7. Relay Agent/Server Message Formats

Relay Agent/Server Message Formats

Relay agents exchange messages with servers to relay messages between clients and servers that are not connected to the same link.

All values in the message header and in options are in network byte order.

Options are stored serially in the options field, with no padding between the options. Options are byte-aligned but are not aligned in any other way such as on 2 or 4 byte boundaries.

There are two relay agent messages, which share the following format:
```
0                   1                   2                   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
```
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | msg-type | hop-count | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | link-address | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | peer-address | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | . . . options (variable number and length) .... . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The following sections describe the use of the Relay Agent message header.

7.1. Relay-forward Message

The following table defines the use of message fields in a Relay- forward message.

  msg-type       RELAY-FORW

  hop-count      Number of relay agents that have relayed this
                 message.

  link-address   A global or site-local address that will be used by
                 the server to identify the link on which the client
                 is located.

  peer-address   The address of the client or relay agent from which
                 the message to be relayed was received.

  options        MUST include a "Relay Message option" (see
                 section 22.10); MAY include other options added by
                 the relay agent.

7.2. Relay-reply Message

The following table defines the use of message fields in a Relay-reply message.

  msg-type       RELAY-REPL

  hop-count      Copied from the Relay-forward message

  link-address   Copied from the Relay-forward message

  peer-address   Copied from the Relay-forward message

  options        MUST include a "Relay Message option"; see
                 section 22.10; MAY include other options