RIP, Routing Information Protocol

Protocol suite:	TCP/IP.
Protocol type:	Application layer, interior gateway protocol, distance vector.
Multicast addresses:	224.0.0.9.
Port:	520 (UDP).
MIME subtype:
SNMP MIBs:	iso.org.dod.internet.mgmt.mib-2.rip-2 (1.3.6.1.2.1.23).
Working groups:	rip, Routing Information Protocol.
Links:

RIP version 1 (RIPv1).

This is a simple distance vector protocol. It has been enhanced with various techniques, including Split Horizon and Poison Reverse in order to enable it to perform better in somewhat complicated networks.

The longest path cannot exceed 15 hops.
RIP uses static metrics to compare routes.

The maximum datagram size is 512 bytes not including the IP or UDP headers.

RIP version 2 (RIPv2).

This version added several new features.

External route tags.
Subnet masks.
Next hop router addresses.
Authentication.
Multicast support.

RFC 2453, section 3.2:

The protocol is limited to networks whose longest path is 15 hops. The designers believe that the basic protocol design is inappropriate for larger networks. Note that this statement of the limit assumes that a cost of 1 is used for each network. This is the way RIP is normally configured. If the system administrator chooses to use larger costs, the upper bound of 15 can easily become a problem.

The protocol depends upon "counting to infinity" to resolve certain unusual situations. If the system of networks has several hundred networks, and a routing loop was formed involving all of them, the resolution of the loop would require either much time (if the frequency of routing updates were limited) or bandwidth (if updates were sent whenever changes were detected). Such a loop would consume a large amount of network bandwidth before the loop was corrected. We believe that in realistic cases, this will not be a problem except on slow lines. Even then, the problem will be fairly unusual, since various precautions are taken that should prevent these problems in most cases.

This protocol uses fixed "metrics" to compare alternative routes. It is not appropriate for situations where routes need to be chosen based on real-time parameters such a measured delay, reliability, or load. The obvious extensions to allow metrics of this type are likely to introduce instabilities of a sort that the protocol is not designed to handle.

MAC header

IP header

UDP header

RIP header

Data :::

RIPv1 header:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Command								Version								0
RIPv1 entry table :::

RIPv2 header:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Command								Version								0
RIPv2 entry table :::

Command. 8 bits.

Value	Description	References
1	Request. A request for the responding system to send all or part of its routing table.
2	Response. A message containing all or part of the sender's routing table. This message may be sent in response to a request, or it may be an unsolicited routing update generated by the sender.
3	Trace on.Obsolete.
4	Trace off.Obsolete.
5	SUN reserved.
6	Triggered request.	RFC 1582
7	Triggered response.	RFC 1582
8	Triggered acknowledgement.	RFC 1582
9	Update Request.	RFC 2091
10	Update Response.	RFC 2091
11	Update Acknowledge.	RFC 2091

Version. 8 bits.
The RIP protocol version.

RIPv1 entry table. Variable length.
An array of RIPv1 entries. Each RIPv1 entry contains the following structure:

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Address family 0

IPv4 address

0

0

Metric

RIPv2 entry table. Variable length.
An array of RIPv2 entries. Each RIPv2 entry contains the following structure:

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Address family Route tag

IPv4 address

Subnet mask

Next hop

Metric

Address family. 16 bits.

Route tag. 16 bits.
Cleared to 0 for RIPv1. For RIPv2, this is an attribute assigned to a route which must be preserved and readvertised with a route. The intended use of this tag is to provide a method of separating routes for networks within the RIP routing domain from routes which may have been imported from an EGP or another IGP.

IPv4 address. 32 bits.

Subnet mask. 32 bits.
Cleared to 0 for RIPv1. For RIPv2, this is the subnet mask that can be applied to the IPv4 address to resolve the network portion of the address. If the field is cleared to 0, no subnet mask is specified.

Next hop. 32 bits.
Cleared to 0 for RIPv1. For RIPv2, this is the IP where packets for this entry should be forwarded to.

Metric. 32 bits.
Contains a value from 1 to 15 which specifies the current metric for the destination. A value of 16 indicates that the destination is not reachable.

Glossary:

Circuit Manager.
A circuit manager provides an interface between the connectionless and connection oriented network layers. The circuit manager takes datagrams from the connectionless network layer protocols and as necessary opens a virtual circuit to the next hop router.

Demand RIP.
Has an upper limit of 255 fragments in an update. This sets an upper limit on the sizes of routing and service advertising databases which can be propagated.

Triggered RIP.
Routing updates are only transmitted for the following conditions:

A request for a routing update has been received.
The routing database is modified by new information from another interface.
The circuit manager indicates that a destination has changed from an unreachable to a reachable state.
When a unit is first powered on to ensure that at least one update is sent. This can be thought of as a transition from unreachable to reachable. It MAY contain no routes or services, and is used to flush routes or services from the peer's database.

Fragmentation is not allowed in an update.

Triggered updates.

RFCs:

[RFC 1058] Routing Information Protocol.