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What is CSeq Header Field In SIP? 

CSeq header field is a necessary header field in every request. It has a decimal number that increases for each request.

1. Regularly, it increases by 1 for each new request, with the exclusion of CANCEL and ACK requests, which make use of the CSeq number of the INVITE request to which it refers.

2. The CSeq count is used by UASs to decide out-of-sequence requests or to distinguish between new requests (different CSeq) or a retransmission (same CSeq).

3. The CSeq header field is used by UACs to correspond a response to the request it references.

4. For example, a UAC that sends an INVITE request then a CANCEL request can distinguish by the process in the CSeq of a 200 OK response if it is a response to the invitation or termination request.

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