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rfc:rfc1058

Network Working Group C. Hedrick Request for Comments: 1058 Rutgers University

                                                             June 1988
                    Routing Information Protocol

Status of this Memo

 This RFC describes an existing protocol for exchanging routing
 information among gateways and other hosts.  It is intended to be
 used as a basis for developing gateway software for use in the
 Internet community.  Distribution of this memo is unlimited.
                           Table of Contents
 1. Introduction                                                     2
      1.1. Limitations of the protocol                               4
      1.2. Organization of this document                             4
 2. Distance Vector Algorithms                                       5
      2.1. Dealing with changes in topology                         11
      2.2. Preventing instability                                   12
           2.2.1. Split horizon                                     14
           2.2.2. Triggered updates                                 15
 3. Specifications for the protocol                                 16
      3.1. Message formats                                          18
      3.2. Addressing considerations                                20
      3.3. Timers                                                   23
      3.4. Input processing                                         24
           3.4.1. Request                                           25
           3.4.2. Response                                          26
      3.5. Output Processing                                        28
      3.6. Compatibility                                            31
 4. Control functions                                               31

Overview

 This memo is intended to do the following things:
  1. Document a protocol and algorithms that are currently in

wide use for routing, but which have never been formally

      documented.
  1. Specify some improvements in the algorithms which will

improve stability of the routes in large networks. These

      improvements do not introduce any incompatibility with
      existing implementations.  They are to be incorporated into

Hedrick [Page 1] RFC 1058 Routing Information Protocol June 1988

      all implementations of this protocol.
  1. Suggest some optional features to allow greater

configurability and control. These features were developed

      specifically to solve problems that have shown up in actual
      use by the NSFnet community.  However, they should have more
      general utility.
 The Routing Information Protocol (RIP) described here is loosely
 based on the program "routed", distributed with the 4.3 Berkeley
 Software Distribution.  However, there are several other
 implementations of what is supposed to be the same protocol.
 Unfortunately, these various implementations disagree in various
 details.  The specifications here represent a combination of features
 taken from various implementations.  We believe that a program
 designed according to this document will interoperate with routed,
 and with all other implementations of RIP of which we are aware.
 Note that this description adopts a different view than most existing
 implementations about when metrics should be incremented.  By making
 a corresponding change in the metric used for a local network, we
 have retained compatibility with other existing implementations.  See
 section 3.6 for details on this issue.

1. Introduction

 This memo describes one protocol in a series of routing protocols
 based on the Bellman-Ford (or distance vector) algorithm.  This
 algorithm has been used for routing computations in computer networks
 since the early days of the ARPANET.  The particular packet formats
 and protocol described here are based on the program "routed", which
 is included with the Berkeley distribution of Unix.  It has become a
 de facto standard for exchange of routing information among gateways
 and hosts.  It is implemented for this purpose by most commercial
 vendors of IP gateways.  Note, however, that many of these vendors
 have their own protocols which are used among their own gateways.
 This protocol is most useful as an "interior gateway protocol".  In a
 nationwide network such as the current Internet, it is very unlikely
 that a single routing protocol will used for the whole network.
 Rather, the network will be organized as a collection of "autonomous
 systems".  An autonomous system will in general be administered by a
 single entity, or at least will have some reasonable degree of
 technical and administrative control.  Each autonomous system will
 have its own routing technology.  This may well be different for
 different autonomous systems.  The routing protocol used within an
 autonomous system is referred to as an interior gateway protocol, or
 "IGP".  A separate protocol is used to interface among the autonomous

Hedrick [Page 2] RFC 1058 Routing Information Protocol June 1988

 systems.  The earliest such protocol, still used in the Internet, is
 "EGP" (exterior gateway protocol).  Such protocols are now usually
 referred to as inter-AS routing protocols.  RIP was designed to work
 with moderate-size networks using reasonably homogeneous technology.
 Thus it is suitable as an IGP for many campuses and for regional
 networks using serial lines whose speeds do not vary widely.  It is
 not intended for use in more complex environments.  For more
 information on the context into which RIP is expected to fit, see
 Braden and Postel [3].
 RIP is one of a class of algorithms known as "distance vector
 algorithms".  The earliest description of this class of algorithms
 known to the author is in Ford and Fulkerson [6].  Because of this,
 they are sometimes known as Ford-Fulkerson algorithms.  The term
 Bellman-Ford is also used.  It comes from the fact that the
 formulation is based on Bellman's equation, the basis of "dynamic
 programming".  (For a standard introduction to this area, see [1].)
 The presentation in this document is closely based on [2].  This text
 contains an introduction to the mathematics of routing algorithms.
 It describes and justifies several variants of the algorithm
 presented here, as well as a number of other related algorithms.  The
 basic algorithms described in this protocol were used in computer
 routing as early as 1969 in the ARPANET.  However, the specific
 ancestry of this protocol is within the Xerox network protocols.  The
 PUP protocols (see [4]) used the Gateway Information Protocol to
 exchange routing information.  A somewhat updated version of this
 protocol was adopted for the Xerox Network Systems (XNS)
 architecture, with the name Routing Information Protocol.  (See [7].)
 Berkeley's routed is largely the same as the Routing Information
 Protocol, with XNS addresses replaced by a more general address
 format capable of handling IP and other types of address, and with
 routing updates limited to one every 30 seconds.  Because of this
 similarity, the term Routing Information Protocol (or just RIP) is
 used to refer to both the XNS protocol and the protocol used by
 routed.
 RIP is intended for use within the IP-based Internet.  The Internet
 is organized into a number of networks connected by gateways.  The
 networks may be either point-to-point links or more complex networks
 such as Ethernet or the ARPANET.  Hosts and gateways are presented
 with IP datagrams addressed to some host.  Routing is the method by
 which the host or gateway decides where to send the datagram.  It may
 be able to send the datagram directly to the destination, if that
 destination is on one of the networks that are directly connected to
 the host or gateway.  However, the interesting case is when the
 destination is not directly reachable.  In this case, the host or
 gateway attempts to send the datagram to a gateway that is nearer the
 destination.  The goal of a routing protocol is very simple: It is to

Hedrick [Page 3] RFC 1058 Routing Information Protocol June 1988

 supply the information that is needed to do routing.

1.1. Limitations of the protocol

 This protocol does not solve every possible routing problem.  As
 mentioned above, it is primary intended for use as an IGP, in
 reasonably homogeneous networks of moderate size.  In addition, the
 following specific limitations should be mentioned:
  1. The protocol is limited to networks whose longest path

involves 15 hops. The designers believe that the basic

      protocol design is inappropriate for larger networks.  Note
      that this statement of the limit assumes that a cost of 1
      is used for each network.  This is the way RIP is normally
      configured.  If the system administrator chooses to use
      larger costs, the upper bound of 15 can easily become a
      problem.
  1. The protocol depends upon "counting to infinity" to resolve

certain unusual situations. (This will be explained in the

      next section.)  If the system of networks has several
      hundred networks, and a routing loop was formed involving
      all of them, the resolution of the loop would require
      either much time (if the frequency of routing updates were
      limited) or bandwidth (if updates were sent whenever
      changes were detected).  Such a loop would consume a large
      amount of network bandwidth before the loop was corrected.
      We believe that in realistic cases, this will not be a
      problem except on slow lines.  Even then, the problem will
      be fairly unusual, since various precautions are taken that
      should prevent these problems in most cases.
  1. This protocol uses fixed "metrics" to compare alternative

routes. It is not appropriate for situations where routes

      need to be chosen based on real-time parameters such a
      measured delay, reliability, or load.  The obvious
      extensions to allow metrics of this type are likely to
      introduce instabilities of a sort that the protocol is not
      designed to handle.

1.2. Organization of this document

 The main body of this document is organized into two parts, which
 occupy the next two sections:
    2   A conceptual development and justification of distance vector
        algorithms in general.

Hedrick [Page 4] RFC 1058 Routing Information Protocol June 1988

    3   The actual protocol description.
 Each of these two sections can largely stand on its own.  Section 2
 attempts to give an informal presentation of the mathematical
 underpinnings of the algorithm.  Note that the presentation follows a
 "spiral" method.  An initial, fairly simple algorithm is described.
 Then refinements are added to it in successive sections.  Section 3
 is the actual protocol description.  Except where specific references
 are made to section 2, it should be possible to implement RIP
 entirely from the specifications given in section 3.

