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

Paul Kyzivat <> Wed, 24 February 2016 20:48 UTC

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Subject: Re: [MMUSIC] WGLC on draft-ietf-mmusic-dtls-sdp-06.txt
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On 2/24/16 3:26 PM, Christer Holmberg wrote:
> Hi,
> ...
>   --------------------
>>>> * 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".

That would work. But maybe this should wait for clarification on the use 
of multiple fingerprints.

>> 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.

I sent a separate reply on this, asking Jonathan to comment.

> --------------------
>>>> * Section 5.3:
>>>> If the offer specified 'existing' but the answerer changes to 'new',
>>>> should the setup attribute settings remain as they had been? (Which
>>>> then determines which end sends the ClientHello message.)
>>> Not sure I understand. If a new DTLS association is to be established, shouldn't it be allowed
>>> to re-negotiate the roles (read: change the value of the 'setup' attribute)?
>>> The problem is that the offerer specified "existing", and so followed the rules for doing that. So it had
>>> to use the last negotiated setup value. But the answerer has decided to switch to "new". At this point
>>> there are limits on what it can do - there can't be a full negotiation.
>> If the offer contained "active" or "passive" then the answerer is stuck with what was previously negotiated. If the
>> offer contained "actpass" then maybe the answerer could choose either "active" or "passive" independent of what
>> was previously negotiated. I think this needs to be explained and clarified as to what is allowed.
> Ok, now I understand.
> The text does say that the 'setup' attribute must be set according to RFC 4572, so I think that covers what values are allowed.

Now that we have discussed it, I agree that this is sufficient.

> But, perhaps we could add a note, saying that if the answerer is not able to able to select a 'setup' value that reflects it capabilities, it must reject the m- line. That of course also applies to the handling of the initial offer.

Yes. I'm not sure if this is needed.

When setup is used with TCP/SCTP there may be cases where passive can't 
be supported, because incoming connection requests are blocked. (That 
was one of the reasons for negotiating this.) But I don't think that 
holds for DTLS if the transport has already been established. So AFAIK 
nothing should prevent either end from taking either role. So it might 
not need to be mentioned.