SIP CSeq for INFO and INVITE Methods

Question

Consider this sample SIP dialog

    A-->--INVITE-->--B CSeq 101
    A--<--TRYING--<--B CSeq 101
    A--<--200 OK--<--B CSeq 101
    A-->-- ACK  -->--B CSeq 101
    A-->-- INFO -->--B CSeq 2
    A--<-- 500  --<--B CSeq 2
    ...

When working with SIP processing code, we put the validation for the CSeq SIP INFO message so that the dialogue is larger than the one sent for INVITE. However, as shown in the SIP stream above, one of the remote SIP gateways sends it below, i.e. 2 instead of the expected 102 or higher.

RFC https://www.ietf.org/rfc/rfc3261.txt states that

The queries in the dialog box MUST contain strictly monotonically increasing and adjacent consecutive CSeq numbers (increasing by one) in each direction

So, is the observed behavior a violation of the RFC?

Answer 1

, . .

RFC SIP INFO CSeq, RFC3261:

Info Package    . Info CSeq [RFC3261]     , INFO .

, CSeq-, , (https://tools.ietf.org/html/rfc3261#section-12.2.2):

    CSeq ,     . , UAS     CSeq ,     , . UAS         CSeq .

- , UAC, UAC          .          CSeq. UAS         , , , CSeq.          .