On Fri, 2010-01-29 at 19:30 +0900, OKUMURA Shinji wrote: > Consider the following scenario. > > | INVITE(CSeq:1) | > |--------------------------->| > | | > | 18x | > |<---------------------------| > | | > | PRACK(CSeq:2) | > |--------------------------->| > | | > | 200 OK(PRACK) | > |<---------------------------| > | | > | 415 | > |<---------------------------| > | | > | ACK | > |--------------------------->| > | | > | INVITE(CSeq:2) | > |--------------------------->| > | with the same Call-ID | > | | > > > according to the section 8.1.3.5 in RFC3261, 2nd INVITE > "SHOULD have the same value of the > Call-ID, To, and From of the previous request, but the CSeq should > contain a new sequence number that is one higher than the previous." > > therefore, I think the CSeq of 2nd INVITE SHOULD be "2".
To be exact, the CSeq of the 2nd INVITE MUST be 2 or higher, and SHOULD be 2. The analysis is messy, and the rules that are needed to make SIP work are not clearly stated in RFC 3261. The principle is that the initial INVITE is part of a "preliminary dialog", that is, the sequence of messages with the given Call-Id and from-tag, but no to-tag. Each early (or established) dialog, that is, each sequence of messages with the given Call-Id and from-tag but with a specific to-tag, *continues* the preliminary dialog, from the point of the INVITE that was responded to, but the early/established dialogs are *independent of each other*. Think of them as branches growing out of the trunk of a tree. This interpretation is the only way that allows the (potentially many) UASs to not interfere with each other or with any needed retries of the INVITE. So the 1st and 2nd INVITEs are in the preliminary dialog, and the PRACK is in the early dialog created by the 18x response to the 1st INVITE. Since the 2nd INVITE and the PRACK are not within the same preliminary/early/confirmed dialog, they take their CSeq's from different counters. They both have CSeq 2, because for both of them, the preceding request is the 1st INVITE. Dale _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
