[Sip-implementors] sip fields

[vishal]

Hi , according to sip ,some fields of sip message can be modified by proxies ,redirect servers and registrar for ex. via,max-forward ,route etc but some fields are never modified in sip message for ex. call id,content-length,content-type etc. we can classify sip message into following categories 1. read/write fields(these can be modified by proxy,redirect servers) 2. read only (these can never be modified by intermediate entities ,only UAS,UAC can modify these) 3. Message body READ/WRITE fileds SHOULD always be at starting of SIP message . SIP should discourage use of intermingled read/write & read only fields . call-id field SHOULD act as separation between read/write and read only fields. Advantages: if read/write fields are in starting of sip message then proxy dont have to search for particular field till end of message. It saves processing time at proxies and improves call setup time as well. It also reduces risk of message curruption . for ex: following sip message INVITE sip:[EMAIL PROTECTED] SIP/2.0 Via: SIP/2.0/UDP home.domain.com:5060;branch=fffcdr45.1 (read/write) Via: SIP/2.0/UDP lab.domain.com:5060;branch=ghf456.1 (read/write) From: USERB<[EMAIL PROTECTED]>;tag=89349 To: USERA<[EMAIL PROTECTED]> contact: <[EMAIL PROTECTED]> (read/write) Max-forward: 70 (read/write) Call-id: [EMAIL PROTECTED] Content-type: application/sdp content-length: 150 (sdp not shown here )   CAN BE WRITTEN AS   INVITE sip:[EMAIL PROTECTED] SIP/2.0 Via: SIP/2.0/UDP home.domain.com:5060;branch=fffcdr45.1 (read/write) Via: SIP/2.0/UDP lab.domain.com:5060;branch=ghf456.1 (read/write) contact: <[EMAIL PROTECTED]> (read/write) Max-forward: 70 (read/write) Call-id: [EMAIL PROTECTED] (read only) From: USERB<[EMAIL PROTECTED]>;tag=89349 (read only ) To: USERA<[EMAIL PROTECTED]> (read only, can be modified by UAS) Content-type: application/sdp (read only ) content-length: 150 (read only ) (sdp not shown here ) any comments are welcome , regards, vishal Vishal , Knowledge Systems Pvt Ltd  ,INDIA 470, East End Main Road  9th Block Jayanagar , Bangalore 560 069  Telephone: +91-80-6545252