Point-toPoint Routing in the Internet in Java

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Point-toPoint Routing in the Internet
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Figure 45-7: Hierarchically structured OSPF AS with four areas A diagram of a hierarchically structured OSPF network is shown in Figure 44-5 We can identify four types of OSPF routers in Figure 45-7:
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internal routers These routers, shown in black, are in a non-backbone areas and only perform intra-AS routing area border routers These routers, shown in blue, belong to both an area and the backbone backbone routers (non border routers) These routers, shown in gray, perform routing within the backbone but themselves are not area border routers Within a non-backbone area, internal routers learn of the existence of routes to other areas from information (essentially a link state advertisement, but advertising the cost of a route to another area, rather than a link cost) broadcast within the area by its backbone routers boundary routers A boundary router, shown in blue, exchanges routing information with routers belonging to other autonomous systems This router might, for example, use BGP to perform inter-AS routing It is through such a boundary router that other routers learn about paths to external networks
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IGRP: Internal Gateway Routing Protocol The Interior Gateway Routing Protocol (IGRP) [Cisco97] is a proprietary routing algorithm developed by Cisco Systems, Inc in the mid-1980's as a successor for RIP IGRP is a distance vector protocol Several cost metrics (including delay, bandwidth, reliability, and load) can be used in making routing decisions, with the weight given to each of the metrics being determined by the network administrator This ability to use administrator-defined costs in making route selections is an important difference from RIP; we will see shortly that so-called policy-based interdomain Internet routing protocols such as BGP
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Point-toPoint Routing in the Internet
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also allow administratively defined routing decisions to be made Other important differences from RIP include the use of a reliable transport protocol to communicate routing information, the use of update messages that are sent only when routing table costs change (rather than periodically) , and the use of a distributed diffusing update routing algorithm [Garcia-Luna-Aceves 1991] to quickly compute loop free routing paths
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452 Inter-Autonomous System Routing
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The Border Gateway Protocol version 4, specified in RFC 1771 (see also RFC 1772, RFC 1773), is the de facto standard interdomain routing protocol in today's Internet It is commonly referred to as BGP4 or simply as BGP As an inter-autonomous system routing protocol, it provides for routing between autonomous systems (that is, administrative domains) While BGP has the same general flavor as the distance vector protocol that we studied in Section 42, it is more appropriately characterized as a path vector protocol This is because BGP in a router does not propagate cost information (eg, number of hops to a destination), but instead propagates path information, such as the sequence of ASs on a route to a destination AS We will examine the path information in detail shortly We note though that while this information includes the names of the ASs on a route to the destination, they do not contain cost information Nor does BGP specify how a specific route to a particular destination should be chosen among the routes that have been advertised That decision is a policy decision that is left up to the domain's administrator Each domain can thus choose its routes according to whatever criteria it chooses (and need not even inform its neighbors of its policy!) -- allowing a significant degree of autonomy in route selection In essence, BGP provides the mechanisms to distribute path information among the interconnected autonomous systems, but leaves the policy for making the actual route selections up to the network administrator Let's begin with a grossly simplified description of how BGP works This will help us see the forest through the trees As discussed in Section 43, as far as BGP is concerned, the whole Internet is a graph of ASs, and each AS is identified by an AS number At any instant of time, a given AS X may, or may not, know of a path of ASs that lead to a given destination AS Z As an example, suppose X has listed in its BGP table such a path XY1Y2Y3Z from itself to Z This means that X knows that it can send datagrams to Z through the ASs X, Y1, Y2 and Y3, Z When X sends updates to its BGP neighbors (ie, the neighbors in the graph), X actually sends the enitre path information, XY1Y2Y3Z, to its neighbors (as well as other paths to other ASs) If, for example, W is a neighbor of X, and W receives an advertisement that includes the path XY1Y2Y3Z, then W can list a new entry WXY1Y2Y3Z in its BGP table However, we should keep in mind that W may decide to not create this new entry for one of several reasons For example, W would not create this entry if W is equal to (say) Y2, thereby creating an undesirable loop in the routing; or if W already has a path to Z in its tables, and this existing path is preferable (with respect to the metric used by BGP at W) to WXY1Y2Y3Z ; or, finally, if W has a policy decision to not forward datagrams through (say) Y2
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