2. Distance Vector Algorithms

 Routing is the task of finding a path from a sender to a desired
 destination.  In the IP "Catenet model" this reduces primarily to a
 matter of finding gateways between networks.  As long as a message
 remains on a single network or subnet, any routing problems are
 solved by technology that is specific to the network.  For example,
 the Ethernet and the ARPANET each define a way in which any sender
 can talk to any specified destination within that one network.  IP
 routing comes in primarily when messages must go from a sender on one
 such network to a destination on a different one.  In that case, the
 message must pass through gateways connecting the networks.  If the
 networks are not adjacent, the message may pass through several
 intervening networks, and the gateways connecting them.  Once the
 message gets to a gateway that is on the same network as the
 destination, that network's own technology is used to get to the
 destination.
 Throughout this section, the term "network" is used generically to
 cover a single broadcast network (e.g., an Ethernet), a point to
 point line, or the ARPANET.  The critical point is that a network is
 treated as a single entity by IP.  Either no routing is necessary (as
 with a point to point line), or that routing is done in a manner that
 is transparent to IP, allowing IP to treat the entire network as a
 single fully-connected system (as with an Ethernet or the ARPANET).
 Note that the term "network" is used in a somewhat different way in
 discussions of IP addressing.  A single IP network number may be
 assigned to a collection of networks, with "subnet" addressing being
 used to describe the individual networks.  In effect, we are using
 the term "network" here to refer to subnets in cases where subnet
 addressing is in use.
 A number of different approaches for finding routes between networks
 are possible.  One useful way of categorizing these approaches is on
 the basis of the type of information the gateways need to exchange in
 order to be able to find routes.  Distance vector algorithms are
 based on the exchange of only a small amount of information.  Each

Hedrick [Page 5] RFC 1058 Routing Information Protocol June 1988

 entity (gateway or host) that participates in the routing protocol is
 assumed to keep information about all of the destinations within the
 system.  Generally, information about all entities connected to one
 network is summarized by a single entry, which describes the route to
 all destinations on that network.  This summarization is possible
 because as far as IP is concerned, routing within a network is
 invisible.  Each entry in this routing database includes the next
 gateway to which datagrams destined for the entity should be sent.
 In addition, it includes a "metric" measuring the total distance to
 the entity.  Distance is a somewhat generalized concept, which may
 cover the time delay in getting messages to the entity, the dollar
 cost of sending messages to it, etc.  Distance vector algorithms get
 their name from the fact that it is possible to compute optimal
 routes when the only information exchanged is the list of these
 distances.  Furthermore, information is only exchanged among entities
 that are adjacent, that is, entities that share a common network.
 Although routing is most commonly based on information about
 networks, it is sometimes necessary to keep track of the routes to
 individual hosts.  The RIP protocol makes no formal distinction
 between networks and hosts.  It simply describes exchange of
 information about destinations, which may be either networks or
 hosts.  (Note however, that it is possible for an implementor to
 choose not to support host routes.  See section 3.2.)  In fact, the
 mathematical developments are most conveniently thought of in terms
 of routes from one host or gateway to another.  When discussing the
 algorithm in abstract terms, it is best to think of a routing entry
 for a network as an abbreviation for routing entries for all of the
 entities connected to that network.  This sort of abbreviation makes
 sense only because we think of networks as having no internal
 structure that is visible at the IP level.  Thus, we will generally
 assign the same distance to every entity in a given network.
 We said above that each entity keeps a routing database with one
 entry for every possible destination in the system.  An actual
 implementation is likely to need to keep the following information
 about each destination:
  1. address: in IP implementations of these algorithms, this

will be the IP address of the host or network.

  1. gateway: the first gateway along the route to the

destination.

  1. interface: the physical network which must be used to reach

the first gateway.

  1. metric: a number, indicating the distance to the

Hedrick [Page 6] RFC 1058 Routing Information Protocol June 1988

      destination.
  1. timer: the amount of time since the entry was last updated.
 In addition, various flags and other internal information will
 probably be included.  This database is initialized with a
 description of the entities that are directly connected to the
 system.  It is updated according to information received in messages
 from neighboring gateways.
 The most important information exchanged by the hosts and gateways is
 that carried in update messages.  Each entity that participates in
 the routing scheme sends update messages that describe the routing
 database as it currently exists in that entity.  It is possible to
 maintain optimal routes for the entire system by using only
 information obtained from neighboring entities.  The algorithm used
 for that will be described in the next section.
 As we mentioned above, the purpose of routing is to find a way to get
 datagrams to their ultimate destinations.  Distance vector algorithms
 are based on a table giving the best route to every destination in
 the system.  Of course, in order to define which route is best, we
 have to have some way of measuring goodness.  This is referred to as
 the "metric".
 In simple networks, it is common to use a metric that simply counts
 how many gateways a message must go through.  In more complex
 networks, a metric is chosen to represent the total amount of delay
 that the message suffers, the cost of sending it, or some other
 quantity which may be minimized.  The main requirement is that it
 must be possible to represent the metric as a sum of "costs" for
 individual hops.
 Formally, if it is possible to get from entity i to entity j directly
 (i.e., without passing through another gateway between), then a cost,
 d(i,j), is associated with the hop between i and j.  In the normal
 case where all entities on a given network are considered to be the
 same, d(i,j) is the same for all destinations on a given network, and
 represents the cost of using that network.  To get the metric of a
 complete route, one just adds up the costs of the individual hops
 that make up the route.  For the purposes of this memo, we assume
 that the costs are positive integers.
 Let D(i,j) represent the metric of the best route from entity i to
 entity j.  It should be defined for every pair of entities.  d(i,j)
 represents the costs of the individual steps.  Formally, let d(i,j)
 represent the cost of going directly from entity i to entity j.  It
 is infinite if i and j are not immediate neighbors. (Note that d(i,i)

Hedrick [Page 7] RFC 1058 Routing Information Protocol June 1988

 is infinite.  That is, we don't consider there to be a direct
 connection from a node to itself.)  Since costs are additive, it is
 easy to show that the best metric must be described by
           D(i,i) = 0,                      all i
           D(i,j) = min [d(i,k) + D(k,j)],  otherwise
                     k
 and that the best routes start by going from i to those neighbors k
 for which d(i,k) + D(k,j) has the minimum value.  (These things can
 be shown by induction on the number of steps in the routes.)  Note
 that we can limit the second equation to k's that are immediate
 neighbors of i.  For the others, d(i,k) is infinite, so the term
 involving them can never be the minimum.
 It turns out that one can compute the metric by a simple algorithm
 based on this.  Entity i gets its neighbors k to send it their
 estimates of their distances to the destination j.  When i gets the
 estimates from k, it adds d(i,k) to each of the numbers.  This is
 simply the cost of traversing the network between i and k.  Now and
 then i compares the values from all of its neighbors and picks the
 smallest.
 A proof is given in [2] that this algorithm will converge to the
 correct estimates of D(i,j) in finite time in the absence of topology
 changes.  The authors make very few assumptions about the order in
 which the entities send each other their information, or when the min
 is recomputed.  Basically, entities just can't stop sending updates
 or recomputing metrics, and the networks can't delay messages
 forever.  (Crash of a routing entity is a topology change.)  Also,
 their proof does not make any assumptions about the initial estimates
 of D(i,j), except that they must be non-negative.  The fact that
 these fairly weak assumptions are good enough is important.  Because
 we don't have to make assumptions about when updates are sent, it is
 safe to run the algorithm asynchronously.  That is, each entity can
 send updates according to its own clock.  Updates can be dropped by
 the network, as long as they don't all get dropped.  Because we don't
 have to make assumptions about the starting condition, the algorithm
 can handle changes.  When the system changes, the routing algorithm
 starts moving to a new equilibrium, using the old one as its starting
 point.  It is important that the algorithm will converge in finite
 time no matter what the starting point.  Otherwise certain kinds of
 changes might lead to non-convergent behavior.
 The statement of the algorithm given above (and the proof) assumes
 that each entity keeps copies of the estimates that come from each of
 its neighbors, and now and then does a min over all of the neighbors.
 In fact real implementations don't necessarily do that.  They simply

