RFC1722 - RIP Version 2 Protocol Applicability Statement

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　　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 (RFC1264), 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 RFC1721 [1].

The RIP-2 protocol description is defined in RFC1723 [2]. This memo
obsoletes RFC1388, which specifies an update to the "Routing
Information Protocol" RFC1058 (STD 34).

The RIP-2 MIB description is defined in RFC1724 [3]. This memo will
obsolete RFC1389.

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 RFC1058
[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

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

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
---------------------------------------

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.

5. References

[1] Malkin, G., "RIP Version 2 Protocol Analysis", RFC1721,
Xylogics, Inc., November 1994.

[2] Malkin, G., "RIP Version 2 - Carrying Additional Information",
RFC1723, 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

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