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Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt

Christer Holmberg <christer.holmberg@ericsson.com> Wed, 24 February 2016 20:44 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>, Jonathan Lennox <jonathan@vidyo.com>, "Eric Rescorla (ekr@rtfm.com)" <ekr@rtfm.com>, "Martin Thomson (martin.thomson@gmail.com)" <martin.thomson@gmail.com>, "Justin Uberti (juberti@google.com)" <juberti@google.com>
Thread-Topic: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt
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Date: Wed, 24 Feb 2016 20:43:46 +0000
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Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt
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Adding Ekr, Martin and Justin, because I think they (or, at least some of them :) were behind the idea of allowing multiple fingerprints.

Regards,

Christer

-----Original Message-----
From: Paul Kyzivat [mailto:pkyzivat@alum.mit.edu] 
Sent: 24 February 2016 22:37
To: Jonathan Lennox <jonathan@vidyo.com>
Cc: Christer Holmberg <christer.holmberg@ericsson.com>; mmusic@ietf.org
Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt

Jonathan,

Can you (as the author of RFC4572) explain the use of multiple fingerprints? The dtls-sdp draft talks about the possibility of multiple fingerprints, but I can find no explanation of the semantics of that.

	Thanks,
	Paul

On 2/24/16 3:26 PM, Christer Holmberg wrote:

>>>> * Section 5.1:
>>>>
>>>>      The certificate received during the DTLS handshake MUST match the
>>>>      fingerprint received in the SDP "fingerprint" attribute.  If the
>>>>      fingerprint does not match the hashed certificate, then the endpoint
>>>>      MUST tear down the media session immediately.  ...
>>>>
>>>> This talks about *the* fingerprint. But, IIUC, multiple fingerprints may be supplied. What is the required processing in that case?
>>>
>>> We try to clarify that in section 3.4, which says:
>>>
>>>      "It is possible to associate multiple SDP fingerprint attribute values
>>>      to an 'm-' line.  If any of the attribute values associated with an
>>>      'm-' line are removed, or if any new attribute values are added, it
>>>      is considered a fingerprint value change."
>>
>> Right. But AFAIK that is the only place it is mentioned. At the least, most places that reference "the fingerprint"
>> should acknowledge that there may be more than one.
>
> I guess we could use "one or more SDP 'fingerprint' attributes" terminology instead of "an SDP 'fingerprint' attribute".
>
>> And *someplace* needs to say what the semantics of use are when there 
>> is more than one. (Maybe it is defined in one of the references?) Am 
>> I correct in assuming that providing multiple fingerprints is for the convenience of the recipient, who can then pick the one that it prefers to verify?
>
> I don't think the semantics are defined anywhere, unless it is defined in some RTCWEB document... I remember that it was agreed at some point that multiple fingerprints were allowed, but I am not sure whether anything was written down in a specificaiton...
>
> In my opinion, the semantics belong to RFC 4572.

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