RFC1009
From RFC-Wiki
Network Working Group R. Braden Request for Comments: 1009 J. Postel Obsoletes: 985 ISI
June 1987
Requirements for Internet Gateways
Status of this Memo
This document is a formal statement of the requirements to be met by gateways used in the Internet system. As such, it is an official specification for the Internet community. Distribution of this memo is unlimited.
This RFC summarizes the requirements for gateways to be used between networks supporting the Internet protocols. While it was written specifically to support National Science Foundation research programs, the requirements are stated in a general context and are applicable throughout the Internet community.
The purpose of this document is to present guidance for vendors offering gateway products that might be used or adapted for use in an Internet application. It enumerates the protocols required and gives references to RFCs and other documents describing the current specifications. In a number of cases the specifications are evolving and may contain ambiguous or incomplete information. In these cases further discussion giving specific guidance is included in this document. Specific policy issues relevant to the NSF scientific networking community are summarized in an Appendix. As other specifications are updated this document will be revised. Vendors are encouraged to maintain contact with the Internet research community.
Contents
Introduction
The following material is intended as an introduction and background for those unfamiliar with the Internet architecture and the Internet gateway model. General background and discussion on the Internet architecture and supporting protocol suite can be found in the DDN Protocol Handbook [25] and ARPANET Information Brochure [26], see also [19, 28, 30, 31].
The Internet protocol architecture was originally developed under DARPA sponsorship to meet both military and civilian communication requirements [32]. The Internet system presently supports a variety of government and government-sponsored operational and research activities. In particular, the National Science Foundation (NSF) is building a major extension to the Internet to provide user access to
national supercomputer centers and other national scientific resources, and to provide a computer networking capability to a large number of universities and colleges.
In this document there are many terms that may be obscure to one unfamiliar with the Internet protocols. There is not much to be done about that but to learn, so dive in. There are a few terms that are much abused in general discussion but are carefully and intentionally used in this document. These few terms are defined here.
Packet A packet is the unit of transmission on a physical
network.
Datagram A datagram is the unit of transmission in the IP
protocol. To cross a particular network a datagram is
encapsulated inside a packet.
Router A router is a switch that receives data transmission
units from input interfaces and, depending on the
addresses in those units, routes them to the
appropriate output interfaces. There can be routers
at different levels of protocol. For example,
Interface Message Processors (IMPs) are packet-level
routers.
Gateway In the Internet documentation generally, and in this
document specifically, a gateway is an IP-level
router. In the Internet community the term has a long
history of this usage [32].
1.1. The DARPA Internet Architecture
1.1.1. Internet Protocols
The Internet system consists of a number of interconnected
packet networks supporting communication among host computers
using the Internet protocols. These protocols include the
Internet Protocol (IP), the Internet Control Message Protocol
(ICMP), the Transmission Control Protocol (TCP), and
application protocols depending upon them [22].
All Internet protocols use IP as the basic data transport
mechanism. IP [1,31] is a datagram, or connectionless,
internetwork service and includes provision for addressing,
type-of-service specification, fragmentation and reassembly,
and security information. ICMP [2] is considered an integral
part of IP, although it is architecturally layered upon IP.
ICMP provides error reporting, flow control and first-hop
gateway redirection.
Reliable data delivery is provided in the Internet protocol
suite by transport-level protocols such as the Transmission
Control Protocol (TCP), which provides end-end retransmission,
resequencing and connection control. Transport-level
connectionless service is provided by the User Datagram
Protocol (UDP).
1.1.2. Networks and Gateways
The constituent networks of the Internet system are required
only to provide packet (connectionless) transport. This
requires only delivery of individual packets. According to the
IP service specification, datagrams can be delivered out of
order, be lost or duplicated and/or contain errors. Reasonable
performance of the protocols that use IP (e.g., TCP) requires
an IP datagram loss rate of less than 5%. In those networks
providing connection-oriented service, the extra reliability
provided by virtual circuits enhances the end-end robustness of
the system, but is not necessary for Internet operation.
Constituent networks may generally be divided into two classes:
* Local-Area Networks (LANs)
LANs may have a variety of designs, typically based upon
buss, ring, or star topologies. In general, a LAN will
cover a small geographical area (e.g., a single building or
plant site) and provide high bandwidth with low delays.
* Wide-Area Networks (WANs)
Geographically-dispersed hosts and LANs are interconnected
by wide-area networks, also called long-haul networks.
These networks may have a complex internal structure of
lines and packet-routers (typified by ARPANET), or they may
be as simple as point-to-point lines.
In the Internet model, constituent networks are connected
together by IP datagram forwarders which are called "gateways"
or "IP routers". In this document, every use of the term
"gateway" is equivalent to "IP router". In current practice,
gateways are normally realized with packet-switching software
executing on a general-purpose CPU, but special-purpose
hardware may also be used (and may be required for future
higher-throughput gateways).
A gateway is connected to two or more networks, appearing to
each of these networks as a connected host. Thus, it has a
physical interface and an IP address on each of the connected
networks. Forwarding an IP datagram generally requires the
gateway to choose the address of the next-hop gateway or (for
the final hop) the destination host. This choice, called
"routing", depends upon a routing data-base within the gateway.
This routing data-base should be maintained dynamically to
reflect the current topology of the Internet system; a gateway
normally accomplishes this by participating in distributed
routing and reachability algorithms with other gateways.
Gateways provide datagram transport only, and they seek to
minimize the state information necessary to sustain this
service in the interest of routing flexibility and robustness.
Routing devices may also operate at the network level; in this
memo we will call such devices MAC routers (informally called
"level-2 routers", and also called "bridges"). The name
derives from the fact that MAC routers base their routing
decision on the addresses in the MAC headers; e.g., in IEEE
802.3 networks, a MAC router bases its decision on the 48-bit
addresses in the MAC header. Network segments which are
connected by MAC routers share the same IP network number,
i.e., they logically form a single IP network.
Another variation on the simple model of networks connected
with gateways sometimes occurs: a set of gateways may be
interconnected with only serial lines, to effectively form a
network in which the routing is performed at the internetwork
(IP) level rather than the network level.
1.1.3. Autonomous Systems
For technical, managerial, and sometimes political reasons, the
gateways of the Internet system are grouped into collections
called "autonomous systems" [35]. The gateways included in a
single autonomous system (AS) are expected to:
* Be under the control of a single operations and
maintenance (O&M) organization;
* Employ common routing protocols among themselves, to
maintain their routing data-bases dynamically.
A number of different dynamic routing protocols have been
developed (see Section 4.1); the particular choice of routing
protocol within a single AS is generically called an interior
gateway protocol or IGP.
An IP datagram may have to traverse the gateways of two or more
ASs to reach its destination, and the ASs must provide each
other with topology information to allow such forwarding. The
Exterior Gateway Protocol (EGP) is used for this purpose,
between gateways of different autonomous systems.
1.1.4. Addresses and Subnets
An IP datagram carries 32-bit source and destination addresses,
each of which is partitioned into two parts -- a constituent
network number and a host number on that network.
Symbolically:
IP-address ::= { <Network-number>, <Host-number> }
To finally deliver the datagram, the last gateway in its path
must map the host-number (or "rest") part of an IP address into
the physical address of a host connection to the constituent
network.
This simple notion has been extended by the concept of
"subnets", which were introduced in order to allow arbitrary
complexity of interconnected LAN structures within an
organization, while insulating the Internet system against
explosive growth in network numbers and routing complexity.
Subnets essentially provide a two-level hierarchical routing
structure for the Internet system. The subnet extension,
described in RFC-950 [21], is now a required part of the
Internet architecture. The basic idea is to partition the
<host number> field into two parts: a subnet number, and a true
host number on that subnet.
IP-address ::=
{ <Network-number>, <Subnet-number>, <Host-number> }
The interconnected LANs of an organization will be given the
same network number but different subnet numbers. The
distinction between the subnets of such a subnetted network
must not be visible outside that network. Thus, wide-area
routing in the rest of the Internet will be based only upon the
<Network-number> part of the IP destination address; gateways
outside the network will lump <Subnet-number> and <Host-number>
together to form an uninterpreted "rest" part of the 32-bit IP
address. Within the subnetted network, the local gateways must
route on the basis of an extended network number:
{ <Network-number>, <Subnet-number> }.
The bit positions containing this extended network number are
indicated by a 32-bit mask called the "subnet mask" [21]; it is
recommended but not required that the <Subnet-number> bits be
contiguous and fall between the <Network-number> and the
<Host-number> fields. No subnet should be assigned the value
zero or -1 (all one bits).
Flexible use of the available address space will be
increasingly important in coping with the anticipated growth of
the Internet. Thus, we allow a particular subnetted network to
use more than one subnet mask. Several campuses with very
large LAN configurations are also creating nested hierarchies
of subnets, sub-subnets, etc.
There are special considerations for the gateway when a
connected network provides a broadcast or multicast capability;
these will be discussed later.
1.2. The Internet Gateway Model
There are two basic models for interconnecting local-area networks and wide-area (or long-haul) networks in the Internet. In the first, the local-area network is assigned a network number and all gateways in the Internet must know how to route to that network. In the second, the local-area network shares (a small part of) the address space of the wide-area network. Gateways that support this second model are called "address sharing gateways" or "transparent gateways". The focus of this memo is on gateways that support the first model, but this is not intended to exclude the use of transparent gateways.
1.2.1. Internet Gateways
An Internet gateway is an IP-level router that performs the
following functions:
1. Conforms to specific Internet protocols specified in
this document, including the Internet Protocol (IP),
Internet Control Message Protocol (ICMP), and others as
necessary. See Section 2 (Protocols Required).
2. Interfaces to two or more packet networks. For each
connected network the gateway must implement the
functions required by that network. These functions
typically include:
a. encapsulating and decapsulating the IP datagrams with
the connected network framing (e.g., an Ethernet
header and checksum);
b. sending and receiving IP datagrams up to the maximum
size supported by that network, this size is the
network's "Maximum Transmission Unit" or "MTU";
c. translating the IP destination address into an
appropriate network-level address for the connected
network (e.g., an Ethernet hardware address);
d. responding to the network flow control and error
indication, if any.
See Section 3 (Constituent Network Interface), for
details on particular constituent network interfaces.