Hedrick [Page 8] RFC 1058 Routing Information Protocol June 1988

 remember the best metric seen so far, and the identity of the
 neighbor that sent it.  They replace this information whenever they
 see a better (smaller) metric.  This allows them to compute the
 minimum incrementally, without having to store data from all of the
 neighbors.
 There is one other difference between the algorithm as described in
 texts and those used in real protocols such as RIP: the description
 above would have each entity include an entry for itself, showing a
 distance of zero.  In fact this is not generally done.  Recall that
 all entities on a network are normally summarized by a single entry
 for the network.  Consider the situation of a host or gateway G that
 is connected to network A.  C represents the cost of using network A
 (usually a metric of one).  (Recall that we are assuming that the
 internal structure of a network is not visible to IP, and thus the
 cost of going between any two entities on it is the same.)  In
 principle, G should get a message from every other entity H on
 network A, showing a cost of 0 to get from that entity to itself.  G
 would then compute C + 0 as the distance to H.  Rather than having G
 look at all of these identical messages, it simply starts out by
 making an entry for network A in its table, and assigning it a metric
 of C.  This entry for network A should be thought of as summarizing
 the entries for all other entities on network A.  The only entity on
 A that can't be summarized by that common entry is G itself, since
 the cost of going from G to G is 0, not C.  But since we never need
 those 0 entries, we can safely get along with just the single entry
 for network A.  Note one other implication of this strategy: because
 we don't need to use the 0 entries for anything, hosts that do not
 function as gateways don't need to send any update messages.  Clearly
 hosts that don't function as gateways (i.e., hosts that are connected
 to only one network) can have no useful information to contribute
 other than their own entry D(i,i) = 0.  As they have only the one
 interface, it is easy to see that a route to any other network
 through them will simply go in that interface and then come right
 back out it.  Thus the cost of such a route will be greater than the
 best cost by at least C.  Since we don't need the 0 entries, non-
 gateways need not participate in the routing protocol at all.
 Let us summarize what a host or gateway G does.  For each destination
 in the system, G will keep a current estimate of the metric for that
 destination (i.e., the total cost of getting to it) and the identity
 of the neighboring gateway on whose data that metric is based.  If
 the destination is on a network that is directly connected to G, then
 G simply uses an entry that shows the cost of using the network, and
 the fact that no gateway is needed to get to the destination.  It is
 easy to show that once the computation has converged to the correct
 metrics, the neighbor that is recorded by this technique is in fact
 the first gateway on the path to the destination.  (If there are

Hedrick [Page 9] RFC 1058 Routing Information Protocol June 1988

 several equally good paths, it is the first gateway on one of them.)
 This combination of destination, metric, and gateway is typically
 referred to as a route to the destination with that metric, using
 that gateway.
 The method so far only has a way to lower the metric, as the existing
 metric is kept until a smaller one shows up.  It is possible that the
 initial estimate might be too low.  Thus, there must be a way to
 increase the metric.  It turns out to be sufficient to use the
 following rule: suppose the current route to a destination has metric
 D and uses gateway G.  If a new set of information arrived from some
 source other than G, only update the route if the new metric is
 better than D.  But if a new set of information arrives from G
 itself, always update D to the new value.  It is easy to show that
 with this rule, the incremental update process produces the same
 routes as a calculation that remembers the latest information from
 all the neighbors and does an explicit minimum.  (Note that the
 discussion so far assumes that the network configuration is static.
 It does not allow for the possibility that a system might fail.)
 To summarize, here is the basic distance vector algorithm as it has
 been developed so far.  (Note that this is not a statement of the RIP
 protocol.  There are several refinements still to be added.)  The
 following procedure is carried out by every entity that participates
 in the routing protocol.  This must include all of the gateways in
 the system.  Hosts that are not gateways may participate as well.
  1. Keep a table with an entry for every possible destination

in the system. The entry contains the distance D to the

      destination, and the first gateway G on the route to that
      network.  Conceptually, there should be an entry for the
      entity itself, with metric 0, but this is not actually
      included.
  1. Periodically, send a routing update to every neighbor. The

update is a set of messages that contain all of the

      information from the routing table.  It contains an entry
      for each destination, with the distance shown to that
      destination.
  1. When a routing update arrives from a neighbor G', add the

cost associated with the network that is shared with G'.

      (This should be the network over which the update arrived.)
      Call the resulting distance D'.  Compare the resulting
      distances with the current routing table entries.  If the
      new distance D' for N is smaller than the existing value D,
      adopt the new route.  That is, change the table entry for N
      to have metric D' and gateway G'.  If G' is the gateway

Hedrick [Page 10] RFC 1058 Routing Information Protocol June 1988

      from which the existing route came, i.e., G' = G, then use
      the new metric even if it is larger than the old one.

2.1. Dealing with changes in topology

 The discussion above assumes that the topology of the network is
 fixed.  In practice, gateways and lines often fail and come back up.
 To handle this possibility, we need to modify the algorithm slightly.
 The theoretical version of the algorithm involved a minimum over all
 immediate neighbors.  If the topology changes, the set of neighbors
 changes.  Therefore, the next time the calculation is done, the
 change will be reflected.  However, as mentioned above, actual
 implementations use an incremental version of the minimization.  Only
 the best route to any given destination is remembered.  If the
 gateway involved in that route should crash, or the network
 connection to it break, the calculation might never reflect the
 change.  The algorithm as shown so far depends upon a gateway
 notifying its neighbors if its metrics change.  If the gateway
 crashes, then it has no way of notifying neighbors of a change.
 In order to handle problems of this kind, distance vector protocols
 must make some provision for timing out routes.  The details depend
 upon the specific protocol.  As an example, in RIP every gateway that
 participates in routing sends an update message to all its neighbors
 once every 30 seconds.  Suppose the current route for network N uses
 gateway G.  If we don't hear from G for 180 seconds, we can assume
 that either the gateway has crashed or the network connecting us to
 it has become unusable.  Thus, we mark the route as invalid.  When we
 hear from another neighbor that has a valid route to N, the valid
 route will replace the invalid one.  Note that we wait for 180
 seconds before timing out a route even though we expect to hear from
 each neighbor every 30 seconds.  Unfortunately, messages are
 occasionally lost by networks.  Thus, it is probably not a good idea
 to invalidate a route based on a single missed message.
 As we will see below, it is useful to have a way to notify neighbors
 that there currently isn't a valid route to some network.  RIP, along
 with several other protocols of this class, does this through a
 normal update message, by marking that network as unreachable.  A
 specific metric value is chosen to indicate an unreachable
 destination; that metric value is larger than the largest valid
 metric that we expect to see.  In the existing implementation of RIP,
 16 is used.  This value is normally referred to as "infinity", since
 it is larger than the largest valid metric.  16 may look like a
 surprisingly small number.  It is chosen to be this small for reasons
 that we will see shortly.  In most implementations, the same
 convention is used internally to flag a route as invalid.

Hedrick [Page 11] RFC 1058 Routing Information Protocol June 1988

2.2. Preventing instability

 The algorithm as presented up to this point will always allow a host
 or gateway to calculate a correct routing table.  However, that is
 still not quite enough to make it useful in practice.  The proofs
 referred to above only show that the routing tables will converge to
 the correct values in finite time.  They do not guarantee that this
 time will be small enough to be useful, nor do they say what will
 happen to the metrics for networks that become inaccessible.
 It is easy enough to extend the mathematics to handle routes becoming
 inaccessible.  The convention suggested above will do that.  We
 choose a large metric value to represent "infinity".  This value must
 be large enough that no real metric would ever get that large.  For
 the purposes of this example, we will use the value 16.  Suppose a
 network becomes inaccessible.  All of the immediately neighboring
 gateways time out and set the metric for that network to 16.  For
 purposes of analysis, we can assume that all the neighboring gateways
 have gotten a new piece of hardware that connects them directly to
 the vanished network, with a cost of 16.  Since that is the only
 connection to the vanished network, all the other gateways in the
 system will converge to new routes that go through one of those
 gateways.  It is easy to see that once convergence has happened, all
 the gateways will have metrics of at least 16 for the vanished
 network.  Gateways one hop away from the original neighbors would end
 up with metrics of at least 17; gateways two hops away would end up
 with at least 18, etc.  As these metrics are larger than the maximum
 metric value, they are all set to 16.  It is obvious that the system
 will now converge to a metric of 16 for the vanished network at all
 gateways.
 Unfortunately, the question of how long convergence will take is not
 amenable to quite so simple an answer.  Before going any further, it
 will be useful to look at an example (taken from [2]).  Note, by the
 way, that what we are about to show will not happen with a correct
 implementation of RIP.  We are trying to show why certain features
 are needed.  Note that the letters correspond to gateways, and the
 lines to networks.
          A-----B
           \   / \
            \ /  |
             C  /    all networks have cost 1, except
             | /     for the direct link from C to D, which
             |/      has cost 10
             D
             |<=== target network

