rfc1583.txt

<VC++网络游戏建摸与实现>源代码

        potentially be multiple routing table entries, one for each IP
        TOS. The IP TOS values are represented in OSPF exactly as they
        appear in the IP packet header.

        Up to this point, all examples shown have assumed that routes do
        not vary on TOS.  In order to differentiate routes based on TOS,
        separate interface costs can be configured for each TOS.  For
        example, in Figure 2 there could be multiple costs (one for each
        TOS) listed for each interface.  A cost for TOS 0 must always be
        specified.

        When interface costs vary based on TOS, a separate shortest path



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        tree is calculated for each TOS (see Section 2.1).  In addition,
        external costs can vary based on TOS.  For example, in Figure 2
        Router RT7 could advertise a separate type 1 external metric for
        each TOS.  Then, when calculating the TOS X distance to Network
        N15 the cost of the shortest TOS X path to RT7 would be added to
        the TOS X cost advertised by RT7 for Network N15 (see Section
        2.2).

        All OSPF implementations must be capable of calculating routes
        based on TOS.  However, OSPF routers can be configured to route
        all packets on the TOS 0 path (see Appendix C), eliminating the
        need to calculate non-zero TOS paths.  This can be used to
        conserve routing table space and processing resources in the
        router.  These TOS-0-only routers can be mixed with routers that
        do route based on TOS.  TOS-0-only routers will be avoided as
        much as possible when forwarding traffic requesting a non-zero
        TOS.

        It may be the case that no path exists for some non-zero TOS,
        even if the router is calculating non-zero TOS paths.  In that
        case, packets requesting that non-zero TOS are routed along the
        TOS 0 path (see Section 11.1).


3.  Splitting the AS into Areas

    OSPF allows collections of contiguous networks and hosts to be
    grouped together.  Such a group, together with the routers having
    interfaces to any one of the included networks, is called an area.
    Each area runs a separate copy of the basic link-state routing
    algorithm.  This means that each area has its own topological
    database and corresponding graph, as explained in the previous
    section.

    The topology of an area is invisible from the outside of the area.
    Conversely, routers internal to a given area know nothing of the
    detailed topology external to the area.  This isolation of knowledge
    enables the protocol to effect a marked reduction in routing traffic
    as compared to treating the entire Autonomous System as a single
    link-state domain.

    With the introduction of areas, it is no longer true that all
    routers in the AS have an identical topological database.  A router
    actually has a separate topological database for each area it is
    connected to.  (Routers connected to multiple areas are called area
    border routers).  Two routers belonging to the same area have, for
    that area, identical area topological databases.




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    Routing in the Autonomous System takes place on two levels,
    depending on whether the source and destination of a packet reside
    in the same area (intra-area routing is used) or different areas
    (inter-area routing is used).  In intra-area routing, the packet is
    routed solely on information obtained within the area; no routing
    information obtained from outside the area can be used.  This
    protects intra-area routing from the injection of bad routing
    information.  We discuss inter-area routing in Section 3.2.


    3.1.  The backbone of the Autonomous System

        The backbone consists of those networks not contained in any
        area, their attached routers, and those routers that belong to
        multiple areas.  The backbone must be contiguous.

        It is possible to define areas in such a way that the backbone
        is no longer contiguous.  In this case the system administrator
        must restore backbone connectivity by configuring virtual links.

        Virtual links can be configured between any two backbone routers
        that have an interface to a common non-backbone area.  Virtual
        links belong to the backbone.  The protocol treats two routers
        joined by a virtual link as if they were connected by an
        unnumbered point-to-point network.  On the graph of the
        backbone, two such routers are joined by arcs whose costs are
        the intra-area distances between the two routers.  The routing
        protocol traffic that flows along the virtual link uses intra-
        area routing only.

        The backbone is responsible for distributing routing information
        between areas.  The backbone itself has all of the properties of
        an area.  The topology of the backbone is invisible to each of
        the areas, while the backbone itself knows nothing of the
        topology of the areas.


