Subject: [AVT] RTP - Last sequence number vs highest sequence number

From: "pieter.demuytere" <[email-masked]>
Date: Mon, 11 Jun 2007 15:33:42 +0200

Hi all,

In RFC 3550, the number of packets expected is defined to be the 
extended last sequence number received less the initial sequence number 
received. However, in the example code given in appendix A.1, the 
(extended) highest sequence number is used, and not the (extended) last 
sequence number. My feeling is that the last sequence number received is 
not automatically equal to the highest sequence number received, due to 
packet reordering. Is it allowed to use the (extended) highest sequence 
number for the calculation of the number of packets expected?

Thanks
Pieter Demuytere

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Stephen Casner <[email-masked]>
Date: Mon, 11 Jun 2007 08:08:50 -0700 (PDT)

On Mon, 11 Jun 2007, pieter.demuytere wrote:

> In RFC 3550, the number of packets expected is defined to be the extended last
> sequence number received less the initial sequence number received. However,
> in the example code given in appendix A.1, the (extended) highest sequence
> number is used, and not the (extended) last sequence number. My feeling is
> that the last sequence number received is not automatically equal to the
> highest sequence number received, due to packet reordering. Is it allowed to
> use the (extended) highest sequence number for the calculation of the number
> of packets expected?

This appears to be the result of incomplete editing on my part.  I had
not noticed the problem before your question, and it did not come up
during the revision from RFC1889 to RFC3550.  An earlier version of
the text may have used "last" everywhere, then to clarify exactly the
point you raise, the word "last" was changed to "highest", but not
everywhere.

If you'll notice, the definition of "cumulative number of packets
lost" says, in part:

      The number of packets expected is defined to be the extended
      last sequence number received, as defined next, less the initial
      sequence number received.  This may be calculated as shown in
      Appendix A.3.

The next definition is "extended highest sequence number received",
not "extended last sequence number received".  Thus, these phrases are
both intended to mean the same thing, i.e., the highest number.

Furthermore, Appendix A.3 calculates "extended_max", which is the
highest sequence number received.

                                                        -- Steve