between a legacy node and a DS-compliant node is that the legacy node may or may not interpret bits 3-6 of the TOS octet as defined in RFC1349 (the "DTRC" bits); in practice it will not interpret these bit as specified in [DSFIELD]. We assume that the use of the TOS markings defined in RFC1349 is deprecated. Nodes which are non-DS-compliant and which are not legacy nodes may exhibit unpredictable forwarding behaviors for packets with non-zero DS codepoints.

Differentiated services depend on the resource allocation mechanisms provided by per-hop behavior implementations in nodes. The quality or statistical assurance level of a service may break down in the event that traffic transits a non-DS-compliant node, or a non-DS- capable domain.

We will examine two separate cases. The first case concerns the use of non-DS-compliant nodes within a DS domain. Note that PHB forwarding is primarily useful for allocating scarce node and link resources in a controlled manner. On high-speed, lightly loaded links, the worst-case packet delay, jitter, and loss may be

negligible, and the use of a non-DS-compliant node on the upstream end of such a link may not result in service degradation. In more realistic circumstances, the lack of PHB forwarding in a node may make it impossible to offer low-delay, low-loss, or provisioned bandwidth services across paths which traverse the node. However, use of a legacy node may be an acceptable alternative, assuming that the DS domain restricts itself to using only the Class Selector Codepoints defined in [DSFIELD], and assuming that the particular precedence implementation in the legacy node provides forwarding behaviors which are compatible with the services offered along paths which traverse that node. Note that it is important to restrict the codepoints in use to the Class Selector Codepoints, since the legacy node may or may not interpret bits 3-5 in accordance with RFC1349, thereby resulting in unpredictable forwarding results.

The second case concerns the behavior of services which traverse non-DS-capable domains. We assume for the sake of argument that a non-DS-capable domain does not deploy traffic conditioning functions on domain boundary nodes; therefore, even in the event that the domain consists of legacy or DS-compliant interior nodes, the lack of traffic enforcement at the boundaries will limit the ability to consistently deliver some types of services across the domain. A DS domain and a non-DS-capable domain may negotiate an agreement which governs how egress traffic from the DS-domain should be marked before entry into the non-DS-capable domain. This agreement might be monitored for compliance by traffic sampling instead of by rigorous traffic conditioning. Alternatively, where there is knowledge that the non-DS-capable domain consists of legacy nodes, the upstream DS domain may opportunistically re-mark differentiated services traffic to one or more of the Class Selector Codepoints. Where there is no knowledge of the traffic management capabilities of the downstream domain, and no agreement in place, a DS domain egress node may choose to re-mark DS codepoints to zero, under the assumption that the non- DS-capable domain will treat the traffic uniformly with best-effort service.

In the event that a non-DS-capable domain peers with a DS domain, traffic flowing from the non-DS-capable domain should be conditioned at the DS ingress node of the DS domain according to the appropriate SLA or policy.

Multicast Considerations

Use of differentiated services by multicast traffic introduces a number of issues for service provisioning. First, multicast packets which enter a DS domain at an ingress node may simultaneously take multiple paths through some segments of the domain due to multicast packet replication. In this way they consume more network resources

than unicast packets. Where multicast group membership is dynamic, it is difficult to predict in advance the amount of network resources that may be consumed by multicast traffic originating from an upstream network for a particular group. A consequence of this uncertainty is that it may be difficult to provide quantitative service guarantees to multicast senders. Further, it may be necessary to reserve codepoints and PHBs for exclusive use by unicast traffic, to provide resource isolation from multicast traffic.

