RFC975

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Network Working Group D. L. MillsRequest for Comments: 975 M/A-COM Linkabit February 1986

                   Autonomous Confederations

Status of This Memo

This RFC proposes certain enhancements of the Exterior Gateway Protocol (EGP) to support a simple, multiple-level routing capability while preserving the robustness features of the current EGP model. It requests discussion and suggestions for improvements. Distribution of this memo is unlimited.

Overview

The enhancements, which do not require retrofits in existing implementations in order to interoperate with enhanced implementations, in effect generalize the concept of core system to include multiple communities of autonomous systems, called autonomous confederations. Autonomous confederations maintain a higher degree of mutual trust than that assumed between autonomous systems in general, including reasonable protection against routing loops between the member systems, but allow the routing restrictions of the current EGP model to be relaxed.

The enhancements involve the "hop count" or distance field of the EGP Update message, the interpretation of which is not covered by the current EGP model. This field is given a special interpretation within each autonomous confederation to support up to three levels of routing, one within the autonomous system, a second within the autonomous confederation and an optional third within the universe of confederations.

Introduction and Background

The historical development of Internet exterior-gateway routing algorithms began with a rather rigid and restricted topological model which emphasized robustness and stability at the expense of routing dynamics and flexibility. Evolution of robust and dynamic routing algorithms has since proved extraordinarily difficult, probably due more to varying perceptions of service requirements than to engineering problems.

The original exterior-gateway model suggested in RFC-827 [1] and subsequently refined in RFC-888 [2] severely restricted the Internet topology essentially to a tree structure with root represented by the BBN-developed "core" gateway system. The most important characteristic of the model was that debilitating resource-consuming routing loops between clusters of gateways (called autonomous



Autonomous Confederations


systems) could not occur in a tree-structured topology. However, the administrative and enforcement difficulties involved, not to mention the performance liabilities, made widespread implementation impractical.

1.1. The Exterior Gateway Protocol

  Requirements for near-term interoperability between the BBN core
  gateways and the remainder of the gateway population implemented
  by other organizations required that an interim protocol be
  developed with the capability of exchanging reachability
  information, but not necessarily the capability to function as a
  true routing algorithm. This protocol is called the Exterior
  Gateway Protocol (EGP) and is documented in RFC-904 [3].
  EGP was not designed as a routing algorithm, since no agreement
  could be reached on a trusted, common metric.  However, EGP was
  designed to provide high-quality reachability information, both
  about neighbor gateways and about routes to non-neighbor gateways.
  At the present state of development, dynamic routes are computed
  only by the core system and provided to non-core gateways using
  EGP only as an interface mechanism.  Non-core gateways can provide
  routes to the core system and even to other non-core gateways, but
  cannot pass on "third-party" routes computed using data received
  from other gateways.
  As operational experience with EGP has accumulated, it has become
  clear that a more decentralized dynamic routing capability is
  needed in order to avoid resource-consuming suboptimal routes.  In
  addition, there has long been resistance to the a-priori
  assumption of a single core system, with implications of
  suboptimal performance, administrative problems, impossible
  enforcement and possible subversion.  Whether or not this
  resistance is real or justified, the important technical question
  remains whether a more dynamic, distributed approach is possible
  without significantly diluting stability and robustness.
  This document proposes certain enhancements of EGP which
  generalize the concept of core system to include multiple
  communities of autonomous systems, called autonomous
  confederations.  Autonomous confederations maintain a higher
  degree of mutual trust than that assumed between autonomous
  systems in general, including reasonable protection against
  routing loops between the member systems.  The enhancements
  involve the "hop count" or distance field of the EGP Update




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  message, which is given a special interpretation as described
  later.  Note that the interpretation of this field is not
  specified in RFC-904, but is left as a matter for further study.
  The interpretation of the distance field involves three levels of
  metrics, in which the lowest level is available to the interior
  gateway protocol (IGP) of the autonomous system itself to extend
  the interior routes to the autonomous system boundary.  The next
  higher level selects preferred routes within the autonomous system
  to those outside, while the third and highest selects preferred
  routes within the autonomous confederation to those outside.
  The proposed model is believed compatible with the current
  specifications and practices used in the Internet.  In fact, the
  entire present conglomeration of autonomous systems, including the
  core system, can be represented as a single autonomous
  confederation, with new confederations being formed from existing
  and new systems as necessary.

