Why does addTransceiver() send SDP a=recvonly causing an extra signalling round-trip?

[Richard Chan]

MDN have recently updated the WebRTC demo webrtc-from-chat
https://github.com/mdn/samples-server/commits/master
to use addTransceiver. What is observed is that call setup requires an
extra roundtrip because the
answer SDP contains a=recvonly

Caller: video-offer
Callee: video-answer with a=recvonly
--- call not connected ---
Callee: onnegotiationneeded is fired
Callee: video-offer
Caller: video-answer
--- call is connected

Using addTrack() the flow is much simpler

Caller: video-offer
Callee: video-answer
--- call is connected ---

Is this now the canonical way to do call setup in WebRTC 1.0 with
transceivers?
Is there anyway to avoid the extra signalling round-trip?

[Byron Campen]

   This is a rough part of the RTCRTPTransceiver specification. The
rules in the spec say that a transceiver created at the answerer does
not "pair up" with anything in an offer, _unless_ that transceiver was
created with addTrack:

https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.10

*snip*

+ If the m= section is sendrecv or recvonly, and there are
RtpTransceivers of the same type that were added to the
PeerConnection by addTrack and are not associated with any
m= section and are not stopped, find the first (according to
the canonical order described inSection 5.2.1 <https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-26#section-5.2.1>) such
RtpTransceiver.

+ If no RtpTransceiver was found in the previous step, create
one with a recvonly direction.
*snip*


Best regards,
Byron Campen

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[SPChan]

Thanks -that explains it!

Do you know of any good guides on when to use addTrack() vs addTransceiver()?

[Byron Campen]

I recommend using addTrack at all times unless you care about the
particulars of how send/receive tracks are paired up, or unless you have to
set the sendEncodings on the transceiver (eg; you want to use simulcast).

Best regards,
Byron Campen