





Network Working Group                                          G. Malkin
Request for Comments: 1722                                Xylogics, Inc.
Category: Standards Track                                  November 1994


             RIP Version 2 Protocol Applicability Statement

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

   As required by Routing Protocol Criteria (RFC 1264), this report
   defines the applicability of the RIP-2 protocol within the Internet.
   This report is a prerequisite to advancing RIP-2 on the standards
   track.

1.  Protocol Documents

   The RIP-2 protocol analysis is documented in RFC 1721 [1].

   The RIP-2 protocol description is defined in RFC 1723 [2].  This memo
   obsoletes RFC 1388, which specifies an update to the "Routing
   Information Protocol" RFC 1058 (STD 34).

   The RIP-2 MIB description is defined in RFC 1724 [3].  This memo will
   obsolete RFC 1389.

2.  Introduction

   This report describes how RIP-2 may be useful within the Internet.
   In essence, the environments in which RIP-2 is the IGP of choice is a
   superset of the environments in which RIP-1, as defined in RFC 1058
   [1], has traditionally been used.  It is important to remember that
   RIP-2 is an extension to RIP-1; RIP-2 is not a new protocol.  Thus,
   the operational aspects of distance-vector routing protocols, and
   RIP-1 in particular, within an autonomous system are well understood.

   It should be noted that RIP-2 is not intended to be a substitute for
   OSPF in large autonomous systems; the restrictions on AS diameter and
   complexity which applied to RIP-1 also apply to RIP-2.  Rather, RIP-2
   allows the smaller, simpler, distance-vector protocol to be used in
   environments which require authentication or the use of variable



Malkin                                                          [Page 1]

RFC 1722                  RIP-2 Applicability              November 1994


   length subnet masks, but are not of a size or complexity which
   require the use of the larger, more complex, link-state protocol.

   The remainder of this report describes how each of the extensions to
   RIP-1 may be used to increase the overall usefullness of RIP-2.

3.  Extension Applicability

3.1 Subnet Masks

   The original impetus behind the creation of RIP-2 was the desire to
   include subnet masks in the routing information exchanged by RIP.
   This was needed because subnetting was not defined when RIP was first
   created.  As long as the subnet mask was fixed for a network, and
   well known by all the nodes on that network, a heuristic could be
   used to determine if a route was a subnet route or a host route.
   With the advent of variable length subnetting, CIDR, and
   supernetting, it was no longer possible for a heuristic to reasonably
   distinguish between network, subnet, and host routes.

   By using the 32-bit field immediately following the IP address in a
   RIP routing entry, it became possible to positively identify a
   route's type.  In fact, one could go so far as to say that the
   inclusion of the subnet mask effictively creates a 64-bit address
   which eliminates the network, subnet, host distinction.

   Therefore, the inclusion of subnet masks in RIP-2 allows it to be
   used in an AS which requires precise knowledge of the subnet mask for
   a given route, but does not otherwise require OSPF.

3.2. Next Hop

   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.
   Consider the following example topology:

      -----   -----         -----   -----
      |IR1|   |IR2|         |XR1|   |XR2|
      --+--   --+--         --+--   --+--
        |       |             |       |
      --+-------+-------------+-------+--
        |--------RIP-2--------|

   The Internal Routers (IR1 and IR2) are only running RIP-2.  The
   External Routers (XR1 and XR2) are both running BGP, for example;
   however, only XR1 is running BGP and RIP-2.  Since XR2 is not running
   RIP-2, the IRs will not know of its existance and will never use it



Malkin                                                          [Page 2]

RFC 1722                  RIP-2 Applicability              November 1994


   as a next hop, even if it is a better next hop than XR1.  Of course,
   XR1 knows this and can indicate, via the Next Hop field, that XR2 is
   the better next hop for some routes.

