rfc1322.txt

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   domains to have exactly the same information; this clearly
   contradicts the notion of selective information hiding.  That is, the
   requirement to perform selective information hiding is unsatisfiable
   with LS hop-by-hop routing.

3.7 Commonality between NR and SDR Components

   In [Breslau-Estrin91] we argued for the use of LS in conjunction with
   SDR.  Therefore there is some preference for using LS with NR.
   However, there are several reasons why NR and SDR would not use
   exactly the same routing information, even if they did use the same
   algorithm.  Moreover, there are several opportunities for unifying
   the management (distribution and storage) of routing and forwarding
   information, even if dissimilar algorithms are used.



Estrin, Rekhter & Hotz                                         [Page 20]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992


   In considering the differences between NR and SDR we must address
   several areas:

     1. Routing information and distribution protocol: LS for SDR is
        quite different from the LS in NR.  For example, SDR LS need
        not aggregate domains; to the contrary SDR LS  requires detailed
        information to generate special routes.

        In addition, consistency requirements (essential for NR) are
        unnecessary for the SDR component.  Therefore LS information for
        the SDR component can be retrieved on-demand, while the NR
        component must use flooding of topology information.

     2. Route computation algorithm: It is not clear whether route
        computation algorithm(s)  can be shared between the SDR and NR
        components, given the difficulty of supporting  heterogeneous
        route selection policies in NR.

     3. Forwarding information: The use of dissimilar route computation
        algorithms does not preclude common handling of packet
        forwarding.  Even if LS were used for NR, the requirement would
        be the same, i.e., that the forwarding agent can determine
        whether to use a NR  precomputed route or an SDR installed route
        to forward a particular data packet.

   In conclusion, using similar algorithms and mechanisms for SDR and NR
   components would have benefits.  However, these benefits do not
   dominate the other factors as discussed before.























Estrin, Rekhter & Hotz                                         [Page 21]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992


3.8 Summary

   Given the performance complexity issues associated with global
   routing, aggregation of routing information is essential; at the same
   time we have argued that such aggregation must be flexible.  Given
   the difficulties of supporting LS hop-by-hop routing in the presence
   of (a) flexible aggregation, (b) heterogeneous route selection
   policies, and (c) incomplete or inconsistent routing information, we
   see no alternative but to employ PV for the NR component of our
   architecture.

   Based on the above tradeoffs, our NR component employs a PV
   architecture, where route computation and installation is done in a
   distributed fashion by the routers participating in the NR component
   [Footnote: Packet forwarding along routes produced by the NR
   component can be accomplished by either source routing or hop-by-hop
   routing.  The latter is the primary choice because it reduces the
   amount of state in routers (if route setup is employed), or avoids
   encoding an explicit source route in network layer packets.  However,
   the architecture does not preclude the use of source routing (or
   route setup) along the routes computed, but not installed, by the NR
   component.].

   The distributed algorithm combines some of the features of link state
   with those of distance vector algorithms; in addition to next hop
   information, the NR component maintains path attributes for each
   route (e.g., the list of domains or routing domain confederations
   that the routing information has traversed so far).  The path
   attributes that are carried along with a route express a variety of
   routing policies, and make explicit the entire route to the
   destination.  With aggregation, this is a superset of the domains
   that form the path to the destination.



















Estrin, Rekhter & Hotz                                         [Page 22]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992


4.0 Source-demand routing (SDR)

   Inter-domain routers participating in the SDR component forward
   packets according to routing information computed and installed by
   the domain that originates the traffic (source routing domain).

   It is important to realize that requiring route installation by the
   source routing domain is not a matter of choice, but rather a
   necessity.  If a particular route is used by a small number of
   domains (perhaps only one) then it is more appropriate to have the
   source compute and install the special route instead of burdening the
   intermediate nodes with the task of looking for and selecting a route
   with the specialized requirements.  In addition, if the demand for
   the route is unpredictable, and thus can be determined only by the
   source, it should be up to the source to install the route.

   In general, information that is used by source routing domains for
   computing source-demand routes reflects administrative (but not
   operational) status of the routing facilities (i.e., configured
   topology and policy) [Footnote: If SDR uses NR information then
   operational status could be considered in some route selection.].
   Consequently, it is possible for a source routing domain to compute a
   route that is not operational at route installation time.  The SDR
   component attempts to notify the source domain of failures when route
   installation is attempted.  Similarly, the SDR component provides
   mechanisms for the source routing domain to be notified of failures
   along previously-installed active routes.  In other words, the SDR
   component performs routing that is adaptive to topological changes;
   however, the adaptability is achieved as a consequence of the route
   installation and route management mechanisms.  This is different from
   the NR component, where status changes are propagated and
   incorporated by nodes as soon as possible.  Therefore, to allow
   faster adaptation to changes in the operational status of routing
   facilities, the SDR component allows the source domain to switch to a
   route computed by the NR component, if failure along the source-
   demand route is detected (either during the route installation phase,
   or after the route is installed), and if policy permits use of the NR
   route.

