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Network Working Group                                          G. MalkinRequest for Comments: 2453                                  Bay NetworksObsoletes:1723,1388                                      November 1998STD: 56Category: Standards TrackRIP Version 2Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.Abstract   This document specifies an extension of the Routing Information   Protocol (RIP), as defined in [1], to expand the amount of useful   information carried in RIP messages and to add a measure of security.   A companion document will define the SNMP MIB objects for RIP-2 [2].   An additional document will define cryptographic security   improvements for RIP-2 [3].Acknowledgements   I would like to thank the IETF RIP Working Group for their help in   improving the RIP-2 protocol. Much of the text for the background   discussions about distance vector protocols and some of the   descriptions of the operation of RIP were taken from "Routing   Information Protocol" by C. Hedrick [1]. Some of the final editing on   the document was done by Scott Bradner.Malkin                      Standards Track                     [Page 1]

RFC 2453                     RIP Version 2                 November 1998Table of Contents1.  Justification  . . . . . . . . . . . . . . . . . . . . . . . .32.  Current RIP  . . . . . . . . . . . . . . . . . . . . . . . . .33.  Basic Protocol . . . . . . . . . . . . . . . . . . . . . . . .33.1   Introduction   . . . . . . . . . . . . . . . . . . . . . . .33.2   Limitations of the Protocol  . . . . . . . . . . . . . . . .53.3.  Organization of this document  . . . . . . . . . . . . . . .63.4   Distance Vector Algorithms . . . . . . . . . . . . . . . . .63.4.1    Dealing with changes in topology  . . . . . . . . . . . .123.4.2    Preventing instability  . . . . . . . . . . . . . . . . .133.4.3    Split horizon . . . . . . . . . . . . . . . . . . . . . .153.4.4    Triggered updates . . . . . . . . . . . . . . . . . . . .173.5   Protocol Specification   . . . . . . . . . . . . . . . . . .183.6   Message Format . . . . . . . . . . . . . . . . . . . . . . .203.7   Addressing Considerations  . . . . . . . . . . . . . . . . .223.8   Timers . . . . . . . . . . . . . . . . . . . . . . . . . . .243.9   Input Processing . . . . . . . . . . . . . . . . . . . . . .253.9.1    Request Messages  . . . . . . . . . . . . . . . . . . . .253.9.2    Response Messages . . . . . . . . . . . . . . . . . . . .263.10  Output Processing  . . . . . . . . . . . . . . . . . . . . .283.10.1   Triggered Updates . . . . . . . . . . . . . . . . . . . .293.10.2   Generating Response Messages. . . . . . . . . . . . . . .304.  Protocol Extensions  . . . . . . . . . . . . . . . . . . . . .314.1   Authentication . . . . . . . . . . . . . . . . . . . . . . .314.2   Route Tag  . . . . . . . . . . . . . . . . . . . . . . . . .324.3   Subnet Mask  . . . . . . . . . . . . . . . . . . . . . . . .324.4   Next Hop . . . . . . . . . . . . . . . . . . . . . . . . . .334.5   Multicasting . . . . . . . . . . . . . . . . . . . . . . . .334.6   Queries  . . . . . . . . . . . . . . . . . . . . . . . . . .335.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . . .345.1   Compatibility Switch . . . . . . . . . . . . . . . . . . . .345.2   Authentication . . . . . . . . . . . . . . . . . . . . . . .345.3   Larger Infinity  . . . . . . . . . . . . . . . . . . . . . .355.4   Addressless Links  . . . . . . . . . . . . . . . . . . . . .356.  Interaction between version 1 and version 2  . . . . . . . . .357.  Security Considerations  . . . . . . . . . . . . . . . . . . .36   Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . .37   References . . . . . . . . . . . . . . . . . . . . . . . . . . . .37   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .38   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . .39Malkin                      Standards Track                     [Page 2]

RFC 2453                     RIP Version 2                 November 19981.  Justification   With the advent of OSPF and IS-IS, there are those who believe that   RIP is obsolete.  While it is true that the newer IGP routing   protocols are far superior to RIP, RIP does have some advantages.   Primarily, in a small network, RIP has very little overhead in terms   of bandwidth used and configuration and management time.  RIP is also   very easy to implement, especially in relation to the newer IGPs.   Additionally, there are many, many more RIP implementations in the   field than OSPF and IS-IS combined.  It is likely to remain that way   for some years yet.   Given that RIP will be useful in many environments for some period of   time, it is reasonable to increase RIP's usefulness.  This is   especially true since the gain is far greater than the expense of the   change.2. Current RIP   The current RIP-1 message contains the minimal amount of information   necessary for routers to route messages through a network.  It also   contains a large amount of unused space, owing to its origins.   The current RIP-1 protocol does not consider autonomous systems and   IGP/EGP interactions, subnetting [11], and authentication since   implementations of these postdate RIP-1.  The lack of subnet masks is   a particularly serious problem for routers since they need a subnet   mask to know how to determine a route.  If a RIP-1 route is a network   route (all non-network bits 0), the subnet mask equals the network   mask.  However, if some of the non-network bits are set, the router   cannot determine the subnet mask.  Worse still, the router cannot   determine if the RIP-1 route is a subnet route or a host route.   Currently, some routers simply choose the subnet mask of the   interface over which the route was learned and determine the route   type from that.3.  Basic Protocol3.1 Introduction   RIP is a routing protocol 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.Malkin                      Standards Track                     [Page 3]