3. Receives and forwards Internet datagrams. Important
issues are buffer management, congestion control, and
fairness. See Section 4 (Gateway Algorithms).
a. Recognizes various error conditions and generates
ICMP error and information messages as required.
b. Drops datagrams whose time-to-live fields have
reached zero.
c. Fragments datagrams when necessary to fit into the
MTU of the next network.
4. Chooses a next-hop destination for each IP datagram,
based on the information in its routing data-base. See
Section 4 (Gateway Algorithms).
5. Supports an interior gateway protocol (IGP) to carry out
distributed routing and reachability algorithms with the
other gateways in the same autonomous system. In
addition, some gateways will need to support the
Exterior Gateway Protocol (EGP) to exchange topological
information with other autonomous systems. See
Section 4 (Gateway Algorithms).
6. Provides system support facilities, including loading,
debugging, status reporting, exception reporting and
control. See Section 5 (Operation and Maintenance).
1.2.2. Embedded Gateways
A gateway may be a stand-alone computer system, dedicated to
its IP router functions. Alternatively, it is possible to
embed gateway functionality within a host operating system
which supports connections to two or more networks. The
best-known example of an operating system with embedded gateway
code is the Berkeley BSD system. The embedded gateway feature
seems to make internetting easy, but it has a number of hidden
pitfalls:
1. If a host has only a single constituent-network
interface, it should not act as a gateway.
For example, hosts with embedded gateway code that
gratuitously forward broadcast packets or datagrams on
the same net often cause packet avalanches.
2. If a (multihomed) host acts as a gateway, it must
implement ALL the relevant gateway requirements
contained in this document.
For example, the routing protocol issues (see Sections
2.6 and 4.1) and the control and monitoring problems are
as hard and important for embedded gateways as for
stand-alone gateways.
Since Internet gateway requirements and
specifications may change independently of operating
system changes, an administration that operates an
embedded gateway in the Internet is strongly advised
to have an ability to maintain and update the gateway
code (e.g., this might require gateway code source).
3. Once a host runs embedded gateway code, it becomes part
of the Internet system. Thus, errors in software or
configuration of such a host can hinder communication
between other hosts. As a consequence, the host
administrator must lose some autonomy.
In many circumstances, a host administrator will need to
disable gateway coded embedded in the operating system,
and any embedded gateway code must be organized so it
can be easily disabled.
4. If a host running embedded gateway code is concurrently
used for other services, the O&M (operation and
maintenance) requirements for the two modes of use may
be in serious conflict.
For example, gateway O&M will in many cases be performed
remotely by an operations center; this may require
privileged system access which the host administrator
would not normally want to distribute.
1.2.3. Transparent Gateways
The basic idea of a transparent gateway is that the hosts on
the local-area network behind such a gateway share the address
space of the wide-area network in front of the gateway. In
certain situations this is a very useful approach and the
limitations do not present significant drawbacks.
The words "in front" and "behind" indicate one of the
limitations of this approach: this model of interconnection is
suitable only for a geographically (and topologically) limited
stub environment. It requires that there be some form of
logical addressing in the network level addressing of the
wide-area network (that is, all the IP addresses in the local
environment map to a few (usually one) physical address in the
wide-area network, in a way consistent with the { IP address
<-> network address } mapping used throughout the wide-area
network).
Multihoming is possible on one wide-area network, but may
present routing problems if the interfaces are geographically
or topologically separated. Multihoming on two (or more)
wide-area networks is a problem due to the confusion of
addresses.
The behavior that hosts see from other hosts in what is
apparently the same network may differ if the transparent
gateway cannot fully emulate the normal wide-area network
service. For example, if there were a transparent gateway
between the ARPANET and an Ethernet, a remote host would not
receive a Destination Dead message [3] if it sent a datagram to
an Ethernet host that was powered off.
1.3. Gateway Characteristics
Every Internet gateway must perform the functions listed above. However, a vendor will have many choices on power, complexity, and features for a particular gateway product. It may be helpful to observe that the Internet system is neither homogeneous nor fully-connected. For reasons of technology and geography, it is growing into a global-interconnect system plus a "fringe" of LANs around the "edge".
* The global-interconnect system is comprised of a number of
wide-area networks to which are attached gateways of several
ASs; there are relatively few hosts connected directly to
it. The global-interconnect system includes the ARPANET,
the NSFNET "backbone", the various NSF regional and
consortium networks, other ARPA sponsored networks such as
the SATNET and the WBNET, and the DCA sponsored MILNET. It
is anticipated that additional networks sponsored by these
and other agencies (such as NASA and DOE) will join the
global-interconnect system.
* Most hosts are connected to LANs, and many organizations
have clusters of LANs interconnected by local gateways.
Each such cluster is connected by gateways at one or more
points into the global-interconnect system. If it is
connected at only one point, a LAN is known as a "stub"
network.
Gateways in the global-interconnect system generally require:
* Advanced routing and forwarding algorithms
These gateways need routing algorithms which are highly
dynamic and also offer type-of-service routing. Congestion
is still not a completely resolved issue [24]. Improvements
to the current situation will be implemented soon, as the
research community is actively working on these issues.
* High availability
These gateways need to be highly reliable, providing 24 hour
a day, 7 days a week service. In case of failure, they must
recover quickly.
* Advanced O&M features
These gateways will typically be operated remotely from a
regional or national monitoring center. In their
interconnect role, they will need to provide sophisticated
means for monitoring and measuring traffic and other events
and for diagnosing faults.
* High performance
Although long-haul lines in the Internet today are most
frequently 56 Kbps, DS1 lines (1.5 Mbps) are of increasing
importance, and even higher speeds are likely in the future.
Full-duplex operation is provided at any of these speeds.
The average size of Internet datagrams is rather small, of
the order of 100 bytes. At DS1 line speeds, the
per-datagram processing capability of the gateways, rather
than the line speed, is likely to be the bottleneck. To
fill a DS1 line with average-sized Internet datagrams, a
gateway would need to pass -- receive, route, and send --
2,000 datagrams per second per interface. That is, a
gateway which supported 3 DS1 lines and and Ethernet
interface would need to be able to pass a dazzling 2,000
datagrams per second in each direction on each of the
interfaces, or a aggregate throughput of 8,000 datagrams per
second, in order to fully utilize DS1 lines. This is beyond
the capability of current gateways.
Note: some vendors count input and output operations
separately in datagrams per second figures; for these
vendors, the above example would imply 16,000 datagrams
per second !
Gateways used in the "LAN fringe" (e.g., campus networks) will generally have to meet less stringent requirements for performance, availability, and maintenance. These may be high or medium-performance devices, probably competitively procured from several different vendors and operated by an internal organization (e.g., a campus computing center). The design of these gateways should emphasize low average delay and good burst performance, together with delay and type-of-service sensitive resource management. In this environment, there will be less formal O&M, more hand-crafted static configurations for special cases, and more need for inter-operation with gateways of other vendors. The routing mechanism will need to be very flexible, but need not be so highly dynamic as in the global-interconnect system.
It is important to realize that Internet gateways normally operate in an unattended mode, but that equipment and software faults can have a wide-spread (sometimes global) effect. In any environment,
a gateway must be highly robust and able to operate, possibly in a degraded state, under conditions of extreme congestion or failure of network resources.
Even though the Internet system is not fully-interconnected, many parts of the system do need to have redundant connectivity. A rich connectivity allows reliable service despite failures of communication lines and gateways, and it can also improve service by shortening Internet paths and by providing additional capacity. The engineering tradeoff between cost and reliability must be made for each component of the Internet system.
Protocols Required in Gateways
The Internet architecture uses datagram gateways to interconnect constituent networks. This section describes the various protocols which a gateway needs to implement.
2.1. Internet Protocol (IP)
IP is the basic datagram protocol used in the Internet system [19, 31]. It is described in RFC-791 [1] and also in MIL-STD-1777 [5] as clarified by RFC-963 [36] ([1] and [5] are intended to describe the same standard, but in quite different words). The subnet extension is described in RFC-950 [21].
With respect to current gateway requirements the following IP features can be ignored, although they may be required in the future: Type of Service field, Security option, and Stream ID option. However, if recognized, the interpretation of these quantities must conform to the standard specification.
It is important for gateways to implement both the Loose and Strict Source Route options. The Record Route and Timestamp options are useful diagnostic tools and must be supported in all gateways.
The Internet model requires that a gateway be able to fragment datagrams as necessary to match the MTU of the network to which they are being forwarded, but reassembly of fragmented datagrams is generally left to the destination hosts. Therefore, a gateway will not perform reassembly on datagrams it forwards.
However, a gateway will generally receive some IP datagrams addressed to itself; for example, these may be ICMP Request/Reply messages, routing update messages (see Sections 2.3 and 2.6), or for monitoring and control (see Section 5). For these datagrams, the gateway will be functioning as a destination host, so it must implement IP reassembly in case the datagrams have been fragmented by some transit gateway. The destination gateway must have a reassembly buffer which is at least as large as the maximum of the MTU values for its network interfaces and 576. Note also that it is possible for a particular protocol implemented by a host or gateway to require a lower bound on reassembly buffer size which is larger than 576. Finally, a datagram which is addressed to a gateway may use any of that gateway's IP addresses as destination address, regardless of which interface the datagram enters.
There are five classes of IP addresses: Class A through Class E [23]. Of these, Class D and Class E addresses are
reserved for experimental use. A gateway which is not participating in these experiments must ignore all datagrams with a Class D or Class E destination IP address. ICMP Destination Unreachable or ICMP Redirect messages must not result from receiving such datagrams.
There are certain special cases for IP addresses, defined in the latest Assigned Numbers document [23]. These special cases can be concisely summarized using the earlier notation for an IP address:
IP-address ::= { <Network-number>, <Host-number> }
or
IP-address ::= { <Network-number>, <Subnet-number>,
<Host-number> }
if we also use the notation "-1" to mean the field contains all 1 bits. Some common special cases are as follows:
(a) {0, 0}
This host on this network. Can only be used as a source
address (see note later).
(b) {0, <Host-number>}
Specified host on this network. Can only be used as a
source address.
(c) { -1, -1}
Limited broadcast. Can only be used as a destination
address, and a datagram with this address must never be
forwarded outside the (sub-)net of the source.