Hedrick [Page 12] RFC 1058 Routing Information Protocol June 1988

 Each gateway will have a table showing a route to each network.
 However, for purposes of this illustration, we show only the routes
 from each gateway to the network marked at the bottom of the diagram.
          D:  directly connected, metric 1
          B:  route via D, metric 2
          C:  route via B, metric 3
          A:  route via B, metric 3
 Now suppose that the link from B to D fails.  The routes should now
 adjust to use the link from C to D.  Unfortunately, it will take a
 while for this to this to happen.  The routing changes start when B
 notices that the route to D is no longer usable.  For simplicity, the
 chart below assumes that all gateways send updates at the same time.
 The chart shows the metric for the target network, as it appears in
 the routing table at each gateway.
      time ------>
      D: dir, 1   dir, 1   dir, 1   dir, 1  ...  dir, 1   dir, 1
      B: unreach  C,   4   C,   5   C,   6       C,  11   C,  12
      C: B,   3   A,   4   A,   5   A,   6       A,  11   D,  11
      A: B,   3   C,   4   C,   5   C,   6       C,  11   C,  12
      dir = directly connected
      unreach = unreachable
 Here's the problem:  B is able to get rid of its failed route using a
 timeout mechanism.  But vestiges of that route persist in the system
 for a long time.  Initially, A and C still think they can get to D
 via B.  So, they keep sending updates listing metrics of 3.  In the
 next iteration, B will then claim that it can get to D via either A
 or C.  Of course, it can't.  The routes being claimed by A and C are
 now gone, but they have no way of knowing that yet.  And even when
 they discover that their routes via B have gone away, they each think
 there is a route available via the other.  Eventually the system
 converges, as all the mathematics claims it must.  But it can take
 some time to do so.  The worst case is when a network becomes
 completely inaccessible from some part of the system.  In that case,
 the metrics may increase slowly in a pattern like the one above until
 they finally reach infinity.  For this reason, the problem is called
 "counting to infinity".
 You should now see why "infinity" is chosen to be as small as
 possible.  If a network becomes completely inaccessible, we want
 counting to infinity to be stopped as soon as possible.  Infinity
 must be large enough that no real route is that big.  But it

Hedrick [Page 13] RFC 1058 Routing Information Protocol June 1988

 shouldn't be any bigger than required.  Thus the choice of infinity
 is a tradeoff between network size and speed of convergence in case
 counting to infinity happens.  The designers of RIP believed that the
 protocol was unlikely to be practical for networks with a diameter
 larger than 15.
 There are several things that can be done to prevent problems like
 this.  The ones used by RIP are called "split horizon with poisoned
 reverse", and "triggered updates".

2.2.1. Split horizon

 Note that some of the problem above is caused by the fact that A and
 C are engaged in a pattern of mutual deception.  Each claims to be
 able to get to D via the other.  This can be prevented by being a bit
 more careful about where information is sent.  In particular, it is
 never useful to claim reachability for a destination network to the
 neighbor(s) from which the route was learned.  "Split horizon" is a
 scheme for avoiding problems caused by including routes in updates
 sent to the gateway from which they were learned.  The "simple split
 horizon" scheme omits routes learned from one neighbor in updates
 sent to that neighbor.  "Split horizon with poisoned reverse"
 includes such routes in updates, but sets their metrics to infinity.
 If A thinks it can get to D via C, its messages to C should indicate
 that D is unreachable.  If the route through C is real, then C either
 has a direct connection to D, or a connection through some other
 gateway.  C's route can't possibly go back to A, since that forms a
 loop.  By telling C that D is unreachable, A simply guards against
 the possibility that C might get confused and believe that there is a
 route through A.  This is obvious for a point to point line.  But
 consider the possibility that A and C are connected by a broadcast
 network such as an Ethernet, and there are other gateways on that
 network.  If A has a route through C, it should indicate that D is
 unreachable when talking to any other gateway on that network.  The
 other gateways on the network can get to C themselves.  They would
 never need to get to C via A.  If A's best route is really through C,
 no other gateway on that network needs to know that A can reach D.
 This is fortunate, because it means that the same update message that
 is used for C can be used for all other gateways on the same network.
 Thus, update messages can be sent by broadcast.
 In general, split horizon with poisoned reverse is safer than simple
 split horizon.  If two gateways have routes pointing at each other,
 advertising reverse routes with a metric of 16 will break the loop
 immediately.  If the reverse routes are simply not advertised, the
 erroneous routes will have to be eliminated by waiting for a timeout.
 However, poisoned reverse does have a disadvantage: it increases the

Hedrick [Page 14] RFC 1058 Routing Information Protocol June 1988

 size of the routing messages.  Consider the case of a campus backbone
 connecting a number of different buildings.  In each building, there
 is a gateway connecting the backbone to a local network.  Consider
 what routing updates those gateways should broadcast on the backbone
 network.  All that the rest of the network really needs to know about
 each gateway is what local networks it is connected to.  Using simple
 split horizon, only those routes would appear in update messages sent
 by the gateway to the backbone network.  If split horizon with
 poisoned reverse is used, the gateway must mention all routes that it
 learns from the backbone, with metrics of 16.  If the system is
 large, this can result in a large update message, almost all of whose
 entries indicate unreachable networks.
 In a static sense, advertising reverse routes with a metric of 16
 provides no additional information.  If there are many gateways on
 one broadcast network, these extra entries can use significant
 bandwidth.  The reason they are there is to improve dynamic behavior.
 When topology changes, mentioning routes that should not go through
 the gateway as well as those that should can speed up convergence.
 However, in some situations, network managers may prefer to accept
 somewhat slower convergence in order to minimize routing overhead.
 Thus implementors may at their option implement simple split horizon
 rather than split horizon with poisoned reverse, or they may provide
 a configuration option that allows the network manager to choose
 which behavior to use.  It is also permissible to implement hybrid
 schemes that advertise some reverse routes with a metric of 16 and
 omit others.  An example of such a scheme would be to use a metric of
 16 for reverse routes for a certain period of time after routing
 changes involving them, and thereafter omitting them from updates.

2.2.2. Triggered updates

 Split horizon with poisoned reverse will prevent any routing loops
 that involve only two gateways.  However, it is still possible to end
 up with patterns in which three gateways are engaged in mutual
 deception.  For example, A may believe it has a route through B, B
 through C, and C through A.  Split horizon cannot stop such a loop.
 This loop will only be resolved when the metric reaches infinity and
 the network involved is then declared unreachable.  Triggered updates
 are an attempt to speed up this convergence.  To get triggered
 updates, we simply add a rule that whenever a gateway changes the
 metric for a route, it is required to send update messages almost
 immediately, even if it is not yet time for one of the regular update
 message.  (The timing details will differ from protocol to protocol.
 Some distance vector protocols, including RIP, specify a small time
 delay, in order to avoid having triggered updates generate excessive
 network traffic.)  Note how this combines with the rules for
 computing new metrics.  Suppose a gateway's route to destination N

Hedrick [Page 15] RFC 1058 Routing Information Protocol June 1988

 goes through gateway G.  If an update arrives from G itself, the
 receiving gateway is required to believe the new information, whether
 the new metric is higher or lower than the old one.  If the result is
 a change in metric, then the receiving gateway will send triggered
 updates to all the hosts and gateways directly connected to it.  They
 in turn may each send updates to their neighbors.  The result is a
 cascade of triggered updates.  It is easy to show which gateways and
 hosts are involved in the cascade.  Suppose a gateway G times out a
 route to destination N.  G will send triggered updates to all of its
 neighbors.  However, the only neighbors who will believe the new
 information are those whose routes for N go through G.  The other
 gateways and hosts will see this as information about a new route
 that is worse than the one they are already using, and ignore it.
 The neighbors whose routes go through G will update their metrics and
 send triggered updates to all of their neighbors.  Again, only those
 neighbors whose routes go through them will pay attention.  Thus, the
 triggered updates will propagate backwards along all paths leading to
 gateway G, updating the metrics to infinity.  This propagation will
 stop as soon as it reaches a portion of the network whose route to
 destination N takes some other path.
 If the system could be made to sit still while the cascade of
 triggered updates happens, it would be possible to prove that
 counting to infinity will never happen.  Bad routes would always be
 removed immediately, and so no routing loops could form.
 Unfortunately, things are not so nice.  While the triggered updates
 are being sent, regular updates may be happening at the same time.
 Gateways that haven't received the triggered update yet will still be
 sending out information based on the route that no longer exists.  It
 is possible that after the triggered update has gone through a
 gateway, it might receive a normal update from one of these gateways
 that hasn't yet gotten the word.  This could reestablish an orphaned
 remnant of the faulty route.  If triggered updates happen quickly
 enough, this is very unlikely.  However, counting to infinity is
 still possible.