RFC 2453                     RIP Version 2                 November 1998   In an international network, such as the Internet, it is very   unlikely that a single routing protocol will used for the entire   network.  Rather, the network will be organized as a collection of   Autonomous Systems (AS), each of which will, in general, be   administered by a single entity.  Each AS will have its own routing   technology, which may differ among AS's.  The routing protocol used   within an AS is referred to as an Interior Gateway Protocol (IGP).  A   separate protocol, called an Exterior Gateway Protocol (EGP), is used   to transfer routing information among the AS's.  RIP was designed to   work as an IGP in moderate-size AS's.  It is not intended for use in   more complex environments.  For information on the context into which   RIP-1 is expected to fit, see Braden and Postel [6].   RIP uses one of a class of routing algorithms known as Distance   Vector algorithms.  The earliest description of this class of   algorithms known to the author is in Ford and Fulkerson [8].  Because   of this, they are sometimes known as Ford-Fulkerson algorithms.  The   term Bellman-Ford is also used, and derives from the fact that the   formulation is based on Bellman's equation [4].  The presentation in   this document is closely based on [5].  This document contains a   protocol specification.  For an introduction to the mathematics of   routing algorithms, see [1].  The basic algorithms used by 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 [7] 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 [9].  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 IPv4 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 special purpose   gateways known as routers.  The networks may be either point-to-point   links or more complex networks such as Ethernet or token ring.  Hosts   and routers are presented with IP datagrams addressed to some host.   Routing is the method by which the host or router 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 router.  However, the   interesting case is when the destination is not directly reachable.Malkin                      Standards Track                     [Page 4]

RFC 2453                     RIP Version 2                 November 1998   In this case, the host or router attempts to send the datagram to a   router that is nearer the destination.  The goal of a routing   protocol is very simple: It is to supply the information that is   needed to do routing.3.2 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 networks   of moderate size.  In addition, the following specific limitations   are be mentioned:   - The protocol is limited to networks whose longest path (the     network's diameter) is 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.   - 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.   - 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.Malkin                      Standards Track                     [Page 5]

RFC 2453                     RIP Version 2                 November 19983.3. Organization of this document   The main body of this document is organized into two parts, which   occupy the next two sections:        A conceptual development and justification of distance vector        algorithms in general.        The actual protocol description.   Each of these two sections can largely stand on its own.Section 3.4   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 3.5   is the actual protocol description.  Except where specific references   are made tosection 3.4, it should be possible to implement RIP   entirely from the specifications given insection 3.5.3.4 Distance Vector Algorithms   Routing is the task of finding a path from a sender to a desired   destination.  In the IP "Internet model" this reduces primarily to a   matter of finding a series of routers between the source and   destination networks.  As long as a message or datagram remains on a   single network or subnet, any forwarding problems are the   responsibility of technology that is specific to the network.  For   example, 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 network to a destination on a different one.  In that case, the   message must pass through one or more routers connecting the   networks.  If the networks are not adjacent, the message may pass   through several intervening networks, and the routers connecting   them.  Once the message gets to a router 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 forwarding decision is   necessary (as with a point to point line), or that forwarding 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.  We are using   the term "network" here to refer to subnets in cases where subnetMalkin                      Standards Track                     [Page 6]

RFC 2453                     RIP Version 2                 November 1998   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 routers 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   entity (router 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   router 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.  Seesection 3.2.)  In fact, the   mathematical developments are most conveniently thought of in terms   of routes from one host or router 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:Malkin                      Standards Track                     [Page 7]

RFC 2453                     RIP Version 2                 November 1998   - address: in IP implementations of these algorithms, this will be     the IP address of the host or network.   - router: the first router along the route to the destination.   - interface: the physical network which must be used to reach the     first router.   - metric: a number, indicating the distance to the destination.   - 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 routers.   The most important information exchanged by the hosts and routers is   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 in each router listing 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 routers 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 router 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 hopsMalkin                      Standards Track                     [Page 8]

RFC 2453                     RIP Version 2                 November 1998   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)   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 finiteMalkin                      Standards Track                     [Page 9]

RFC 2453                     RIP Version 2                 November 1998   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   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 router 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 routers don't need to send any update messages.  Clearly   hosts that don't function as routers (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-   routers need not participate in the routing protocol at all.   Let us summarize what a host or router 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 identityMalkin                      Standards Track                    [Page 10]

RFC 2453                     RIP Version 2                 November 1998   of the neighboring router 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 router 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 router on the path to the destination.  (If there are   several equally good paths, it is the first router on one of them.)   This combination of destination, metric, and router is typically   referred to as a route to the destination with that metric, using   that router.   4.ne 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 router 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 routers in the   system.  Hosts that are not routers may participate as well.   - 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 router 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.   - 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.   - 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 resultingMalkin                      Standards Track                    [Page 11]

RFC 2453                     RIP Version 2                 November 1998     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 router G'.  If G' is     the router from which the existing route came, i.e., G' = G, then     use the new metric even if it is larger than the old one.3.4.1 Dealing with changes in topology   The discussion above assumes that the topology of the network is   fixed.  In practice, routers 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 router   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 router notifying its neighbors if its   metrics change.  If the router 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 router that   participates in routing sends an update message to all its neighbors   once every 30 seconds.  Suppose the current route for network N uses   router G.  If we don't hear from G for 180 seconds, we can assume   that either the router 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", sinceMalkin                      Standards Track                    [Page 12]

