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Versions: 00 01 02 03 04 05 rfc2453                                     
Draft-ietf-ripv2-protocol-v2-00.txt                            G. Malkin
Obsoletes RFCs 1723, 1388                                   Bay Networks
                                                          September 1996


                             RIP Version 2
                    Carrying Additional Information


Abstract

   This document specifies an extension of the Routing Information
   Protocol (RIP), as defined in [1,2], 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 [3].
   An additional document will define cryptographic security
   improvements for RIP-2 [10].


Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other
   Internet Draft.

   It is intended that this document will be submitted to the IESG for
   consideration as a standards document.  Distribution of this document
   is unlimited.


Acknowledgements

   I would like to thank the IETF RIP Working Group for their help in
   improving the RIP-2 protocol.





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                             Table of Contents


   1.  Justification  . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Current RIP  . . . . . . . . . . . . . . . . . . . . . . . . .  3

   3.  Basic Protocol . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1   Introduction   . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2   Limitations of the Protocol  . . . . . . . . . . . . . . . .  4
   3.3   Protocol Specification . . . . . . . . . . . . . . . . . . .  5
   3.4   Message Format . . . . . . . . . . . . . . . . . . . . . . .  6
   3.5   Addressing Considerations  . . . . . . . . . . . . . . . . .  8
   3.6   Timers . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.7   Input Processing . . . . . . . . . . . . . . . . . . . . . . 11
   3.8   Output Processing  . . . . . . . . . . . . . . . . . . . . . 15
   3.9   Split Horizon  . . . . . . . . . . . . . . . . . . . . . . . 16

   4.  Protocol Extensions  . . . . . . . . . . . . . . . . . . . . . 17
   4.1   Authentication . . . . . . . . . . . . . . . . . . . . . . . 17
   4.2   Route Tag  . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.3   Subnet Mask  . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.4   Next Hop . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.5   Multicasting . . . . . . . . . . . . . . . . . . . . . . . . 19
   4.6   Queries  . . . . . . . . . . . . . . . . . . . . . . . . . . 20

   5.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.1   Compatibility Switch . . . . . . . . . . . . . . . . . . . . 20
   5.2   Authentication . . . . . . . . . . . . . . . . . . . . . . . 20
   5.3   Larger Infinity  . . . . . . . . . . . . . . . . . . . . . . 21
   5.4   Addressless Links  . . . . . . . . . . . . . . . . . . . . . 21

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21

   Appendicies  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23












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1.  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, 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 Protocol

   Much of the material in this section has been taken from [1].

3.1 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



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   ARPANET.  The particular packet formats and protocol described here
   are based on the program "routed," which is included with the
   Berkeley distribution of Unix.

   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.

   An introduction to the theory and math behind Distance Vector
   protocols is provided in [1].  It has not been incorporated in this
   document for the sake of brevity.

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



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   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 (see section 2.2 in [1]).  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.

3.3 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.




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   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.  See section 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).

   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.

3.4 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



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   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)                        ~
      |                                                               |
      +---------------+---------------+---------------+---------------+

   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:




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   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 (0x0800) 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.

3.5 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 gateways, then there is no reason to
   mention individual hosts in the routing tables.  However, networks
   that include point-to-point lines sometimes require gateways to keep
   track of routes to certain 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 (see section 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)




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   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,
   routes to a subnet must not be sent outside the network of which the
   subnet is a part.  RIP-2 (see section 4) eliminates the subnet/host
   ambiguity by including the subnet mask in the routing entry.

   This 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 gateway 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 special address 0.0.0.0 is used to describe a default route.  A



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   default route is used when it is not convenient to list every
   possible network in the RIP updates, and when one or more closely-
   connected gateways in the system are prepared to handle traffic to
   the networks that are not listed explicitly.  These gateways should
   create RIP entries for the address 0.0.0.0, just as if it were a
   network to which they are connected.  The decision as to how gateways
   create entries for 0.0.0.0 is left to the implementor.  Most
   commonly, the system administrator will be provided with a way to
   specify which gateways should create entries for 0.0.0.0; however,
   other mechanisms are possible.  For example, an implementor might
   decide that any gateway which speaks BGP should be declared to be a
   default gateway.  It may be useful to allow the network administrator
   to choose the metric to be used in these entries.  If there is more
   than one default gateway, this will make it possible to express a
   preference for one over the other.  The entries for 0.0.0.0 are
   handled by RIP in exactly the same manner as if there were an actual
   network with this address.  System administrators should take care to
   make sure that routes to 0.0.0.0 do not propagate further than is
   intended.  Generally, each autonomous system has its own preferred
   default gateway.  Thus, routes involving 0.0.0.0 should generally not
   leave the boundary of an autonomous system.  The mechanisms for
   enforcing this are not specified in this document.

