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RIP is obsolete.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.
The current RIP message contains the minimal amount of information
necessary for routers to route messages 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
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RFC 1723 RIP Version 2 November 1994
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.
This document does not change the RIP protocol per se. Rather, it
provides extensions to the message format which allows routers to
share important additional information.
The first four octets of a RIP message contain the RIP header. The
remainder of the message is composed of 1 - 25 route entries (20
octets each). The new RIP message 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) | unused |
+---------------+---------------+-------------------------------+
| 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 messages which use authentication or
carry information in any of the newly defined fields. The contents
of the unused field (two octets) shall be ignored.
All fields are coded in IP network byte order (big-endian).
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RFC 1723 RIP Version 2 November 1994
Since authentication is a per message function, and since there is
only one 2-octet field available in the message header, and since any
reasonable authentication scheme will require more than two octets,
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 message 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 message. If
authentication is not in use, then no entries in the message should
have an Address Family Identifier of 0xFFFF. A RIP message which
contains an authentication entry would begin with 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) | unused |
+---------------+---------------+-------------------------------+
| 0xFFFF | Authentication Type (2) |
+-------------------------------+-------------------------------+
~ Authentication (16) ~
+---------------------------------------------------------------+
Currently, the only Authentication Type is simple password and it is
type 2. The remaining 16 octets contain the plain text password. If
the password is under 16 octets, it must be left-justified and padded
to the right with nulls (0x00).
The Route Tag (RT) field is an attribute assigned to a route which
must be preserved and readvertised with a route. The intended use of
the Route Tag is to provide a method of separating "internal" RIP
routes (routes for networks within the RIP routing domain) from
"external" RIP routes, which may have been imported from an EGP or
another IGP.
Routers supporting protocols other than RIP should be configurable to
allow the Route Tag to be configured for routes imported from
different sources. For example, routes imported from EGP or BGP
should be able to have their Route Tag either set to an arbitrary
value, or at least to the number of the Autonomous System from which
the routes were learned.
Other uses of the Route Tag are valid, as long as all routers in the
RIP domain use it consistently. This allows for the possibility of a
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RFC 1723 RIP Version 2 November 1994
BGP-RIP protocol interactions document, which would describe methods
for synchronizing routing in a transit network.
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 rules apply:
1) information internal to one network must never be advertised into
another network,
2) information about a more specific subnet may not be advertised
where RIP-1 routers would consider it a host route, and
3) supernet routes (routes with a netmask less specific than the
"natural" network mask) must not be advertised where they could be
misinterpreted by RIP-1 routers.
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
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. If
the received Next Hop is not directly reachable, it should be treated
as 0.0.0.0.
In order to reduce unnecessary load on those hosts which are not
listening to RIP-2 messages, 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.
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RFC 1723 RIP Version 2 November 1994
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.
If a RIP-2 router receives a RIP-1 Request, it should respond with a
RIP-1 Response. If the router is configured to send only RIP-2
messages, it should not respond to a RIP-1 Request.
RFC 1058 showed considerable forethought in its specification of the
handling of version numbers. It specifies that RIP messages of
version 0 are to be discarded, that RIP messages of version 1 are to
be discarded if any Must Be Zero (MBZ) field is non-zero, and that
RIP messages 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.
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). This switch should be configurable
on a per-interface basis.
The switch has four settings: RIP-1, in which only RIP-1 messages are
sent; RIP-1 compatibility, in which RIP-2 messages are broadcast;
RIP-2, in which RIP-2 messages are multicast; and "none", which
disables the sending of RIP messages. The recommended default for
this switch is RIP-1 compatibility.
For completeness, routers should also implement a receive control
switch which would determine whether to accept, RIP-1 only, RIP-2
only, both, or none. It should also be configurable on a per-
interface basis.
The following algorithm should be used to authenticate a RIP message.
If the router is not configured to authenticate RIP-2 messages, then
RIP-1 and unauthenticated RIP-2 messages will be accepted;
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RFC 1723 RIP Version 2 November 1994
authenticated RIP-2 messages shall be discarded. If the router is
configured to authenticate RIP-2 messages, then RIP-1 messages and
RIP-2 messages which pass authentication testing shall be accepted;
unauthenticated and failed authentication RIP-2 messages shall be
discarded. For maximum security, RIP-1 messages should be ignored
when authentication is in use (see section 4.1).
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 messages. If desired, this may be done using
multicasting, as described in sections 3.5 and 4.1.
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 octet and
reuse the high three octets, but this would break any implementations
which treat the metric as a 4-octet entity.
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.
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RFC 1723 RIP Version 2 November 1994
Appendix A
This is a simple example of the use of the next hop field in a rip
entry.
----- ----- ----- ----- ----- -----
|IR1| |IR2| |IR3| |XR1| |XR2| |XR3|
--+-- --+-- --+-- --+-- --+-- --+--
| | | | | |
--+-------+-------+---------------+-------+-------+--
<-------------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 necessary for
XR2 and XR3 to also participate in the RIP-2 protocol to eliminate
extra hops.
References
[1] Hedrick, C., "Routing Information Protocol", STD 34, RFC 1058,
Rutgers University, June 1988.
[2] Malkin, G., "RIP Version 2 - Carrying Additional Information",
RFC 1388, Xylogics, Inc., January 1993.
[3] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
1724, Xylogics, Inc., Cisco Systems, November 1994.
[4] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
Xylogics, Inc., November 1994.
[5] Malkin, G., "RIP Version 2 Protocol Applicability Statement", RFC
1722, Xylogics, Inc., November 1994.
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RFC 1723 RIP Version 2 November 1994
Author's Address
Gary Scott Malkin
Xylogics, Inc.
53 Third Avenue
Burlington, MA 01803
Phone: (617) 272-8140
EMail: gmalkin@Xylogics.COM
Malkin [Page 9]