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4.1. AVP Header

The fields in the AVP header MUST be sent in network byte order. The format of the header 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           AVP Code                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V M P r r r r r|                  AVP Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Vendor-ID (opt)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Data ...
   +-+-+-+-+-+-+-+-+

AVP Code​

The AVP Code, combined with the Vendor-Id field, identifies the attribute uniquely. AVP numbers 1 through 255 are reserved for reuse of RADIUS attributes, without setting the Vendor-Id field. AVP numbers 256 and above are used for Diameter, which are allocated by IANA (see Section 11.1.1).

AVP Flags​

The AVP Flags field informs the receiver how each attribute must be handled. New Diameter applications SHOULD NOT define additional AVP Flag bits. However, note that new Diameter applications MAY define additional bits within the AVP header, and an unrecognized bit SHOULD be considered an error. The sender of the AVP MUST set 'R' (reserved) bits to 0 and the receiver SHOULD ignore all 'R' (reserved) bits. The 'P' bit has been reserved for future usage of end-to-end security. At the time of writing, there are no end-to-end security mechanisms specified; therefore, the 'P' bit SHOULD be set to 0.

The 'M' bit, known as the Mandatory bit, indicates whether the receiver of the AVP MUST parse and understand the semantics of the AVP including its content. The receiving entity MUST return an appropriate error message if it receives an AVP that has the M-bit set but does not understand it. An exception applies when the AVP is embedded within a Grouped AVP. See Section 4.4 for details. Diameter relay and redirect agents MUST NOT reject messages with unrecognized AVPs.

The 'M' bit MUST be set according to the rules defined in the application specification that introduces or reuses this AVP. Within a given application, the M-bit setting for an AVP is defined either for all command types or for each command type.

AVPs with the 'M' bit cleared are informational only; a receiver that receives a message with such an AVP that is not supported, or whose value is not supported, MAY simply ignore the AVP.

The 'V' bit, known as the Vendor-Specific bit, indicates whether the optional Vendor-ID field is present in the AVP header. When set, the AVP Code belongs to the specific vendor code address space.

AVP Length​

The AVP Length field is three octets, and indicates the number of octets in this AVP including the AVP Code field, AVP Length field, AVP Flags field, Vendor-ID field (if present), and the AVP Data field. If a message is received with an invalid attribute length, the message MUST be rejected.

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