As you read this section, consider the following questions: What is RIP? What are the features of the RIP protocol? How does RIP meet the objectives of intradomain routing? What are its weaknesses?
The Routing Information Protocol (RIP) is the simplest routing protocol that was standardised for the TCP/IP protocol suite. RIP is defined in RFC 2453. Additional information about RIP may be found in [Malkin1999]
RIP routers periodically exchange RIP messages. The format of these messages is shown below. A RIP message is sent inside a UDP segment whose destination port is set to 521. A RIP message contains several fields. The Cmd field indicates whether the RIP message is a request or a response. Routers send one of more RIP response messages every 30 seconds. These messages contain the distance vectors that summarize the router’s routing table. The RIP request messages can be used by routers or hosts to query other routers about the content of their routing table. A typical usage is when a router boots and quickly wants to receive the RIP responses from its neighbours to compute its own routing table. The current version of RIP is version 2 defined in RFC 2453 for IPv4 and RFC 2080 for IPv6.
Figure 5.47: RIP message format
The RIP header contains an authentication field. This authentication can be used by network administrators to ensure that only the RIP messages sent by the routers that they manage are used to build the routing tables. RFC 2453 only supports a basic authentication scheme where all routers are configured with the same password and include this password in all RIP messages. This is not very secure since an attacker can know the password by capturing a single RIP message. However, this password can protect against configuration errors. Stronger authentication schemes are described in RFC 2082 and RFC 4822, but the details of these mechanisms are outside the scope of this section.
Each RIP message contains a set of route entries. Each route entry is encoded as a 20 bytes field whose format is shown below. RIP was initially designed to be suitable for different network layer protocols. Some implemen- tations of RIP were used in XNS or IPX networks. The first field of the RIP route entry is the Address Family Identifier (AFI). This identifier indicates the type of address found in the route entry 22. IPv4 uses AFI=1. The other important fields of the route entry are the IPv4 prefix, the netmask that indicates the length of the subnet identifier and is encoded as a 32 bits netmask and the metric. Although the metric is encoded as a 32 bits field, the maximum RIP metric is 15 (for RIP, 16 = ∞)
Figure 5.48: Format of the RIP IPv4 route entries ( RFC 2453)
With a 20 bytes route entry, it was difficult to use the same format as above to support IPv6. Instead of defining a variable length route entry format, the designers of RFC 2080 defined a new format that does not include an AFI field. The format of the route entries used by RFC 2080 is shown below. Plen is the length of the subnet identifier in bits and the metric is encoded as one byte. The maximum metric is still 15.
Figure 5.49: Format of the RIP IPv6 route entries
Note: A note on timers
The first RIP implementations sent their distance vector exactly every 30 seconds. This worked well in most networks, but some researchers noticed that routers were sometimes overloaded because they were processing too many distance vectors at the same time [FJ1994]. They collected packet traces in these networks and found that after some time the routers’ timers became synchronised, i.e. almost all routers were sending their distance vectors at almost the same time. This synchronisation of the transmission times of the distance vectors caused an overload on the routers’ CPU but also increased the convergence time of the protocol in some cases. This was mainly due to the fact that all routers set their timers to the same expiration time after having processed the received distance vectors. Sally Floyd and Van Jacobson proposed in [FJ1994] a simple solution to solve this synchronisation problem. Instead of advertising their distance vector exactly after 30 seconds, a router should send its next distance vector after a delay chosen randomly in the [15,45] interval RFC 2080. This randomisation of the delays prevents the synchronisation that occurs with a fixed delay and is now a recommended practice for protocol designers.
Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
This work is licensed under a Creative Commons Attribution 3.0 License.