(d) {<Network-number>, -1}
Directed broadcast to specified network. Can only be used
as a destination address.
(e) {<Network-number>, <Subnet-number>, -1}
Directed broadcast to specified subnet. Can only be used as
a destination address.
(f) {<Network-number>, -1, -1}
Directed broadcast to all subnets of specified subnetted
network. Can only be used as a destination address.
(g) {127, <any>}
Internal host loopback address. Should never appear outside
a host.
The following two are conventional notation for network numbers, and do not really represent IP addresses. They can never be used in an IP datagram header as an IP source or destination address.
(h) {<Network-number>, 0}
Specified network (no host).
(i) {<Network-number>, <Subnet-number>, 0}
Specified subnet (no host).
Note also that the IP broadcast address, which has primary application to Ethernets and similar technologies that support an inherent broadcast function, has an all-ones value in the host field of the IP address. Some early implementations chose the all-zeros value for this purpose, which is not in conformance with the specification [23, 49, 50].
2.2. Internet Control Message Protocol (ICMP)
ICMP is an auxiliary protocol used to convey advice and error messages and is described in RFC-792 [2].
We will discuss issues arising from gateway handling of particular ICMP messages. The ICMP messages are grouped into two classes: error messages and information messages. ICMP error messages are never sent about ICMP error messages, nor about broadcast or multicast datagrams.
The ICMP error messages are: Destination Unreachable, Redirect,
Source Quench, Time Exceeded, and Parameter Problem.
The ICMP information messages are: Echo, Information,
Timestamp, and Address Mask.
2.2.1. Destination Unreachable
The distinction between subnets of a subnetted network, which
depends on the address mask described in RFC-950 [21], must not
be visible outside that network. This distinction is important
in the case of the ICMP Destination Unreachable message.
The ICMP Destination Unreachable message is sent by a gateway
in response to a datagram which it cannot forward because the
destination is unreachable or down. The gateway chooses one of
the following two types of Destination Unreachable messages to
send:
* Net Unreachable
* Host Unreachable
Net unreachable implies that an intermediate gateway was unable
to forward a datagram, as its routing data-base gave no next
hop for the datagram, or all paths were down. Host Unreachable
implies that the destination network was reachable, but that a
gateway on that network was unable to reach the destination
host. This might occur if the particular destination network
was able to determine that the desired host was unreachable or
down. It might also occur when the destination host was on a
subnetted network and no path was available through the subnets
of this network to the destination. Gateways should send Host
Unreachable messages whenever other hosts on the same
destination network might be reachable; otherwise, the source
host may erroneously conclude that ALL hosts on the network are
unreachable, and that may not be the case.
2.2.2. Redirect
The ICMP Redirect message is sent by a gateway to a host on the
same network, in order to change the gateway used by the host
for routing certain datagrams. A choice of four types of
Redirect messages is available to specify datagrams destined
for a particular host or network, and possibly with a
particular type-of-service.
If the directly-connected network is not subnetted, a gateway
can normally send a network Redirect which applies to all hosts
on a specified remote network. Using a network rather than a
host Redirect may economize slightly on network traffic and on
host routing table storage. However, the saving is not
significant, and subnets create an ambiguity about the subnet
mask to be used to interpret a network Redirect. In a general
subnet environment, it is difficult to specify precisely the
cases in which network Redirects can be used.
Therefore, it is recommended that a gateway send only host (or
host and type-of-service) Redirects.
2.2.3. Source Quench
All gateways must contain code for sending ICMP Source Quench
messages when they are forced to drop IP datagrams due to
congestion. Although the Source Quench mechanism is known to
be an imperfect means for Internet congestion control, and
research towards more effective means is in progress, Source
Quench is considered to be too valuable to omit from production
gateways.
There is some argument that the Source Quench should be sent
before the gateway is forced to drop datagrams [62]. For
example, a parameter X could be established and set to have
Source Quench sent when only X buffers remain. Or, a parameter
Y could be established and set to have Source Quench sent when
only Y per cent of the buffers remain.
Two problems for a gateway sending Source Quench are: (1) the
consumption of bandwidth on the reverse path, and (2) the use
of gateway CPU time. To ameliorate these problems, a gateway
must be prepared to limit the frequency with which it sends
Source Quench messages. This may be on the basis of a count
(e.g., only send a Source Quench for every N dropped datagrams
overall or per given source host), or on the basis of a time
(e.g., send a Source Quench to a given source host or overall
at most once per T millseconds). The parameters (e.g., N or T)
must be settable as part of the configuration of the gateway;
furthermore, there should be some configuration setting which
disables sending Source Quenches. These configuration
parameters, including disabling, should ideally be specifiable
separately for each network interface.
Note that a gateway itself may receive a Source Quench as the
result of sending a datagram targeted to another gateway. Such
datagrams might be an EGP update, for example.
2.2.4. Time Exceeded
The ICMP Time Exceeded message may be sent when a gateway
discards a datagram due to the TTL being reduced to zero. It
may also be sent by a gateway if the fragments of a datagram
addressed to the gateway itself cannot be reassembled before
the time limit.
2.2.5. Parameter Problem
The ICMP Parameter Problem message may be sent to the source
host for any problem not specifically covered by another ICMP
message.
2.2.6. Address Mask
Host and gateway implementations are expected to support the
ICMP Address Mask messages described in RFC-950 [21].
2.2.7. Timestamp
The ICMP Timestamp message has proven to be useful for
diagnosing Internet problems. The preferred form for a
timestamp value, the "standard value", is in milliseconds since
midnight GMT. However, it may be difficult to provide this
value with millisecond resolution. For example, many systems
use clocks which update only at line frequency, 50 or 60 times
per second. Therefore, some latitude is allowed in a
"standard" value:
* The value must be updated at a frequency of at least 30
times per second (i.e., at most five low-order bits of
the value may be undefined).
* The origin of the value must be within a few minutes of
midnight, i.e., the accuracy with which operators
customarily set CPU clocks.
To meet the second condition for a stand-alone gateway, it will
be necessary to query some time server host when the gateway is
booted or restarted. It is recommended that the UDP Time
Server Protocol [44] be used for this purpose. A more advanced
implementation would use NTP (Network Time Protocol) [45] to
achieve nearly millisecond clock synchronization; however, this
is not required.
Even if a gateway is unable to establish its time origin, it
ought to provide a "non-standard" timestamp value (i.e., with
the non-standard bit set), as a time in milliseconds from
system startup.
New gateways, especially those expecting to operate at T1 or
higher speeds, are expected to have at least millisecond
clocks.
2.2.8. Information Request/Reply
The Information Request/Reply pair was intended to support
self-configuring systems such as diskless workstations, to
allow them to discover their IP network numbers at boot time.
However, the Reverse ARP (RARP) protocol [15] provides a better
mechanism for a host to use to discover its own IP address, and
RARP is recommended for this purpose. Information
Request/Reply need not be implemented in a gateway.
2.2.9. Echo Request/Reply
A gateway must implement ICMP Echo, since it has proven to be
an extremely useful diagnostic tool. A gateway must be
prepared to receive, reassemble, and echo an ICMP Echo Request
datagram at least as large as the maximum of 576 and the MTU's
of all of the connected networks. See the discussion of IP
reassembly in gateways, Section 2.1.
The following rules resolve the question of the use of IP
source routes in Echo Request and Reply datagrams. Suppose a
gateway D receives an ICMP Echo Request addressed to itself
from host S.
1. If the Echo Request contained no source route, D should
send an Echo Reply back to S using its normal routing
rules. As a result, the Echo Reply may take a different
path than the Request; however, in any case, the pair
will sample the complete round-trip path which any other
higher-level protocol (e.g., TCP) would use for its data
and ACK segments between S and D.
2. If the Echo Request did contain a source route, D should
send an Echo Reply back to S using as a source route the
return route built up in the source-routing option of
the Echo Request.
2.3. Exterior Gateway Protocol (EGP)
EGP is the protocol used to exchange reachability information between Autonomous Systems of gateways, and is defined in RFC-904 [11]. See also RFC-827 [51], RFC-888 [46], and RFC-975 [27] for background information. The most widely used EGP implementation is described in RFC-911 [13].
When a dynamic routing algorithm is operated in the gateways of an Autonomous System (AS), the routing data-base must be coupled to the EGP implementation. This coupling should ensure that, when a net is determined to be unreachable by the routing algorithm, the net will not be declared reachable to other ASs via EGP. This requirement is designed to minimize spurious traffic to "black holes" and to ensure fair utilization of the resources on other systems.
The present EGP specification defines a model with serious limitations, most importantly a restriction against propagating "third party" EGP information in order to prevent long-lived routing loops [27]. This effectively limits EGP to a two-level hierarchy; the top level is formed by the "core" AS, while the lower level is composed of those ASs which are direct neighbor gateways to the core AS. In practice, in the current Internet, nearly all of the "core gateways" are connected to the ARPANET, while the lower level is composed of those ASs which are directly gatewayed to the ARPANET or MILNET.
RFC-975 [27] suggested one way to generalize EGP to lessen these topology restrictions; it has not been adopted as an official specification, although its ideas are finding their way into the new EGP developments. There are efforts underway in the research community to develop an EGP generalization which will remove these restrictions.
In EGP, there is no standard interpretation (i.e., metric) for the distance fields in the update messages, so distances are comparable only among gateways of the same AS. In using EGP data, a gateway should compare the distances among gateways of the same AS and prefer a route to that gateway which has the smallest distance value.
The values to be announced in the distance fields for particular networks within the local AS should be a gateway configuration parameter; by suitable choice of these values, it will be possible to arrange primary and backup paths from other AS's. There are other EGP parameters, such as polling intervals, which also need to be set in the gateway configuration.
When routing updates become large they must be transmitted in parts. One strategy is to use IP fragmentation, another is to explicitly send the routing information in sections. The Internet Engineering Task Force is currently preparing a recommendation on this and other EGP engineering issues.
2.4. Address Resolution Protocol (ARP)
ARP is an auxiliary protocol used to perform dynamic address translation between LAN hardware addresses and Internet addresses, and is described in RFC-826 [4].
ARP depends upon local network broadcast. In normal ARP usage, the initiating host broadcasts an ARP Request carrying a target IP address; the corresponding target host, recognizing its own IP address, sends back an ARP Reply containing its own hardware interface address.
