Diameter Message Structure and Message Flow

Diameter is Message (Packet) based protocol. There are two types of messages Request Messages and Answer Messages.And the Message structure is of following sort.

Diameter Message Structure

 

Version

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

Message Length 

Contain the length of Message Header + (Data) Avp

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, This bit MUST NOT be set in request messages.

T(Potentially re-transmitted message) - This flag is set after a link Failover procedures to aid the removal of duplicate requests. 
It is set when resending a request that 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 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 is sent again. This flag MUST NOT be set in answer messages.

For instance

Re-Transmission

Client triggers a request to server and at that instance, suppose server was down, so client does not receive the  answer/acknowledgement for that message then after a certain period of time the client will trigger the same request with bit-T=1 and this is called the retransmission. 

if client receive the error-Answer-message then client will not trigger the same request message with bit-T=0,because error-Answer-messgae it self the answer-message or acknowledgemet for that request.

r(eserved) - these flag bits are reserved for future use, and MUST be set to zero, and ignored by the receiver.

Command-Code

To uniquely identify the each diameter message IANA has assigned the unique command code for each Request. The answer of this Request will also have the same command code but have Bit-R=0.The Command-Code field is three octets.

For Example :Command code for  User-Authorization-Request is 300, 301 for Server-Assignment-Request  and  280  for Device-Watchdog-Request etc.

Application-ID

As we know that Base Diameter Protocol is the extensible protocol, so many application/interfaces are built by using the Base Diameter Protocol, Therefore to uniquely identify the each application IANA gives the Application-Id to each application.

There is also an Application-ID Avp, so in one application these two field (Application-ID Avp and Application-ID in structure) will contain the same value. Application-ID is four octets.

For Example : Application Id for S6a/s6d interface is 16777251 and for S13 is 16777252

Hop-by-Hop Identifier

The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in network byte order) and 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 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.Use of Hop-By-Hop Identifier is explain below with figure.

End-to-End Identifier

The End-to-End Identifier is an unsigned 32-bit integer field (in network byte order) and is used to detect duplicate messages. 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.  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 MUST NOT affect any state that was set when the original request was processed. Duplicate answer messages that are to be locally consumed  SHOULD be silently discarded.Use of  End-To-End Identifier is  explained below with Example.

Message Flow Explained

Hop-By-Hop Identifier =H-Id

End-To-End Identifier =E-Id

Message Flow among Diameter Agents

Every Dia-Agent contains the map for each message it receives and forwards to other node, so that answer for any request will come then answer will be sent to proper host(that generated the request). Map shown above is just for understanding purpose.

--> Whenever Diameter agent receives a message(Req/Ans) it matches the End-To-End Identifier.

     If message is Request and received End-To-End Identifier is not in the Table then it will insert the new entry in the table and treat the message as new message. 

     In case of Answer End-To-End Identifier shall not be inserted in TABLE if it is not present because answer shall follow the route followed by request(implies mis-routed message).

--> if End-To-End Id exists in Table then it will check for bit-T, if Bit-t=1 and R=1 that means message is retransmitted so it retransmit the message to the next host.

--> if it receives the message with t=0 and r=0 and End-To-End Id exists in Table i.e. Answer message corresponding to the Request received earlier then it will insert the Proper hop-by-hop Id as shown in figure to the Answer and forwards it and deletes the entry form the Table.

--> if it receives a message with  t=0 and r=1 and End-To-End Id exists in Table then it will treat the message as duplicate message of Looped-message(i.e. Route is looped/circular) and take the appropriate action that was configured during the deployment.

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