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

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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