A variation on this procedure, called "proxy ARP", has been used by gateways attached to broadcast LANs [14]. The gateway sends an ARP Reply specifying its interface address in response to an ARP Request for a target IP address which is not on the directly-connected network but for which the gateway offers an appropriate route. By observing ARP and proxy ARP traffic, a gateway may accumulate a routing data-base [14].
Proxy ARP (also known in some quarters as "promiscuous ARP" or "the ARP hack") is useful for routing datagrams from hosts which do not implement the standard Internet routing rules fully -- for example, host implementations which predate the introduction of subnetting. Proxy ARP for subnetting is discussed in detail in RFC-925 [14].
Reverse ARP (RARP) allows a host to map an Ethernet interface address into an IP address [15]. RARP is intended to allow a self-configuring host to learn its own IP address from a server at boot time.
2.5. Constituent Network Access Protocols
See Section 3.
2.6. Interior Gateway Protocols
Distributed routing algorithms continue to be the subject of research and engineering, and it is likely that advances will be made over the next several years. A good algorithm needs to respond rapidly to real changes in Internet connectivity, yet be stable and insensitive to transients. It needs to synchronize the distributed data-base across gateways of its Autonomous System rapidly (to avoid routing loops), while consuming only a small fraction of the available bandwidth.
Distributed routing algorithms are commonly broken down into the following three components:
A. An algorithm to assign a "length" to each Internet path.
The "length" may be a simple count of hops (1, or infinity
if the path is broken), or an administratively-assigned
cost, or some dynamically-measured cost (usually an average
delay).
In order to determine a path length, each gateway must at
least test whether each of its neighbors is reachable; for
this purpose, there must be a "reachability" or "neighbor
up/down" protocol.
B. An algorithm to compute the shortest path(s) to a given
destination.
C. A gateway-gateway protocol used to exchange path length and
routing information among gateways.
The most commonly-used IGPs in Internet gateways are as follows.
2.6.1. Gateway-to-Gateway Protocol (GGP)
GGP was designed and implemented by BBN for the first
experimental Internet gateways [41]. It is still in use in the
BBN LSI/11 gateways, but is regarded as having serious
drawbacks [58]. GGP is based upon an algorithm used in the
early ARPANET IMPs and later replaced by SPF (see below).
GGP is a "min-hop" algorithm, i.e., its length measure is
simply the number of network hops between gateway pairs. It
implements a distributed shortest-path algorithm, which
requires global convergence of the routing tables after a
change in topology or connectivity. Each gateway sends a GGP
routing update only to its neighbors, but each update includes
an entry for every known network, where each entry contains the
hop count from the gateway sending the update.
2.6.2. Shortest-Path-First (SPF) Protocols
SPF [40] is the name for a class of routing algorithms based on
a shortest-path algorithm of Dijkstra. The current ARPANET
routing algorithm is SPF, and the BBN Butterfly gateways also
use SPF. Its characteristics are considered superior to
GGP [58].
Under SPF, the routing data-base is replicated rather than
distributed. Each gateway will have its own copy of the same
data-base, containing the entire Internet topology and the
lengths of every path. Since each gateway has all the routing
data and runs a shortest-path algorithm locally, there is no
problem of global convergence of a distributed algorithm, as in
GGP. To build this replicated data-base, a gateway sends SPF
routing updates to ALL other gateways; these updates only list
the distances to each of the gateway's neighbors, making them
much smaller than GGP updates. The algorithm used to
distribute SPF routing updates involves reliable flooding.
2.6.3. Routing Information (RIP)
RIP is the name often used for a class of routing protocols
based upon the Xerox PUP and XNS routing protocols. These are
relatively simple, and are widely available because they are
incorporated in the embedded gateway code of Berkeley BSD
systems. Because of this simplicity, RIP protocols have come
the closest of any to being an "Open IGP", i.e., a protocol
which can be used between different vendors' gateways.
Unfortunately, there is no standard, and in fact not even a
good document, for RIP.
As in GGP, gateways using RIP periodically broadcast their
routing data-base to their neighbor gateways, and use a
hop-count as the metric.
A fixed value of the hop-count (normally 16) is defined to be
"infinity", i.e., network unreachable. A RIP implementation
must include measures to avoid both the slow-convergence
phenomen called "counting to infinity" and the formation of
routing loops. One such measure is a "hold-down" rule. This
rule establishes a period of time (typically 60 seconds) during
which a gateway will ignore new routing information about a
given network, once the gateway has learned that network is
unreachable (has hop-count "infinity"). The hold-down period
must be settable in the gateway configuration; if gateways with
different hold-down periods are using RIP in the same
Autonomous System, routing loops are a distinct possibility.
In general, the hold-down period is chosen large enough to
allow time for unreachable status to propagate to all gateways
in the AS.
2.6.4. Hello
The "Fuzzball" software for an LSI/11 developed by Dave Mills
incorporated an IGP called the "Hello" protocol [39]. This IGP
is mentioned here because the Fuzzballs have been widely used
in Internet experimentation, and because they have served as a
testbed for many new routing ideas.
2.7. Monitoring Protocols
See Section 5 of this document.
2.8. Internet Group Management Protocol (IGMP)
An extension to the IP protocol has been defined to provide Internet-wide multicasting, i.e., delivery of copies of the same IP datagram to a set of Internet hosts [47, 48]. This delivery is to be performed by processes known as "multicasting agents", which reside either in a host on each net or (preferably) in the gateways.
The set of hosts to which a datagram is delivered is called a "host group", and there is a host-agent protocol called IGMP, which a host uses to join, leave, or create a group. Each host group is distinguished by a Class D IP address.
This multicasting mechanism and its IGMP protocol are currently experimental; implementation in vendor gateways would be premature at this time. A datagram containing a Class D IP address must be dropped, with no ICMP error message.
Constituent Network Interface
This section discusses the rules used for transmission of IP datagrams on the most common types of constituent networks. A gateway must be able to send and receive IP datagrams of any size up to the MTU of any constituent network to which it is connected.
3.1. Public data networks via X.25
The formats specified for public data networks accessed via X.25 are described in RFC-877 [8]. Datagrams are transmitted over standard level-3 virtual circuits as complete packet sequences. Virtual circuits are usually established dynamically as required and time-out after a period of no traffic. Link-level retransmission, resequencing and flow control are performed by the network for each virtual circuit and by the LAPB link-level protocol. Note that a single X.25 virtual circuit may be used to multiplex all IP traffic between a pair of hosts. However, multiple parallel virtual circuits may be used in order to improve the utilization of the subscriber access line, in spite of small X.25 window sizes; this can result in random resequencing.
The correspondence between Internet and X.121 addresses is usually established by table-lookup. It is expected that this will be replaced by some sort of directory procedure in the future. The table of the hosts on the Public Data Network is in the Assigned Numbers [23].
The normal MTU is 576; however, the two DTE's (hosts or gateways) can use X.25 packet size negotiation to increase this value [8].
3.2. ARPANET via 1822 LH, DH, or HDH
The formats specified for ARPANET networks using 1822 access are described in BBN Report 1822 [3], which includes the procedures for several subscriber access methods. The Distant Host (DH) method is used when the host and IMP (the Defense Communication Agency calls it a Packet Switch Node or PSN) are separated by not more than about 2000 feet of cable, while the HDLC Distant Host (HDH) is used for greater distances where a modem is required. Under HDH, retransmission, resequencing and flow control are performed by the network and by the HDLC link-level protocol.
The IP encapsulation format is simply to include the IP datagram as the data portion of an 1822 message. In addition, the high-order 8 bits of the Message Id field (also known as the "link" field") should be set to 155 [23]. The MTU is 1007 octets.
While the ARPANET 1822 protocols are widely used at present, they are expected to be eventually overtaken by the DDN Standard X.25 protocol (see Section 3.3). The original IP address mapping (RFC-796 [38]) is in the process of being replaced by a new interface specification called AHIP-E; see RFC-1005 [61] for the proposal.
Gateways connected to ARPANET or MILNET IMPs using 1822 access must incorporate features to avoid host-port blocking (i.e., RFNM counting) and to detect and report as ICMP Unreachable messages the failure of destination hosts or gateways (i.e., convert the 1822 error messages to the appropriate ICMP messages).
In the development of a network interface it will be useful to review the IMP end-to-end protocol described in RFC-979 [29].
3.3. ARPANET via DDN Standard X.25
The formats specified for ARPANET networks via X.25 are described in the Defense Data Network X.25 Host Interface Specification [6], which describes two sets of procedures: the DDN Basic X.25, and the DDN Standard X.25. Only DDN Standard X.25 provides the functionality required for interoperability assumptions of the Internet protocol.
The DDN Standard X.25 procedures are similar to the public data network X.25 procedures, except in the address mappings. Retransmission, resequencing and flow control are performed by the network and by the LAPB link-level protocol. Multiple parallel virtual circuits may be used in order to improve the utilization of the subscriber access line; this can result in random resequencing.
Gateways connected to ARPANET or MILNET using Standard X.25 access must detect and report as ICMP Unreachable messages the failure of destination hosts or gateways (i.e., convert the X.25 diagnostic codes to the appropriate ICMP messages).
To achieve compatibility with 1822 interfaces, the effective MTU for a Standard X.25 interface is 1007 octets.
3.4. Ethernet and IEEE 802
The formats specified for Ethernet networks are described in RFC-894 [10]. Datagrams are encapsulated as Ethernet packets with 48-bit source and destination address fields and a 16-bit type field (the type field values are listed in the Assigned Numbers [23]). Address translation between Ethernet addresses and Internet addresses is managed by the Address Resolution Protocol, which is required in all Ethernet implementations. There is no explicit link-level retransmission, resequencing or flow control, although most hardware interfaces will retransmit automatically in case of collisions on the cable.
The IEEE 802 networks use a Link Service Access Point (LSAP) field in much the same way the ARPANET uses the "link" field. Further, there is an extension of the LSAP header called the Sub-Network Access Protocol (SNAP).
The 802.2 encapsulation is used on 802.3, 802.4, and 802.5 network by using the SNAP with an organization code indicating that the following 16 bits specify the Ether-Type code [23].
