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Network Working Group G. Malkin Request for Comments: 2081 Xylogics Category: Informational January 1997

               RIPng Protocol Applicability Statement

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

Abstract

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

1. Protocol Documents

 The RIPng protocol description is defined in RFC 2080.

2. Introduction

 This report describes how RIPng may be useful within the new IPv6
 Internet.  In essence, the environments in which RIPng is the IGP of
 choice is comparable to the environments in which RIP-2 (RFC 1723) is
 used in the IPv4 Internet.  It is important to remember that RIPng is
 a simple extrapolation of RIP-2; RIPng has nothing conceptually new.
 Thus, the operational aspects of distance-vector routing protocols,
 and RIP-2 in particular, within an autonomous system are well
 understood.

 It should be noted that RIPng is not intended to be a substitute for
 OSPFng in large autonomous systems; the restrictions on AS diameter
 and complexity which applied to RIP-2 also apply to RIPng.  Rather,
 RIPng 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 features of
 RIPng is useful within IPv6.

Malkin Informational [Page 1] RFC 2081 RIP-2 Applicability January 1997

3. Applicability

 A goal in developing RIPng was to make the minimum necessary change
 to RIP-2 to produce RIPng.  In essence, the IPv4 address was expanded
 into an IPv6 address, the IPv4 subnet mask was replaced with an IPv6
 prefix length, the next-hop field was eliminated but the
 functionality has been preserved, and authentication was removed.
 The route tag field has been preserved.  The maximum diameter of the
 network (the maximum metric value) is 15; 16 still means infinity
 (unreachable).

 The basic RIP header is unchanged.  However, the size of a routing
 packet is no longer arbitrarily limited.  Because routing updates are
 never forwarded, the routing packet size is now determined by the
 physical media and the sizes of the headers which precede the routing
 data (i.e., media MTU minus the combined header lengths).  The number
 routes which may be included in a routing update is the routing data
 length divided by the size of a routing entry.

3.1 Prefix

 The address field of a routing entry is 128 bits in length, expanded
 from the 32 bits available in RIP-2.  This allows the RIP entry to
 carry an IPv6 prefix.

3.2 Prefix Length

 The 32-bit RIP-2 subnet mask field is replaced by an 8-bit prefix
 length field.  It allows the specification of the number of bits in
 the prefix which form the actual prefix.

3.3 Next Hop

 The ability to specify the next hop, rather than simply allowing the
 recipient of the update to set the next hop to the sender of the
 update, allows for the elimination of unnecessary hops through
 routers which are running multiple routing protocols.  Consider
 following example topology:

—- —– —– —–

|IR1| |IR2| |XR1| |XR2|

-+– –+– –+– –+–

| | | |

-+——-+————-+——-+–

|——–RIPng——–|

Malkin Informational [Page 2] RFC 2081 RIP-2 Applicability January 1997

 The Internal Routers (IR1 and IR2) are only running RIPng.  The
 External Routers (XR1 and XR2) are both running BGP, for example;
 however, only XR1 is running BGP and RIPng.  Since XR2 is not running
 RIPng, 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 mechanism, that XR2
 is the better next hop for some routes.

3.4 Authentication

 Authentication, which was added to RIP-2 because RIP-1 did not have
 it, has been dropped from RIPng.  This is safe to do because IPv6,
 which carries the RIPng packets, has build in security which IPv4 did
 not have.

3.5 Packet Length

 By allowing RIPng routing update packets to be as big as possible,
 the number of packets which must be sent for a complete update is
 greatly reduced.  This in no way affects the operation of the
 distance-vector protocol; it is merely a performance enhancement.

3.6 Diameter and Complexity

 The limit of 15 cost-1 hops is a function of the distance-vector
 protocol, which depends on counting to infinity to resolve some
 routing loops.  If infinity is too high, the time it would take to
 resolve, not to mention the number of routing updates which would be
 sent, would be prohibitive.  If the infinity is too small, the
 protocol becomes useless in a reasonably sized network.  The choice
 of 16 for infinity was made in the earliest of RIP implementations
 and experience has shown it to be a good compromise value.

 RIPng will efficiently support networks of moderate complexity.  That
 is, topologies without too many multi-hop loops.  RIPng also
 effeciently supports topologies which change frequently because
 routing table changes are made incrementally and do not require the
 computation which link-state protocols require to rebuild their maps.

4. Conclusion

 Because the basic protocol is unchanged, RIPng is as correct a
 routing protocol as RIP-2.  RIPng serves the same niche for IPv6 as
 RIP-2 does for IPv4.

5. Security Considerations

 RIPng security is discussed in section 3.4.

Malkin Informational [Page 3] RFC 2081 RIP-2 Applicability January 1997

Author's Address

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

 Phone:  (617) 238-6237
 EMail:  gmalkin@xylogics.com

Malkin Informational [Page 4]