Subject: [AVT] Comments on draft-ietf-avt-dtls-srtp-00

From: "Elwell, John" <[email-masked]>
Date: Wed, 11 Jul 2007 14:16:46 +0100

1. Obviously the ideal situation is that symmetric RTP and multiplexed
RTP/RTCP are used, in which case only a single DTLS-SRTP is needed per
medium. Symmetric RTP is, I think, quite common now, but multiplexed
RTP/RTCP is not. I guess anyone implementing DTLS-SRTP could implement
multiplexed RTP/RTCP at the same time. However, I am curious to know why
it is necessary to have two DTLS-SRTP sessions when RTP and RTCP are on
different ports.

2. "In order to negotiate the use of SRTP data protection, clients MAY
   include an extension of type "use_srtp" in the extended client
hello."
Shouldn't the "MAY" be a "MUST", i.e., how can it be negotiated without
including this extension?

3. "Once the "use_srtp" extension is negotiated, packets of type
   "application_data" in the newly negotiated association (i.e., after
   the change_cipher_spec) MUST be protected using SRTP and packets of
   type "application_data" MUST NOT be sent."
This seems to be contradictory - it first says how to send such packets
and then says you must not send them. Is the latter half of the sentence
supposed to say that they must not use ordinary DTLS framing?

4. Figure 6 has a line missing - SRTP is shown only in one direction
(yet SRTCP is shown in both directions).

John

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