Headers:
...--------+--------+--------+
MAC Header| Length | 802.{3/4/5} MAC
...--------+--------+--------+
+--------+--------+--------+
| DSAP=K1| SSAP=K1| control| 802.2 SAP
+--------+--------+--------+
+--------+--------+--------+--------+--------+
|protocol id or org code=K2| Ether-Type | 802.2 SNAP
+--------+--------+--------+--------+--------+
The total length of the SAP Header and the SNAP header is 8-octets, making the 802.2 protocol overhead come out on a 64-bit boundary.
K1 is 170. The IEEE likes to talk about things in bit transmission order and specifies this value as 01010101. In big-endian order, as used in the Internet specifications, this becomes 10101010 binary, or AA hex, or 170 decimal. K2 is 0 (zero).
The use of the IP LSAP (K1 = 6) is reserved for future development.
The assigned values for the Ether-Type field are the same for either this IEEE 802 encapsulation or the basic Ethernet encapsulation [10].
In either Ethernets or IEEE 802 nets, the IP datagram is the data portion of the packet immediately following the Ether-Type.
The MTU for an Ethernet or its IEEE-standard equivalent (802.3) is 1500 octets.
3.5. Serial-Line Protocols
In some configurations, gateways may be interconnected with each other by means of serial asynchronous or synchronous lines, with or without modems. When justified by the expected error rate and other factors, a link-level protocol may be required on the serial line. While there is no single Internet standard for this protocol, it is suggested that one of the following protocols be used.
* X.25 LAPB (Synchronous Lines)
This is the link-level protocol used for X.25 network
access. It includes HDLC "bit-stuffing" as well as
rotating-window flow control and reliable delivery.
A gateway must be configurable to play the role of either
the DCE or the DTE.
* HDLC Framing (Synchronous Lines)
This is just the bit-stuffing and framing rules of LAPB. It
is the simplest choice, although it provides no flow control
or reliable delivery; however, it does provide error
detection.
* Xerox Synchronous Point-to-Point (Synchronous Lines)
This Xerox protocol is an elaboration upon HDLC framing that
includes negotiation of maximum packet sizes, dial-up or
dedicated circuits, and half- or full-duplex operation [12].
* Serial Line Framing Protocol (Asynchronous Lines)
This protocol is included in the MIT PC/IP package for an
IBM PC and is defined in Appendix I to the manual for that
system [20].
It will be important to make efficient use of the bandwidth available on a serial line between gateways. For example, it is desirable to provide some form of data compression. One possible standard compression algorithm, "Thinwire II", is described in RFC-914 [42]. This and similar algorithms are tuned to the particular types of redundancy which occur in IP and TCP headers; however, more work is necessary to define a standard serial-line compression protocol for Internet gateways. Until a standard has been adopted, each vendor is free to choose a compression algorithm; of course, the result will only be useful on a serial line between two gateways using the same compression algorithm.
Another way to ensure maximum use of the bandwidth is to avoid unnecessary retransmissions at the link level. For some kinds of IP traffic, low delay is more important than reliable delivery. The serial line driver could distinguish such datagrams by their IP TOS field, and place them on a special high-priority, no-retransmission queue.
A serial point-to-point line between two gateways may be considered to be a (particularly simple) network, a "null net". Considered in this way, a serial line requires no special considerations in the routing algorithms of the connected gateways, but does need an IP network number. To avoid the wholesale consumption of Internet routing data-base space by null nets, we strongly recommend that subnetting be used for null net numbering, whenever possible.
For example, assume that network 128.203 is to be constructed
of gateways joined by null nets; these nets are given (sub-)net
numbers 128.203.1, 128.203.2, etc., and the two interfaces on
each end of null net 128.203.s might have IP addresses
128.203.s.1 and 128.203.s.2.
An alternative model of a serial line is that it is not a network, but rather an internal communication path joining two "half gateways". It is possible to design an IGP and routing algorithm that treats a serial line in this manner [39, 52].
Gateway Algorithms
Gateways are general packet-switches that forward packets according to the IP address, i.e., they are IP routers. While it is beyond the scope of this document to specify the details of the mechanisms used in any particular, perhaps proprietary, gateway architecture, there are a number of basic algorithms which must be provided by any acceptable design.
4.1. Routing Algorithm
The routing mechanism is fundamental to Internet operation. In all but trivial network topologies, robust Internet service requires some degree of routing dynamics, whether it be effected by manual or automatic means or by some combination of both. In particular, if routing changes are made manually, it must be possible to make these routing changes from a remote Network Operation Center (NOC) without taking down the gateway for reconfiguration. If static routes are used, there must be automatic fallback or rerouting features.
Handling unpredictable changes in Internet connectivity must be considered the normal case, so that systems of gateways will normally be expected to have a routing algorithm with the capability of reacting to link and other gateway failures and changing the routing automatically.
This document places no restriction on the type of routing algorithm, e.g., node-based, link-based or any other algorithm, or on the routing distance metric, e.g., delay or hop-count. However, the following features are considered necessary for a successful gateway routing algorithm:
1. The algorithm must sense the failure or restoration of a
link or other gateway and switch to appropriate paths. A
design objective is to switch paths within an interval less
than the typical TCP user time-out (one minute is a safe
assumption).
2. The algorithm must suppress routing loops between neighbor
gateways and must contain provisions to avoid or suppress
routing loops that may form between non-neighbor gateways.
A design objective is for no loop to persist for longer
than an interval greater than the typical TCP user
time-out.
3. The control traffic necessary to operate the routing
algorithm must not significantly degrade or disrupt normal
network operation. Changes in state which might
momentarily disrupt normal operation in a local-area must
not cause disruption in remote areas of the network.
4. As the size of the network increases, the demand on
resources must be controlled in an efficient way. Table
lookups should be hashed, for example, and data-base
updates handled piecemeal, with only incremental changes
broadcast over a wide-area.
5. The size of the routing data-base must not be allowed to
exceed a constant, independent of network topology, times
the number of nodes times the mean connectivity (average
number of incident links). An advanced design might not
require that the entire routing data-base be kept in any
particular gateway, so that discovery and caching
techniques would be necessary.
6. Reachability and delay metrics, if used, must not depend on
direct connectivity to all other gateways or on the use of
network-specific broadcast mechanisms. Polling procedures
(e.g., for consistency checking) must be used only
sparingly and in no case introduce an overhead exceeding a
constant, independent of network topology, times the
longest non-looping path.
7. Default routes (generally intended as a means to reduce the
size of the routing data-base) must be used with care,
because of the many problems with multiple paths, loops,
and mis-configurations which routing defaults have caused.
The most common application of defaults is for routing
within an Internet region which is connected in a strictly
hierarchical fashion and is a stub from the rest of the
Internet system. In this case, the default is used for
routing "up" the tree. Unfortunately, such restricted
topology seldom lasts very long, and defaults cease to
work.
More generally, defaults could be used for initial routing
guesses, with final routes to be discovered and cached from
external or internal data-bases via the routing algorithm
or EGP.
4.2. Subnets and Routing
We will call a gateway "subnetted" if at least one of its interfaces is connected to a subnet; the set of gateways directly connected to subnets of the same network will be referred to as a "subnet cluster". For example, in the following diagram, network 2 is subnetted, with subnets 2.1 and 2.2, but network 1 is not; gateways 1, 2, and 3 are subnetted and are members of the same subnet cluster.
(Net 1) === [Gwy 1] === (Net 2.1) === [Gwy 2] === (Net 2.2)
| |
| |
=================== [Gwy 3] =======================
Subnets have the following effects on gateway routing:
A. Non-subnetted gateways are not affected at all.
B. The routing data-base in a subnetted gateway must consider
the address mask for subnet entries.
C. Routing updates among the gateways in the same subnet
cluster must include entries for the various subnets. The
corresponding address mask(s) may be implicit, but for full
generality the mask needs to be given explicitly for each
entry. Note that if the routing data-base included a full
32-bit mask for every IP network, the gateway could deal
with networks and subnets in a natural way. This would
also handle the case of multiple subnet masks for the same
subnetted network.
D. Routing updates from a subnetted gateway to a gateway
outside the cluster can contain nets, never subnets.
E. If a subnetted gateway (e.g., gateway 2 above) is unable to
forward a datagram from one subnet to another subnet of the
same network, then it must return a Host Unreachable, not a
Net Unreachable, as discussed in Section 2.2.1.
When considering the choice of routing protocol, a gateway builder must consider how that protocol generalizes for subnets. For some routing protocols it will be possible to use the same procedures in a regular gateway and a subnetted gateway, with only a change of parameters (e.g., address masks).
A different subnet address mask must be configurable for each
interface of a given gateway. This will allow a subnetted gateway to connect to two different subnetted networks, or to connect two subnets of the same network with different masks.
4.3 Resource Allocation
In order to perform its basic datagram-forwarding functions, a gateway must allocate resources; its packet buffers and CPU time must be allocated to packets it receives from connected networks, while the bandwidth to each of the networks must also be allocated for sending packets. The choice of allocation strategies will be critical when a particular resource is scarce. The most obvious allocation strategy, first-come-first-served (FCFS), may not be appropriate under overload conditions, for reasons which we will now explore.
A first example is buffer allocation. It is important for a gateway to allocate buffers fairly among all of its connected networks, even if these networks have widely varying bandwidths. A high-speed interface must not be allowed to starve slower interfaces of buffers. For example, consider a gateway with a 10 Mbps Ethernet connection and two 56 Kbps serial lines. A buggy host on the Ethernet may spray that gateway interface with packets at high speed. Without careful algorithm design in the gateway, this could tie up all the gateway buffers in such a way that transit traffic between the serial lines would be completely stopped.
Allocation of output bandwidth may also require non-FCFS strategies. In an advanced gateway design, allocation of output bandwidth may depend upon Type-of-Service bits in the IP headers. A gateway may also want to give priority to datagrams for its own up/down and routing protocols.
Finally, Nagle [24] has suggested that gateways implement "fair queueing", i.e., sharing output bandwidth equitably among the current traffic sources. In his scheme, for each network interface there would be a dynamically-built set of output queues, one per IP source address; these queues would be serviced in a round-robin fashion to share the bandwidth. If subsequent research shows fair queueing to be desirable, it will be added to a future version of this document as a universal requirement.