3. Specifications for the protocol

 RIP is intended to allow hosts and gateways to exchange information
 for computing routes through an IP-based network.  RIP is a distance
 vector protocol.  Thus, it has the general features described in
 section 2.  RIP may be implemented by both hosts and gateways.  As in
 most IP documentation, the term "host" will be used here to cover
 either.  RIP is used to convey information about routes to
 "destinations", which may be individual hosts, networks, or a special
 destination used to convey a default route.

Hedrick [Page 16] RFC 1058 Routing Information Protocol June 1988

 Any host that uses RIP is assumed to have interfaces to one or more
 networks.  These are referred to as its "directly-connected
 networks".  The protocol relies on access to certain information
 about each of these networks.  The most important is its metric or
 "cost".  The metric of a network is an integer between 1 and 15
 inclusive.  It is set in some manner not specified in this protocol.
 Most existing implementations always use a metric of 1.  New
 implementations should allow the system administrator to set the cost
 of each network.  In addition to the cost, each network will have an
 IP network number and a subnet mask associated with it.  These are to
 be set by the system administrator in a manner not specified in this
 protocol.
 Note that the rules specified in section 3.2 assume that there is a
 single subnet mask applying to each IP network, and that only the
 subnet masks for directly-connected networks are known.  There may be
 systems that use different subnet masks for different subnets within
 a single network.  There may also be instances where it is desirable
 for a system to know the subnets masks of distant networks.  However,
 such situations will require modifications of the rules which govern
 the spread of subnet information.  Such modifications raise issues of
 interoperability, and thus must be viewed as modifying the protocol.
 Each host that implements RIP is assumed to have a routing table.
 This table has one entry for every destination that is reachable
 through the system described by RIP.  Each entry contains at least
 the following information:
  1. The IP address of the destination.
  1. A metric, which represents the total cost of getting a

datagram from the host to that destination. This metric is

      the sum of the costs associated with the networks that
      would be traversed in getting to the destination.
  1. The IP address of the next gateway along the path to the

destination. If the destination is on one of the

      directly-connected networks, this item is not needed.
  1. A flag to indicate that information about the route has

changed recently. This will be referred to as the "route

      change flag."
  1. Various timers associated with the route. See section 3.3

for more details on them.

 The entries for the directly-connected networks are set up by the
 host, using information gathered by means not specified in this

Hedrick [Page 17] RFC 1058 Routing Information Protocol June 1988

 protocol.  The metric for a directly-connected network is set to the
 cost of that network.  In existing RIP implementations, 1 is always
 used for the cost.  In that case, the RIP metric reduces to a simple
 hop-count.  More complex metrics may be used when it is desirable to
 show preference for some networks over others, for example because of
 differences in bandwidth or reliability.
 Implementors may also choose to allow the system administrator to
 enter additional routes.  These would most likely be routes to hosts
 or networks outside the scope of the routing system.
 Entries for destinations other these initial ones are added and
 updated by the algorithms described in the following sections.
 In order for the protocol to provide complete information on routing,
 every gateway in the system must participate in it.  Hosts that are
 not gateways need not participate, but many implementations make
 provisions for them to listen to routing information in order to
 allow them to maintain their routing tables.

3.1. Message formats

 RIP is a UDP-based protocol.  Each host that uses RIP has a routing
 process that sends and receives datagrams on UDP port number 520.
 All communications directed at another host's RIP processor are sent
 to port 520.  All routing update messages are sent from port 520.
 Unsolicited routing update messages have both the source and
 destination port equal to 520.  Those sent in response to a request
 are sent to the port from which the request came.  Specific queries
 and debugging requests may be sent from ports other than 520, but
 they are directed to port 520 on the target machine.
 There are provisions in the protocol to allow "silent" RIP processes.
 A silent process is one that normally does not send out any messages.
 However, it listens to messages sent by others.  A silent RIP might
 be used by hosts that do not act as gateways, but wish to listen to
 routing updates in order to monitor local gateways and to keep their
 internal routing tables up to date.  (See [5] for a discussion of
 various ways that hosts can keep track of network topology.)  A
 gateway that has lost contact with all but one of its networks might
 choose to become silent, since it is effectively no longer a gateway.
 However, this should not be done if there is any chance that
 neighboring gateways might depend upon its messages to detect that
 the failed network has come back into operation.  (The 4BSD routed
 program uses routing packets to monitor the operation of point-to-
 point links.)

Hedrick [Page 18] RFC 1058 Routing Information Protocol June 1988

 The packet format is shown in Figure 1.
    Format of datagrams containing network information.  Field sizes
    are given in octets.  Unless otherwise specified, fields contain
    binary integers, in normal Internet order with the most-significant
    octet first.  Each tick mark represents one bit.
     0                   1                   2                   3 3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | command (1)   | version (1)   |      must be zero (2)         |
    +---------------+---------------+-------------------------------+
    | address family identifier (2) |      must be zero (2)         |
    +-------------------------------+-------------------------------+
    |                         IP address (4)                        |
    +---------------------------------------------------------------+
    |                        must be zero (4)                       |
    +---------------------------------------------------------------+
    |                        must be zero (4)                       |
    +---------------------------------------------------------------+
    |                          metric (4)                           |
    +---------------------------------------------------------------+
                                    .
                                    .
                                    .
    The portion of the datagram from address family identifier through
    metric may appear up to 25 times.  IP address is the usual 4-octet
    Internet address, in network order.
                        Figure 1.   Packet format
 Every datagram contains a command, a version number, and possible
 arguments.  This document describes version 1 of the protocol.
 Details of processing the version number are described in section
 3.4.  The command field is used to specify the purpose of this
 datagram.  Here is a summary of the commands implemented in version
 1:
 1 - request     A request for the responding system to send all or
                 part of its routing table.
 2 - response    A message containing all or part of the sender's
                 routing table.  This message may be sent in response
                 to a request or poll, or it may be an update message
                 generated by the sender.
 3 - traceon     Obsolete.  Messages containing this command are to be
                 ignored.

Hedrick [Page 19] RFC 1058 Routing Information Protocol June 1988

 4 - traceoff    Obsolete.  Messages containing this command are to be
                 ignored.
 5 - reserved    This value is used by Sun Microsystems for its own
                 purposes.  If new commands are added in any
                 succeeding version, they should begin with 6.
                 Messages containing this command may safely be
                 ignored by implementations that do not choose to
                 respond to it.
 For request and response, the rest of the datagram contains a list of
 destinations, with information about each.  Each entry in this list
 contains a destination network or host, and the metric for it.  The
 packet format is intended to allow RIP to carry routing information
 for several different protocols.  Thus, each entry has an address
 family identifier to indicate what type of address is specified in
 that entry.  This document only describes routing for Internet
 networks.  The address family identifier for IP is 2.  None of the
 RIP implementations available to the author implement any other type
 of address.  However, to allow for future development,
 implementations are required to skip entries that specify address
 families that are not supported by the implementation.  (The size of
 these entries will be the same as the size of an entry specifying an
 IP address.) Processing of the message continues normally after any
 unsupported entries are skipped.  The IP address is the usual
 Internet address, stored as 4 octets in network order.  The metric
 field must contain a value between 1 and 15 inclusive, specifying the
 current metric for the destination, or the value 16, which indicates
 that the destination is not reachable.  Each route sent by a gateway
 supercedes any previous route to the same destination from the same
 gateway.
 The maximum datagram size is 512 octets.  This includes only the
 portions of the datagram described above.  It does not count the IP
 or UDP headers.  The commands that involve network information allow
 information to be split across several datagrams.  No special
 provisions are needed for continuations, since correct results will
 occur if the datagrams are processed individually.

