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rfc:rfc1723

Network Working Group G. Malkin Request for Comments: 1723 Xylogics, Inc. Obsoletes: 1388 November 1994 Updates: 1058 Category: Standards Track

                           RIP Version 2
                  Carrying Additional Information

Status 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.

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.
 This memo obsoletes RFC 1388, which specifies an update to the
 "Routing Information Protocol" STD 34, RFC 1058.
 The RIP-2 protocol analysis is documented in RFC 1721 [4].
 The RIP-2 applicability statement is document in RFC 1722 [5].
 The RIP-2 MIB description is defined in RFC 1724 [3].  This memo
 obsoletes RFC 1389.

Acknowledgements

 I would like to thank the IETF ripv2 Working Group for their help in
 improving the RIP-2 protocol.

Malkin [Page 1] RFC 1723 RIP Version 2 November 1994

Table of Contents

 1.  Justification . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Current RIP . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . . . 3
 3.1   Authentication  . . . . . . . . . . . . . . . . . . . . . . . 4
 3.2   Route Tag . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 3.3   Subnet Mask . . . . . . . . . . . . . . . . . . . . . . . . . 5
 3.4   Next Hop  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 3.5   Multicasting  . . . . . . . . . . . . . . . . . . . . . . . . 5
 3.6   Queries . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
 4.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . 6
 4.1   Compatibility Switch  . . . . . . . . . . . . . . . . . . . . 6
 4.2   Authentication  . . . . . . . . . . . . . . . . . . . . . . . 6
 4.3   Larger Infinity . . . . . . . . . . . . . . . . . . . . . . . 7
 4.4   Addressless Links . . . . . . . . . . . . . . . . . . . . . . 7
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
 Appendix A  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
 References  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9

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 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 protocol does not consider autonomous systems and
 IGP/EGP interactions, subnetting, and authentication since
 implementations of these postdate RIP.  The lack of subnet masks is a

Malkin [Page 2] RFC 1723 RIP Version 2 November 1994

 particularly serious problem for routers since they need a subnet
 mask to know how to determine a route.  If a RIP 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 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. Protocol Extensions

 This document 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.  The
 remainder of the message is composed of 1 - 25 route entries (20
 octets each).  The new RIP message 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Command (1)   | Version (1)   |           unused              |
 +---------------+---------------+-------------------------------+
 | Address Family Identifier (2) |        Route Tag (2)          |
 +-------------------------------+-------------------------------+
 |                         IP Address (4)                        |
 +---------------------------------------------------------------+
 |                         Subnet Mask (4)                       |
 +---------------------------------------------------------------+
 |                         Next Hop (4)                          |
 +---------------------------------------------------------------+
 |                         Metric (4)                            |
 +---------------------------------------------------------------+
 The Command, Address Family Identifier (AFI), IP Address, and Metric
 all have the meanings defined in RFC 1058.  The Version field will
 specify version number 2 for RIP messages which use authentication or
 carry information in any of the newly defined fields.  The contents
 of the unused field (two octets) shall be ignored.
 All fields are coded in IP network byte order (big-endian).

Malkin [Page 3] RFC 1723 RIP Version 2 November 1994

3.1 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:
  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).

3.2 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

Malkin [Page 4] RFC 1723 RIP Version 2 November 1994

 BGP-RIP protocol interactions document, which would describe methods
 for synchronizing routing in a transit network.

3.3 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.

3.4 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.

3.5 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.

Malkin [Page 5] RFC 1723 RIP Version 2 November 1994

 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.

3.6 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.

4. 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.

4.1 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.

4.2 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;

Malkin [Page 6] RFC 1723 RIP Version 2 November 1994

 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 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.

4.3 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.

4.4 Addressless Links

 As in RIP-1, addressless links will not be supported by RIP-2.

5. 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 3.1 and 4.2.

Malkin [Page 7] RFC 1723 RIP Version 2 November 1994

Appendix A

 This is a simple example of the use of the next hop field in a rip
 entry.
  1. —- —– —– —– —– —–

|IR1| |IR2| |IR3| |XR1| |XR2| |XR3|

  1. -+– –+– –+– –+– –+– –+–

| | | | | |

  1. -+——-+——-+—————+——-+——-+–

←————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., "RIP Version 2 - Carrying Additional Information",
     RFC 1388, Xylogics, Inc., January 1993.
 [3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
     1724, Xylogics, Inc., Cisco Systems, November 1994.
 [4] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
     Xylogics, Inc., November 1994.
 [5] Malkin, G., "RIP Version 2 Protocol Applicability Statement", RFC
     1722, Xylogics, Inc., November 1994.

Malkin [Page 8] RFC 1723 RIP Version 2 November 1994

Author's Address

 Gary Scott Malkin
 Xylogics, Inc.
 53 Third Avenue
 Burlington, MA 01803
 Phone:  (617) 272-8140
 EMail:  gmalkin@Xylogics.COM

Malkin [Page 9]

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