Expected behavior when receiving media packets from unannounced SSRCs

[Jeremy Ong]

Hi discuss-webrtc,

I was curious about the expected client side behavior of receiving multiple media packets on a single PeerConnection tagged with different SSRCs if the offering SDP didn't advertise any SSRC attributes. Should the client mix the packets and respect the SSRCs as individual sources? Should the client ignore the SSRC field altogether and try to mash all the frames on a single track? If the latter, what is the correct approach of having the client do the former? Do the SSRCs need to be known ahead of time or renegotiated constantly?

Thanks,
Jeremy

[Alexandre GOUAILLARD]

there are two cases here:

1. the O/A cycle has been completed, and suddenly packets with an unannounced SSRC is received at the IP:port assigned to an existing peer connection. My understanding is that you should drop the packets.

2. The offer has been sent, but the caller hasn't received the answer yet. suddenly packet start flowing to the caller. This is called "Early media" and was added to the webrtc specs in november 2015. I do not know what the browser support is today. You can create a transceiver to handle early media. There are some subtleties about encryption and codec settings, but you can refer to the specs for the details.

https://webrtcstandards.info/webrtc-early-media/

I do not think webrtc ever require you to MIX media. You can do it at the application level, or within a media server if you want though.

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Alex. Gouaillard, PhD, PhD, MBA

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Principal Architect - Citrix, San Francisco

President - CoSMo Software Consulting, Singapore

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[Simon Perreault]

Le 2016-08-11 à 05:44, Alexandre GOUAILLARD a écrit :
> 1. the O/A cycle has been completed, and suddenly packets with an
> unannounced SSRC is received at the IP:port assigned to an existing peer
> connection. My understanding is that you should drop the packets.

This suggestion goes against the specs. See this excerpt from
https://tools.ietf.org/html/draft-ietf-rtcweb-rtp-usage-26:

While support for signalled SSRC identifiers is mandated, their use
in an RTP session is OPTIONAL. Implementations MUST be prepared to
accept RTP and RTCP packets using SSRCs that have not been explicitly
signalled ahead of time.

Simon

[Alexandre GOUAILLARD]

I stand corrected. thanks.

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