   The NR component will group domains into confederations to achieve
   its scaling goals (see [IDRP91]).  In contrast, SDR will allow an
   AD-level route to be used by an individual domain without allowing
   use by the entire confederation to which the domain belongs.
   Similarly, a single transit domain may support a policy or special
   TOS that is not supported by other domains in its confederation(s).
   In other words, the architecture uses SDR to support non-
   hierarchical, non-aggregated policies where and when needed.
   Consequently, SDR by itself does not have the scaling properties of



Estrin, Rekhter & Hotz                                         [Page 23]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992


   NR.  In compensation, SDR does not require a complete, global domain
   map if portions of the world are never traversed or communicated
   with.  As a result of the looser routing structure, SDR does not
   guarantee that a participating source routing domain will always have
   sufficient information to compute a route to a destination.  In
   addition, if the domain does have sufficient information, it is
   possible that the quantity may be large enough to preclude storage
   and/or route computation in a timely fashion.  However, despite the
   lack of guarantees, it is a goal of the architecture to provide
   efficient methods whereby sources can obtain the information needed
   to compute desired routes [Footnote: The primary goal of policy or
   TOS routing is to compute a route that satisfies a set of specialized
   requirements, and these requirements take precedence over optimality.
   In other words, even if a routing domain that participates in SDR or
   NR has sufficient information to compute a route, given a particular
   set of requirements, the architecture does not guarantee that the
   computed route is optimal.].

   Essential to SDR is the assumption that the routes installed on
   demand will be used sparingly.  The architecture assumes that at any
   given moment the set of all source-demand routes installed in an
   internet forms a small fraction of the total number of source-demand
   routes that can potentially be installed by all the routing domains.
   It is an assumption of the architecture that the number of routes
   installed in a BR by the SDR component should be on the order of log
   N (where N is the total number of routing domains in the Internet),
   so that the scaling properties of the SDR component are comparable
   with the scaling properties of the NR component.  The NR component
   achieves this property as a result of hierarchy.

   Note that the above requirement does not imply that only a few
   domains can participate in SDR, or that routes installed by the SDR
   component must have short life times.  What the requirement does
   imply, is that the product of the number of routes specified by
   domains that participate in SDR, times the average SDR-route life
   time, is bounded.  For example, the architecture allows either a
   small number of SDR routes with relatively long average life times,
   or a large number of SDR routes with relatively short average life
   times.  But the architecture clearly prohibits a large number of SDR
   routes with relatively long average life times.  The number of SDR
   routes is a function of the number of domains using SDR routes and
   the number of routes used per source domain.

   In summary, SDR is well suited for traffic that (1) is not widely-
   used enough (or is not sufficiently predictable or steady) to justify
   computation and maintenance by the NR component, and (2) whose
   duration is significantly longer than the time it takes to perform
   the route installation procedure.



Estrin, Rekhter & Hotz                                         [Page 24]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992


   The architecture does not require all domains in the Internet to
   participate in SDR.  Therefore, issues of scalability (with respect
   to the size of the Internet) become less crucial (though still
   important) to the SDR component.  Instead, the primary focus of the
   SDR component is shifted towards the ability to compute routes that
   satisfy specialized requirements, where we assume that the total
   number of domains requiring special routes simultaneously through the
   same part of the network is small relative to the total population.

4.1 Path Vector vs. Link State for SDR

   It is feasible to use either a distance vector or link state method
   of route computation along with source routing.  One could imagine,
   for instance, a protocol like BGP in which the source uses the full
   AD path information it receives in routing updates to create a source
   route. Such a protocol could address some of the deficiencies
   identified with distance vector, hop-by-hop designs.  However, we opt
   against further discussion of such a protocol because there is less
   to gain by using source routing without also using a link state
   scheme.  The power of source routing, in the context of inter-AD
   policy routing, is in giving the source control over the entire
   route.  This goal cannot be realized fully when intermediate nodes
   control which legal routes are advertised to neighbors, and therefore
   to a source.

   In other words,