3.2. Addressing considerations

 As indicated in section 2, distance vector routing can be used to
 describe routes to individual hosts or to networks.  The RIP protocol
 allows either of these possibilities.  The destinations appearing in
 request and response messages can be networks, hosts, or a special
 code used to indicate a default address.  In general, the kinds of
 routes actually used will depend upon the routing strategy used for
 the particular network.  Many networks are set up so that routing

Hedrick [Page 20] RFC 1058 Routing Information Protocol June 1988

 information for individual hosts is not needed.  If every host on a
 given network or subnet is accessible through the same gateways, then
 there is no reason to mention individual hosts in the routing tables.
 However, networks that include point to point lines sometimes require
 gateways to keep track of routes to certain hosts.  Whether this
 feature is required depends upon the addressing and routing approach
 used in the system.  Thus, some implementations may choose not to
 support host routes.  If host routes are not supported, they are to
 be dropped when they are received in response messages.  (See section
 3.4.2.)
 The RIP packet formats do not distinguish among various types of
 address.  Fields that are labeled "address" can contain any of the
 following:
    host address
    subnet number
    network number
    0, indicating a default route
 Entities that use RIP are assumed to use the most specific
 information available when routing a datagram.  That is, when routing
 a datagram, its destination address must first be checked against the
 list of host addresses.  Then it must be checked to see whether it
 matches any known subnet or network number.  Finally, if none of
 these match, the default route is used.
 When a host evaluates information that it receives via RIP, its
 interpretation of an address depends upon whether it knows the subnet
 mask that applies to the net.  If so, then it is possible to
 determine the meaning of the address.  For example, consider net
 128.6.  It has a subnet mask of 255.255.255.0.  Thus 128.6.0.0 is a
 network number, 128.6.4.0 is a subnet number, and 128.6.4.1 is a host
 address.  However, if the host does not know the subnet mask,
 evaluation of an address may be ambiguous.  If there is a non-zero
 host part, there is no clear way to determine whether the address
 represents a subnet number or a host address.  As a subnet number
 would be useless without the subnet mask, addresses are assumed to
 represent hosts in this situation.  In order to avoid this sort of
 ambiguity, hosts must not send subnet routes to hosts that cannot be
 expected to know the appropriate subnet mask.  Normally hosts only
 know the subnet masks for directly-connected networks.  Therefore,
 unless special provisions have been made, routes to a subnet must not
 be sent outside the network of which the subnet is a part.
 This filtering is carried out by the gateways at the "border" of the
 subnetted network.  These are gateways that connect that network with
 some other network.  Within the subnetted network, each subnet is

Hedrick [Page 21] RFC 1058 Routing Information Protocol June 1988

 treated as an individual network.  Routing entries for each subnet
 are circulated by RIP.  However, border gateways send only a single
 entry for the network as a whole to hosts in other networks.  This
 means that a border gateway will send different information to
 different neighbors.  For neighbors connected to the subnetted
 network, it generates a list of all subnets to which it is directly
 connected, using the subnet number.  For neighbors connected to other
 networks, it makes a single entry for the network as a whole, showing
 the metric associated with that network.  (This metric would normally
 be the smallest metric for the subnets to which the gateway is
 attached.)
 Similarly, border gateways must not mention host routes for hosts
 within one of the directly-connected networks in messages to other
 networks.  Those routes will be subsumed by the single entry for the
 network as a whole.  We do not specify what to do with host routes
 for "distant" hosts (i.e., hosts not part of one of the directly-
 connected networks).  Generally, these routes indicate some host that
 is reachable via a route that does not support other hosts on the
 network of which the host is a part.
 The special address 0.0.0.0 is used to describe a default route.  A
 default route is used when it is not convenient to list every
 possible network in the RIP updates, and when one or more closely-
 connected gateways in the system are prepared to handle traffic to
 the networks that are not listed explicitly.  These gateways should
 create RIP entries for the address 0.0.0.0, just as if it were a
 network to which they are connected.  The decision as to how gateways
 create entries for 0.0.0.0 is left to the implementor.  Most
 commonly, the system administrator will be provided with a way to
 specify which gateways should create entries for 0.0.0.0.  However,
 other mechanisms are possible.  For example, an implementor might
 decide that any gateway that speaks EGP should be declared to be a
 default gateway.  It may be useful to allow the network administrator
 to choose the metric to be used in these entries.  If there is more
 than one default gateway, this will make it possible to express a
 preference for one over the other.  The entries for 0.0.0.0 are
 handled by RIP in exactly the same manner as if there were an actual
 network with this address.  However, the entry is used to route any
 datagram whose destination address does not match any other network
 in the table.  Implementations are not required to support this
 convention.  However, it is strongly recommended.  Implementations
 that do not support 0.0.0.0 must ignore entries with this address.
 In such cases, they must not pass the entry on in their own RIP
 updates.  System administrators should take care to make sure that
 routes to 0.0.0.0 do not propagate further than is intended.
 Generally, each autonomous system has its own preferred default
 gateway.  Thus, routes involving 0.0.0.0 should generally not leave

Hedrick [Page 22] RFC 1058 Routing Information Protocol June 1988

 the boundary of an autonomous system.  The mechanisms for enforcing
 this are not specified in this document.

3.3. Timers

 This section describes all events that are triggered by timers.
 Every 30 seconds, the output process is instructed to generate a
 complete response to every neighboring gateway.  When there are many
 gateways on a single network, there is a tendency for them to
 synchronize with each other such that they all issue updates at the
 same time.  This can happen whenever the 30 second timer is affected
 by the processing load on the system.  It is undesirable for the
 update messages to become synchronized, since it can lead to
 unnecessary collisions on broadcast networks.  Thus, implementations
 are required to take one of two precautions.
  1. The 30-second updates are triggered by a clock whose rate

is not affected by system load or the time required to

      service the previous update timer.
  1. The 30-second timer is offset by addition of a small random

time each time it is set.

 There are two timers associated with each route, a "timeout" and a
 "garbage-collection time".  Upon expiration of the timeout, the route
 is no longer valid.  However, it is retained in the table for a short
 time, so that neighbors can be notified that the route has been
 dropped.  Upon expiration of the garbage-collection timer, the route
 is finally removed from the tables.
 The timeout is initialized when a route is established, and any time
 an update message is received for the route.  If 180 seconds elapse
 from the last time the timeout was initialized, the route is
 considered to have expired, and the deletion process which we are
 about to describe is started for it.
 Deletions can occur for one of two reasons: (1) the timeout expires,
 or (2) the metric is set to 16 because of an update received from the
 current gateway.  (See section 3.4.2 for a discussion processing
 updates from other gateways.)  In either case, the following events
 happen:
  1. The garbage-collection timer is set for 120 seconds.
  1. The metric for the route is set to 16 (infinity). This

causes the route to be removed from service.

Hedrick [Page 23] RFC 1058 Routing Information Protocol June 1988

  1. A flag is set noting that this entry has been changed, and

the output process is signalled to trigger a response.

 Until the garbage-collection timer expires, the route is included in
 all updates sent by this host, with a metric of 16 (infinity).  When
 the garbage-collection timer expires, the route is deleted from the
 tables.
 Should a new route to this network be established while the garbage-
 collection timer is running, the new route will replace the one that
 is about to be deleted.  In this case the garbage-collection timer
 must be cleared.
 See section 3.5 for a discussion of a delay that is required in
 carrying out triggered updates.  Although implementation of that
 delay will require a timer, it is more natural to discuss it in
 section 3.5 than here.

3.4. Input processing

 This section will describe the handling of datagrams received on UDP
 port 520.  Before processing the datagrams in detail, certain general
 format checks must be made.  These depend upon the version number
 field in the datagram, as follows:
    0   Datagrams whose version number is zero are to be ignored.
        These are from a previous version of the protocol, whose
        packet format was machine-specific.
    1   Datagrams whose version number is one are to be processed
        as described in the rest of this specification.  All fields
        that are described above as "must be zero" are to be checked.
        If any such field contains a non-zero value, the entire
        message is to be ignored.
    >1  Datagrams whose version number are greater than one are
        to be processed as described in the rest of this
        specification.  All fields that are described above as
        "must be zero" are to be ignored.  Future versions of the
        protocol may put data into these fields.  Version 1
        implementations are to ignore this extra data and process
        only the fields specified in this document.
 After checking the version number and doing any other preliminary
 checks, processing will depend upon the value in the command field.