4.4. Special Addresses and Filters
Section 2.1 contained a list of the 32-bit IP addresses which have special meanings. They do not in general represent unique IP addresses of Internet hosts, and there are restrictions on their use in IP headers.
We can distinguish two classes of these special cases. The first class (specifically, cases (a), (b), (c), (g), (h), and (i) in section 2.1) contains addresses which should never appear in the destination address field of any IP datagram, so a gateway should never be asked to route to one of these addresses. However, in the real world of imperfect implementations and configuration errors, such bad destination addresses do occur. It is the responsibility of a gateway to avoid propagating such erroneous addresses; this is especially important for gateways included in the global interconnect system. In particular, a gateway which receives a datagram with one of these forbidden addresses should:
1. Avoid inserting that address into its routing database, and
avoid including it in routing updates to any other gateway.
2. Avoid forwarding a datagram containing that address as a
destination.
To enforce these restrictions, it is suggested that a gateway include a configurable filter for datagrams and routing updates. A typical filter entry might consist of a 32-bit mask and value pair. If the logical AND of the given address with the mask equals the value, a match has been found. Since filtering will consume gateway resources, it is vital that the gateway configuration be able to control the degree of filtering in use.
There is a second class of special case addresses (cases (d), (e), and (f) in section 2.1), the so-called "directed broadcasts". A directed broadcast is a datagram to be forwarded normally to the specified destination (sub-)net and then broadcast on the final hop. An Internet gateway is permitted, but not required, to filter out directed broadcasts destined for any of its locally-connected networks. Hence, it should be possible to configure the filter to block the delivery of directed broadcasts.
Finally, it will also be useful for Internet O&M to have a configurable filter on the IP source address. This will allow a network manager to temporarily block traffic from a particular misbehaving host, for example.
4.5. Redirects
The ICMP Redirect message is specified only for use by a gateway to update the routing table of a host on the same connected net. However, the Redirect message is sometimes used between gateways, due to the following considerations:
The routing function in a host is very much like that in a
"dumb gateway" (i.e., a gateway having only static routes). It
is desirable to allow the routing tables of a dumb gateway to
be changed under the control of a dynamic gateway (i.e., a
gateway with full dynamic routing) on the same network. By
analogy, it is natural to let the dynamic gateway send ICMP
Redirect messages to dumb gateway.
The use of ICMP Redirect between gateways in this fashion may be considered to be part of the IGP (in fact, the totality of the IGP, as far as the dumb gateway is concerned!) in the particular Autonomous System. Specification of an IGP is outside the scope of this document, so we only note the possibility of using Redirect in this fashion. Gateways are not required to receive and act upon redirects, and in fact dynamic gateways must ignore them. We also note that considerable experience shows that dumb gateways often create problems resulting in "black holes"; a full routing gateway is always preferable.
Routing table entries established by redirect messages must be removed automatically, either by a time-out or when a use count goes to zero.
4.6. Broadcast and Multicast
A host which is connected to a network (generally a LAN) with an intrinsic broadcast capability may want to use this capability to effect multidestination delivery of IP datagrams. The basic Internet model assumes point-to-point messages, and we must take some care when we incorporate broadcasting. It is important to note that broadcast addresses may occur at two protocol levels: the local network header and the IP header.
Incorrect handling of broadcasting has often been the cause of packet avalanches (sometimes dubbed "meltdown") in LANs. These avalanches are generally caused by gratuitous datagram-forwarding by hosts, or by hosts sending ICMP error messages when they discard broadcast datagrams.
Gateways have a responsibility to prevent avalanches, or datagrams which can trigger avalanches, from escaping into another network.
In general, a gateway must not forward a datagram which arrives via local network broadcast, and must not send an ICMP error message when dropping the datagram. A discussion of the rules will be found in Appendix A; see also [50].
As noted in Section 4.4, a gateway is permitted to filter out directed broadcasts. Hence, directed broadcasts will only be useful in limited Internet regions (e.g., the within the subnets of a particular campus) in which delivery is supported by the gateway administrators. Host group multicasting (see Sections 2.8 and 4.6) will soon provide a much more efficient mechanism than directed broadcasting. Gateway algorithms for host group multicasting will be specified in future RFC's.
4.7. Reachability Procedures
The architecture must provide a robust mechanism to establish the operational status of each link and node in the network, including the gateways, the links connecting them and, where appropriate, the hosts as well. Ordinarily, this requires at least a link-level reachability protocol involving a periodic exchange of messages across each link. This function might be intrinsic to the link-level protocols used (e.g., LAPB). However, it is in general ill-advised to assume a host or gateway is operating correctly even if its link-level reachability protocol is operating correctly. Additional confirmation is required in the form of an operating routing algorithm or peer-level reachability protocol (such as used in EGP).
Failure and restoration of a link and/or gateway are considered network events and must be reported to the control center. It is desirable, although not required, that reporting paths not require correct functioning of the routing algorithm itself.
4.8. Time-To-Live
The Time-to-Live (TTL) field of the IP header is defined to be a timer limiting the lifetime of a datagram in the Internet. It is an 8-bit field and the units are seconds. This would imply that for a maximum TTL of 255 a datagram would time-out after about 4 and a quarter minutes. Another aspect of the definition requires each gateway (or other module) that handles a datagram to decrement the TTL by at least one, even if the elapsed time was much less than a second. Since this is very often the case, the TTL effectively becomes a hop count limit on how far a datagram can propagate through the Internet.
As the Internet grows, the number of hops needed to get from one edge to the opposite edge increases, i.e., the Internet diameter grows.
If a gateway holds a datagram for more than one second, it must decrement the TTL by one for each second.
If the TTL is reduced to zero, the datagram must be discarded, and the gateway may send an ICMP Time Exceeded message to the source. A datagram should never be received with a TTL of zero.
When it originates a datagram, a gateway is acting in the role of a host and must supply a realistic initial value for the TTL.
Operation and Maintenance
5.1. Introduction
Facilities to support operation and maintenance (O&M) activities form an essential part of any gateway implementation. The following kinds of activity are included under gateway O&M:
* Diagnosing hardware problems in the gateway processor, in
its network interfaces, or in the connected networks,
modems, or communication lines.
* Installing a new version of the gateway software.
* Restarting or rebooting a gateway after a crash.
* Configuring (or reconfiguring) the gateway.
* Detecting and diagnosing Internet problems such as
congestion, routing loops, bad IP addresses, black holes,
packet avalanches, and misbehaved hosts.
* Changing network topology, either temporarily (e.g., to
diagnose a communication line problem) or permanently.
* Monitoring the status and performance of the gateways and
the connected networks.
* Collecting traffic statistics for use in (Inter-)network
planning.
Gateways, packet-switches, and their connected communication lines are often operated as a system by a centralized O&M organization. This organization will maintain a (Inter-)network operation center, or NOC, to carry out its O&M functions. It is essential that gateways support remote control and monitoring from such a NOC, through an Internet path (since gateways might not be connected to the same network as their NOC). Furthermore, an IP datagram traversing the Internet will often use gateways under the control of more than one NOC; therefore, Internet problem diagnosis will often involve cooperation of personnel of more than one NOC. In some cases, the same gateway may need to be monitored by more than one NOC.
The tools available for monitoring at a NOC may cover a wide range of sophistication. Proposals have included multi-window, dynamic displays of the entire gateway system, and the use of AI techniques for automatic problem diagnosis.
Gateway O&M facilities discussed here are only a part of the large and difficult problem of Internet management. These problems encompass not only multiple management organizations, but also multiple protocol layers. For example, at the current stage of evolution of the Internet architecture, there is a strong coupling between host TCP implementations and eventual IP-level congestion in the gateway system [9]. Therefore, diagnosis of congestion problems will sometimes require the monitoring of TCP statistics in hosts. Gateway algorithms also interact with local network performance, especially through handling of broadcast packets and ARP, and again diagnosis will require access to hosts (e.g., examining ARP caches). However, consideration of host monitoring is beyond the scope of this RFC.
There are currently a number of R&D efforts in progress in the area of Internet management and more specifically gateway O&M. It is hoped that these will lead quickly to Internet standards for the gateway protocols and facilities required in this area. This is also an area in which vendor creativity can make a significant contribution.
5.2. Gateway O&M Models
There is a range of possible models for performing O&M functions on a gateway. At one extreme is the local-only model, under which the O&M functions can only be executed locally, e.g., from a terminal plugged into the gateway machine. At the other extreme, the fully-remote model allows only an absolute minimum of functions to be performed locally (e.g., forcing a boot), with most O&M being done remotely from the NOC. There intermediate models, e.g., one in which NOC personnel can log into the gateway as a host, using the Telnet protocol, to perform functions which can also be invoked locally. The local-only model may be adequate in a few gateway installations, but in general remote operation from a NOC will be required, and therefore remote O&M provisions are required for most gateways.
Remote O&M functions may be exercised through a control agent (program). In the direct approach, the gateway would support remote O&M functions directly from the NOC using standard Internet protocols (e.g., UDP or TCP); in the indirect approach, the control agent would support these protocols and control the gateway itself using proprietary protocols. The direct approach is preferred, although either approach is acceptable. The use of specialized host hardware and/or software requiring significant additional investment is discouraged; nevertheless, some vendors may elect to provide the control agent as an integrated part of the network in which the gateways are a part. If this is the
case, it is required that a means be available to operate the control agent from a remote site using Internet protocols and paths and with equivalent functionality with respect to a local agent terminal.
It is desirable that a control agent and any other NOC software tools which a vendor provides operate as user programs in a standard operating system. The use of the standard Internet protocols UDP and TCP for communicating with the gateways should facilitate this.
Remote gateway monitoring and (especially) remote gateway control present important access control problems which must be addressed. Care must also be taken to ensure control of the use of gateway resources for these functions. It is not desirable to let gateway monitoring take more than some limited fraction of the gateway CPU time, for example. On the other hand, O&M functions must receive priority so they can be exercised when the gateway is congested, i.e., when O&M is most needed.
There are no current Internet standards for the control and monitoring protocols, although work is in progress in this area. The Host Monitoring Protocol (HMP) [7] could be used as a model until a standard is developed; however, it is strongly recommended that gateway O&M protocol be built on top of one of the standard Internet end-to-end protocols UDP or TCP. An example of a very simple but effective approach to gateway monitoring is contained in RFC-996 [43].
