RFC 3261, "SIP: Session Initiation Protocol", June 2002

Note: This RFC has been updated by RFC 3265, RFC 3853, RFC 4320, RFC 4916, RFC 5393, RFC 5621, RFC 5626, RFC 5630, RFC 5922, RFC 5954, RFC 6026, RFC 6141, RFC 6665, RFC 6878, RFC 7462, RFC 7463, RFC 8217, RFC 8591, RFC 8760, RFC 8898, RFC 8996

Errata ID: 6793
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Mojtaba Esfandiari.S
Date Reported: 2021-12-21

Section 8.1.1.5 says:

The CSeq header field serves as a way to identify and order
   transactions.  It consists of a sequence number and a method.  The
   method MUST match that of the request.  For non-REGISTER requests
   outside of a dialog, the sequence number value is arbitrary.  The
   sequence number value MUST be expressible as a 32-bit unsigned
   integer and MUST be less than 2**31.  As long as it follows the above
   guidelines, a client may use any mechanism it would like to select
   CSeq header field values.

It should say:

The CSeq header field serves as a way to identify and order
   transactions.  It consists of a sequence number and a method.  The
   method MUST match that of the request.  For non-REGISTER requests
   outside of a dialog, the sequence number value is arbitrary. For requests with its credentials after receiving a
   401 (Unauthorized) or 407 (Proxy Authentication Required) response,
  the CSeq header field value MUST increment. The
   sequence number value MUST be expressible as a 32-bit unsigned
   integer and MUST be less than 2**31.  As long as it follows the above
   guidelines, a client may use any mechanism it would like to select
   CSeq header field values.

Notes:

For more clarity and more understanding, It would be nice to have a quiet bit of update in RFC 3261 section 8.1.1.5. Because, the word "arbitrary" in the CSeq value may be confused with the same value as before.

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