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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
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+Malkin                       Informational                      [Page 4]
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