SCTP and webrtc

Question

I saw that SCTP is going to be used for Data Channels starting in Chrome 31, and officially RTP based channels are going to be deprecated sometimes in February 2014 according to this google group thread:

https://groups.google.com/forum/#!topic/discuss-webrtc/y2A97iCByTU

Does this also mean that webrtc audio and video channels are also going to be transported over SCTP eventually(if not already in the works)?

How does RTP fit in the whole SCTP transport effort? Does that mean SRTP packets will flow over SCTP data channel? Or perhaps just the payload will be sent over SCTP transport protocol.

If I am reading info on SCTP, it combines best of TCP and UDP protocols; but it does not include encryption by default; so in mind the traffic flowing over should still be encrypted.

Any additional info would be helpful. Thanks!

Ora

Answer 1

The audio and video will keep going over RTP (actually SRTP which is the secure version or RTP) but the data channel will uses SCTP over DTLS over UDP. Both Firefox and Chrome are implementing this and you can find it described in the IETF draft specifications. For some types of NAT / Firewall traversal, the UDP packets may get tunneled in TURN over TCP.

Glad to provide you pointers to the draft specifications if that helps.

Meddle

If you could add the links to the draft specs that would he helpful. – Ora

Good starting points are tools.ietf.org/id/draft-ietf-rtcweb-rtp-usage and tools.ietf.org/html/draft-ietf-rtcweb-data-channel – Meddle

Is non-native usage of SCTP, i.e. over UDP, a part of specification, or just an implementation detail? – Cand

Answer 2

SCTP stands for Stream Control Transmission Protocol.

SCTP as a protocol can be seen as a hybrid of UDP and TCP.

At its core, SCTP holds the following characteristics:

Connection oriented. Similar to TCP, SCTP is connection oriented. It also offers a multi-homing capability that isn’t used by WebRTC Optional reliability. Reliability is optional in SCTP and is up to the implementer using SCTP to decide if he needs this capability or not Optional ordering. Ordering of packets sent via SCTP is optional and is left for the implementer to decide if this is necessary for him or not Message oriented. SCTP makes sure that each message sent is properly parsed on the receiver end in the same manner in which it was sent Flow control. Similar to TCP, SCTP provides a flow control mechanism that makes sure the network doesn’t get congested SCTP is not implemented by all operating systems. In such cases, an application level implementation of SCTP will usually be used.

SCTP is used in WebRTC for the implementation and delivery of the Data Channel.

Google is experimenting with the QUIC protocol as a future replacement to SCTP

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