The second issue is the selection of the DS codepoint for a multicast packet arriving at a DS ingress node. Because that packet may exit the DS domain at multiple DS egress nodes which peer with multiple downstream domains, the DS codepoint used should not result in the request for a service from a downstream DS domain which is in violation of a peering SLA. When establishing classifier and traffic conditioner state at an DS ingress node for an aggregate of traffic receiving a differentiated service which spans across the egress boundary of the domain, the identity of the adjacent downstream transit domain and the specifics of the corresponding peering SLA can be factored into the configuration decision (subject to routing policy and the stability of the routing infrastructure). In this way peering SLAs with downstream DS domains can be partially enforced at the ingress of the upstream domain, reducing the classification and traffic conditioning burden at the egress node of the upstream domain. This is not so easily performed in the case of multicast traffic, due to the possibility of dynamic group membership. The result is that the service guarantees for unicast traffic may be impacted. One means of addressing this problem is to establish a separate peering SLA for multicast traffic, and to either utilize a particular set of codepoints for multicast packets, or to implement the necessary classification and traffic conditioning mechanisms in the DS egress nodes to provide preferential isolation for unicast traffic in conformance with the peering SLA with the downstream domain.

Security and Tunneling Considerations

This section addresses security issues raised by the introduction of differentiated services, primarily the potential for denial-of- service attacks, and the related potential for theft of service by unauthorized traffic (Sec. 6.1). In addition, the operation of differentiated services in the presence of IPsec and its interaction with IPsec are also discussed (Sec. 6.2), as well as auditing requirements (Sec. 6.3). This section considers issues introduced by the use of both IPsec and non-IPsec tunnels.

Theft and Denial of Service

The primary goal of differentiated services is to allow different levels of service to be provided for traffic streams on a common network infrastructure. A variety of resource management techniques may be used to achieve this, but the end result will be that some packets receive different (e.g., better) service than others. The mapping of network traffic to the specific behaviors that result in different (e.g., better or worse) service is indicated primarily by the DS field, and hence an adversary may be able to obtain better service by modifying the DS field to codepoints indicating behaviors used for enhanced services or by injecting packets with the DS field set to such codepoints. Taken to its limits, this theft of service becomes a denial-of-service attack when the modified or injected traffic depletes the resources available to forward it and other traffic streams. The defense against such theft- and denial-of- service attacks consists of the combination of traffic conditioning at DS boundary nodes along with security and integrity of the network infrastructure within a DS domain.

As described in Sec. 2, DS ingress nodes must condition all traffic entering a DS domain to ensure that it has acceptable DS codepoints. This means that the codepoints must conform to the applicable TCA(s) and the domain's service provisioning policy. Hence, the ingress nodes are the primary line of defense against theft- and denial-of- service attacks based on modified DS codepoints (e.g., codepoints to which the traffic is not entitled), as success of any such attack constitutes a violation of the applicable TCA(s) and/or service provisioning policy. An important instance of an ingress node is that any traffic-originating node in a DS domain is the ingress node for that traffic, and must ensure that all originated traffic carries acceptable DS codepoints.

Both a domain's service provisioning policy and TCAs may require the ingress nodes to change the DS codepoint on some entering packets (e.g., an ingress router may set the DS codepoint of a customer's traffic in accordance with the appropriate SLA). Ingress nodes must condition all other inbound traffic to ensure that the DS codepoints are acceptable; packets found to have unacceptable codepoints must either be discarded or must have their DS codepoints modified to acceptable values before being forwarded. For example, an ingress node receiving traffic from a domain with which no enhanced service agreement exists may reset the DS codepoint to the Default PHB codepoint [DSFIELD]. Traffic authentication may be required to validate the use of some DS codepoints (e.g., those corresponding to enhanced services), and such authentication may be performed by technical means (e.g., IPsec) and/or non-technical means (e.g., the inbound link is known to be connected to exactly one customer site).

An inter-domain agreement may reduce or eliminate the need for ingress node traffic conditioning by making the upstream domain partly or completely responsible for ensuring that traffic has DS codepoints acceptable to the downstream domain. In this case, the ingress node may still perform redundant traffic conditioning checks to reduce the dependence on the upstream domain (e.g., such checks can prevent theft-of-service attacks from propagating across the domain boundary). If such a check fails because the upstream domain is not fulfilling its responsibilities, that failure is an auditable event; the generated audit log entry should include the date/time the packet was received, the source and destination IP addresses, and the DS codepoint that caused the failure. In practice, the limited gains from such checks need to be weighed against their potential performance impact in determining what, if any, checks to perform under these circumstances.

