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rfc:rfc1388

Network Working Group G. Malkin Request for Comments: 1388 Xylogics, Inc. Updates: RFC 1058 January 1993

                           RIP Version 2
                  Carrying Additional Information

Status of this Memo

 This RFC specifies an IAB standards track protocol for the Internet
 community, and requests discussion and suggestions for improvements.
 Please refer to the current edition of the "IAB Official Protocol
 Standards" 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], to expand the amount of useful
 information carried in RIP packets and to add a measure of security.
 A companion document will define the SNMP MIB objects for RIP-2 [2].

Acknowledgements

 I would like to thank the following for their contributions to this
 document: Fred Baker, Noel Chiappa and Vince Fuller.  This memo is a
 product of the RIP-2 Working Group of the Internet Engineering Task
 Force (IETF).

Table of Contents

 1.  Justification . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Current RIP . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . . . 2
 3.1   Authentication  . . . . . . . . . . . . . . . . . . . . . . . 3
 3.2   Routing Domain  . . . . . . . . . . . . . . . . . . . . . . . 4
 3.3   Route Tag . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 3.4   Subnet Mask . . . . . . . . . . . . . . . . . . . . . . . . . 4
 3.5   Next Hop  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 3.6   Multicasting  . . . . . . . . . . . . . . . . . . . . . . . . 5
 4.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . 5
 4.1   Compatibility Switch  . . . . . . . . . . . . . . . . . . . . 5
 4.2   Authentication  . . . . . . . . . . . . . . . . . . . . . . . 6
 4.3   Larger Infinity . . . . . . . . . . . . . . . . . . . . . . . 6
 4.4   Addressless Links . . . . . . . . . . . . . . . . . . . . . . 6
 Appendix A  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
 References  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Malkin [Page 1] RFC 1388 RIP Version 2 January 1993

 Security Considerations . . . . . . . . . . . . . . . . . . . . . . 7
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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 packet contains the minimal amount of information
 necessary for routers to route packets 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
 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 datagram format which allows routers to
 share important additional information.

Malkin [Page 2] RFC 1388 RIP Version 2 January 1993

 The new RIP datagram 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)   |       Routing Domain (2)      |
 +---------------+---------------+-------------------------------+
 | 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 datagrams which use authentication
 or carry information in any of the newly defined fields.
 All fields are coded in IP network byte order (big-endian).

3.1 Authentication

 Since authentication is a per packet function, and since there is
 only one 2-byte field available in the packet header, and since any
 reasonable authentication scheme will require more than two bytes,
 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 packet 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 packet.  If
 authentication is not in use, then no entries in the packet should
 have an Address Family Identifier of 0xFFFF.  A RIP packet which
 contains an authentication entry would have 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)   |       Routing Domain (2)      |
 +---------------+---------------+-------------------------------+
 |             0xFFFF            |    Authentication Type (2)    |
 +-------------------------------+-------------------------------+
 ~                       Authentication (16)                     ~
 +---------------------------------------------------------------+

Malkin [Page 3] RFC 1388 RIP Version 2 January 1993

 Currently, the only Authentication Type is simple password and it is
 type 2.  The remaining 16 bytes contain the plain text password.  If
 the password is under 16 bytes, it must be left-justified and padded
 to the right with nulls (0x00).

3.2 Routing Domain

 The Routing Domain (RD) number is the number of the routing process
 to which this update belongs.  This field is used to associate the
 routing update to a specific routing process on the receiving router.
 The RD is needed to allow multiple, independent RIP "clouds" to co-
 exist on the same physical wire.  This gives administrators the
 ability to run multiple, possibly parallel, instances of RIP in order
 to implement simple policy.  This means that a router operating
 within one routing domain, or a set of routing domains, should ignore
 RIP packets which belong to another routing domain.  RD 0 is the
 default routing domain.

3.3 Route Tag

 The Route Tag (RT) field exists as a support for EGPs.  The contents
 and use of this field are outside the scope of this protocol.
 However, it is expected that the field will be used to carry
 Autonomous System numbers for EGP and BGP.  Any RIP system which
 receives a RIP entry which contains a non-zero RT value must re-
 advertise that value.  Those routes which have no RT value must
 advertise an RT value of zero.

3.4 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 two rules apply:
 1) information internal to one network must never be advertised into
    another network, and
 2) information about a more specific subnet may not be advertised
    where RIP-1 routers would consider it a host route.

3.5 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

Malkin [Page 4] RFC 1388 RIP Version 2 January 1993

 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.

3.6 Multicasting

 In order to reduce unnecessary load on those hosts which are not
 listening to RIP-2 packets, 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.

4. Compatibility

 RFC 1058 showed considerable forethought in its specification of the
 handling of version numbers.  It specifies that RIP packets of
 version 0 are to be discarded, that RIP packets of version 1 are to
 be discarded if any Must Be Zero (MBZ) field is non-zero, and that
 RIP packets 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).
 The switch has three settings: RIP-1, in which only RIP-1 packets are
 sent; RIP-1 compatibility, in which RIP-2 packets are broadcast; and
 RIP-2, in which RIP-2 packets are multicast.  The recommended default
 for this switch is RIP-1 compatibility.

Malkin [Page 5] RFC 1388 RIP Version 2 January 1993

4.2 Authentication

 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 packets.  If desired, this may be done using
 multicasting, as described in sections 3.6 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 byte and
 reuse the high three bytes, but this would break any implementations
 which treat the metric as a long.

4.4 Addressless Links

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

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

Malkin [Page 6] RFC 1388 RIP Version 2 January 1993

 necessary for XR2 and XR3 to also participate in the RIP-2 protocol
 to eliminate extra hops.

References

 [1] Hedrick, C., "Routing Information Protocol", RFC 1058, Rutgers
     University, June 1988.
 [2] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
     1389, Xylogics, Inc., Advanced Computer Communications, January
     1993.
 [3] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1387,
     Xylogics, Inc., January 1993.

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.

Author's Address

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

Malkin [Page 7]

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