   Another use for Next Hop has also been found.  Consider the following
   example topology:

           -----
           |COR|
           -----
           /   \
          /     \
      -----     -----     -----
      |RO1|-----|RO2|=====| R |
      -----     -----     -----

   The three links between the Central Office Router (COR) and the
   Remote Office routers (RO1 and RO2) are all Dial-On-Demand (DOD)
   links.  The link between RO2 and R is a fixed link.  Once all of the
   routers have been initialized, the only routes they know about are
   the configured static routes for the DOD links.  Assume that
   connections between COR and RO1, and COR and RO2 are established and
   RIP information is passing between the routers.  RO1 will ignore
   COR's route to RO2 because it already has a better one; however, it
   will learn to reach R via COR.

   If we assume that RO1 and RO2 are only capable of establishing one
   link at a time, then RO1 will not be able to reach RO2; however, RO1
   will be able to reach R.  Worse still, if we assume that traffic
   stops and the DOD links drop due to inactivity, an attempt by RO1 to
   reach R will trigger the dialing of two links (through COR).  Of
   course, once RO1 establishes a link to RO2, the problem corrects
   itself because the new route to R is one hop shorter.

   To correct this problem, the routers may use the Next Hop field to
   indicate their next hop.  Consider the following route advertisements
   during the period described above (before the RO1/RO2 link has ever
   been established):

      Sender  Recvr   Route   NextHop  Metric
      =======================================
      RO2     COR     R       0        1
      ---------------------------------------
      COR     RO1     RO2     0        1
                      R       RO2      2
      ---------------------------------------





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RFC 1722                  RIP-2 Applicability              November 1994


   When R01 receives the two routes from COR, it will ignore the route
   for RO2, as mentioned above.  However, since R is not in RO1's
   routing table, it will add it using a next hop of RO2 (because RO2 is
   directly connected, after a fashion).  Note that COR does count
   itself in R's metric; this is less than accurate, but entirely safe
   and correctable (when the RO1/RO2 link comes up).  Suppose, now, that
   the RO1/RO2 link did not exist.  RO1 would ignore the specification
   of RO2 as the next hop to R and use COR, as it would if no Next Hop
   had been specified.

   Note that this is not a recursive algorithm; it only works to
   eliminate a single extra hop from the path.  There are methods by
   which this mechanism might be extended to include larger
   optimizations, but the potential to create routing loops has not been
   sufficiently analyzed to specify them here.

3.3 Authentication

   The need for authentication in a routing protocol is obvious.  It is
   not usually important to conceal the information in the routing
   messages, but it is essential to prevent the insertion of bogus
   routing information into the routers.  So, while the authentication
   mechanism specified in RIP-2 is less than ideal, it does prevent
   anyone who cannot directly access the network (i.e., someone who
   cannot sniff the routing packets to determine the password) from
   inserting bogus routing information.

   However, the specification does allow for additional types of
   authentication to be incorporated into the protocol.  Unfortunately,
   because of the original format of RIP packets, the amount of space
   available for providing authentication information is only 16 octets.

3.4 Multicasting

   The RIP-2 protocol provides for the IP multicasting of periodic
   advertisements.  This feature was added to decrease the load on
   systems which do not support RIP-2.  It also provides a mechanism
   whereby RIP-1 routers will never receive RIP-2 routes.  This is a
   feature when correct use of an advertised route depends on knowing
   the precise subnet mask, which would be ignored by a RIP-1 router.

4.  Conclusion

   Because the basic protocol is unchanged, RIP-2 is as correct a
   routing protocol as RIP-1.  The enhancements make RIP-2 useful in
   environments which RIP-1 could not handle, but which do not
   necessitate the use of OSPF by virtue of requirements which RIP-2
   does not satisfy.



Malkin                                                          [Page 4]

RFC 1722                  RIP-2 Applicability              November 1994


5.  References

   [1] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
       Xylogics, Inc., November 1994.

   [2] Malkin, G., "RIP Version 2 - Carrying Additional Information",
       RFC 1723, Xylogics, Inc., November 1994.

   [3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
       1724, Xylogics, Inc., Cisco Systems, November 1994.

6.  Security Considerations

   Security issues are not discussed in this memo.

7.  Author's Address

   Gary Scott Malkin
   Xylogics, Inc.
   53 Third Avenue
   Burlington, MA 01803

   Phone:  (617) 272-8140
   EMail:  gmalkin@Xylogics.COM



























Malkin                                                          [Page 5]

