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

From: Euchner Martin ICN M SR 3 <[email-masked]>
Date: Thu, 23 May 2002 10:19:15 +0200

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?

With kind regards

Martin Euchner.
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From: "David A. Mcgrew" <[email-masked]>
Date: Tue, 28 May 2002 05:37:50 -0700

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
>
>
>

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