Subject: RE: [AVT] WG last call on draft-ietf-avt-srtp-04.txt - choice of initial sequence number

From: Euchner Martin ICN M SR 3 <[email-masked]>
Date: Wed, 29 May 2002 11:26:09 +0200

David,

thanks for the reply.

As Elisabetta already answered and as I understand, the algorithm for
handling the sequence number at receiver side is a local matter and there
are certainly more intelligent procedures possible than the simple one given
in the SRTP spec. So I do not see any necessity for a change request in that
area.

However, it is certainly useful to point these matters out such that the
reader understands, that the shown algorithm is merely an example, is not
perfectly robust etc. and leaves plenty opportunities of improvements....

On the other side, it is certainly not a bad idea to provide an improved
estimation algorithm as an example, especially since the SRTP implementation
features one and has considered these issues. If it wouldn't involve too
many changes to the example, you may want to reflect the improved estimation
algorithm in the spec.

Finally, I agree that the sequence number should not be fixed, even that
might make things easier.

With kind regards

Martin Euchner.
-----------------------------------------------------------------------
| Dipl.-Inf.                     Rapporteur Q.G/SG16
| Martin Euchner                 Phone: +49 89 722 55790
| Siemens AG.....................Fax  : +49 89 722 47713
| ICN M SR 3                     mailto:[email-masked]
|                                mailto:[email-masked]
| Hofmannstr. 51                 Intranet:
http://intranet.icn.siemens.de/marketing/cs27/topics/security/
| D-81359 Muenchen               Internet: http://www.siemens.de/
| __________________
| Germany     
-----------------------------------------------------------------------

 -----Original Message-----
From: 	David A. Mcgrew [mailto:[email-masked]] 
Sent:	Tuesday, May 28, 2002 2:38 PM
To:	Euchner Martin  ICN M SR 3; [email-masked]; [email-masked];
[email-masked]; [email-masked];
[email-masked]; [email-masked]
Subject:	RE: [AVT] WG last call on draft-ietf-avt-srtp-04.txt -
choice of initial sequence number

Martin,

good comment, more below.

> -----Original Message-----
> From: Euchner Martin ICN M SR 3 [mailto:[email-masked]]
> Sent: Thursday, May 23, 2002 1:19 AM
> To: Euchner Martin ICN M SR 3; [email-masked]'; [email-masked]';
> [email-masked]'; [email-masked]';
> [email-masked]'; [email-masked]';
> [email-masked]'
> Subject: [AVT] WG last call on draft-ietf-avt-srtp-04.txt - choice of
> initial sequence number
>
>
>
> SRTP allows to use arbitrary initial sequence number from the
> 16-bit range.
> It has been observed that not each potential initial sequence
> number may be
> equally well suited. For example, there is concern that the choice of an
> initial sequence number that would immediately wrap around with the next
> packet, might lead to problems for the receiver in estimating the sequence
> number, especially on error prone links where is initial RTP packet gets
> lost. Thus, it is probably better (more reliable) to choose an initial
> sequence number that is a certain distance away from the
> wrap-around point.
>
> Has this been taken already into consideration?

yes.  It is possible to correctly estimate the packet index even when the
initial RTP sequence number is chosen at random.  This can be done by adding
logic to the index guessing algorithm to handle the initial packet
differently.  This logic is done properly in the rdbx_estimate_index()
function in the srtp reference implementation online at
http://srtp.sourceforge.net/srtp.html  Do you think that it would be useful
to add the initial-case logic to Appendix A of the SRTP draft?

The SRTP packet processing could be slightly simpler if we could assume that
the initial RTP sequence number were deterministic.  However, it's a major
goal of SRTP to work with existing RTP implementations, so it makes more
sense to just add the logic to SRTP.

Sorry for the late reply, I was at the beach last week :-)

David

>
>
>
> With kind regards
>
> Martin Euchner.
> -----------------------------------------------------------------------
> | Dipl.-Inf.                     Rapporteur Q.G/SG16
> | Martin Euchner                 Phone: +49 89 722 55790
> | Siemens AG.....................Fax  : +49 89 722 47713
> | ICN M SR 3                     mailto:[email-masked]
> |                                mailto:[email-masked]
> | Hofmannstr. 51                 Intranet:
> http://intranet.icn.siemens.de/marketing/cs27/topics/security/
> | D-81359 Muenchen               Internet: http://www.siemens.de/
> | __________________
> | Germany
> -----------------------------------------------------------------------
>

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