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9.1: Distance-Vector Routing-Update Algorithm

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    11193
  • Distance-vector is the simplest routing-update algorithm, used by the Routing Information Protocol, or RIP. Version 2 of the protocol is specified in RFC 2453 [https://tools.ietf.org/html/rfc2453.html].

    Routers identify their router neighbors (through some sort of neighbor-discovery mechanism), and add a third column to their forwarding tables representing the total cost for delivery to the corresponding destination. These costs are the “distance” of the algorithm name. Forwarding-table entries are now of the form ⟨destination,next_hop,cost⟩.

    Costs are administratively assigned to each link, and the algorithm then calculates the total cost to a destination as the sum of the link costs along the path. The simplest case is to assign a cost of 1 to each link, in which case the total cost to a destination will be the number of links to that destination. This is known as the “hopcount” metric; it is also possible to assign link costs that reflect each link’s bandwidth, or delay, or whatever else the network administrators wish. Thoughtful cost assignments are a form of traffic engineering and sometimes play a large role in network performance.

    At this point, each router then reports the ⟨destination,cost⟩ portion of its table to its neighboring routers at regular intervals; these table portions are the “vectors” of the algorithm name. It does not matter if neighbors exchange reports at the same time, or even at the same rate.

    Each router also monitors its continued connectivity to each neighbor; if neighbor N becomes unreachable then its reachability cost is set to infinity.

    In a real IP network, actual destinations would be subnets attached to routers; one router might be directly connected to several such destinations. In the following, however, we will identify all a router’s directly connected subnets with the router itself. That is, we will build forwarding tables to reach every router. While it is possible that one destination subnet might be reachable by two or more routers, thus breaking our identification of a router with its set of attached subnets, in practice this is of little concern. See exercise 4.0 for an example in which subnets are not identified with adjacent routers.

    In 18.5   IP Routers With Simple Distance-Vector Implementation we present a simplified working implementation of RIP using the Mininet network emulator.

    9.1.1 Distance-Vector Update Rules

    Let A be a router receiving a report ⟨D,cD⟩ from neighbor N at cost cN. Note that this means A can reach D via N with cost c = cD + cN. A updates its own table according to the following three rules:

    1. New destination: D is a previously unknown destination. A adds ⟨D,N,c⟩ to its forwarding table.

    2. Lower cost: D is a known destination with entry ⟨D,M,cold⟩, but the new total cost c is less than cold. A switches to the cheaper route, updating its entry for D to ⟨D,N,c⟩. It is possible that M=N, meaning that N is now reporting a cost decrease to D. (If c = cold, A ignores the new report; see exercise 5.5.)

    3. Next_hop increase: A has an existing entry ⟨D,N,cold⟩, and the new total cost c is greater than cold. Because this is a cost increase from the neighbor N that A is currently using to reach D, A must incorporate the increase in its table. A updates its entry for D to ⟨D,N,c⟩.

    The first two rules are for new destinations and a shorter path to existing destinations. In these cases, the cost to each destination monotonically decreases (at least if we consider all unreachable destinations as being at cost ∞). Convergence is automatic, as the costs cannot decrease forever.

    The third rule, however, introduces the possibility of instability, as a cost may also go up. It represents the bad-news case, in that neighbor N has learned that some link failure has driven up its own cost to reach D, and is now passing that “bad news” on to A, which routes to D via N.

    The next_hop-increase case only passes bad news along; the very first cost increase must always come from a router discovering that a neighbor N is unreachable, and thus updating its cost to N to ∞. Similarly, if router A learns of a next_hop increase to destination D from neighbor B, then we can follow the next_hops back until we reach a router C which is either the originator of the cost=∞ report, or which has learned of an alternative route through one of the first two rules.

    9.1.2 Example 1

    For our first example, no links will break and thus only the first two rules above will be used. We will start out with the network below with empty forwarding tables; all link costs are 1.

    fivenodes1.svg

    After initial neighbor discovery, here are the forwarding tables. Each node has entries only for its directly connected neighbors:

    A: ⟨B,B,1⟩ ⟨C,C,1⟩ ⟨D,D,1⟩
    B: ⟨A,A,1⟩ ⟨C,C,1⟩
    C: ⟨A,A,1⟩ ⟨B,B,1⟩ ⟨E,E,1⟩
    D: ⟨A,A,1⟩ ⟨E,E,1⟩
    E: ⟨C,C,1⟩ ⟨D,D,1⟩

    Now let D report to A; it sends records ⟨A,1⟩ and ⟨E,1⟩. A ignores D’s ⟨A,1⟩ record, but ⟨E,1⟩ represents a new destination; A therefore adds ⟨E,D,2⟩ to its table. Similarly, let A now report to D, sending ⟨B,1⟩ ⟨C,1⟩ ⟨D,1⟩ ⟨E,2⟩ (the last is the record we just added). D ignores A’s records ⟨D,1⟩ and ⟨E,2⟩ but A’s records ⟨B,1⟩ and ⟨C,1⟩ cause D to create entries ⟨B,A,2⟩ and ⟨C,A,2⟩. A and D’s tables are now, in fact, complete.