1.2. Routing Restrictions

  It was the intent in RFC-904 that the stipulated routing
  restrictions superceded all previous documents, including RFC-827
  and RFC-888.  The notion that a non-core gateway must not pass on
  third-party information was suggested in planning meetings that
  occured after the previous documents had been published and before
  RFC-904 was finalized.  This effectively obsoletes prior notions
  of "stub" or any other asymmetry other than the third-party rule.
  Thus, the only restrictions placed on a non-core gateway is that
  in its EGP messages (a) a gateway can be listed only if it belongs
  to the same autonomous system (internal neighbor) and (b) a net
  can be listed only if it is reachable via gateways belonging to
  that system.  There are no other restrictions, overt or implied.
  The specification does not address the design of the core system
  or its gateways.
  The restrictions imply that, to insure full connectivity, every
  non-core gateway must run EGP with a core gateway.  Since the
  present core-gateway implementation disallows other gateways on
  EGP-neighbor paths, this further implies that every non-core
  gateway must share a net in common with at least one core gateway.
  Note that there is no a-priori prohibition on using EGP as an IGP,
  or even on using EGP with a gateway of another non-core system,




Autonomous Confederations


  providing that the third-party rule is observed.  If a gateway in
  each system ran EGP with a gateway in every other system, the
  notion of core system would be unneccessary and superflous.
  At one time during the evolution of the EGP model a strict
  hierarchical topology (tree structure) of autonomous systems was
  required, but this is not the case now.  At one time it was
  forbidden for two nets to be connected by gateways of two or more
  systems, but this is not the case now.  Autonomous systems are
  sets of gateways, not nets or hosts, so that a given net or host
  can be reachable via more than one system;  however, every gateway
  belongs to exactly one system.

1.3. Examples and Problems

  Consider the common case of two local-area nets A and B connected
  to the ARPANET by gateways of different systems.  Now assume A and
  B are connected to each other by a gateway A-B belonging to the
  same system as the A-ARPANET gateway, which could then list itself
  and both the A and B nets in EGP messages sent to any other
  gateway, since both are now reachable in its system.  However, the
  B-ARPANET gateway could list itself and only the B net, since the
  A-B gateway is not in its system.
  In principle, we could assume the existence of a second gateway
  B-A belonging to the same system as the B-ARPANET gateway, which
  would entitle it to list the A net as well;  however, it may be
  easier for both systems to sign a treaty and consider the A-B
  gateway under joint administration.  The implementation of the
  treaty may not be trivial, however, since the joint gateway must
  appear to other gateways as two distinct gateways, each with its
  own autonomous-system number.
  Another case occurs when for some reason or other a system has no
  path to a core gateway other than via another non-core system.
  Consider a third local-are net C, together with gateway C-A
  belonging to a system other than the A-ARPANET and B-ARPANET
  gateways.  According to the restrictions above, gateway C-A could
  list net C in EGP messages sent to A-ARPANET, while A-ARPANET
  could list ARPANET in messages sent to C-A, but not other nets
  which it may learn about from the core.  Thus, gateway C-A cannot
  acquire full routing information unless it runs EGP directly with
  a core gateway.





Autonomous Confederations


Autonomous Systems and Confederations

The second example above illustrates the need for a mechanism in which arbitrary routing information can be exchanged between non-core gateways without degrading the degree of robustness relative to a mutually agreed security model. One way of doing this is is to extend the existing single-core autonomous-system model to include multiple core systems. This requires both a topological model which can be used to define the scope of these systems together with a global, trusted metric that can be used to drive the routing computations. An appropriate topological model is described in the next section, while an appropriate metric is suggested in the following section.

2.1. Topological Models

  An "autonomous system" consists of a set of gateways, each of
  which can reach any other gateway in the same system using paths
  via gateways only in that system.  The gateways of a system
  cooperatively maintain a routing data base using an interior
  gateway protocol (IGP) and a intra-system trusted routing
  mechanism of no further concern here.  The IGP is expected to
  include security mechanisms to insure that only gateways of the
  same system can acquire each other as neighbors.
  One or more gateways in an autonomous system can run EGP with one
  or more gateways in a neighboring system.  There is no restriction
  on the number or configuration of EGP neighbor paths, other than
  the requirement that each path involve only gateways of one system
  or the other and not intrude on a third system.  It is
  specifically not required that EGP neighbors share a common
  network, although most probably will.
  An "autonomous confederation" consists of a set of autonomous
  systems sharing a common security model;  that is, they trust each
  other to compute routes to other systems in the same
  confederation.  Each gateway in a confederation can reach any
  other gateway in the same confederation using paths only in that
  confederation.  Although there is no restriction on the number or
  configuration of EGP paths other than the above, it is expected
  that some mechanism be available so that potential EGP neighbors
  can discover whether they are in the same confederation.  This
  could be done by access-control lists, for example, or by
  partitioning the set of system numbers.
  A network is "directly reachable" from an autonomous system if a
  gateway in that system has an interface to it.  Every gateway in



