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7.6: The Classless IP Delivery Algorithm

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    Recall from Chapter 1 that any IPv4 address can be divided into a net portion IPnet and a host portion IPhost; the division point was determined by whether the IPv4 address was a Class A, a Class B, or a Class C. We also indicated in Chapter 1 that the division point was not always so clear-cut; we now present the delivery algorithm, for both hosts and routers, that does not assume a globally predeclared division point of the input IPv4 address into net and host portions. We will, for the time being, punt on the question of forwarding-table lookup and assume there is a lookup() method available that, when given a destination address, returns the next_hop neighbor.

    Instead of class-based divisions, we will assume that each of the IPv4 addresses assigned to a node’s interfaces is configured with an associated length of the network prefix; following the slash notation of 1.10 IP - Internet Protocol, if B is an address and the prefix length is k = kB then the prefix itself is B/k. As usual, an ordinary host may have only one IP interface, while a router will always have multiple interfaces.

    Let D be the given IPv4 destination address; we want to decide if D is local or nonlocal. The host or router involved may have multiple IP interfaces, but for each interface the length of the network portion of the address will be known. For each network address B/k assigned to one of the host’s interfaces, we compare the first k bits of B and D; that is, we ask if D matches B/k.

    • If one of these comparisons yields a match, delivery is local; the host delivers the packet to its final destination via the LAN connected to the corresponding interface. This means looking up the LAN address of the destination, if applicable, and sending the packet to that destination via the interface.
    • If there is no match, delivery is nonlocal, and the host passes D to the lookup() routine of the forwarding table and sends to the associated next_hop (which must represent a physically connected neighbor). It is now up to lookup() routine to make any necessary determinations as to how D might be split into Dnet and Dhost; the split cannot be made outside of lookup().

    The forwarding table is, abstractly, a set of network addresses – now also with lengths – each of the form B/k, with an associated next_hop destination for each. The lookup() routine will, in principle, compare D with each table entry B/k, looking for a match (that is, equality of the first k = kB bits). As with the local-delivery interfaces check above, the net/host division point (that is, k) will come from the table entry; it will not be inferred from D or from any other information borne by the packet. There is, in fact, no place in the IPv4 header to store a net/host division point, and furthermore different routers along the path may use different values of k with the same destination address D. Routers receive the prefix length /k for a destination B/k as part of the process by which they receive ⟨destination,next_hop⟩ pairs; see 9 Routing-Update Algorithms.

    In 10 Large-Scale IP Routing we will see that in some cases multiple matches in the forwarding table may exist, eg and The longest-match rule will be introduced for such cases to pick the best match.

    Here is a simple example for a router with immediate neighbors A-E:








    The IPv4 addresses and both route to A. The addresses, and route to B, C and D respectively. Finally, matches both A and E, but the E match is longer so the packet is routed that way.

    The forwarding table may also contain a default entry for the next_hop, which it may return in cases when the destination D does not match any known network. We take the view here that returning such a default entry is a valid result of the routing-table lookup() operation, rather than a third option to the algorithm above; one approach is for the default entry to be the next_hop corresponding to the destination, which does indeed match everything (use of this would definitely require the above longest-match rule, though).

    Default routes are hugely important in keeping leaf forwarding tables small. Even backbone routers sometimes expend considerable effort to keep the network address prefixes in their forwarding tables as short as possible, through consolidation.

    At a site with a single ISP and with no Internet customers (that is, which is not itself an ISP for others), the top-level forwarding table usually has a single external route: its default route to its ISP. If a site has more than one ISP, however, the top-level forwarding table can expand in a hurry. For example, Internet2 [] is a consortium of research sites with very-high-bandwidth internal interconnections, acting as a sort of “parallel Internet”. Before Internet2, Loyola’s top-level forwarding table had the usual single external default route. After Internet2, we in effect had a second ISP and had to divide traffic between the commercial ISP and the Internet2 ISP. The default route still pointed to the commercial ISP, but the top-level forwarding table now had to have an entry for every individual Internet2 site, so that traffic to any of these sites would be forwarded via the Internet2 ISP. See exercise 5.0.

    Routers may also be configured to allow passing quality-of-service information to the lookup() method, as mentioned in Chapter 1, to support different routing paths for different kinds of traffic (eg bulk file-transfer versus real-time).

    For a modest exception to the local-delivery rule described here, see below in 7.12 Unnumbered Interfaces.

    This page titled 7.6: The Classless IP Delivery Algorithm is shared under a CC BY-NC-ND license and was authored, remixed, and/or curated by Peter Lars Dordal.

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