3.1. Message formats

RIP is a UDP-based protocol. Each host that uses RIP has a routing process that sends and receives datagrams on UDP port number 520. All communications directed at another host's RIP processor are sent to port 520. All routing update messages are sent from port 520. Unsolicited routing update messages have both the source and destination port equal to 520. Those sent in response to a request are sent to the port from which the request came. Specific queries and debugging requests may be sent from ports other than 520, but they are directed to port 520 on the target machine.

There are provisions in the protocol to allow "silent" RIP processes. A silent process is one that normally does not send out any messages. However, it listens to messages sent by others. A silent RIP might be used by hosts that do not act as gateways, but wish to listen to routing updates in order to monitor local gateways and to keep their internal routing tables up to date. (See [5] for a discussion of various ways that hosts can keep track of network topology.) A gateway that has lost contact with all but one of its networks might choose to become silent, since it is effectively no longer a gateway.

However, this should not be done if there is any chance that neighboring gateways might depend upon its messages to detect that the failed network has come back into operation. (The 4BSD routed program uses routing packets to monitor the operation of point-to- point links.)

The packet format is shown in Figure 1.

      Format of datagrams containing network information.  Field sizes
      are given in octets.  Unless otherwise specified, fields contain
      binary integers, in normal Internet order with the most-significant
      octet first.  Each tick mark represents one bit.

       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)   |      must be zero (2)         |
      +---------------+---------------+-------------------------------+
      | address family identifier (2) |      must be zero (2)         |
      +-------------------------------+-------------------------------+
      |                         IP address (4)                        |
      +---------------------------------------------------------------+
      |                        must be zero (4)                       |
      +---------------------------------------------------------------+
      |                        must be zero (4)                       |
      +---------------------------------------------------------------+
      |                          metric (4)                           |
      +---------------------------------------------------------------+
                                      .
                                      .
                                      .
      The portion of the datagram from address family identifier through
      metric may appear up to 25 times.  IP address is the usual 4-octet
      Internet address, in network order.

                          Figure 1.   Packet format

Every datagram contains a command, a version number, and possible arguments. This document describes version 1 of the protocol. Details of processing the version number are described in section 3.4. The command field is used to specify the purpose of this datagram. Here is a summary of the commands implemented in version 1:

   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 poll, or it may be an update message
                   generated by the sender.

   3 - traceon     Obsolete.  Messages containing this command are to be
                   ignored.
   4 - traceoff    Obsolete.  Messages containing this command are to be
                   ignored.

   5 - reserved    This value is used by Sun Microsystems for its own
                   purposes.  If new commands are added in any
                   succeeding version, they should begin with 6.
                   Messages containing this command may safely be
                   ignored by implementations that do not choose to
                   respond to it.

For request and response, the rest of the datagram contains a list of destinations, with information about each. Each entry in this list contains a destination network or host, and the metric for it. The packet format is intended to allow RIP to carry routing information for several different protocols. Thus, each entry has an address family identifier to indicate what type of address is specified in that entry. This document only describes routing for Internet networks. The address family identifier for IP is 2. None of the RIP implementations available to the author implement any other type of address. However, to allow for future development, implementations are required to skip entries that specify address families that are not supported by the implementation. (The size of these entries will be the same as the size of an entry specifying an IP address.) Processing of the message continues normally after any unsupported entries are skipped. The IP address is the usual Internet address, stored as 4 octets in network order. The metric field must contain a value between 1 and 15 inclusive, specifying the current metric for the destination, or the value 16, which indicates that the destination is not reachable. Each route sent by a gateway supercedes any previous route to the same destination from the same gateway.

The maximum datagram size is 512 octets. This includes only the portions of the datagram described above. It does not count the IP or UDP headers. The commands that involve network information allow information to be split across several datagrams. No special provisions are needed for continuations, since correct results will occur if the datagrams are processed individually.