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

Christer Holmberg <> Wed, 19 October 2016 13:06 UTC

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From: Christer Holmberg <>
To: Alan Ford <>
Thread-Topic: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value
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Date: Wed, 19 Oct 2016 13:06:32 +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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>>>>>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
>>>>> ANY candidate).
>>>> That would certainly work in this scenario - different from the SCTP
>>>> text, but would permit this behaviour, whilst still providing clear
>>>> guidance.
>>> We would update the SCTP text too.
>>>> However, I fear that would go against the ICE spec; specifically, 5245
>>>> says:
>>>>  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
>>> is known whether the peers support ICE or not.
>>> In any case, I don¹t think there should be different rules for BFCF,
>>> etc. This should be defined in draft-ietf-mmusic-ice-sip-sdp as a
>>> rule.
>> 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
>> 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.
>That still implies they need to support the transport in question, so
>it’s not dissimilar to the SCTP text about default candidate.

The idea was that, if the answerer doesn’t support the transport in the m-
line of the offer, it would have to reject the m- line (as you pointed out

>Question really is - and this is probably something more for icebis than
>here - would it be legitimate to relax that requirement in the text I
>quoted earlier for the answerer? (The way it is written in 5245 suggests
>it applies to both offerer and answerer). I don’t see it being a problem
>for endpoints, but I’d be worried some proxies may expect behaviour here
>which isn’t true.

I DID send an e-mail to the MMUSIC list (I think it is more related to
SIP/SDP-usage of ICE than ICE in general) about this a few days ago, but
nobody has replied. Feel free to jump on the discussion :)

Obviously, if you e.g., include TCP in the m- line of the answer (because
the offer contained TCP), but you don’t actually support TCP, the m- line
port value would only be a “dummy value”.