RFC 2453                     RIP Version 2                 November 1998   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.3.4.2 Preventing instability   The algorithm as presented up to this point will always allow a host   or router 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   routers time out and set the metric for that network to 16.  For   purposes of analysis, we can assume that all the neighboring routers   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 routers in the   system will converge to new routes that go through one of those   routers.  It is easy to see that once convergence has happened, all   the routers will have metrics of at least 16 for the vanished   network.  Routers one hop away from the original neighbors would end   up with metrics of at least 17; routers 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   routers.   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 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.  In the following example the letters correspond to   routers, and the lines to networks.Malkin                      Standards Track                    [Page 13]

RFC 2453                     RIP Version 2                 November 1998     A-----B      \   / \       \ /  |        C  /    all networks have cost 1, except        | /     for the direct link from C to D, which        |/      has cost 10        D        |<=== target network   Each router will have a table showing a route to each network.   However, for purposes of this illustration, we show only the routes   from each router 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 routers send updates at the same time.   The chart shows the metric for the target network, as it appears in   the routing table at each router.       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 becomesMalkin                      Standards Track                    [Page 14]

RFC 2453                     RIP Version 2                 November 1998   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   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".3.4.3 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 router 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   router.  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 routers on that   network.  If A has a route through C, it should indicate that D is   unreachable when talking to any other router on that network.  The   other routers 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 router on that network needs to know that A can reach D.   This is fortunate, because it means that the same update message thatMalkin                      Standards Track                    [Page 15]

RFC 2453                     RIP Version 2                 November 1998   is used for C can be used for all other routers 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 routers 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   size of the routing messages.  Consider the case of a campus backbone   connecting a number of different buildings.  In each building, there   is a router connecting the backbone to a local network.  Consider   what routing updates those routers should broadcast on the backbone   network.  All that the rest of the network really needs to know about   each router is what local networks it is connected to.  Using simple   split horizon, only those routes would appear in update messages sent   by the router to the backbone network.  If split horizon with   poisoned reverse is used, the router 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 routers 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   router 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.   The router requirements RFC [11] specifies that all implementation of   RIP must use split horizon and should also use split horizon with   poisoned reverse, although there may be a knob to disable poisoned   reverse.Malkin                      Standards Track                    [Page 16]

RFC 2453                     RIP Version 2                 November 19983.4.4  Triggered updates   Split horizon with poisoned reverse will prevent any routing loops   that involve only two routers.  However, it is still possible to end   up with patterns in which three routers 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 router 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 router's route to destination N   goes through router G.  If an update arrives from G itself, the   receiving router 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 router will send triggered   updates to all the hosts and routers 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 routers and   hosts are involved in the cascade.  Suppose a router 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   routers 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   router 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.   Routers that haven't received the triggered update yet will still be   sending out information based on the route that no longer exists.  ItMalkin                      Standards Track                    [Page 17]

RFC 2453                     RIP Version 2                 November 1998   is possible that after the triggered update has gone through a   router, it might receive a normal update from one of these routers   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.   The router requirements RFC [11] specifies that all implementation of   RIP must implement triggered update for deleted routes and may   implement triggered updates for new routes or change of routes.  RIP   implementations must also limit the rate which of triggered updates   may be trandmitted. (seesection 3.10.1)3.5 Protocol Specification   RIP is intended to allow routers to exchange information for   computing routes through an IPv4-based network.  Any router that uses   RIP is assumed to have interfaces to one or more networks, otherwise   it isn't really a router.  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 of which   is its metric.  The RIP metric of a network is an integer between 1   and 15, inclusive.  It is set in some manner not specified in this   protocol; however, given the maximum path limit of 15, a value of 1   is usually used.  Implementations should allow the system   administrator to set the metric of each network.  In addition to the   metric, each network will have an IPv4 destination address and subnet   mask associated with it.  These are to be set by the system   administrator in a manner not specified in this protocol.   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   IPv4 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 insection 3.7 assume that there is a   single subnet mask applying to each IPv4 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 desirableMalkin                      Standards Track                    [Page 18]

RFC 2453                     RIP Version 2                 November 1998   for a system to know the subnets masks of distant networks. Network-   wide distribution of routing information which contains different   subnet masks is permitted if all routers in the network are running   the extensions presented in this document. However, if all routers in   the network are not running these extensions distribution of routing   information containing different subnet masks must be limited to   avoid interoperability problems. See sections3.7 and4.3 for the   rules governing subnet distribution.   Each router that implements RIP is assumed to have a routing table.   This table has one entry for every destination that is reachable   throughout the system operating RIP.  Each entry contains at least   the following information:   - The IPv4 address of the destination.   - A metric, which represents the total cost of getting a datagram     from the router to that destination.  This metric is the sum of the     costs associated with the networks that would be traversed to get     to the destination.   - The IPv4 address of the next router along the path to the     destination (i.e., the next hop).  If the destination is on one of     the directly-connected networks, this item is not needed.   - A flag to indicate that information about the route has changed     recently.  This will be referred to as the "route change flag."   - Various timers associated with the route.  Seesection 3.6 for more     details on timers.   The entries for the directly-connected networks are set up by the   router using information gathered by means not specified in this   protocol.  The metric for a directly-connected network is set to the   cost of that network.  As mentioned, 1 is the usual 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 (e.g., to indicate of differences in bandwidth   or reliability).   To support the extensions detailed in this document, each entry must   additionally contain a subnet mask. The subnet mask allows the router   (along with the IPv4 address of the destination) to identify the   different subnets within a single network as well as the subnets   masks of distant networks.Malkin                      Standards Track                    [Page 19]

