rfc1126.txt

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RFC 1126            Inter-Autonomous System Routing         October 1989


   determines the boundary gateway which will serve as the exit gateway.
   This may require that the entrance boundary gateway add a source
   route to the packet, or encapsulate the packet in another level of IP
   or gateway-to-gateway header.  This allows boundary gateways to
   forward data traffic using the appropriate tunnelling technique.

   Finally, with solution (3), the interior gateways have some knowledge
   of Inter-AS Routing.  At a minimum, the interior gateways would need
   to know the identity of each boundary gateway, the address(es) that
   can be reached by that gateway, and the Inter-AS metric associated
   with the route to that address(es).  If the IARP allows for separate
   routing for multiple TOS classes, then the information that the
   interior gateways need to know includes a separate Inter-AS metric
   for each TOS class.  The Inter-AS metrics are necessary to allow
   gateways to choose among multiple possible exit boundary gateways.
   In general, it is not necessary for the Inter-AS metrics to have any
   relationship with the metric used within an AS for interior routing.
   The interior gateways do not need to know how to interpret the
   exterior metrics, except to know that each metric is to be
   interpreted as an unsigned integer and a lesser value is preferable
   to a greater value.  It would be possible, but not necessary, for the
   interior gateways to have full knowledge of the IARP.

   It is not necessary for the Inter-AS Routing architecture to specify
   which of these solutions are to be used for any particular AS.
   Rather, it is possible for individual AS's to choose which scheme or
   combination of schemes to use.  Independence of the IARP from the
   internal operation of each AS implies that this decision be left up
   to the internal protocols used in each AS.  The IARP must be able to
   operate as if the boundary gateways were directly connected.

A.2.6.2  Forwarding of Data from the AS

   The scheme used for forwarding transit traffic across an AS also has
   implications for the forwarding of traffic which originates within an
   AS, but whose destination is reachable only from other AS's.  If
   either of solutions (1) or (2) in Section A.2.6.1 is followed, then
   it will be sufficient for an interior gateway to forward such traffic
   to any boundary gateway.  Greater efficiency may optionally be
   achieved in some cases by providing interior gateways with additional
   information which will allow them to choose the "best" boundary
   gateway in some sense.  If solution (3) is followed, then the
   information passed to interior gateways to allow them to forward
   transit traffic will also be sufficient to forward traffic
   originating within that AS.






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RFC 1126            Inter-Autonomous System Routing         October 1989


A.2.6.3  Forwarding of Data to a Destination in the AS

   If a packet whose destination is reachable from an AS arrives at that
   AS, then it is desired that the interior routing protocol used in
   that AS be able to successfully deliver the packet without reliance
   on Inter-AS Routing.  This does not preclude that the Inter-AS
   Routing protocol will deal with partitioned AS's.

   An AS may have access control, security, and policy restrictions that
   restrict which data packets may enter or leave the AS. However, for
   any data packet which is allowed access to the AS, the AS is expected
   to deliver the packet to its destination without further restrictions
   between parts of the AS.  In this sense, the internal structure of
   the AS should not be visible to the IARP.

A.3  Policy Issues

   The interconnection of multiple heterogeneous networks and multiple
   heterogeneous autonomous systems owned and operated by multiple
   administrations into a single combined internet is a very complex
   task.  It is expected that the administrations associated with such
   an internet will wish to impose a variety of constraints on the
   operation of the internet.  Specifically, externally imposed routing
   constraints may include a variety of transit access control,
   administrative policy, and security constraints.

   Transit access control refers to those access control restrictions
   which an AS may impose to restrict which traffic the AS is willing to
   forward.  There are a large number of access control restrictions
   which one could envision being used.  For example, some AS's may wish
   to operate only as "non-transit" AS's, that is, they will only
   forward data traffic which either originates or terminates within
   that AS.  Other AS's may restrict transit traffic to datagrams which
   originate within a specified set of source hosts. Restrictions may
   require that datagrams be associated with specific applications (such
   as electronic mail traffic only), or that datagrams be associated
   with specific classes of users.

   Policy restrictions may allow either the source of datagrams, or the
   organization that is paying for transmission of a datagram, to limit
   which AS's the datagrams may transit.  For example, an organization
   may wish to specify that certain traffic originating within their AS
   should not transit any AS administered by its main competitor.

   Security restrictions on traffic relates to the official security
   classification level of traffic.  As an example, an AS may specify
   that all classified traffic is not allowed to leave its AS.




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RFC 1126            Inter-Autonomous System Routing         October 1989


   The main problem with producing a routing scheme which is sensitive
   to transit access control, administrative policy, and security
   constraints is the associated potential for exponential growth of
   routes.  For example, suppose that there are 20 packets arriving at a
   particular gateway, each for the same destination, but subject to a
   different combination of access control, policy, and security
   constraints.  It is possible that all 20 packets would need to follow
   different routes to the destination.

   This explosive growth of routes leads to the question: "Is it
   practically feasible to deal with complete general external routing
   constraints?" One approach would allow only a smaller subset of
   constraints, chosen to provide some useful level of control while
   allowing minimal impact on the routing protocol.  Further work is
   needed to determine the feasibility of this approach.

