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RIP is obsolete.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 packet contains the minimal amount of information
necessary for routers to route packets 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
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 datagram format which allows routers to
share important additional information.
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RFC 1388 RIP Version 2 January 1993
The new RIP datagram 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) | Routing Domain (2) |
+---------------+---------------+-------------------------------+
| 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 datagrams which use authentication
or carry information in any of the newly defined fields.
All fields are coded in IP network byte order (big-endian).
Since authentication is a per packet function, and since there is
only one 2-byte field available in the packet header, and since any
reasonable authentication scheme will require more than two bytes,
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 packet 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 packet. If
authentication is not in use, then no entries in the packet should
have an Address Family Identifier of 0xFFFF. A RIP packet which
contains an authentication entry would have 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) | Routing Domain (2) |
+---------------+---------------+-------------------------------+
| 0xFFFF | Authentication Type (2) |
+-------------------------------+-------------------------------+
~ Authentication (16) ~
+---------------------------------------------------------------+
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RFC 1388 RIP Version 2 January 1993
Currently, the only Authentication Type is simple password and it is
type 2. The remaining 16 bytes contain the plain text password. If
the password is under 16 bytes, it must be left-justified and padded
to the right with nulls (0x00).
The Routing Domain (RD) number is the number of the routing process
to which this update belongs. This field is used to associate the
routing update to a specific routing process on the receiving router.
The RD is needed to allow multiple, independent RIP "clouds" to co-
exist on the same physical wire. This gives administrators the
ability to run multiple, possibly parallel, instances of RIP in order
to implement simple policy. This means that a router operating
within one routing domain, or a set of routing domains, should ignore
RIP packets which belong to another routing domain. RD 0 is the
default routing domain.
The Route Tag (RT) field exists as a support for EGPs. The contents
and use of this field are outside the scope of this protocol.
However, it is expected that the field will be used to carry
Autonomous System numbers for EGP and BGP. Any RIP system which
receives a RIP entry which contains a non-zero RT value must re-
advertise that value. Those routes which have no RT value must
advertise an RT value of zero.
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 two rules apply:
1) information internal to one network must never be advertised into
another network, and
2) information about a more specific subnet may not be advertised
where RIP-1 routers would consider it a host route.
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
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RFC 1388 RIP Version 2 January 1993
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.
In order to reduce unnecessary load on those hosts which are not
listening to RIP-2 packets, 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.
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.
RFC 1058 showed considerable forethought in its specification of the
handling of version numbers. It specifies that RIP packets of
version 0 are to be discarded, that RIP packets of version 1 are to
be discarded if any Must Be Zero (MBZ) field is non-zero, and that
RIP packets 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).
The switch has three settings: RIP-1, in which only RIP-1 packets are
sent; RIP-1 compatibility, in which RIP-2 packets are broadcast; and
RIP-2, in which RIP-2 packets are multicast. The recommended default
for this switch is RIP-1 compatibility.
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RFC 1388 RIP Version 2 January 1993
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 packets. If desired, this may be done using
multicasting, as described in sections 3.6 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 byte and
reuse the high three bytes, but this would break any implementations
which treat the metric as a long.
As in RIP-1, addressless links will not be supported by RIP-2.
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
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RFC 1388 RIP Version 2 January 1993
necessary for XR2 and XR3 to also participate in the RIP-2 protocol
to eliminate extra hops.
References
[1] Hedrick, C., "Routing Information Protocol", RFC 1058, Rutgers
University, June 1988.
[2] Malkin, G., and F. Baker, "RIP Version 2 MIB Extension", RFC
1389, Xylogics, Inc., Advanced Computer Communications, January
1993.
[3] Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1387,
Xylogics, Inc., January 1993.
Security Considerations
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.
Author's Address
Gary Scott Malkin
Xylogics, Inc.
53 Third Avenue
Burlington, MA 01803
Phone: (617) 272-8140
EMail: gmalkin@Xylogics.COM
Malkin [Page 7]