RFC 2453                     RIP Version 2                 November 1998   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.  They are   referred to as "static routes."  Entries for destinations other than   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 router in the AS must participate in the protocol.  In cases   where multiple IGPs are in use, there must be at least one router   which can leak routing information between the protocols.3.6 Message Format   RIP is a UDP-based protocol.  Each router that uses RIP has a routing   process that sends and receives datagrams on UDP port number 520, the   RIP-1/RIP-2 port.  All communications intended for another routers's   RIP process are sent to the RIP port.  All routing update messages   are sent from the RIP port.  Unsolicited routing update messages have   both the source and destination port equal to the RIP port.  Update   messages sent in response to a request are sent to the port from   which the request came.  Specific queries may be sent from ports   other than the RIP port, but they must be directed to the RIP port on   the target machine.   The RIP packet format is:       0                   1                   2                   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)        |      +---------------+---------------+-------------------------------+      |                                                               |      ~                         RIP Entry (20)                        ~      |                                                               |      +---------------+---------------+---------------+---------------+Malkin                      Standards Track                    [Page 20]

RFC 2453                     RIP Version 2                 November 1998   There may be between 1 and 25 (inclusive) RIP entries.  A RIP-1 entry   has the following format:       0                   1                   2                   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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      | address family identifier (2) |      must be zero (2)         |      +-------------------------------+-------------------------------+      |                        IPv4 address (4)                       |      +---------------------------------------------------------------+      |                        must be zero (4)                       |      +---------------------------------------------------------------+      |                        must be zero (4)                       |      +---------------------------------------------------------------+      |                           metric (4)                          |      +---------------------------------------------------------------+   Field sizes are given in octets.  Unless otherwise specified, fields   contain binary integers, in network byte order, with the most-   significant octet first (big-endian).  Each tick mark represents one   bit.   Every message contains a RIP header which consists of a command and a   version number.  This section of the document describes version 1 of   the protocol;section 4 describes the version 2 extensions.  The   command field is used to specify the purpose of this message.  The   commands implemented in version 1 and 2 are:   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 it may be an unsolicited routing                  update generated by the sender.   For each of these message types, in version 1, the remainder of the   datagram contains a list of Route Entries (RTEs).  Each RTE in this   list contains an Address Family Identifier (AFI), destination IPv4   address, and the cost to reach that destination (metric).   The AFI is the type of address.  For RIP-1, only AF_INET (2) is   generally supported.   The metric field contains a value between 1 and 15 (inclusive) which   specifies the current metric for the destination; or the value 16   (infinity), which indicates that the destination is not reachable.Malkin                      Standards Track                    [Page 21]

RFC 2453                     RIP Version 2                 November 19983.7 Addressing Considerations   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 information for individual   hosts is not needed.  If every node on a given network or subnet is   accessible through the same routers, then there is no reason to   mention individual hosts in the routing tables.  However, networks   that include point-to-point lines sometimes require routers to keep   track of routes to certain nodes.  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 (seesection 3.7.2).   The RIP-1 packet format does not distinguish among various types of   address.  Fields that are labeled "address" can contain any of the   following:   host address subnet number network number zero (default route)   Entities which use RIP-1 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 node 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 node evaluates information that it receives via RIP-1, 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 node   address.  However, if the node does not know the subnet mask,   evaluation of an address may be ambiguous.  If there is a non-zero   node part, there is no clear way to determine whether the address   represents a subnet number or a node address.  As a subnet number   would be useless without the subnet mask, addresses are assumed to   represent nodes in this situation.  In order to avoid this sort of   ambiguity, when using version 1, nodes must not send subnet routes to   nodes 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,Malkin                      Standards Track                    [Page 22]

RFC 2453                     RIP Version 2                 November 1998   routes to a subnet must not be sent outside the network of which the   subnet is a part.  RIP-2 (seesection 4) eliminates the subnet/host   ambiguity by including the subnet mask in the routing entry.   This "subnet filtering" is carried out by the routers at the "border"   of the subnetted network.  These are routers which connect that   network with some other network.  Within the subnetted network, each   subnet is treated as an individual network.  Routing entries for each   subnet are circulated by RIP.  However, border routers send only a   single entry for the network as a whole to nodes in other networks.   This means that a border router 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 router is   attached.   Similarly, border routers must not mention host routes for nodes   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.   The router requirements RFC [11] specifies that all implementation of   RIP should support host routes but if they do not then they must   ignore any received host routes.   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 routers in the system are prepared to handle traffic to the   networks that are not listed explicitly.  These routers 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 routers 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   routers should create entries for 0.0.0.0; however, other mechanisms   are possible.  For example, an implementor might decide that any   router which speaks BGP should be declared to be a default router.   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 router, 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.  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 defaultMalkin                      Standards Track                    [Page 23]

RFC 2453                     RIP Version 2                 November 1998   router.  Thus, routes involving 0.0.0.0 should generally not leave   the boundary of an autonomous system.  The mechanisms for enforcing   this are not specified in this document.3.8 Timers   This section describes all events that are triggered by timers.   Every 30 seconds, the RIP process is awakened to send an unsolicited   Response message containing the complete routing table (seesection3.9 on Split Horizon) to every neighboring router.  When there are   many routers 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.  Therefore,   implementations are required to take one of two precautions:   - 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.   - The 30-second timer is offset by a small random time (+/- 0 to 5     seconds) each time it is set.  (Implementors may wish to consider     even larger variation in the light of recent research results [10])   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 routing 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 routing table.   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 described below   begins for that route.   Deletions can occur for one of two reasons: the timeout expires, or   the metric is set to 16 because of an update received from the   current router (seesection 3.7.2 for a discussion of processing   updates from other routers).  In either case, the following events   happen:Malkin                      Standards Track                    [Page 24]

