sd-dhcp6: Introduce vendor specific information

sd-dhcp6: Introduce vendor specific information

RFC: 8415
21.17.  Vendor-specific Information Option

   This option is used by clients and servers to exchange vendor-
   specific information.

   The format of the Vendor-specific Information option is:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      OPTION_VENDOR_OPTS       |           option-len          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       enterprise-number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                       vendor-option-data                      .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 30: Vendor-specific Information Option Format

      option-code          OPTION_VENDOR_OPTS (17).

      option-len           4 + length of vendor-option-data field.

      enterprise-number    The vendor's registered Enterprise Number as
                           maintained by IANA [IANA-PEN].  A 4-octet
                           field containing an unsigned integer.

      vendor-option-data   Vendor options, interpreted by
                           vendor-specific code on the clients and
                           servers.  A variable-length field (4 octets
                           less than the value in the option-len field).

 The definition of the information carried in this option is vendor
   specific.  The vendor is indicated in the enterprise-number field.
   Use of vendor-specific information allows enhanced operation,
   utilizing additional features in a vendor's DHCP implementation.  A
   DHCP client that does not receive requested vendor-specific
   information will still configure the node's IPv6 stack to be
   functional.

   The vendor-option-data field MUST be encoded as a sequence of
   code/length/value fields of format identical to the DHCP options (see
   Section 21.1).  The sub-option codes are defined by the vendor
   identified in the enterprise-number field and are not managed by
   IANA.  Each of the sub-options is formatted as follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          sub-opt-code         |         sub-option-len        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                        sub-option-data                        .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 31: Vendor-specific Options Format

      sub-opt-code         The code for the sub-option.  A 2-octet
                           field.

      sub-option-len       An unsigned integer giving the length of the
                           sub-option-data field in this sub-option in
                           octets.  A 2-octet field.

      sub-option-data      The data area for the sub-option.  The
                           length, in octets, is specified by
                           sub-option-len.

   Multiple instances of the Vendor-specific Information option may
   appear in a DHCP message.  Each instance of the option is interpreted
   according to the option codes defined by the vendor identified by the
   Enterprise Number in that option.  Servers and clients MUST NOT send
   more than one instance of the Vendor-specific Information option with
   the same Enterprise Number.  Each instance of the Vendor-specific
   Information option MAY contain multiple sub-options.

A client that is interested in receiving a Vendor-specific
   Information option:

   -  MUST specify the Vendor-specific Information option in an Option
      Request option.

   -  MAY specify an associated Vendor Class option (see Section 21.16).

   -  MAY specify the Vendor-specific Information option with
      appropriate data.

   Servers only return the Vendor-specific Information options if
   specified in Option Request options from clients and:

   -  MAY use the Enterprise Numbers in the associated Vendor Class
      options to restrict the set of Enterprise Numbers in the
      Vendor-specific Information options returned.

   -  MAY return all configured Vendor-specific Information options.

   -  MAY use other information in the packet or in its configuration to
      determine which set of Enterprise Numbers in the Vendor-specific
      Information options to return.