5.3. Gateway O&M Functions
The following O&M functions need to be performed in a gateway:
A. Maintenance -- Hardware Diagnosis
Each gateway must operate as a stand-alone device for the
purposes of local hardware maintenance. Means must be
available to run diagnostic programs at the gateway site
using only on-site tools, which might be only a diskette or
tape and local terminal. It is desirable, although not
required, to be able to run diagnostics or dump the gateway
via the network in case of fault. Means should be provided
to allow remote control from the NOC of of modems attached
to the gateway. The most important modem control capability
is entering and leaving loopback mode, to diagnose line
problems.
B. Control -- Dumping and Rebooting
It must be possible to dump and reboot a stand-alone gateway
upon command from the NOC. In addition, a stand-alone
gateway must include a watchdog timer that either initiates
a reboot automatically or signals a remote control site if
not reset periodically by the software. It is desirable
that the boot data involved reside at an Internet host
(e.g., the NOC host) and be transmitted via the net;
however, the use of local devices at the gateway site is
acceptable.
C. Control -- Configuring the Gateway
Every gateway will have a number of configuration parameters
which must be set (see the next section for examples). It
must be possible to update the parameters without rebooting
the gateway; at worst, a restart may be required.
D. Monitoring -- Status and Performance
A mechanism must be provided for retrieving status and
statistical information from a gateway. A gateway must
supply such information in response to a polling message
from the NOC. In addition, it may be desirable to configure
a gateway to transmit status spontaneously and periodically
to a NOC (or set of NOCs), for recording and display.
Examples of interesting status information include: link
status, queue lengths, buffer availability, CPU and memory
utilization, the routing data-base, error counts, and packet
counts. Counts should be kept for dropped datagrams,
separated by reason. Counts of ICMP datagrams should be
kept by type and categorized into those originating at the
gateway, and those destined for the gateway. It would be
useful to maintain many of these statistics by network
interface, by source/destination network pair, and/or by
source/destination host pair.
Note that a great deal of useful monitoring data is often to
be found in the routing data-base. It is therefore useful
to be able to tap into this data-base from the NOC.
E. Monitoring -- Error Logging
A gateway should be capable of asynchronously sending
exception ("trap") reports to one or more specified Internet
addresses, one of which will presumably be the NOC host.
There must also be a mechanism to limit the frequency of
such trap reports, and the parameters controlling this
frequency must be settable in the gateway configuration.
Examples of conditions which should result in traps include:
datagrams discarded because of TTL expiration (an indicator
of possible routing loops); resource shortages; or an
interface changing its up/down status.
5.4. Gateway Configuration Parameters
Every gateway will have a set of configuration parameters controlling its operation. It must be possible to set these parameters remotely from the NOC or locally at any time, without taking the gateway down.
The following is a partial but representative list of possible configuration parameters for a full-function gateway. The items marked with "(i)" should be settable independently for each network interface.
* (i) IP (sub-) network address
* (i) Subnet address mask
* (i) MTU of local network
* (i) Hardware interface address
* (i) Broadcast compatibility option (0s or 1s)
* EGP parameters -- neighbors, Autonomous System number,
and polling parameters
* Static and/or default routes, if any
* Enable/Disable Proxy ARP
* Source Quench parameters
* Address filter configuration
* Boot-host address
* IP address of time server host
* IP address(es) of logging host(s)
* IP address(es) of hosts to receive traps
* IP address(es) of hosts authorized to issue control
commands
* Error level for logging
* Maximum trap frequency
* Hold-down period (if any)
Appendix A. Technical Details
This Appendix collects a number of technical details and rules concerning datagram forwarding by gateways and datagram handling by hosts, especially in the presence of broadcasting and subnets.
A.1. Rules for Broadcasting
The following rules define how to handle broadcasts of packets and datagrams [50]:
a. Hosts (which do not contain embedded gateways) must NEVER
forward any datagrams received from a connected network,
broadcast or not.
When a host receives an IP datagram, if the destination
address identifies the host or is an IP broadcast address,
the host passes the datagram to its appropriate
higher-level protocol module (possibly sending ICMP
protocol unreachable, but not if the IP address was a
broadcast address). Any other IP datagram must simply be
discarded, without an ICMP error message. Hosts never send
redirects.
b. All packets containing IP datagrams which are sent to the
local-network packet broadcast address must contain an IP
broadcast address as the destination address in their IP
header. Expressed in another way, a gateway (or host) must
not send in a local-network broadcast packet an IP datagram
that has a specific IP host address as its destination
field.
c. A gateway must never forward an IP datagram that arrives
addressed to the IP limited broadcast address {-1,-1}.
Furthermore, it must must not send an ICMP error message
about discarding such a datagram.
d. A gateway must not forward an IP datagram addressed to
network zero, i.e., {0, *}.
e. A gateway may forward a directed broadcast datagram, i.e.,
a datagram with the IP destination address:
{ <Network-number>, -1}.
However, it must not send such a directed broadcast out the
same interface it came in, if this interface has
<Network-number> as its network number. If the code in the
gateway making this decision does not know what interface
the directed-broadcast datagram arrived on, the gateway
cannot support directed broadcast to this connected network
at all.
f. A gateway is permitted to protect its connected networks by
discarding directed broadcast datagrams.
A gateway will broadcast an IP datagram on a connected network if it is a directed broadcast destined for that network. Some gateway-gateway routing protocols (e.g., RIP) also require broadcasting routing updates on the connected networks. In either case, the datagram must have an IP broadcast address as its destination.
Note: as observed earlier, some host implementations (those
based on Berkeley 4.2BSD) use zero rather than -1 in the host
field. To provide compatibility during the period until these
systems are fixed or retired, it may be useful for a gateway to
be configurable to send either choice of IP broadcast address
and accept both if received.
A.2. ICMP Redirects
A gateway will generate an ICMP Redirect if and only if the destination IP address is reachable from the gateway (as determined by the routing algorithm) and the next-hop gateway is on the same (sub-)network as the source host. Redirects must not be sent in response to an IP network or subnet broadcast address or in response to a Class D or Class E IP address.
A host must discard an ICMP Redirect if the destination IP address is not its own IP address, or the new target address is not on the same (sub-)network. An accepted Redirect updates the routing data-base for the old target address. If there is no route associated with the old target address, the Redirect is ignored. If the old route is associated with a default gateway, a new route associated with the new target address is inserted in the data-base.
Appendix B. NSFNET Specific Requirements
The following sections discuss certain issues of special concern to the NSF scientific networking community. These issues have primary relevance in the policy area, but also have ramifications in the technical area.
B.1. Proprietary and Extensibility Issues
Although hosts, gateways and networks supporting Internet technology have been in continuous operation for several years, vendors users and operators must understand that not all networking issues are fully resolved. As a result, when new needs or better solutions are developed for use in the NSF networking community, it may be necessary to field new protocols or augment existing ones. Normally, these new protocols will be designed to interoperate in all practical respects with existing protocols; however, occasionally it may happen that existing systems must be upgraded to support these new or augmented protocols.
NSF systems procurements may favor those vendors who undertake a commitment to remain aware of current Internet technology and be prepared to upgrade their products from time to time as appropriate. As a result, vendors are strongly urged to consider extensibility and periodic upgrades as fundamental characteristics of their products. One of the most productive and rewarding ways to do this on a long-term basis is to participate in ongoing Internet research and development programs in partnership with the academic community.
B.2. Interconnection Technology
In order to ensure network-level interoperability of different vendor's gateways within the NSFNET context, we specify that a gateway must at a minimum support Ethernet connections and serial line protocol connections.
Currently the most important common interconnection technology between Internet systems of different vendors is Ethernet. Among the reasons for this are the following:
1. Ethernet specifications are well-understood and mature.
2. Ethernet technology is in almost all aspects vendor
independent.
3. Ethernet-compatible systems are common and becoming more
so.
These advantages combined favor the use of Ethernet technology as the common point of demarcation between NSF network systems supplied by different vendors, regardless of technology. It is a requirement of NSF gateways that, regardless of the possibly proprietary switching technology used to implement a given vendor-supplied network, its gateways must support an Ethernet attachment to gateways of other vendors.
It is expected that future NSF gateway requirements will specify other interconnection technologies. The most likely candidates are those based on X.25 or IEEE 802, but other technologies including broadband cable, optical fiber, or other media may also be considered.
B.3. Routing Interoperability
The Internet does not currently have an "open IGP" standard, i.e., a common IGP which would allow gateways from different vendors to form a single Autonomous System. Several approaches to routing interoperability are currently in use among vendors and the NSF networking community.
* Proprietary IGP
At least one gateway vendor has implemented a proprietary IGP
and uses EGP to interface to the rest of the Internet.
* RIP
Although RIP is undocumented and various implementations of it
differ in subtle ways, it has been used successfully for
interoperation among multiple vendors as an IGP.
* Gateway Daemon
The NSF networking community has built a "gateway daemon"
program which can mediate among multiple routing protocols to
create a mixed-IGP Autonomous System. In particular, the
prototype gateway daemon executes on a 4.3BSD machine acting as
a gateway and exchanges routing information with other
gateways, speaking both RIP and Hello protocols; in addition,
it supports EGP to other Autonomous Systems.
B.4. Multi-Protocol Gateways
The present NSF gateway requirements specify only the Internet protocol IP. However, in a few years the Internet will begin a gradual transition to the functionally-equivalent subset of the ISO protocols [17]. In particular, an increasing percentage of the traffic will use the ISO Connectionless Mode Network Service (CLNS, but commonly called "ISO IP") [33] in place of IP. It is expected that the ISO suite will eventually become the dominant one; however, it is also expected that requirements to support Internet IP will continue, perhaps indefinitely.
To support the transition to ISO protocols and the coexistence stage, it is highly desirable that a gateway design provide for future extensions to support more than one protocol simultaneous, and in particular both IP and CLNS [18].
Present NSF gateway requirements do not include protocols above the network layer, such as TCP, unless necessary for network monitoring or control. Vendors should recognize that future requirements to interwork between Internet and ISO applications, for example, may result in an opportunity to market gateways supporting multiple protocols at all levels up through the application level [16]. It is expected that the network-level NSF gateway requirements summarized in this document will be incorporated in the requirements document for these application-level gateways.