RFC 2453                     RIP Version 2                 November 1998   - The garbage-collection timer is set for 120 seconds.   - The metric for the route is set to 16 (infinity).  This causes the     route to be removed from service.   - The route change flag is set to indicate that this entry has been     changed.   - 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 router.  When the garbage-collection timer   expires, the route is deleted from the routing table.   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.   Triggered updates also use a small timer; however, this is best   described insection 3.9.1.3.9 Input Processing   This section will describe the handling of datagrams received on the   RIP port.  Processing will depend upon the value in the command   field.   See sections4.6 and5.1 for details on handling version numbers.3.9.1 Request Messages   A Request is used to ask for a response containing all or part of a   router's routing table.  Normally, Requests are sent as broadcasts   (multicasts for RIP-2), from the RIP port, by routers which have just   come up and are seeking to fill in their routing tables as quickly as   possible.  However, there may be situations (e.g., router monitoring)   where the routing table of only a single router is needed.  In this   case, the Request should be sent directly to that router from a UDP   port other than the RIP port.  If such a Request is received, the   router responds directly to the requestor's address and port.   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, and it has an address family identifier of   zero and a metric of infinity (i.e., 16), then 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 requestingMalkin                      Standards Track                    [Page 25]

RFC 2453                     RIP Version 2                 November 1998   address/port.  Except for this special case, processing is quite   simple.  Examine the list of RTEs in the Request one by one.  For   each entry, look up the destination in the router's routing database   and, if there is a route, put that route's metric in the metric field   of the RTE.  If there is no explicit route to the specified   destination, put infinity in the metric field.  Once all the entries   have been filled in, change the command from Request to Response and   send the datagram back to the requestor.   Note that there is a difference in metric handling for specific and   whole-table requests.  If the request is for a complete routing   table, normal output processing is done, including Split Horizon (seesection 3.9 on Split Horizon).  If the request is for specific   entries, they are looked up in the routing table and the information   is returned as is; no Split Horizon processing is done.  The reason   for this distinction is the expectation that these requests are   likely to be used for different purposes.  When a router first comes   up, it multicasts a Request on every connected network asking for a   complete routing table.  It is assumed that these complete routing   tables are to be used to update the requestor's routing table.  For   this reason, Split Horizon must be done.  It is further assumed that   a Request for specific networks is made only by diagnostic software,   and is not used for routing.  In this case, the requester would want   to know the exact contents of the routing table and would not want   any information hidden or modified.3.9.2 Response Messages   A Response can be received for one of several different reasons:   - response to a specific query   - regular update (unsolicited response)   - triggered update caused by a route change   Processing is the same no matter why the Response was generated.   Because processing of a Response may update the router's routing   table, the Response must be checked carefully for validity.  The   Response must be ignored if it is not from the RIP port.  The   datagram's IPv4 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 router's own addresses.   Interfaces on broadcast networks may receive copies of their own   broadcasts/multicasts immediately.  If a router processes its own   output as new input, confusion is likely so such datagrams must be   ignored.Malkin                      Standards Track                    [Page 26]

RFC 2453                     RIP Version 2                 November 1998   Once the datagram as a whole has been validated, process the RTEs in   the Response one by one.  Again, start by doing validation.   Incorrect metrics and other format errors usually indicate   misbehaving neighbors and should probably be brought to the   administrator's attention.  For example, if the metric is greater   than infinity, ignore the entry but log the event.  The basic   validation tests are:   - is the destination address valid (e.g., unicast; not net 0 or 127)   - is the metric valid (i.e., between 1 and 16, inclusive)   If any check fails, ignore that entry and proceed to the next.   Again, logging the error is probably a good idea.   Once the entry has been validated, update the metric by adding the   cost of the network on which the message arrived.  If the result is   greater than infinity, use infinity.  That is,   metric = MIN (metric + cost, infinity)   Now, check to see whether there is already an explicit route for the   destination address.  If there is no such route, add this route to   the routing table, unless the metric is infinity (there is no point   in adding a route which is unusable).  Adding a route to the routing   table consists of:   - Setting the destination address to the destination address in the     RTE   - Setting the metric to the newly calculated metric (as described     above)   - Set the next hop address to be the address of the router from which     the datagram came   - Initialize the timeout for the route.  If the garbage-collection     timer is running for this route, stop it (seesection 3.6 for a     discussion of the timers)   - Set the route change flag   - Signal the output process to trigger an update (seesection 3.8.1)   If there is an existing route, compare the next hop address to the   address of the router from which the datagram came.  If this datagram   is from the same router as the existing route, reinitialize the   timeout.  Next, compare the metrics.  If the datagram is from the   same router as the existing route, and the new metric is differentMalkin                      Standards Track                    [Page 27]

