Extra m=audio line in Chrome generated SDP

[Nick Tindall]

Hi All,

I'm noticing that when I create an offer in Chrome after creating ONLY a data channel I get an m=audio line in the local description SDP.

I could be wrong, but it looks like due to the declared audio channel chrome is producing more ICE candidates (i.e. one of each type for each channel) as some say "audio" and others say "data".

The only reason this concerns me is the added overhead of allocating ports on the TURN server to accommodate these audio channels that weren't created and will never be used.

My code basically goes something like this...

var conn = new RTCPeerConnection()

var chan = conn.createDataChannel("blah")

conn.createOffer(...)

As I'm only interested in establishing a single data connection. Typical SDPs for Chrome (35) and Firefox (30) are included below.

As you can see the firefox SDP only contains a single m=application (I'm assuming that's the data channel) but the Chrome SDP also contains an audio channel.

It's not a massive concern, although if I'm right about the extra ICE candidates generated (i.e. extra allocations on the TURN server) it seems to be an unnecessary overhead.

Sorry if this has been asked before, I searched and couldn't see anything. Any advice is sincerely appreciated. 

Cheers,

Nick T

Chrome SDP

v=0

o=- 2389257510285150519 2 IN IP4 127.0.0.1

s=-

t=0 0

a=group:BUNDLE audio data

a=msid-semantic: WMS

m=audio 1 RTP/SAVPF 111 103 104 0 8 106 105 13 126

c=IN IP4 0.0.0.0.

a=rtcp:1 IN IP4 0.0.0.0

a=ice-ufrag:UkxA/lEciQHSuacs

a=ice-pwd:EPAm/cICAn1iYAXeY1pnFUzs

a=ice-options:google-ice

a=fingerprint:sha-256 95:C2:CF:37:88:1D:EE:A7:B3:D5:4D:E2:62:3F:21:0A:D1:DD:99:2E:32:83:FD:AD:94:BF:8E:EC:19:9D:E6:D7

a=setup:actpass

a=mid:audio

a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level

a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

a=recvonly

a=rtcp-mux

a=rtpmap:111 opus/48000/2

a=fmtp:111 minptime=10

a=rtpmap:103 ISAC/16000

a=rtpmap:104 ISAC/32000

a=rtpmap:0 PCMU/8000

a=rtpmap:8 PCMA/8000

a=rtpmap:106 CN/32000

a=rtpmap:105 CN/16000

a=rtpmap:13 CN/8000

a=rtpmap:126 telephone-event/8000

a=maxptime:60

m=application 1 DTLS/SCTP 5000

c=IN IP4 0.0.0.0

a=ice-ufrag:UkxA/lEciQHSuacs

a=ice-pwd:EPAm/cICAn1iYAXeY1pnFUzs

a=ice-options:google-ice

a=fingerprint:sha-256 95:C2:CF:37:88:1D:EE:A7:B3:D5:4D:E2:62:3F:21:0A:D1:DD:99:2E:32:83:FD:AD:94:BF:8E:EC:19:9D:E6:D7

a=setup:actpass

a=mid:data

a=sctpmap:5000 webrtc-datachannel 1024

Firefox SDP

v=0

o=Mozilla-SIPUA-30.0 1679 0 IN IP4 0.0.0.0 

s=SIP Call 

t=0 0 

a=ice-ufrag:d13f90f9 

a=ice-pwd:0cb536eaff1e1763efc904afd900e96f 

a=fingerprint:sha-256 1D:1F:C2:97:65:65:73:87:17:2E:96:D6:0E:F5:6D:94:9B:03:98:9C:8C:6A:21:D6:FE:15:FE:FA:FF:16:04:2C

m=application 51367 DTLS/SCTP 5000 

c=IN IP4 192.168.1.2 

a=sctpmap:5000 webrtc-datachannel 16 

a=setup:actpass 

a=candidate:0 1 UDP 2122252543 192.168.1.2 51367 typ host

[Philipp Hancke]

setting the createOffer constraints to {OfferToReceiveAudio: false} should get rid of that -- it defaults to true in chrome.

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[Justin Uberti]

Can you file a bug to clean that up? Shouldn't be the case.

[Philipp Hancke]

already did a while ago, issue #2108

[Justin Uberti]

That is a separate issue. This is about the default if OfferToCreateAudio is not specified (and hopefully much simpler)

[Vikas]

Filed issue 3508 to track.

/Vikas

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[Nick Tindall]

Thanks guys!

[hao wu]

do you sure your IP adress is right?