Interior nodes in a DS domain may rely on the DS field to associate differentiated services traffic with the behaviors used to implement enhanced services. Any node doing so depends on the correct operation of the DS domain to prevent the arrival of traffic with unacceptable DS codepoints. Robustness concerns dictate that the arrival of packets with unacceptable DS codepoints must not cause the failure (e.g., crash) of network nodes. Interior nodes are not responsible for enforcing the service provisioning policy (or individual SLAs) and hence are not required to check DS codepoints before using them. Interior nodes may perform some traffic conditioning checks on DS codepoints (e.g., check for DS codepoints that are never used for traffic on a specific link) to improve security and robustness (e.g., resistance to theft-of-service attacks based on DS codepoint modifications). Any detected failure of such a check is an auditable event and the generated audit log entry should include the date/time the packet was received, the source and destination IP addresses, and the DS codepoint that caused the failure. In practice, the limited gains from such checks need to be weighed against their potential performance impact in determining what, if any, checks to perform at interior nodes.

Any link that cannot be adequately secured against modification of DS codepoints or traffic injection by adversaries should be treated as a boundary link (and hence any arriving traffic on that link is treated as if it were entering the domain at an ingress node). Local security policy provides the definition of "adequately secured," and such a definition may include a determination that the risks and consequences of DS codepoint modification and/or traffic injection do not justify any additional security measures for a link. Link security can be enhanced via physical access controls and/or software means such as tunnels that ensure packet integrity.

IPsec and Tunneling Interactions

The IPsec protocol, as defined in [ESP, AH], does not include the IP header's DS field in any of its cryptographic calculations (in the case of tunnel mode, it is the outer IP header's DS field that is not included). Hence modification of the DS field by a network node has no effect on IPsec's end-to-end security, because it cannot cause any IPsec integrity check to fail. As a consequence, IPsec does not provide any defense against an adversary's modification of the DS field (i.e., a man-in-the-middle attack), as the adversary's modification will also have no effect on IPsec's end-to-end security. In some environments, the ability to modify the DS field without affecting IPsec integrity checks may constitute a covert channel; if it is necessary to eliminate such a channel or reduce its bandwidth, the DS domains should be configured so that the required processing (e.g., set all DS fields on sensitive traffic to a single value) can be performed at DS egress nodes where traffic exits higher security domains.

IPsec's tunnel mode provides security for the encapsulated IP header's DS field. A tunnel mode IPsec packet contains two IP headers: an outer header supplied by the tunnel ingress node and an encapsulated inner header supplied by the original source of the packet. When an IPsec tunnel is hosted (in whole or in part) on a differentiated services network, the intermediate network nodes operate on the DS field in the outer header. At the tunnel egress node, IPsec processing includes stripping the outer header and forwarding the packet (if required) using the inner header. If the inner IP header has not been processed by a DS ingress node for the tunnel egress node's DS domain, the tunnel egress node is the DS ingress node for traffic exiting the tunnel, and hence must carry out the corresponding traffic conditioning responsibilities (see Sec. 6.1). If the IPsec processing includes a sufficiently strong cryptographic integrity check of the encapsulated packet (where sufficiency is determined by local security policy), the tunnel egress node can safely assume that the DS field in the inner header has the same value as it had at the tunnel ingress node. This allows a tunnel egress node in the same DS domain as the tunnel ingress node, to safely treat a packet passing such an integrity check as if it had arrived from another node within the same DS domain, omitting the DS ingress node traffic conditioning that would otherwise be required. An important consequence is that otherwise insecure links internal to a DS domain can be secured by a sufficiently strong IPsec tunnel.