   There is another problem with dealing with transit access control,
   policy, and security restrictions in a fully general way.
   Specifically, it is not clear just what the total set of possible
   restrictions should be.  Efforts to study this issue are currently
   underway, but are not expected to produce definitive results within a
   short to medium time frame.  It is therefore not practical to wait
   for this effort to be finished before defining the next generation of
   Inter-AS Routing.

A.4  Service Features

   The following paragraphs discuss issues concerning the services an
   Inter-AS Routing Protocol may provide.  This is not a complete list
   of service issues but does address many of those services which are
   of concern to a significant portion of the community.

A.4.1  Cost on Toll Networks

   Consideration must be given to the use of routing protocols with toll
   (i.e., charge) networks.  Although uncommon in the Internet at the
   moment, they will become more common in the future, and thought needs
   to be given to allowing their inclusion in an efficient and effective
   manner.

   There are two areas in which concerns of cost intrude.  First,
   provision must be made to include in the routing information
   distributed throughout the system the information that certain links
   cost money, so that traffic patterns may account for the cost.
   Second, the actual operation of the algorithm, in terms of the
   messages that must be exchanged to operate the algorithm, must
   recognize that fact that on certain links, the exchange may have an
   associated cost which must be taken into account.  These areas often



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RFC 1126            Inter-Autonomous System Routing         October 1989


   involve policy questions on the part of the user.  It is a
   requirement of the algorithm that facilities be available to allow
   different groups to answer these questions in different ways.  The
   first area is related to type-of-service routing, and is discussed in
   Section A.4.2.  The second area is discussed below.

   Previous attempts at providing these sorts of controls were
   incomplete because they were not thought through fully; a new effort
   must avoid these pitfalls.  For instance, even though the Hello rate
   in EGP may be adjusted, turning the rate down too low (to control the
   costs) could cause the route to be dropped from databases through
   timeout.

   In a large internet, changes will be occurring constantly; a
   simplistic mechanism might mean that a site which is only connected
   by toll networks has to either accept having an old picture of the
   network, or spend more to keep a more current picture of things.
   However, that is not necessarily the case if incomplete knowledge and
   cache-based techniques are used more. For instance, if a site
   connected only by toll links keeps an incomplete or less up-to-date
   map of the situation, an agreement with a neighbor which does not
   labor under these restrictions might allow it to get up-to-date
   information only when needed.

A.4.2  Type-of-Service Routing

   The need for type-of-service (TOS) has been increasing as networks
   become more heterogeneous in physical channel components, high-level
   applications, and administrative management.  For instance, some
   recently installed fiber cables provide abundant communication
   bandwidths, while old narrow-band channels will still be with us for
   a long time period.  Electronic mail traffic tolerates delivery
   delays and low throughput.  New image transmissions are coming up;
   these require high bandwidths but are not effected by a few bit
   errors.  Furthermore, some networks may soon install accounting
   functions to charge users, while others may still provide free
   services.

   Considering the long life span of a new routing architecture, it is
   mandatory that it be built with mechanisms to provide TOS routing.
   Unfortunately, we have had very little experience with TOS routing,
   even with a single network.  No TOS routing system has ever been
   field-tested on a large-scale basis.

   We foresee the need for TOS routing and recognize the possible
   complexities and difficulties in design and implementation.  We also
   consider that new applications coming along may require novel
   services that are unforeseeable today.  We feel a practical approach



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RFC 1126            Inter-Autonomous System Routing         October 1989


   is to provide a small set of TOS routing functions as a first step
   while the design of the architecture should be such that new classes
   of TOS can be easily added later and incrementally deployed.  The
   Inter-AS Routing Architecture should allow both globally and locally
   defined TOS classes.

   We intend to address TOS routing based on the following metrics:

      -  Delay

      -  Throughput

      -  Cost

   Delay and throughput are the main network performance concerns.
   Considering that some networks may soon start charging users for the
   transmission services provided, the cost should also be added as a
   factor in route selection.

   Reliability is not included in the above list.  Different
   applications with different reliability requirements will differ in
   terms of what Transport Protocol they use.  However, IP offers only a
   "moderate" level of reliability, suitable to applications such as
   voice, or possibly even less than that required by voice. The level
   of reliability offered by IP will not differ substantially based on
   the application.  Thus the requested level of reliability will not
   affect Inter-AS Routing.

   Delay and throughput will be measured from the physical
   characteristics of communication channels, without considering
   instantaneous load.  This is necessary in order to provide stable
   routes, and to minimize the overhead caused by the Inter-AS Routing
   scheme.

   A number of TOS service classes may be defined according to these
   metrics.  Each class will present defined requirements for each of
   the TOS metrics.  For example, one class may require low delay,
   require only low throughput, and require low cost.

A.4.3  Multipath Routing

   There are two types of multipath routing which are useful for Inter-
   AS Routing: (1) there may be multiple gateways between any two
   neighboring AS's; (2) there may be multiple AS-to-AS paths between
   any pair of source and destination AS's.  Both forms of multipath are
   useful in order to allow for loadsplitting.  Provision for multipath
   routing in the IARP is desirable, but not an absolute requirement.




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RFC 1126            Inter-Autonomous System Routing         October 1989