routing by preference
Martha Steenstrup <msteenst@bbn.com> Sun, 14 November 1993 23:32 UTC
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To: murayama@wide.ad.jp
cc: idpr-wg@bbn.com
Subject: routing by preference
Date: Sun, 14 Nov 1993 18:19:17 -0500
Sender: ietf-archive-request@IETF.CNRI.Reston.VA.US
From: Martha Steenstrup <msteenst@bbn.com>
Message-ID: <9311141832.aa19466@CNRI.Reston.VA.US>
Hello Yuko, After viewing your video, I have the following comments about your "routing by preference" problem and some suggestions for handling it within the context of IDPR. It seems as if routing by preference, i.e., specifying the set of "unfavorable" domains and taking the "shortest" path through such domains can be problematic. Generating the shortest (according to some metric such as distance, delay, monetary cost) path between source and destination may not be consistent with routing by preference. That is, if routing by preference is strictly enforced, and one always selects the shortest routes across unfavorable domains, one may end up with a path which is not the shortest path between source and destination. The following example illustrates this. Suppose we want to generate a route between Hx in domain X and Hy in domain Y. Suppose that S and T are two intervening domains and S is unfavorable from the perspective of both Hx and Hy. Suppose that S has two virtual gateways, T1 and T2, to T and one virtual gateway, X1, to X. Furthermore, suppose that the distance from X1 to T1 is 25, and the distance from X1 to T2 is 50. Moreover, suppose that the distance from T1 to the virtual gateway Y1 connecting T to Y is 100, and the distance from T2 to the virtual gateway Y1 is 40. With strict routing by preference one would choose the route through X1, T1, and Y1, with distance 125. However, the shortest route is through X1, T2, and Y1 with distance 90. I think you have to be careful when deciding when you actually want to route by preference. With strict routing by preference, the local optimizations in unfavorable domains do not necessarily lead to end-to-end optimization between source and destination domains. Hence, you must decide when the shortest end-to-end path takes precedence over selecting the shortest routes through unfavorable domains. Moreover, if the source requires that the qualities of service and the monetary cost of service for a route be kept within specified constraints, then satisfaction of these constraints should take precedence over selecting the shortest routes through unfavorable domains. Here are some suggestions for using IDPR to do routing by preference. IDPR version 1 supports source policies and thus source "preferences". These source policies specify, for a route between a set of source hosts and destination hosts: - the qualities of service desired, - the monetary cost of service desired, and - the domains to exclude, favor, or avoid if possible. One can treat "avoid if possible" as synonymous with "unfavorable". Currently, source policies are configurable and cannot be dynamically updated by hosts themselves. You could include as part of the source policies, the preferences of the destinations listed in the source policy. For example, if destination Hz considers that domain W should be avoided if possible, this information will be incorporated into the source policies involving destination Hz. Hence, these sorts of destination preferences could be accommodated without modifying IDPR. A route server generating a route on behalf of a source host accounts for the source policies when generating and selecting a route. Each route server executes its own route generation procedure. Note that IDPR provides as part of the domain transit policies the qualities and monetary costs of service for traversing a domain between pairs of virtual gateways. Hence, when routing by preference, a route server may select routes across a domain according to any metrics available in domain transit policies. For strict routing by preference, your implementation of the route generation procedure should exhibit the following behavior. The route server should try to select the route that minimizes the "cost" attributed to traversing unfavorable domains. However, this minimization should be subject to the service constraints specified in the source policy. That is, the route selected should obey these service constraints. You also mentioned the ability to factor in the "cost" in travelling from source or destination hosts to virtual gateways in the same domains in order to choose a favored exit virtual gateway from the source domain or entry virtual gateway at the destination domain. Currently, IDPR does not handle this sort of information. Hosts have no information about IDPR, and virtual gateways have no information about which hosts in their domain favor them. Moreover, path setup messages contain no information about source and destination hosts, only source and destination domains. IDPR currently supports multiple source and destination host flows over a single path. Hence, a favored entry or exit gateway for one host may not be the same for all hosts using the path. So the notion of favorite gateway entry or exit virtual gateway does not always make sense. Nevertheless, for a single source host you could do the following. At the source domain, if a host were configured with information about its favorite exit virtual gateway, it could loose source route all of its inter-domain traffic to this favorite virtual gateway. The virtual gateway in trun would select an IDPR policy route to the destination. However, this implies that the host also knows which of its traffic is intra-domain and which is inter-domain. Perhaps the first-hop router for the host could make this determination and could force loose source routing to the preferred virtual gateway, on behalf of the host. The favorite entry virtual gateway at the destination is much more difficult to handle. IDPR path setup messages would have to carry information about destination hosts, and entry virtual gateways would have to know which hosts in the domain favored which virtual gateways. When a path agent in the destination domain received a path setup message, that path agent (a member of a virtual gateway) could return in its path accept message information about preferred virtual gateways for the given destination hosts, if the given virtual gateway were not the preferred entry virtual gateway. (However, there may not be a single preferred virtual gateway if there are many destination hosts for the path.) The path agent in the source domain, upon receiving a path accept message with suggested alternate virtual gateway entry points could request a route server to try to generate a feasible (i.e., constraint satisfying) route that includes one of the favored entry virtual gateways at the destination. If successful, the source path agent would tear down the original path and set up the new path. If unsuccessful, the source path agent would use the original path. This solution requires some changes to the IDPR path control protocol. m
- routing by preference Martha Steenstrup
- Re: routing by preference 村山優子
- routing by preference Martha Steenstrup
- Re: routing by preference 村山優子