This analysis and its implications apply to any tunneling protocol that performs integrity checks, but the level of assurance of the inner header's DS field depends on the strength of the integrity

check performed by the tunneling protocol. In the absence of sufficient assurance for a tunnel that may transit nodes outside the current DS domain (or is otherwise vulnerable), the encapsulated packet must be treated as if it had arrived at a DS ingress node from outside the domain.

The IPsec protocol currently requires that the inner header's DS field not be changed by IPsec decapsulation processing at a tunnel egress node. This ensures that an adversary's modifications to the DS field cannot be used to launch theft- or denial-of-service attacks across an IPsec tunnel endpoint, as any such modifications will be discarded at the tunnel endpoint. This document makes no change to that IPsec requirement.

If the IPsec specifications are modified in the future to permit a tunnel egress node to modify the DS field in an inner IP header based on the DS field value in the outer header (e.g., copying part or all of the outer DS field to the inner DS field), then additional considerations would apply. For a tunnel contained entirely within a single DS domain and for which the links are adequately secured against modifications of the outer DS field, the only limits on inner DS field modifications would be those imposed by the domain's service provisioning policy. Otherwise, the tunnel egress node performing such modifications would be acting as a DS ingress node for traffic exiting the tunnel and must carry out the traffic conditioning responsibilities of an ingress node, including defense against theft- and denial-of-service attacks (See Sec. 6.1). If the tunnel enters the DS domain at a node different from the tunnel egress node, the tunnel egress node may depend on the upstream DS ingress node having ensured that the outer DS field values are acceptable. Even in this case, there are some checks that can only be performed by the tunnel egress node (e.g., a consistency check between the inner and outer DS codepoints for an encrypted tunnel). Any detected failure of such a check is an auditable event and the generated audit log entry should include the date/time the packet was received, the source and destination IP addresses, and the DS codepoint that was unacceptable.

An IPsec tunnel can be viewed in at least two different ways from an architectural perspective. If the tunnel is viewed as a logical single hop "virtual wire", the actions of intermediate nodes in forwarding the tunneled traffic should not be visible beyond the ends of the tunnel and hence the DS field should not be modified as part of decapsulation processing. In contrast, if the tunnel is viewed as a multi-hop participant in forwarding traffic, then modification of the DS field as part of tunnel decapsulation processing may be desirable. A specific example of the latter situation occurs when a tunnel terminates at an interior node of a DS domain at which the domain administrator does not wish to deploy traffic conditioning

logic (e.g., to simplify traffic management). This could be supported by using the DS codepoint in the outer IP header (which was subject to traffic conditioning at the DS ingress node) to reset the DS codepoint in the inner IP header, effectively moving DS ingress traffic conditioning responsibilities from the IPsec tunnel egress node to the appropriate upstream DS ingress node (which must already perform that function for unencapsulated traffic).

Auditing

Not all systems that support differentiated services will implement auditing. However, if differentiated services support is incorporated into a system that supports auditing, then the differentiated services implementation should also support auditing. If such support is present the implementation must allow a system administrator to enable or disable auditing for differentiated services as a whole, and may allow such auditing to be enabled or disabled in part.

For the most part, the granularity of auditing is a local matter. However, several auditable events are identified in this document and for each of these events a minimum set of information that should be included in an audit log is defined. Additional information (e.g., packets related to the one that triggered the auditable event) may also be included in the audit log for each of these events, and additional events, not explicitly called out in this specification, also may result in audit log entries. There is no requirement for the receiver to transmit any message to the purported sender in response to the detection of an auditable event, because of the potential to induce denial of service via such action.

Acknowledgements

This document has benefitted from earlier drafts by Steven Blake, David Clark, Ed Ellesson, Paul Ferguson, Juha Heinanen, Van Jacobson, Kalevi Kilkki, Kathleen Nichols, Walter Weiss, John Wroclawski, and Lixia Zhang.

