Re: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value

Christer Holmberg <> Wed, 19 October 2016 12:08 UTC

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From: Christer Holmberg <>
To: Christer Holmberg <>, Alan Ford <>
Thread-Topic: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value
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Date: Wed, 19 Oct 2016 12:08:20 +0000
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Cc: "" <>, "Charles Eckel \(eckelcu\)" <>, Roman Shpount <>
Subject: Re: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value
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>>>>While the text is fine and probably closest to current behaviour, it
>>>>means we can¹t get the benefits from ICE if one end supported only one
>>>>of UDP or TCP.
>>>> Today if you offer only TCP or UDP as the proto value on the m-line,
>>>>and the far end doesn¹t, then the far end will zero-port the m-line and
>>>> either re-INVITE if you support the other, or just run without BFCP.
>>>>That same behaviour would continue with the proposed text.
>>>> However, the real benefit of ICE in this circumstance would allow the
>>>>offerer to offer TCP
>>>> and UDP candidates under a TCP m-line, but the answerer, if they
>>>>supported only UDP, could then only offer back only UDP
>>>> candidates (whilst matching the offerer¹s proto). But given the text
>>>>requires a default candidate to be present matching the proto on the
>>>>m-line, then if the answerer did not support TCP, they would have to
>>>>zero-port the line.
>>>> However, I would assume that breaking the semantics of the offered
>>>>proto field is probably not a good thing, and defining a
>>>>transport-agnostic proto isn¹t practical, so borrowing the proposed
>>>>text (with the photo -> proto typo fixed) is probably
>>>> sufficient at this stage. Unless anyone can think of a neater way of
>>>>solving this?
>>> I guess one solution would be to allow the answerer to use a m- line
>>>proto value that does NOT match the default candidate (or, doesn¹t match
>>>ANY candidate).
>> That would certainly work in this scenario - different from the SCTP
>>text, but would permit this behaviour, whilst still providing clear
>We would update the SCTP text too.
>> However, I fear that would go against the ICE spec; specifically, 5245
>>   The transport addresses that will be the default destination for
>>   media when communicating with non-ICE peers MUST also be present as
>>   candidates in one or more a=candidate lines.
>> So we¹d no longer be adhering to that in the answer.
>The text is certainly valid for the offer, but when the answer is sent it
>is known whether the peers support ICE or not.
>In any case, I don¹t think there should be different rules for BFCF, SCTP
>etc. This should be defined in draft-ietf-mmusic-ice-sip-sdp as a generic

I was thinking a little more about this: maybe indicating a transport in
the m- line that you don’t support isn’t a very good idea - even if it
won’t be used with ICE.

Maybe it would be better to say that the m- line shall contain a transport
that the peer is “most likely” to support. In case of BFCP, I guess
neither TCP or UDP is mandatory to support, but in other cases there is
often a mandatory transport.