Hedrick [Page 24] RFC 1058 Routing Information Protocol June 1988

3.4.1. Request

 Request is used to ask for a response containing all or part of the
 host's routing table.  [Note that the term host is used for either
 host or gateway, in most cases it would be unusual for a non-gateway
 host to send RIP messages.]  Normally, requests are sent as
 broadcasts, from a UDP source port of 520.  In this case, silent
 processes do not respond to the request.  Silent processes are by
 definition processes for which we normally do not want to see routing
 information.  However, there may be situations involving gateway
 monitoring where it is desired to look at the routing table even for
 a silent process.  In this case, the request should be sent from a
 UDP port number other than 520.  If a request comes from port 520,
 silent processes do not respond.  If the request comes from any other
 port, processes must respond even if they are silent.
 The request is processed entry by entry.  If there are no entries, no
 response is given.  There is one special case.  If there is exactly
 one entry in the request, with an address family identifier of 0
 (meaning unspecified), and a metric of infinity (i.e., 16 for current
 implementations), this is a request to send the entire routing table.
 In that case, a call is made to the output process to send the
 routing table to the requesting port.
 Except for this special case, processing is quite simple.  Go down
 the list of entries in the request one by one.  For each entry, look
 up the destination in the host's routing database.  If there is a
 route, put that route's metric in the metric field in the datagram.
 If there isn't a route to the specified destination, put infinity
 (i.e., 16) in the metric field in the datagram.  Once all the entries
 have been filled in, set the command to response and send the
 datagram back to the port from which it came.
 Note that there is a difference in handling depending upon whether
 the request is for a specified set of destinations, or for a complete
 routing table.  If the request is for a complete host table, normal
 output processing is done.  This includes split horizon (see section
 2.2.1) and subnet hiding (section 3.2), so that certain entries from
 the routing table will not be shown.  If the request is for specific
 entries, they are looked up in the host table and the information is
 returned.  No split horizon processing is done, and subnets are
 returned if requested.  We anticipate that these requests are likely
 to be used for different purposes.  When a host first comes up, it
 broadcasts requests on every connected network asking for a complete
 routing table.  In general, we assume that complete routing tables
 are likely to be used to update another host's routing table.  For
 this reason, split horizon and all other filtering must be used.
 Requests for specific networks are made only by diagnostic software,

Hedrick [Page 25] RFC 1058 Routing Information Protocol June 1988

 and are not used for routing.  In this case, the requester would want
 to know the exact contents of the routing database, and would not
 want any information hidden.

3.4.2. Response

 Responses can be received for several different reasons:
    response to a specific query
    regular updates
    triggered updates triggered by a metric change
 Processing is the same no matter how responses were generated.
 Because processing of a response may update the host's routing table,
 the response must be checked carefully for validity.  The response
 must be ignored if it is not from port 520.  The IP source address
 should be checked to see whether the datagram is from a valid
 neighbor.  The source of the datagram must be on a directly-connected
 network.  It is also worth checking to see whether the response is
 from one of the host's own addresses.  Interfaces on broadcast
 networks may receive copies of their own broadcasts immediately.  If
 a host processes its own output as new input, confusion is likely,
 and such datagrams must be ignored (except as discussed in the next
 paragraph).
 Before actually processing a response, it may be useful to use its
 presence as input to a process for keeping track of interface status.
 As mentioned above, we time out a route when we haven't heard from
 its gateway for a certain amount of time.  This works fine for routes
 that come from another gateway.  It is also desirable to know when
 one of our own directly-connected networks has failed.  This document
 does not specify any particular method for doing this, as such
 methods depend upon the characteristics of the network and the
 hardware interface to it.  However, such methods often involve
 listening for datagrams arriving on the interface.  Arriving
 datagrams can be used as an indication that the interface is working.
 However, some caution must be used, as it is possible for interfaces
 to fail in such a way that input datagrams are received, but output
 datagrams are never sent successfully.
 Now that the datagram as a whole has been validated, process the
 entries in it one by one.  Again, start by doing validation.  If the
 metric is greater than infinity, ignore the entry.  (This should be
 impossible, if the other host is working correctly.  Incorrect
 metrics and other format errors should probably cause alerts or be
 logged.)  Then look at the destination address.  Check the address
 family identifier.  If it is not a value which is expected (e.g., 2

Hedrick [Page 26] RFC 1058 Routing Information Protocol June 1988

 for Internet addresses), ignore the entry.  Now check the address
 itself for various kinds of inappropriate addresses.  Ignore the
 entry if the address is class D or E, if it is on net 0 (except for
 0.0.0.0, if we accept default routes) or if it is on net 127 (the
 loopback network).  Also, test for a broadcast address, i.e.,
 anything whose host part is all ones on a network that supports
 broadcast, and ignore any such entry.  If the implementor has chosen
 not to support host routes (see section 3.2), check to see whether
 the host portion of the address is non-zero; if so, ignore the entry.
 Recall that the address field contains a number of unused octets.  If
 the version number of the datagram is 1, they must also be checked.
 If any of them is nonzero, the entry is to be ignored.  (Many of
 these cases indicate that the host from which the message came is not
 working correctly.  Thus some form of error logging or alert should
 be triggered.)
 Update the metric by adding the cost of the network on which the
 message arrived.  If the result is greater than 16, use 16.  That is,
    metric = MIN (metric + cost, 16)
 Now look up the address to see whether this is already a route for
 it.  In general, if not, we want to add one.  However, there are
 various exceptions.  If the metric is infinite, don't add an entry.
 (We would update an existing one, but we don't add new entries with
 infinite metric.)  We want to avoid adding routes to hosts if the
 host is part of a net or subnet for which we have at least as good a
 route.  If neither of these exceptions applies, add a new entry to
 the routing database.  This includes the following actions:
  1. Set the destination and metric to those from the datagram.
  1. Set the gateway to be the host from which the datagram

came.

  1. Initialize the timeout for the route. If the garbage-

collection timer is running for this route, stop it. (See

      section 3.3 for a discussion of the timers.)
  1. Set the route change flag, and signal the output process to

trigger an update (see 3.5).

 If there is an existing route, first compare gateways.  If this
 datagram is from the same gateway as the existing route, reinitialize
 the timeout.  Next compare metrics.  If the datagram is from the same
 gateway as the existing route and the new metric is different than
 the old one, or if the new metric is lower than the old one, do the

Hedrick [Page 27] RFC 1058 Routing Information Protocol June 1988

 following actions:
  1. adopt the route from the datagram. That is, put the new

metric in, and set the gateway to be the host from which

      the datagram came.
  1. Initialize the timeout for the route.
  1. Set the route change flag, and signal the output process to

trigger an update (see 3.5).

  1. If the new metric is 16 (infinity), the deletion process is

started.

 If the new metric is 16 (infinity), this starts the process for
 deleting the route.  The route is no longer used for routing packets,
 and the deletion timer is started (see section 3.3).  Note that a
 deletion is started only when the metric is first set to 16.  If the
 metric was already 16, then a new deletion is not started.  (Starting
 a deletion sets a timer.  The concern is that we do not want to reset
 the timer every 30 seconds, as new messages arrive with an infinite
 metric.)
 If the new metric is the same as the old one, it is simplest to do
 nothing further (beyond reinitializing the timeout, as specified
 above).  However, the 4BSD routed uses an additional heuristic here.
 Normally, it is senseless to change to a route with the same metric
 as the existing route but a different gateway.  If the existing route
 is showing signs of timing out, though, it may be better to switch to
 an equally-good alternative route immediately, rather than waiting
 for the timeout to happen.  (See section 3.3 for a discussion of
 timeouts.)  Therefore, if the new metric is the same as the old one,
 routed looks at the timeout for the existing route.  If it is at
 least halfway to the expiration point, routed switches to the new
 route.  That is, the gateway is changed to the source of the current
 message.  This heuristic is optional.
 Any entry that fails these tests is ignored, as it is no better than
 the current route.

3.5. Output Processing

 This section describes the processing used to create response
 messages that contain all or part of the routing table.  This
 processing may be triggered in any of the following ways:
  1. by input processing when a request is seen. In this case,

the resulting message is sent to only one destination.

