Roman, Bernard, right, RFC 5764 is too vague on that aspect. Thanks for confirming the number of DTLS sessions, which is inline with our understanding. Would appreciate if this could be somewhere fixed in an rtcweb draft due to significant side effects. This topic is also an ongoing FAQ.
The most simple case is given by a scenario with usage of bundling plus usage of RTP/RTCP transport multiplexing, leading to a single DTLS session/DTLS connection, which could be then also shared for WebRTC data. Both capabilities are mandated in the rtp usage draft: https://tools.ietf.org/html/draft-ietf-rtcweb-rtp-usage-25#section-4.4 => bundling https://tools.ietf.org/html/draft-ietf-rtcweb-rtp-usage-25#section-4.5 => RTP/RTCP transport multiplexing Now, IF bundling is not used OR RTP/RTCP transport multiplexing is not used THEN there will be more than one DTLS session/DTLS connection (either 2 or 4 in case of audio & video). Raising the question which DTLS connection is used for additional WebRTC data traffic? - Because https://tools.ietf.org/html/draft-ietf-rtcweb-transports-09#section-3.5 indicates the sharing option. Would then be a 3rd (or 5th) self-contained DTLS session/DTLS connection for WebRTC data traffic? Proposal: Add explicit text to clause https://tools.ietf.org/html/draft-ietf-rtcweb-transports-09#section-3.5 about (in red), something like: WebRTC implementations MUST support multiplexing of DTLS and RTP over the same port pair, as described in the DTLS-SRTP specification [RFC5764], section 5.1.2<https://tools.ietf.org/html/rfc5764#section-5.1.2>. All application layer protocol payloads over this DTLS connection are SCTP packets. Note 1: There will be two DTLS sessions/DTLS connections when RTP/RTCP transport multiplexing is not applied. WebRTC data traffic could still share one of these DTLS connections … (“which one?”) … or There should be then a separate, self-contained DTLS session/DTLS connection established exclusively for WebRTC data. Note 2: There are similar considerations in case of bundling. Protocol identification MUST be supplied as part of the DTLS handshake, as specified in [I-D.ietf-rtcweb-alpn<https://tools.ietf.org/html/draft-ietf-rtcweb-transports-09#ref-I-D.ietf-rtcweb-alpn>]. Comments? Regards, Albrecht PS Using (D)TLS terminology according to http://tools.ietf.org/id/draft-guballa-tls-terminology-01.txt From: Bernard Aboba [mailto:bernard.ab...@gmail.com] Sent: Mittwoch, 5. August 2015 04:04 To: Roman Shpount Cc: Asveren, Tolga; Christer Holmberg; Eric Rescorla; Schwarz, Albrecht (Albrecht); Rauschenbach, Uwe (Nokia - DE/Munich); <rtc...@ietf.org> Subject: Re: [rtcweb] What the gateway draft should say about mux/non-mux On Aug 4, 2015, at 16:33, Roman Shpount <ro...@telurix.com<mailto:ro...@telurix.com>> wrote: Most of the people implement this wrong, since you need to create two DTLS sessions: one for RTP and another for RTCP. And then use different keys or possibly even encryption profiles for RTP and RTCP. I commonly see one session for RTP and keys negotiated there used for both RTP and RTCP, which is wrong. [BA] Yes, that is only one of several common mistakes. Unfortunately, RFC 5764 does not rule out all of these and the security documents are not crystal clear on how things must be done. It is much harder to mess up RTP/RTCP mux. Given what I've seen so far, non-mux could become a support nightmare.
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