RFC 2453                     RIP Version 2                 November 1998   than the old one; or, if the new metric is lower than the old one; do   the following actions:   - Adopt the route from the datagram (i.e., put the new metric in and     adjust the next hop address, if necessary).   - Set the route change flag and signal the output process to trigger     an update   - If the new metric is infinity, start the deletion process     (described above); otherwise, re-initialize the timeout   If the new metric is infinity, the deletion process begins for the   route, which is no longer used for routing packets.  Note that the   deletion process is started only when the metric is first set to   infinity.  If the metric was already infinity, then a new deletion   process is not started.   If the new metric is the same as the old one, it is simplest to do   nothing further (beyond re-initializing the timeout, as specified   above); but, there is a heuristic which could be applied.  Normally,   it is senseless to replace a route if the new route has the same   metric as the existing route; this would cause the route to bounce   back and forth, which would generate an intolerable number of   triggered updates.  However, if the existing route is showing signs   of timing out, it may be better to switch to an equally-good   alternative route immediately, rather than waiting for the timeout to   happen.  Therefore, if the new metric is the same as the old one,   examine the timeout for the existing route.  If it is at least   halfway to the expiration point, switch to the new route.  This   heuristic is optional, but highly recommended.   Any entry that fails these tests is ignored, as it is no better than   the current route.3.10 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:   - By input processing, when a Request is received (this Response is     unicast to the requestor; seesection 3.7.1)   - By the regular routing update (broadcast/multicast every 30     seconds) router.   - By triggered updates (broadcast/multicast when a route changes)Malkin                      Standards Track                    [Page 28]

RFC 2453                     RIP Version 2                 November 1998   When a Response is to be sent to all neighbors (i.e., a regular or   triggered update), a Response message is directed to the router at   the far end of each connected point-to-point link, and is broadcast   (multicast for RIP-2) on all connected networks which support   broadcasting.  Thus, one Response is prepared for each directly-   connected network, and sent to the appropriate address (direct or   broadcast/multicast).  In most cases, this reaches all neighboring   routers.  However, there are some cases where this may not be good   enough.  This may involve a network that is not a broadcast network   (e.g., the ARPANET), or a situation involving dumb routers.  In such   cases, it may be necessary to specify an actual list of neighboring   routers 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.3.10.1 Triggered Updates   Triggered updates require special handling for two reasons.  First,   experience shows that triggered updates can cause excessive load on   networks with limited capacity or networks with many routers on them.   Therefore, 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 interval 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.  The timer is then reset to another random value between 1   and 5 seconds.  A triggered update should 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 which have changed need to be   included.  Therefore, 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, at the   discretion of the implementor; however, sending complete routing   updates 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 (seesection 3.9).  If, after Split   Horizon processing for a given network, a changed route will appear   unchanged on that network (e.g., it appears with an infinite metric),   the route need not be sent.  If no routes need be sent on that   network, the update may be omitted.  Once all of the triggered   updates have been generated, the route change flags should be   cleared.Malkin                      Standards Track                    [Page 29]

RFC 2453                     RIP Version 2                 November 1998   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 remaining mechanisms, described in the next section, must be   applied to all updates.3.10.2  Generating Response Messages   This section describes how a Response message is generated for a   particular directly-connected network:   Set the version number to either 1 or 2.  The mechanism for deciding   which version to send is implementation specific; however, if this is   the Response to a Request, the Response version should match the   Request version.  Set the command to Response.  Set the bytes labeled   "must be zero" to zero.  Start filling in RTEs.  Recall that there is   a limit of 25 RTEs to a Response; if there are more, send the current   Response and start a new one.  There is no defined limit to the   number of datagrams which make up a Response.   To fill in the RTEs, examine each route in the routing table.  If a   triggered update is being generated, only entries whose route change   flags are set need be included.  If, after Split Horizon processing,   the route should not be included, skip it.  If the route is to be   included, then the destination address and metric are put into the   RTE.  Routes must be included in the datagram even if their metrics   are infinite.Malkin                      Standards Track                    [Page 30]

RFC 2453                     RIP Version 2                 November 19984. Protocol Extensions   This section does not change the RIP protocol per se.  Rather, it   provides extensions to the message format which allows routers to   share important additional information.   The same header format is used for RIP-1 and RIP-2 messages (seesection 3.4).  The format for the 20-octet route entry (RTE) for   RIP-2 is:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   | Address Family Identifier (2) |        Route Tag (2)          |   +-------------------------------+-------------------------------+   |                         IP Address (4)                        |   +---------------------------------------------------------------+   |                         Subnet Mask (4)                       |   +---------------------------------------------------------------+   |                         Next Hop (4)                          |   +---------------------------------------------------------------+   |                         Metric (4)                            |   +---------------------------------------------------------------+   The Address Family Identifier, IP Address, and Metric all have the   meanings defined insection 3.4.  The Version field will specify   version number 2 for RIP messages which use authentication or carry   information in any of the newly defined fields.4.1 Authentication   Since authentication is a per message function, and since there is   only one 2-octet field available in the message header, and since any   reasonable authentication scheme will require more than two octets,   the authentication scheme for RIP version 2 will use the space of an   entire RIP entry.  If the Address Family Identifier of the first (and   only the first) entry in the message is 0xFFFF, then the remainder of   the entry contains the authentication.  This means that there can be,   at most, 24 RIP entries in the remainder of the message.  If   authentication is not in use, then no entries in the message should   have an Address Family Identifier of 0xFFFF.  A RIP message which   contains an authentication entry would begin with the following   format:Malkin                      Standards Track                    [Page 31]

