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3. Diameter Header

3. Diameter Header​

A summary of the Diameter header format is shown below. The fields are transmitted in network byte order.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |                 Message Length                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Command Flags |                  Command Code                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Application-ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Hop-by-Hop Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      End-to-End Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  AVPs ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

Version

This Version field MUST be set to 1 to indicate Diameter Version 1.

Message Length

The Message Length field is three octets and indicates the length of the Diameter message including the header fields and the padded AVPs. Thus, the Message Length field is always a multiple of 4.

Command Flags

The Command Flags field is eight bits. The following bits are assigned:

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |R P E T r r r r|
     +-+-+-+-+-+-+-+-+

R(equest)

If set, the message is a request. If cleared, the message is an answer.

P(roxiable)

If set, the message MAY be proxied, relayed, or redirected. If cleared, the message MUST be locally processed.

E(rror)

If set, the message contains a protocol error, and the message will not conform to the CCF described for this command. Messages with the 'E' bit set are commonly referred to as error messages. This bit MUST NOT be set in request messages (see Section 7.2).

T(Potentially retransmitted message)

This flag is set after a link failover procedure, to aid the removal of duplicate requests. It is set when resending requests not yet acknowledged, as an indication of a possible duplicate due to a link failure. This bit MUST be cleared when sending a request for the first time; otherwise, the sender MUST set this flag. Diameter agents only need to be concerned about the number of requests they send based on a single received request; retransmissions by other entities need not be tracked. Diameter agents that receive a request with the T flag set, MUST keep the T flag set in the forwarded request. This flag MUST NOT be set if an error answer message (e.g., a protocol error) has been received for the earlier message. It can be set only in cases where no answer has been received from the server for a request, and the request has been sent again. This flag MUST NOT be set in answer messages.

r(eserved)

These flag bits are reserved for future use; they MUST be set to zero and ignored by the receiver.

Command Code

The Command Code field is three octets and is used in order to communicate the command associated with the message. The 24-bit address space is managed by IANA (see Section 3.1). Command Code values 16,777,214 and 16,777,215 (hexadecimal values FFFFFE-FFFFFF) are reserved for experimental use (see Section 11.2).

Application-ID

Application-ID is four octets and is used to identify for which application the message is applicable. The application can be an authentication application, an accounting application, or a vendor-specific application.

The value of the Application-ID field in the header MUST be the same as any relevant Application-Id AVPs contained in the message.

Hop-by-Hop Identifier

The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in network byte order) that aids in matching requests and replies. The sender MUST ensure that the Hop-by-Hop Identifier in a request is unique on a given connection at any given time, and it MAY attempt to ensure that the number is unique across reboots. The sender of an answer message MUST ensure that the Hop-by-Hop Identifier field contains the same value that was found in the corresponding request. The Hop-by-Hop Identifier is normally a monotonically increasing number, whose start value was randomly generated. An answer message that is received with an unknown Hop-by-Hop Identifier MUST be discarded.

End-to-End Identifier

The End-to-End Identifier is an unsigned 32-bit integer field (in network byte order) that is used to detect duplicate messages. Upon reboot, implementations MAY set the high order 12 bits to contain the low order 12 bits of current time, and the low order 20 bits to a random value. Senders of request messages MUST insert a unique identifier on each message. The identifier MUST remain locally unique for a period of at least 4 minutes, even across reboots. The originator of an answer message MUST ensure that the End-to-End Identifier field contains the same value that was found in the corresponding request. The End-to-End Identifier MUST NOT be modified by Diameter agents of any kind. The combination of the Origin-Host AVP (Section 6.3) and this field is used to detect duplicates. Duplicate requests SHOULD cause the same answer to be transmitted (modulo the Hop-by-Hop Identifier field and any routing AVPs that may be present), and they MUST NOT affect any state that was set when the original request was processed. Duplicate answer messages that are to be locally consumed (see Section 6.2) SHOULD be silently discarded.

AVPs

AVPs are a method of encapsulating information relevant to the Diameter message. See Section 4 for more information on AVPs.

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