EGP Confederations: Autonomous confederations

Network Working Group                                        D. L. Mills
Request 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.

1.  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

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   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,

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      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.

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2.  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

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      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.

3.  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

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   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.

4.  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.

5.  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.