RFC 2453                     RIP Version 2                 November 1998    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)   |            unused             |   +---------------+---------------+-------------------------------+   |             0xFFFF            |    Authentication Type (2)    |   +-------------------------------+-------------------------------+   ~                       Authentication (16)                     ~   +---------------------------------------------------------------+   Currently, the only Authentication Type is simple password and it is   type 2.  The remaining 16 octets contain the plain text password.  If   the password is under 16 octets, it must be left-justified and padded   to the right with nulls (0x00).4.2 Route Tag   The Route Tag (RT) field is an attribute assigned to a route which   must be preserved and readvertised with a route.  The intended use of   the Route Tag is to provide a method of separating "internal" RIP   routes (routes for networks within the RIP routing domain) from   "external" RIP routes, which may have been imported from an EGP or   another IGP.   Routers supporting protocols other than RIP should be configurable to   allow the Route Tag to be configured for routes imported from   different sources.  For example, routes imported from EGP or BGP   should be able to have their Route Tag either set to an arbitrary   value, or at least to the number of the Autonomous System from which   the routes were learned.   Other uses of the Route Tag are valid, as long as all routers in the   RIP domain use it consistently.  This allows for the possibility of a   BGP-RIP protocol interactions document, which would describe methods   for synchronizing routing in a transit network.4.3 Subnet mask   The Subnet Mask field contains the subnet mask which is applied to   the IP address to yield the non-host portion of the address.  If this   field is zero, then no subnet mask has been included for this entry.   On an interface where a RIP-1 router may hear and operate on the   information in a RIP-2 routing entry the following rules apply:   1) information internal to one network must never be advertised into      another network,Malkin                      Standards Track                    [Page 32]

RFC 2453                     RIP Version 2                 November 1998   2) information about a more specific subnet may not be advertised      where RIP-1 routers would consider it a host route, and   3) supernet routes (routes with a netmask less specific than the      "natural" network mask) must not be advertised where they could be      misinterpreted by RIP-1 routers.4.4 Next Hop   The immediate next hop IP address to which packets to the destination   specified by this route entry should be forwarded.  Specifying a   value of 0.0.0.0 in this field indicates that routing should be via   the originator of the RIP advertisement.  An address specified as a   next hop must, per force, be directly reachable on the logical subnet   over which the advertisement is made.   The purpose of the Next Hop field is to eliminate packets being   routed through extra hops in the system.  It is particularly useful   when RIP is not being run on all of the routers on a network.  A   simple example is given inAppendix A.  Note that Next Hop is an   "advisory" field.  That is, if the provided information is ignored, a   possibly sub-optimal, but absolutely valid, route may be taken.  If   the received Next Hop is not directly reachable, it should be treated   as 0.0.0.0.4.5 Multicasting   In order to reduce unnecessary load on those hosts which are not   listening to RIP-2 messages, an IP multicast address will be used for   periodic broadcasts.  The IP multicast address is 224.0.0.9.  Note   that IGMP is not needed since these are inter-router messages which   are not forwarded.   On NBMA networks, unicast addressing may be used.  However, if a   response addressed to the RIP-2 multicast address is received, it   should be accepted.   In order to maintain backwards compatibility, the use of the   multicast address will be configurable, as described insection 5.1.   If multicasting is used, it should be used on all interfaces which   support it.4.6 Queries   If a RIP-2 router receives a RIP-1 Request, it should respond with a   RIP-1 Response.  If the router is configured to send only RIP-2   messages, it should not respond to a RIP-1 Request.Malkin                      Standards Track                    [Page 33]

RFC 2453                     RIP Version 2                 November 19985. Compatibility   RFC [1] showed considerable forethought in its specification of the   handling of version numbers.  It specifies that RIP messages of   version 0 are to be discarded, that RIP messages of version 1 are to   be discarded if any Must Be Zero (MBZ) field is non-zero, and that   RIP messages of any version greater than 1 should not be discarded   simply because an MBZ field contains a value other than zero.  This   means that the new version of RIP is totally backwards compatible   with existing RIP implementations which adhere to this part of the   specification.5.1 Compatibility Switch   A compatibility switch is necessary for two reasons.  First, there   are implementations of RIP-1 in the field which do not follow RFC [1]   as described above.  Second, the use of multicasting would prevent   RIP-1 systems from receiving RIP-2 updates (which may be a desired   feature in some cases).  This switch should be configurable on a   per-interface basis.   The switch has four settings: RIP-1, in which only RIP-1 messages are   sent; RIP-1 compatibility, in which RIP-2 messages are broadcast;   RIP-2, in which RIP-2 messages are multicast; and "none", which   disables the sending of RIP messages.  It is recommended that the   default setting be either RIP-1 or RIP-2, but not RIP-1   compatibility.  This is because of the potential problems which can   occur on some topologies.  RIP-1 compatibility should only be used   when all of the consequences of its use are well understood by the   network administrator.   For completeness, routers should also implement a receive control   switch which would determine whether to accept, RIP-1 only, RIP-2   only, both, or none.  It should also be configurable on a per-   interface basis.  It is recommended that the default be compatible   with the default chosen for sending updates.5.2 Authentication   The following algorithm should be used to authenticate a RIP message.   If the router is not configured to authenticate RIP-2 messages, then   RIP-1 and unauthenticated RIP-2 messages will be accepted;   authenticated RIP-2 messages shall be discarded.  If the router is   configured to authenticate RIP-2 messages, then RIP-1 messages and   RIP-2 messages which pass authentication testing shall be accepted;   unauthenticated and failed authentication RIP-2 messages shall be   discarded.  For maximum security, RIP-1 messages should be ignoredMalkin                      Standards Track                    [Page 34]