Hedrick [Page 28] RFC 1058 Routing Information Protocol June 1988

  1. by the regular routing update. Every 30 seconds, a

response containing the whole routing table is sent to

      every neighboring gateway.  (See section 3.3.)
  1. by triggered updates. Whenever the metric for a route is

changed, an update is triggered. (The update may be

      delayed; see below.)
 Before describing the way a message is generated for each directly-
 connected network, we will comment on how the destinations are chosen
 for the latter two cases.  Normally, when a response is to be sent to
 all destinations (that is, either the regular update or a triggered
 update is being prepared), a response is sent to the host at the
 opposite end of each connected point-to-point link, and a response is
 broadcast on all connected networks that support broadcasting.  Thus,
 one response is prepared for each directly-connected network and sent
 to the corresponding (destination or broadcast) address.  In most
 cases, this reaches all neighboring gateways.  However, there are
 some cases where this may not be good enough.  This may involve a
 network that does not support broadcast (e.g., the ARPANET), or a
 situation involving dumb gateways.  In such cases, it may be
 necessary to specify an actual list of neighboring hosts and
 gateways, and send a datagram to each one explicitly.  It is left to
 the implementor to determine whether such a mechanism is needed, and
 to define how the list is specified.
 Triggered updates require special handling for two reasons.  First,
 experience shows that triggered updates can cause excessive loads on
 networks with limited capacity or with many gateways on them.  Thus
 the protocol requires that implementors include provisions to limit
 the frequency of triggered updates.  After a triggered update is
 sent, a timer should be set for a random time between 1 and 5
 seconds.  If other changes that would trigger updates occur before
 the timer expires, a single update is triggered when the timer
 expires, and the timer is then set to another random value between 1
 and 5 seconds.  Triggered updates may be suppressed if a regular
 update is due by the time the triggered update would be sent.
 Second, triggered updates do not need to include the entire routing
 table.  In principle, only those routes that have changed need to be
 included.  Thus messages generated as part of a triggered update must
 include at least those routes that have their route change flag set.
 They may include additional routes, or all routes, at the discretion
 of the implementor; however, when full routing updates require
 multiple packets, sending all routes is strongly discouraged.  When a
 triggered update is processed, messages should be generated for every
 directly-connected network.  Split horizon processing is done when
 generating triggered updates as well as normal updates (see below).

Hedrick [Page 29] RFC 1058 Routing Information Protocol June 1988

 If, after split horizon processing, a changed route will appear
 identical on a network as it did previously, the route need not be
 sent; if, as a result, no routes need be sent, the update may be
 omitted on that network.  (If a route had only a metric change, or
 uses a new gateway that is on the same network as the old gateway,
 the route will be sent to the network of the old gateway with a
 metric of infinity both before and after the change.)  Once all of
 the triggered updates have been generated, the route change flags
 should be cleared.
 If input processing is allowed while output is being generated,
 appropriate interlocking must be done.  The route change flags should
 not be changed as a result of processing input while a triggered
 update message is being generated.
 The only difference between a triggered update and other update
 messages is the possible omission of routes that have not changed.
 The rest of the mechanisms about to be described must all apply to
 triggered updates.
 Here is how a response datagram is generated for a particular
 directly-connected network:
 The IP source address must be the sending host's address on that
 network.  This is important because the source address is put into
 routing tables in other hosts.  If an incorrect source address is
 used, other hosts may be unable to route datagrams.  Sometimes
 gateways are set up with multiple IP addresses on a single physical
 interface.  Normally, this means that several logical IP networks are
 being carried over one physical medium.  In such cases, a separate
 update message must be sent for each address, with that address as
 the IP source address.
 Set the version number to the current version of RIP.  (The version
 described in this document is 1.)  Set the command to response.  Set
 the bytes labeled "must be zero" to zero.  Now start filling in
 entries.
 To fill in the entries, go down all the routes in the internal
 routing table.  Recall that the maximum datagram size is 512 bytes.
 When there is no more space in the datagram, send the current message
 and start a new one.  If a triggered update is being generated, only
 entries whose route change flags are set need be included.
 See the description in Section 3.2 for a discussion of problems
 raised by subnet and host routes.  Routes to subnets will be
 meaningless outside the network, and must be omitted if the
 destination is not on the same subnetted network; they should be

Hedrick [Page 30] RFC 1058 Routing Information Protocol June 1988

 replaced with a single route to the network of which the subnets are
 a part.  Similarly, routes to hosts must be eliminated if they are
 subsumed by a network route, as described in the discussion in
 Section 3.2.
 If the route passes these tests, then the destination and metric are
 put into the entry in the output datagram.  Routes must be included
 in the datagram even if their metrics are infinite.  If the gateway
 for the route is on the network for which the datagram is being
 prepared, the metric in the entry is set to 16, or the entire entry
 is omitted.  Omitting the entry is simple split horizon.  Including
 an entry with metric 16 is split horizon with poisoned reverse.  See
 Section 2.2 for a more complete discussion of these alternatives.

3.6. Compatibility

 The protocol described in this document is intended to interoperate
 with routed and other existing implementations of RIP.  However, a
 different viewpoint is adopted about when to increment the metric
 than was used in most previous implementations.  Using the previous
 perspective, the internal routing table has a metric of 0 for all
 directly-connected networks.  The cost (which is always 1) is added
 to the metric when the route is sent in an update message.  By
 contrast, in this document directly-connected networks appear in the
 internal routing table with metrics equal to their costs; the metrics
 are not necessarily 1.  In this document, the cost is added to the
 metrics when routes are received in update messages.  Metrics from
 the routing table are sent in update messages without change (unless
 modified by split horizon).
 These two viewpoints result in identical update messages being sent.
 Metrics in the routing table differ by a constant one in the two
 descriptions.  Thus, there is no difference in effect.  The change
 was made because the new description makes it easier to handle
 situations where different metrics are used on directly-attached
 networks.
 Implementations that only support network costs of one need not
 change to match the new style of presentation.  However, they must
 follow the description given in this document in all other ways.

4. Control functions

 This section describes administrative controls.  These are not part
 of the protocol per se.  However, experience with existing networks
 suggests that they are important.  Because they are not a necessary
 part of the protocol, they are considered optional.  However, we
 strongly recommend that at least some of them be included in every

Hedrick [Page 31] RFC 1058 Routing Information Protocol June 1988

 implementation.
 These controls are intended primarily to allow RIP to be connected to
 networks whose routing may be unstable or subject to errors.  Here
 are some examples:
 It is sometimes desirable to limit the hosts and gateways from which
 information will be accepted.  On occasion, hosts have been
 misconfigured in such a way that they begin sending inappropriate
 information.
 A number of sites limit the set of networks that they allow in update
 messages.  Organization A may have a connection to organization B
 that they use for direct communication.  For security or performance
 reasons A may not be willing to give other organizations access to
 that connection.  In such cases, A should not include B's networks in
 updates that A sends to third parties.
 Here are some typical controls.  Note, however, that the RIP protocol
 does not require these or any other controls.
  1. a neighbor list - the network administrator should be able

to define a list of neighbors for each host. A host would

      accept response messages only from hosts on its list of
      neighbors.
  1. allowing or disallowing specific destinations - the network

administrator should be able to specify a list of

      destination addresses to allow or disallow.  The list would
      be associated with a particular interface in the incoming
      or outgoing direction.  Only allowed networks would be
      mentioned in response messages going out or processed in
      response messages coming in.  If a list of allowed
      addresses is specified, all other addresses are disallowed.
      If a list of disallowed addresses is specified, all other
      addresses are allowed.

REFERENCES and BIBLIOGRAPHY

 [1] Bellman, R. E., "Dynamic Programming", Princeton University
     Press, Princeton, N.J., 1957.
 [2] Bertsekas, D. P., and Gallaher, R. G., "Data Networks",
     Prentice-Hall, Englewood Cliffs, N.J., 1987.
 [3] Braden, R., and Postel, J., "Requirements for Internet Gateways",
     USC/Information Sciences Institute, RFC-1009, June 1987.

Hedrick [Page 32] RFC 1058 Routing Information Protocol June 1988

 [4] Boggs, D. R., Shoch, J. F., Taft, E. A., and Metcalfe, R. M.,
     "Pup: An Internetwork Architecture", IEEE Transactions on
     Communications, April 1980.
 [5] Clark, D. D., "Fault Isolation and Recovery," MIT-LCS, RFC-816,
     July 1982.
 [6] Ford, L. R. Jr., and Fulkerson, D. R., "Flows in Networks",
     Princeton University Press, Princeton, N.J., 1962.
 [7] Xerox Corp., "Internet Transport Protocols", Xerox System
     Integration Standard XSIS 028112, December 1981.

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