    3.2.  Inter-area routing

        When routing a packet between two areas the backbone is used.
        The path that the packet will travel can be broken up into three
        contiguous pieces: an intra-area path from the source to an area
        border router, a backbone path between the source and
        destination areas, and then another intra-area path to the
        destination.  The algorithm finds the set of such paths that
        have the smallest cost.

        Looking at this another way, inter-area routing can be pictured



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        as forcing a star configuration on the Autonomous System, with
        the backbone as hub and each of the areas as spokes.

        The topology of the backbone dictates the backbone paths used
        between areas.  The topology of the backbone can be enhanced by
        adding virtual links.  This gives the system administrator some
        control over the routes taken by inter-area traffic.

        The correct area border router to use as the packet exits the
        source area is chosen in exactly the same way routers
        advertising external routes are chosen.  Each area border router
        in an area summarizes for the area its cost to all networks
        external to the area.  After the SPF tree is calculated for the
        area, routes to all other networks are calculated by examining
        the summaries of the area border routers.


    3.3.  Classification of routers

        Before the introduction of areas, the only OSPF routers having a
        specialized function were those advertising external routing
        information, such as Router RT5 in Figure 2.  When the AS is
        split into OSPF areas, the routers are further divided according
        to function into the following four overlapping categories:


        Internal routers
            A router with all directly connected networks belonging to
            the same area.  Routers with only backbone interfaces also
            belong to this category.  These routers run a single copy of
            the basic routing algorithm.

        Area border routers
            A router that attaches to multiple areas.  Area border
            routers run multiple copies of the basic algorithm, one copy
            for each attached area and an additional copy for the
            backbone.  Area border routers condense the topological
            information of their attached areas for distribution to the
            backbone.  The backbone in turn distributes the information
            to the other areas.

        Backbone routers
            A router that has an interface to the backbone.  This
            includes all routers that interface to more than one area
            (i.e., area border routers).  However, backbone routers do
            not have to be area border routers.  Routers with all
            interfaces connected to the backbone are considered to be
            internal routers.



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        AS boundary routers
            A router that exchanges routing information with routers
            belonging to other Autonomous Systems.  Such a router has AS
            external routes that are advertised throughout the
            Autonomous System.  The path to each AS boundary router is
            known by every router in the AS.  This classification is
            completely independent of the previous classifications: AS
            boundary routers may be internal or area border routers, and
            may or may not participate in the backbone.


    3.4.  A sample area configuration

        Figure 6 shows a sample area configuration.  The first area
        consists of networks N1-N4, along with their attached routers
        RT1-RT4.  The second area consists of networks N6-N8, along with
        their attached routers RT7, RT8, RT10 and RT11.  The third area
        consists of networks N9-N11 and Host H1, along with their
        attached routers RT9, RT11 and RT12.  The third area has been
        configured so that networks N9-N11 and Host H1 will all be
        grouped into a single route, when advertised external to the
        area (see Section 3.5 for more details).

        In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
        internal routers.  Routers RT3, RT4, RT7, RT10 and RT11 are area
        border routers.  Finally, as before, Routers RT5 and RT7 are AS
        boundary routers.

        Figure 7 shows the resulting topological database for the Area
        1.  The figure completely describes that area's intra-area
        routing.  It also shows the complete view of the internet for
        the two internal routers RT1 and RT2.  It is the job of the area
        border routers, RT3 and RT4, to advertise into Area 1 the
        distances to all destinations external to the area.  These are
        indicated in Figure 7 by the dashed stub routes.  Also, RT3 and
        RT4 must advertise into Area 1 the location of the AS boundary
        routers RT5 and RT7.  Finally, external advertisements from RT5
        and RT7 are flooded throughout the entire AS, and in particular
        throughout Area 1.  These advertisements are included in Area
        1's database, and yield routes to Networks N12-N15.

        Routers RT3 and RT4 must also summarize Area 1's topology for
        distribution to the backbone.  Their backbone advertisements are
        shown in Table 4.  These summaries show which networks are
        contained in Area 1 (i.e., Networks N1-N4), and the distance to
        these networks from the routers RT3 and RT4 respectively.





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RFC 1583                     OSPF Version 2                   March 1994



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