Re: [rtcweb] JSEP: Why always offer actpass?

Roman Shpount <roman@telurix.com> Fri, 28 November 2014 14:34 UTC

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Date: Fri, 28 Nov 2014 09:34:41 -0500
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From: Roman Shpount <roman@telurix.com>
To: =?UTF-8?Q?Stefan_H=C3=A5kansson_LK?= <stefan.lk.hakansson@ericsson.com>
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Subject: Re: [rtcweb] JSEP: Why always offer actpass?
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On Fri, Nov 28, 2014 at 8:44 AM, Stefan Håkansson LK <
stefan.lk.hakansson@ericsson.com> wrote:

> In this context we're dealing only with DTLS-SRTP I guess.
>
> I read [5] as that subsequent offers should offer the negotiated role
> for existing connections (and not actpass), but I'm not sure. What is
> your interpretation?
>
>
I actually have a more generic question: In what circumstances would new
DTLS connection handshake be initiated if transport parameters and
fingerprint did not change?

I assume new negotiation is required if subsequent offer contains active or
passive role that does not match the current connection state. What about
actpass? Is getting an actpass in the subsequent offer should mean that the
roles did not change and existing connection should continue to be re-used
or does it mean new negotiation should be started?

Should DTLS-SRTP endpoints be able to handle new DTLS handshake at any
time, without any associated signaling exchange? I do not think any of the
current browser implementations support that, even though such support
would be required for DTLS-SRTP rekey. If DTLS handshake handling at any
time is required, then end-points should only be forced to do a new
handshake by signaling if their roles change. If one of the end-points
turns out to be overzealous and starts an unexpected handshake, it would be
handled no differently then DTLS rekey, as long as roles stayed the same.

There is also a consideration, that the newly generated offer is intended
to be send to a completely new end-point and not to the existing
connection. This would most likely be known in advance for webrtc end
points, since they will need to collect a new set of ICE candidates, but
this can affect SIP end-points.

Since we would probably want to avoid extra handshakes, and that we want to
accommodate both offers intended for the existing and the new destinations,
we can state that in the subsequent offers, the end point should include
the actpass setup attribute. In the answer, if DTLS connection is already
setup, the end point should respond with the currently negotiated role. If
an end-point receives an offer with the non actpass setup attribute, it
should only initiate a new DTLS handshake if the specified attribute does
not match the currently negotiated setup state.
_____________
Roman Shpount