    Now suppose C reports to B; this gives B an entry ⟨E,C,2⟩. If C also reports to E, then E’s table will have ⟨A,C,2⟩ and ⟨B,C,2⟩. The tables are now:

    A: ⟨B,B,1⟩ ⟨C,C,1⟩ ⟨D,D,1⟩ ⟨E,D,2⟩
    B: ⟨A,A,1⟩ ⟨C,C,1⟩ ⟨E,C,2⟩
    C: ⟨A,A,1⟩ ⟨B,B,1⟩ ⟨E,E,1⟩
    D: ⟨A,A,1⟩ ⟨E,E,1⟩ ⟨B,A,2⟩ ⟨C,A,2⟩
    E: ⟨C,C,1⟩ ⟨D,D,1⟩ ⟨A,C,2⟩ ⟨B,C,2⟩

    We have two missing entries: B and C do not know how to reach D. If A reports to B and C, the tables will be complete; B and C will each reach D via A at cost 2. However, the following sequence of reports might also have occurred:

    • E reports to C, causing C to add ⟨D,E,2⟩

    • C reports to B, causing B to add ⟨D,C,3

    In this case we have 100% reachability but B routes to D via the longer-than-necessary path B–C–E–D. However, one more report will fix this: suppose A reports to B. B will received ⟨D,1⟩ from A, and will update its entry ⟨D,C,3⟩ to ⟨D,A,2⟩.

    Note that A routes to E via D while E routes to A via C; this asymmetry was due to indeterminateness in the order of initial table exchanges.

    If all link weights are 1, and if each pair of neighbors exchange tables once before any pair starts a second exchange, then the above process will discover the routes in order of length, ie the shortest paths will be the first to be discovered. This is not, however, a particularly important consideration.

    9.1.3 Example 2

    The next example illustrates link weights other than 1. The first route discovered between A and B is the direct route with cost 8; eventually we discover the longer A–C–D–B route with cost 2+1+3=6.

    2831.svg

    The initial tables are these:

    A: ⟨C,C,2⟩ ⟨B,B,8⟩
    B: ⟨A,A,8⟩ ⟨D,D,3⟩
    C: ⟨A,A,2⟩ ⟨D,D,1⟩
    D: ⟨B,B,3⟩ ⟨C,C,1⟩

    After A and C exchange, A has ⟨D,C,3⟩ and C has ⟨B,A,10⟩. After C and D exchange, C updates its ⟨B,A,10⟩ entry to ⟨B,D,4⟩ and D adds ⟨A,C,3⟩; D receives C’s report of ⟨B,10⟩ but ignores it. Now finally suppose B and D exchange. D ignores B’s route to A, as it has a better one. B, however, gets D’s report ⟨A,3⟩ and updates its entry for A to ⟨A,D,6⟩. At this point the tables are as follows:

    A: ⟨C,C,2⟩ ⟨B,B,8⟩ ⟨D,C,3⟩
    B: ⟨A,D,6⟩ ⟨D,D,3⟩
    C: ⟨A,A,2⟩ ⟨D,D,1⟩ ⟨B,D,4⟩
    D: ⟨B,B,3⟩ ⟨C,C,1⟩ ⟨A,C,3⟩

    We have two more things to fix before we are done: A has an inefficient route to B, and B has no route to C. The first will be fixed when C reports ⟨B,4⟩ to A; A will replace its route to B with ⟨B,C,6⟩. The second will be fixed when D reports to B; if A reports to B first then B will temporarily add the inefficient route ⟨C,A,10⟩; this will change to ⟨C,D,4⟩ when D’s report to B arrives. If we look only at the A–B route, B discovers the lower-cost route to A once, first, C reports to D and, second, after that, D reports to B; a similar sequence leads to A’s discovering the lower-cost route.

    9.1.4 Example 3

    Our third example will illustrate how the algorithm proceeds when a link breaks. We return to the first diagram, with all tables completed, and then suppose the D–E link breaks. This is the “bad-news” case: a link has broken, and is no longer available; this will bring the third rule into play.

    fivenodes_break.svg

    We shall assume, as above, that A reaches E via D, but we will here assume – contrary to Example 1 – that C reaches D via A (see exercise 3.5 for the original case).

    Initially, upon discovering the break, D and E update their tables to ⟨E,-,∞⟩ and ⟨D,-,∞⟩ respectively (whether or not they actually enter ∞ into their tables is implementation-dependent; we may consider this as equivalent to removing their entries for one another; the “-” as next_hop indicates there is no next_hop).

    Eventually D and E will report the break to their respective neighbors A and C. A will apply the “bad-news” rule above and update its entry for E to ⟨E,-,∞⟩. We have assumed that C, however, routes to D via A, and so it will ignore E’s report.

    We will suppose that the next steps are for C to report to E and to A. When C reports its route ⟨D,2⟩ to E, E will add the entry ⟨D,C,3⟩, and will again be able to reach D. When C reports to A, A will add the route ⟨E,C,2⟩. The final step will be when A next reports to D, and D will have ⟨E,A,3⟩. Connectivity is restored.

    9.1.5 Example 4

    The previous examples have had a “global” perspective in that we looked at the entire network. In the next example, we look at how one specific router, R, responds when it receives a distance-vector report from its neighbor S. Neither R nor S nor we have any idea of what the entire network looks like. Suppose R’s table is initially as follows, and the S–R link has cost 1:

    destination

    next_hop

    cost

    A

    S

    3

    B

    T

    4

    C

    S

    5

    D

    U

    6

    S now sends R the following report, containing only destinations and its costs:

    destination

    cost

    A

    2

    B

    3

    C

    5

    D

    4

    E

    2

    R then updates its table as follows:

    destination

    next_hop

    cost

    reason

    A

    S

    3

    No change; S probably sent this report before

    B

    T

    4

    No change; R’s cost via S is tied with R’s cost via T

    C

    S

    6

    Next_hop increase

    D

    S

    5

    Lower-cost route via S

    E

    S

    3

    New destination

    Whatever S’s cost to a destination, R’s cost to that destination via S is one greater.