3.6 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 (see section
   3.9 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.

   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



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   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 (see section 3.7.2 for a discussion of processing
   updates from other routers).  In either case, the following events
   happen:

   - 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 in section 3.9.1.

3.7 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.

3.7.1 Request Messages

   A Request is used to ask for a response containing all or part of a
   routers's routing table.  Normally, Requests are sent as broadcasts



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   (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 a destination address 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 requesting 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 (see
   section 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.

3.7.2 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




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   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 immediately.  If a router processes its own output as new
   input, confusion is likely so such datagrams must be ignored.

   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




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   - Initialize the timeout for the route.  If the garbage-collection
     timer is running for this route, stop it (see section 3.6 for a
     discussion of the timers)

   - Set the route change flag

   - Signal the output process to trigger an update (see section 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 different
   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.




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3.8 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; see section 3.7.1)

   - By the regular routing update (broadcast every 30 seconds) router.

   - By triggered updates (broadcast when a route changes)

   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).  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.

3.8.1 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



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   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 (see section 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.

   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.

3.8.2  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.

3.9  Split Horizon

   Split Horizon is a algorithm for avoiding problems caused by
   including routes in updates sent to the gateway from which they were
   learned.  The basic split horizon algorithm omits routes learned from



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   one neighbor in updates sent to that neighbor.  In the case of a
   broadcast network, all routes learned from any neighbor on that
   network are omitted from updates sent on that network.

   Split Horizon with Poisoned Reverse (more simply, Poison Reverse)
   does include such routes in updates, but sets their metrics to
   infinity.  In effect, advertising the fact that these routes are not
   reachable.  This is the preferred method of operation; however,
   implementations should provide a per-interface control allowing no
   horizoning, split horizoning, and poisoned reverse to be selected.

   For a theoretical discussion of Split Horizon and Poison Reverse, and
   why they are needed, see section 2.1.1 of [1].


4. 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 first four octets of a RIP message contain the RIP header (as
   defined in section 3.4).  The same header format is used for RIP-1
   and RIP-2.  The remainder of the message is composed of 1 - 25
   (inclusive) 20-octet route entries (RTEs).  The RIP-2 RTE format 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 in section 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



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   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:

    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.



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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,

   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 in Appendix 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.

   In order to maintain backwards compatibility, the use of the
   multicast address will be configurable, as described in section 4.1.  If
   multicasting is used, it should be used on all interfaces which support
   it.



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4.5 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.


5. Compatibility

   RFC 1058 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
   1058 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.  The recommended default
   for this switch is RIP-1 compatibility.

   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.

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



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   maximum security, RIP-1 messages should be ignored when authentication
   is in use (see section 4.1).

   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 sections 3.5 and 4.1.

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. 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 sections 4.1 and 5.2.  Security is further
   enhanced by the mechanism described in [10].


















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Appendix 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.


























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References

   [1] Hedrick, C., Routing Information Protocol, Request For Comments
       (RFC) 1058, Rutgers University, June 1988.

   [2] Malkin, G., RIP Version 2 - Carrying Additional Information,
       Request for Comments (RFC) 1723, Xylogics, Inc., July 1994.

   [3] Malkin, G., and F. Baker, RIP Version 2 MIB Extension, Request
       For Comments (RFC) 1389, Xylogics, Inc., Advanced Computer
       Communications, January 1993.

   [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",
       USC/Information Sciences Institute, RFC-1009, June 1987.

   [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] Baker, F., Atkinson, R., "RIP-II MD5 Authentication", draft-
       ietf-ripv2-md5-04.txt, September 1996.

Author's Address

   Gary Scott Malkin
   Bay Networks
   53 Third Avenue
   Burlington, MA 01803

   Phone:  (617) 272-8140
   EMail:  gmalkin@baynetworks.com








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