DIAMETER: Can someone please explain a diameter message ABNF format?

Question

I want to know about the significance of each symbol like [ ,{, < , *[ etc used to define a diameter message. Looking for a detailed description.

Answer 1

I believe you are looking for the ABNF description which is defined in the RFC 2234.

For the summary take the following example

<CER>::= < Diameter Header: 257, REQ >
        { Origin-Host }
        { Origin-Realm }
     1* { Host-IP-Address }
        { Vendor-Id }
        { Product-Name }
        [ Origin-State-Id ]
      * [ Supported-Vendor-Id ]
      * [ Auth-Application-Id ]
      * [ Inband-Security-Id ]
      * [ Acct-Application-Id ]
      * [ Vendor-Specific-Application-Id ]
        [ Firmware-Revision ]
      * [ AVP ]

where

[  - OPTIONAL SEQUENCE 
*  - means variable repetition 
n* - variable repetition but atleast n 
;  - comment 
<> - to define rule name or element name though not mandatory 

Comments

Is there any order constraints among the AVPs within the message? For example: AVP within {} should present first then AVP within [].

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Could not get it??

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<> is used to indicate fixed position. See < Session-Id > in the base command definitions. Typically you would expect to find mandatory fields ({}) before optional fields, but it is not a requirement.