The authors would like to acknowledge the following individuals for their helpful comments and suggestions: Kathleen Nichols, Brian Carpenter, Konstantinos Dovrolis, Shivkumar Kalyana, Wu-chang Feng, Marty Borden, Yoram Bernet, Ronald Bonica, James Binder, Borje Ohlman, Alessio Casati, Scott Brim, Curtis Villamizar, Hamid Ould- Brahi, Andrew Smith, John Renwick, Werner Almesberger, Alan O'Neill, James Fu, and Bob Braden.

References

[802.1p] ISO/IEC Final CD 15802-3 Information technology - Tele- 

           communications and information exchange between systems -
           Local and metropolitan area networks - Common
           specifications - Part 3: Media Access Control (MAC)
           bridges, (current draft available as IEEE P802.1D/D15).

[AH] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 

           2402, November 1998.

[ATM] ATM Traffic Management Specification Version 4.0 <af-tm- 

           0056.000>, ATM Forum, April 1996.

[Bernet] Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang, K. 

           Nichols, and M. Speer, "A Framework for Use of RSVP with
           Diff-serv Networks", Work in Progress.

[DSFIELD] Nichols, K., Blake, S., Baker, F. and D. Black, 

           "Definition of the Differentiated Services Field (DS
           Field) in the IPv4 and IPv6 Headers", RFC 2474, December
           1998.

[EXPLICIT] D. Clark and W. Fang, "Explicit Allocation of Best Effort 

           Packet Delivery Service", IEEE/ACM Trans. on Networking,
           vol. 6, no. 4, August 1998, pp. 362-373.

[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security 

           Payload (ESP)", RFC 2406, November 1998.

[FRELAY] ANSI T1S1, "DSSI Core Aspects of Frame Rely", March 1990.

RFC791 Postel, J., Editor, "Internet Protocol", STD 5, RFC 791, 

           September 1981.

RFC1349 Almquist, P., "Type of Service in the Internet Protocol 

           Suite", RFC 1349, July 1992.

RFC1633 Braden, R., Clark, D. and S. Shenker, "Integrated 

           Services in the Internet Architecture: An Overview", RFC
           1633, July 1994.

RFC1812 Baker, F., Editor, "Requirements for IP Version 4 

           Routers", RFC 1812, June 1995.

[RSVP] Braden, B., Zhang, L., Berson S., Herzog, S. and S. 

           Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
           Functional Specification", RFC 2205, September 1997.

[2BIT] K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit 

           Differentiated Services Architecture for the Internet",
           ftp://ftp.ee.lbl.gov/papers/dsarch.pdf, November 1997.

[TR] ISO/IEC 8802-5 Information technology - 

           Telecommunications and information exchange between
           systems - Local and metropolitan area networks - Common
           specifications - Part 5: Token Ring Access Method and
           Physical Layer Specifications, (also ANSI/IEEE Std 802.5-
           1995), 1995.

Authors' Addresses

Steven Blake Torrent Networking Technologies 3000 Aerial Center, Suite 140 Morrisville, NC 27560

Phone: +1-919-468-8466 x232 EMail: [email protected]

David L. Black EMC Corporation 35 Parkwood Drive Hopkinton, MA 01748

Phone: +1-508-435-1000 x76140 EMail: [email protected]

Mark A. Carlson Sun Microsystems, Inc. 2990 Center Green Court South Boulder, CO 80301

Phone: +1-303-448-0048 x115 EMail: [email protected]

Elwyn Davies Nortel UK London Road Harlow, Essex CM17 9NA, UK

Phone: +44-1279-405498 EMail: [email protected]

Zheng Wang Bell Labs Lucent Technologies 101 Crawfords Corner Road Holmdel, NJ 07733

EMail: [email protected]

Walter Weiss Lucent Technologies 300 Baker Avenue, Suite 100 Concord, MA 01742-2168

EMail: [email protected]

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