Autonomous Confederations


  that system is entitled to list all directly reachable networks in
  EGP messages sent to any other system.  In general, it may happen
  that a particular network is directly reachable from more than one
  system.
  A network is "reachable" from an autonomous system if it is
  directly reachable from an autonomous system belonging to the same
  confederation.  A directly reachable net is always reachable from
  the same system.  Every gateway in that confederation is entitled
  to list all reachable nets in EGP messages sent to any other
  system.  It may happen that a particular net is either directly
  reachable or reachable from different confederations.
  In order to preserve global routing stability in the Internet, it
  is explicitly assumed that routes within an autonomous system to a
  directly reachable net are always preferred over routes outside
  that system and that routes within an autonomous confederation are
  always preferred over routes outside that confederation.  The
  mechanism by which this is assured is described in the next
  section.
  In general, EGP Update messages can include two lists of gateways,
  one for those gateways belonging to the same system (internal
  neighbors) and the other for gateways belonging to different
  systems (external neighbors).  Directly reachable nets must always
  be associated with gateways of the same system, that is, with
  internal neighbors, while non-directly reachable nets can be
  associated with either internal or external neighbors.  Nets that
  are reachable, but not directly reachable, must always be
  associated with gateways of the same confederation.

2.2. Trusted Routing Metrics

  There seems to be a general principle which characterizes
  distributed systems:  The "nearer" a thing is the more dynamic and
  trustable it is, while the "farther" a thing is the more static
  and suspicious it is.  For instance, the concept of network is
  intrinsic to the Internet model, as is the concept of gateways
  which bind them together.  A cluster of gateways "near" each other
  (e.g.  within an autonomous system) typically exchange routing
  information using a high-performance routing algorithm capable of
  sensitive monitoring of, and rapid adaptation to, changing
  performance indicators such as queueing delays and link loading.
  However, clusters of gateways "far" from each other (e.g.  widely
  separated autonomous systems) usually need only coarse routing
  information, possibly only "hints" on the best likely next hop to



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  the general destination area.  On the other hand, mutual suspicion
  increases with distance, so these clusters may need elaborate
  security considerations, including peer authentication,
  confidentiality, secrecy and signature verification.  In addition,
  considerations of efficiency usually dictate that the allowable
  network bandidth consumed by the routing protocol itself decreases
  with distance.  The price paid for both of these things typically
  is in responsiveness, with the effect that the more distant
  clusters are from each other, the less dynamic is the routing
  algorithm.
  The above observations suggest a starting point for the evolution
  of a globally acceptable routing metric.  Assume the metric is
  represented by an integer, with low values representing finer
  distinctions "nearer" the gateway and high values coarser
  distinctions "farther" from it.  Values less than a globally
  agreed constant X are associated with paths confined to the same
  autonomous system as the sender, values greater than X but less
  than another constant Y with paths confined to the autonomous
  confederation of the sender and values greater than Y associated
  with the remaining paths.
  At each of these three levels - autonomous system, autonomous
  confederation and universe of confederations - multiple routing
  algorithms could be operated simultaneously, with each producing
  for each destination net a possibly different subtree and metric
  in the ranges specified above.  However, within each system the
  metric must have the same interpretation, so that other systems
  can mitigate routes between multiple gateways in that system.
  Likewise, within each confederation the metric must have the same
  interpretation, so that other confederations can mitigate routes
  to gateways in that confederation.  Although all confederations
  must agree on a common universe-of-confederations algorithm, not
  all confederations need to use the same confederation-level
  algorithm and not all systems in the same confederation need to
  use the same system-level algorithm.

Implementation Issues

The manner in which the eight-bit "hop count" or distance field in the EGP Update to be used is not specified in RFC-904, but left as a matter for further study. The above model provides both an interpretation of this field, as well as hints on how to design appropriate routing algorithms.

For the sake of illustration, assume the values of X and Y above are 128 and 192 respectively. This means that the gateways in a



Autonomous Confederations


particular system will assign distance values less than 128 for directly-reachable nets and that exterior gateways can compare these values freely in order to select among these gateways. It also means that the gateways in all systems of a particular confederation will assign distance values between 128 and 192 for those nets not directly reachable in the system but reachable in the confederation. In the following it will be assumed that the various confederations can be distinguished by some feature of the 16-bit system-number field, perhaps by reserving a subfield.

