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9. Relay Agent/Server Message Formats

This section preserves the RFC text for DHCPv6, including message exchanges, relay behavior, DUIDs, IA_NA, IA_TA, IA_PD, DHCP options, RKAP authentication, IANA registries, normative requirements, and appendix option-appearance matrices.

Original RFC Text​

9.  Relay Agent/Server Message Formats

   Relay agents exchange messages with other relay agents and 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-byte or 4-byte boundaries).

   There are two relay agent messages (Relay-forward and Relay-reply),
   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)   ....        .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Relay Agent/Server Message Format

   The following sections describe the use of the relay agent message
   header.

9.1.  Relay-forward Message

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

   msg-type:  RELAY-FORW (12).  A 1-octet field.

   hop-count:  Number of relay agents that have already relayed this
      message.  A 1-octet field.

   link-address:  An address that may be used by the server to identify
      the link on which the client is located.  This is typically a
      globally scoped unicast address (i.e., GUA or ULA), but see the
      discussion in Section 19.1.1.  A 16-octet field.

   peer-address:  The address of the client or relay agent from which
      the message to be relayed was received.  A 16-octet field.

   options:  MUST include a Relay Message option (see Section 21.10);
      MAY include other options, such as the Interface-Id option (see
      Section 21.18), added by the relay agent.  A variable-length field
      (34 octets less than the size of the message).

   See Section 13.1 for an explanation of how the link-address field is
   used.

9.2.  Relay-reply Message

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

   msg-type:  RELAY-REPL (13).  A 1-octet field.

   hop-count:  Copied from the Relay-forward message.  A 1-octet field.

   link-address:  Copied from the Relay-forward message.  A 16-octet
      field.

   peer-address:  Copied from the Relay-forward message.  A 16-octet
      field.

   options:  MUST include a Relay Message option (see Section 21.10);
      MAY include other options, such as the Interface-Id option (see
      Section 21.18).  A variable-length field (34 octets less than the
      size of the message).
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