RFC 2453                     RIP Version 2                 November 1998   when authentication is in use (seesection 4.1); otherwise, the   routing information from authenticated messages will be propagated by   RIP-1 routers in an unauthenticated manner.   Since an authentication entry is marked with an Address Family   Identifier of 0xFFFF, a RIP-1 system would ignore this entry since it   would belong to an address family other than IP.  It should be noted,   therefore, that use of authentication will not prevent RIP-1 systems   from seeing RIP-2 messages.  If desired, this may be done using   multicasting, as described in sections4.5 and5.1.5.3 Larger Infinity   While on the subject of compatibility, there is one item which people   have requested: increasing infinity.  The primary reason that this   cannot be done is that it would violate backwards compatibility.  A   larger infinity would obviously confuse older versions of rip.  At   best, they would ignore the route as they would ignore a metric of   16.  There was also a proposal to make the Metric a single octet and   reuse the high three octets, but this would break any implementations   which treat the metric as a 4-octet entity.5.4 Addressless Links   As in RIP-1, addressless links will not be supported by RIP-2.6. Interaction between version 1 and version 2   Because version 1 packets do not contain subnet information, the   semantics employed by routers on networks that contain both version 1   and version 2 networks should be limited to that of version 1.   Otherwise it is possible either to create blackhole routes (i.e.,   routes for networks that do not exist) or to create excessive routing   information in a version 1 environment.   Some implementations attempt to automatically summarize groups of   adjacent routes into single entries, the goal being to reduce the   total number of entries.  This is called auto-summarization.   Specifically, when using both version 1 and version 2 within a   network, a single subnet mask should be used throughout the network.   In addition, auto-summarization mechanisms should be disabled for   such networks, and implementations must provide mechanisms to disable   auto-summarization.Malkin                      Standards Track                    [Page 35]

RFC 2453                     RIP Version 2                 November 19987. Security Considerations   The basic RIP protocol is not a secure protocol.  To bring RIP-2 in   line with more modern routing protocols, an extensible authentication   mechanism has been incorporated into the protocol enhancements.  This   mechanism is described in sections4.1 and5.2.  Security is further   enhanced by the mechanism described in [3].Malkin                      Standards Track                    [Page 36]

RFC 2453                     RIP Version 2                 November 1998Appendix A   This is a simple example of the use of the next hop field in a rip   entry.      -----   -----   -----           -----   -----   -----      |IR1|   |IR2|   |IR3|           |XR1|   |XR2|   |XR3|      --+--   --+--   --+--           --+--   --+--   --+--        |       |       |               |       |       |      --+-------+-------+---------------+-------+-------+--        <-------------RIP-2------------->   Assume that IR1, IR2, and IR3 are all "internal" routers which are   under one administration (e.g. a campus) which has elected to use   RIP-2 as its IGP. XR1, XR2, and XR3, on the other hand, are under   separate administration (e.g. a regional network, of which the campus   is a member) and are using some other routing protocol (e.g. OSPF).   XR1, XR2, and XR3 exchange routing information among themselves such   that they know that the best routes to networks N1 and N2 are via   XR1, to N3, N4, and N5 are via XR2, and to N6 and N7 are via XR3. By   setting the Next Hop field correctly (to XR2 for N3/N4/N5, to XR3 for   N6/N7), only XR1 need exchange RIP-2 routes with IR1/IR2/IR3 for   routing to occur without additional hops through XR1. Without the   Next Hop (for example, if RIP-1 were used) it would be necessary for   XR2 and XR3 to also participate in the RIP-2 protocol to eliminate   extra hops.References   [1] Hedrick, C., "Routing Information Protocol", STD 34,RFC 1058,       Rutgers  University, June 1988.   [2] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension",RFC1389, January 1993.   [3] Baker, F., and R. Atkinson, "RIP-II MD5 Authentication",RFC2082, January 1997.   [4] Bellman, R. E., "Dynamic Programming", Princeton University       Press, Princeton, N.J., 1957.   [5] Bertsekas, D. P., and Gallaher, R. G., "Data Networks",       Prentice-Hall, Englewood Cliffs, N.J., 1987.   [6] Braden, R., and Postel, J., "Requirements for Internet Gateways",       STD 4,RFC 1009, June 1987.Malkin                      Standards Track                    [Page 37]

RFC 2453                     RIP Version 2                 November 1998   [7] Boggs, D. R., Shoch, J. F., Taft, E. A., and Metcalfe, R. M.,       "Pup: An Internetwork Architecture", IEEE Transactions on       Communications, April 1980.   [8] Ford, L. R. Jr., and Fulkerson, D. R., "Flows in Networks",       Princeton University Press, Princeton, N.J., 1962.   [9] Xerox Corp., "Internet Transport Protocols", Xerox System       Integration Standard XSIS 028112, December 1981.   [10] Floyd, S., and V. Jacobson, "The synchronization of Periodic        Routing Messages," ACM Sigcom '93 symposium, September 1993.   [11] Baker, F., "Requirements for IP Version 4 Routers."RFC 1812,        June 1995.Author's Address   Gary Scott Malkin   Bay Networks   8 Federal Street   Billerica, MA 01821   Phone:  (978) 916-4237   EMail:  gmalkin@baynetworks.comMalkin                      Standards Track                    [Page 38]

RFC 2453                     RIP Version 2                 November 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Malkin                      Standards Track                    [Page 39]