3.1. Data-Base Management Functions

  The following implementation model may clarify the above issues,
  as well as present at least one way to organize the gateway data
  base.  The data base is organized as a routing table, the entries
  of which include a net number together with a list of items, where
  each item consists of (a) the gateway address, system number and
  distance provided by an EGP neighbor, (b) a time-to-live counter,
  local routing information and other information as necessary to
  manage the data base.
  The routing table is updated each time an EGP Update message is
  received from a neighbor and possibly by other means, such as the
  system IGP.  The message is first decoded into a list of quads
  consisting of a network number, gateway address, system number and
  distance.  If the gateway address is internal to the neighbor
  system, as determined from the EGP message, the system number of
  the quad is set to that system; while, if not, the system number
  is set to zero, indicating "external."
  Next, a new value of distance is computed from the old value
  provided in the message and subject to the following constraints:
  If the system number matches the local system number, the new
  value is determined by the rules for the system IGP but must be
  less than 128. If not and either the system number belongs to the
  same confederation or the system number is zero and the old
  distance is less than 192, the value is determined by the rules
  for the confederation EGP, but must be at least 128 and less than
  192.  Otherwise, the value is determined by the rules for the
  (global) universe-of-federations EGP, but must be at least 192.
  For each quad in the list the routing table is first searched for
  matching net number and a new entry made if not already there.
  Next, the list of items for that net number is searched for
  matching gateway address and system number and a new entry made if
  not already there. Finally, the distance field is recomputed, the
  time-to-live field reset and local routing information inserted.



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  The time-to-live fields of all items in each list are incremented
  on a regular basis.  If a field exceeds a preset maximum, the item
  is discarded;  while, if all items on a list are discarded, the
  entire entry including net number is discarded.
  When a gateway sends an EGP Update message to a neighbor, it must
  invert the data base in order by gateway address, rather than net
  number.  As part of this process the routing table is scanned and
  the gateway with minimum distance selected for each net number.
  The resulting list is sorted by gateway address and partitioned on
  the basis of internal/external system number.

3.2. Routing Functions

  A gateway encountering a datagram (service unit) searches the
  routing table for matching destination net number and then selects
  the gateway on that list with minimum distance.  As the result of
  the value assignments above, it should be clear that routes at a
  higher level will never be chosen if routes at a lower level
  exist.  It should also be clear that route selection within a
  system cannot affect route selection outside that system, except
  through the intervention of the intra-confederation routing
  algorithm.  If a simple min-system-hop algorithm is used for the
  confederation EGP, the IGP of each system can influence it only to
  the extent of reachability.

3.3. Compatibility Issues

  The proposed interpretation is backwards-compatibile with known
  EGP implementations which do not interpret the distance field and
  with several known EGP implementations that take private liberties
  with this field.  Perhaps the simplest way to evolve the present
  system is to collect the existing implementations that do not
  interpet the distance field at all as a single confederation with
  the present core system and routing restrictions.  All distances
  provided by this confederation would be assumed equal to 192,
  which would provide at least a rudimentary capability for routing
  within the universe of confederations.
  One or more existing or proposed systems in which the distance
  field has a uniform interpretation throughout the system can be
  organized as autonomous confederations.  This might include the
  Butterfly gateways now now being deployed, as well as clones
  elsewhere. These systems provide the capability to select routes
  into the system based on the distance fields for the different
  gateways.  It is anticipated that the distance fields for the
  Butterfly system can be set to at least 128 if the routing



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  information comes from another Butterfly system and to at least
  192 if from a non-Butterfly system presumed outside the
  confederation.
  New systems using an implmentation model such as suggested above
  can select routes into a confederation based on the distance
  field.  For this to work properly, however, it is necessary that
  all systems and confederations adopt a consistent interpretation
  of distance values exceeding 192.

Summary and Conclusions

Taken at face value, this document represents a proposal for an interpretation of the distance field of the EGP Update message, which has previously been assigned no architected interpretation, but has been often used informally. The proposal amounts to ordering the autonomous systems in a hierarchy of systems and confederations, together with an interpretation of the distance field as a three-level metric. The result is to create a corresponding three-level routing community, one prefering routes inside a system, a second preferring routes inside a confederation and the third with no preference.

While the proposed three-level hierarchy can readily be extended to any number of levels, this would create strain on the distance field, which is limited to eight bits in the current EGP model.

The concept of distance can easily be generalized to "administrative distance" as suggested by John Nagle and others.

References

[1] Rosen, E., Exterior Gateway Protocol (EGP), DARPA Network Working Group Report RFC-827, Bolt Beranek and Newman, September 1982. [2] Seamonson, L.J., and E.C., Rosen. "STUB" Exterior Gateway Protocol, DARPA Network Working Group Report RFC-888, BBN Communications, January 1984. [3] Mills, D.L., Exterior Gateway Protocol Formal Specification, DARPA Network Working Group Report RFC-904, M/A-COM Linkabit, April 1984.