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

Christer Holmberg <christer.holmberg@ericsson.com> Mon, 17 October 2016 08:56 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Alan Ford <alan.ford@gmail.com>
Thread-Topic: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value
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Date: Mon, 17 Oct 2016 08:55:25 +0000
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Cc: "bfcpbis@ietf.org" <bfcpbis@ietf.org>, "Charles Eckel \(eckelcu\)" <eckelcu@cisco.com>, Roman Shpount <roman@telurix.com>
Subject: Re: [bfcpbis] BFCPbis: UDP- and TCP candidates and proto value
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Hi,


>>> 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
>>>you¹ll 
>>> 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
>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 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
rule.

Regards,

Christer