Internet gateways function as intermediate systems (IS) with respect to the ISO connectionless network model and incorporate defined packet formats, routing algorithms and related procedures [33, 34]. The ISO ES-IS [37] provides the functions of ARP and ICMP Redirect.
B.5. Access Control and Accounting
There are no requirements for NSF gateways at this time to incorporate specific access-control and accounting mechanisms in the design; however, these important issues are currently under study and will be incorporated into a subsequent edition of this document. Vendors are encouraged to plan for the introduction of these mechanisms into their products. While at this time no definitive common model for access control and accounting has emerged, it is possible to outline some general features such a model is likely to have, among them the following:
1. The primary access control and accounting mechanisms will
be in the service hosts themselves, not the gateways,
packet-switches or workstations.
2. Agents acting on behalf of access control and accounting
mechanisms may be necessary in the gateways, to collect
data, enforce password protection, or mitigate resource
priority and fairness. However, the architecture and
protocols used by these agents may be a local matter and
cannot be specified in advance.
3. NSF gateways may be required to incorporate access control
and accounting mechanisms based on datagram
source/destination address, as well as other fields in the
IP header.
4. NSF gateways may be required to enforce policies on access
to gateway and communication resources. These policies may
be based upon equity ("fairness") or upon inequity
("priority").
Acknowledgments
An earlier version of this document (RFC-985) [60] was prepared by Dave Mills in behalf of the Gateway Requirements Subcommittee of the NSF Network Technical Advisory Group, in cooperation with the Internet Activities Board, Internet Architecture Task Force, and Internet Engineering Task Force. This effort was chaired by Dave Mills, and contributed to by many people.
The authors of current document have also received assistance from many people in the NSF and ARPA networking community. We thank you, one and all.
References and Bibliography
Many of these references are available from the DDN Network Information Center, SRI International, 333 Ravenswood Avenue, Menlo Park, California 94025 (telephone: 800-235-3155).
[1] Postel, J., "Internet Protocol", RFC-791, USC Information
Sciences Institute, September 1981.
[2] Postel, J., "Internet Control Message Protocol", RFC-792, USC
Information Sciences Institute, September 1981.
[3] BBN, "Interface Message Processor - Specifications for the
Interconnection of a Host and an IMP", Report 1822, Bolt
Beranek and Newman, December 1981.
[4] Plummer, D., "An Ethernet Address Resolution Protocol",
RFC-826, Symbolics, September 1982.
[5] DOD, "Military Standard Internet Protocol", Military Standard
MIL-STD-1777, United States Department of Defense, August 1983.
[6] BBN, "Defense Data Network X.25 Host Interface Specification",
Report 5476, Bolt Beranek and Newman, December 1983.
[7] Hinden, R., "A Host Monitoring Protocol", RFC-869, BBN
Communications, December 1983.
[8] Korb, J.T., "A Standard for the Transmission of IP Datagrams
over Public Data Networks", RFC-877, Purdue University,
September 1983.
[9] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC-896, Ford Aerospace, January 1984.
[10] Hornig, C., "A Standard for the Transmission of IP Datagrams
over Ethernet Networks", RFC-894, Symbolics, April 1984.
[11] Mills, D.L., "Exterior Gateway Formal Specification", RFC-904,
M/A-COM Linkabit, April 1984.
[12] Xerox, "Xerox Synchronous Point-to-Point Protocol", Xerox
System Integration Standard 158412, December 1984.
[13] Kirton, P., "EGP Gateway under Berkeley UNIX 4.2", RFC-911, USC
Information Sciences Institute, August 1984.
[14] Postel, J., "Multi-LAN Address Resolution", RFC-925, USC
Information Sciences Institute, October 1984.
[15] Finlayson, R., T. Mann, J. Mogul, and M. Theimer, "A Reverse
Address Resolution Protocol", RFC-904, Stanford University,
June 1984.
[16] NRC, "Transport Protocols for Department of Defense Data
Networks", RFC-942, National Research Council, March 1985.
[17] Postel, J., "DOD Statement on NRC Report", RFC-945, USC
Information Sciences Institute, April 1985.
[18] ISO, "Addendum to the Network Service Definition Covering
Network Layer Addressing", RFC-941, International Standards
Organization, April 1985.
[19] Leiner, B., J. Postel, R. Cole and D. Mills, "The DARPA
Internet Protocol Suite", Proceedings INFOCOM 85, IEEE,
Washington DC, March 1985. Also in: IEEE Communications
Magazine, March 1985. Also available as ISI-RS-85-153.
[20] Romkey, J., "PC/IP Programmer's Manual", MIT Laboratory for
Computer Science, pp. 57-59, April 1986.
[21] Mogul, J., and J. Postel, "Internet Standard Subnetting
Procedure", RFC-950, Stanford University, August 1985.
[22] Reynolds, J., and J. Postel, "Official Internet Protocols",
RFC-1011, USC Information Sciences Institute, May 1987.
[23] Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1010, USC
Information Sciences Institute, May 1987.
[24] Nagle, J., "On Packet Switches with Infinite Storage", RFC-970,
Ford Aerospace, December 1985.
[25] SRI, "DDN Protocol Handbook", NIC-50004, NIC-50005, NIC-50006,
(three volumes), SRI International, December 1985.
[26] SRI, "ARPANET Information Brochure", NIC-50003, SRI
International, December 1985.
[27] Mills, D.L., "Autonomous Confederations", RFC-975, M/A-COM
Linkabit, February 1986.
[28] Jacobsen, O., and J. Postel, "Protocol Document Order
Information", RFC-980, SRI International, March 1986.
[29] Malis, A.G., "PSN End-to-End Functional Specification",
RFC-979, BBN Communications, March 1986.
[30] Postel, J, "Internetwork Applications using the DARPA Protocol
Suite", Proceedings INFOCOM 85, IEEE, Washington DC,
March 1985. Also available as ISI-RS-85-151.
[31] Postel, J, C. Sunshine, and D. Cohen, "The ARPA Internet
Protocol", Computer Networks, Vol. 5, No. 4, July 1981.
[32] Cerf, V., and R. Kahn, "A Protocol for Packet Network
Intercommunication", IEEE Transactions on Communication,
May 1974.
[33] ISO, "Protocol for Providing the Connectionless-mode Network
Service", RFC-994, DIS-8473, International Standards
Organization, March 1986.
[34] ANSI, "Draft Network Layer Routing Architecture", ANSI X3S3.3,
86-215R, April 1987.
[35] Rosen, E., "Exterior Gateway Protocol (EGP)", RFC-827, Bolt
Beranek and Newman, October 1982.
[36] Sidhu, D., "Some Problems with the Specification of the
Military Standard Internet Protocol", RFC-963, Iowa State
University, November 1985.
[37] ISO, "End System to Intermediate System Routing Exchange
Protocol for use in conjunction with ISO 8473", RFC-995,
April 1986.
[38] Postel, J., "Address Mappings", RFC-796, USC/Information
Sciences Institute, September 1981.
[39] Mills, D., "DCN Local Network Protocols", RFC-891, M/A-COM
Linkabit, December 1983.
[40] McQuillan, J. M., I. Richer, and E. C. Rosen, "The New Routing
Algorithm for the ARPANET", IEEE Transactions on
Communications, May 1980.
[41] Hinden, R., and A. Sheltzer, "The DARPA Internet Gateway",
RFC-823, Bolt Beranek and Newman, September 1982.
[42] Farber, D., G. Delp, and T. Conte, "A Thinwire Protocol for
Connecting Personal Computers to the Internet", RFC-914,
University of Delaware, September 1984.
[43] Mills, D., "Statistics Server", RFC-996, University Of
Delaware, February 1987.
[44] Postel, J. and K. Harrenstien, "Time Protocol", RFC-868,
May 1983.
[45] Mills, D., "Network Time Protocol (NTP)", RFC-958, M/A-Com
Linkabit, September 1985.
[46] Seamonson, L., and E. Rosen, "Stub Exterior Gateway Protocol",
RFC-888, Bolt Beranek And Newman, January 1984.
[47] Deering, S., and D. Cheriton, "Host Groups: A Multicast
Extension to the Internet Protocol", RFC-966, Stanford
University, December 1985.
[48] Deering, S., "Host Extensions for IP Multicasting", RFC-988,
Stanford University, July 1986.
[49] Mogul, J., "Broadcasting Internet Datagrams", RFC-919, Stanford
University, October 1984.
[50] Mogul, J., "Broadcasting Internet Datagrams in the Presence of
Subnets", RFC-922, Stanford University, October 1984.
[51] Rosen, E., "Exterior Gateway Protocol", RFC-827, Bolt Beranek
and Newman, October 1982.
[52] Rose, M., "Low Tech Connection into the ARPA Internet: The Raw
Packet Split Gateway", Technical Report 216, Department of
Information and Computer Science, University of California,
Irvine, February 1984.
[53] Rosen, E., "Issues in Buffer Management", IEN-182, Bolt Beranek
and Newman, May 1981.
[54] Rosen, E., "Logical Addressing", IEN-183, Bolt Beranek and
Newman, May 1981.
[55] Rosen, E., "Issues in Internetting - Part 1: Modelling the
Internet", IEN-184, Bolt Beranek and Newman, May 1981.
[56] Rosen, E., "Issues in Internetting - Part 2: Accessing the
Internet", IEN-187, Bolt Beranek and Newman, June 1981.
[57] Rosen, E., "Issues in Internetting - Part 3: Addressing",
IEN-188, Bolt Beranek and Newman, June 1981.
[58] Rosen, E., "Issues in Internetting - Part 4: Routing", IEN-189,
Bolt Beranek and Newman, June 1981.
[59] Sunshine, C., "Comments on Rosen's Memos", IEN-191, USC
Information Sciences Institute, July 1981.
[60] NTAG, "Requirements for Internet Gateways -- Draft", RFC-985,
Network Technical Advisory Group, National Science Foundation,
May 1986.
[61] Khanna, A., and Malis, A., "The ARPANET AHIP-E Host Access
Protocol (Enhanced AHIP)", RFC-1005, BBN Communications,
May 1987
[62] Nagle, J., "Congestion Control in IP/TCP Internetworks", ACM
Computer Communications Review, Vol.14, no.4, October 1984.
