Re: [MMUSIC] Draft new version: draft-holmberg-mmusic-sdp-dtls-01

[Paul Kyzivat <[e-mail]>]

On 6/20/15 6:05 PM, Christer Holmberg wrote:
> Hi Paul,
>
> Thanks for your comments. See inline.
>
>
>> * Section 2.1:
>>
>>     When a new DTLS association is established, an endpoint MUST use a
>>     new set of transport parameters (IP address and port combination).
>>
>> The above seems slightly ambiguous: does "an endpoint" mean "each endpoint" or "one (of the two) endpoints"?
>>
>> IIUC we have established that the important point is that the 5-tuple must change. So at least one side must change the address or port. But if one > side is known to do so, then the other side need not do so. So I suggest changing the above to:
>>
>>     When a new DTLS association is established, one of the endpoints
>>     MUST use a new set of transport parameters (IP address and port
>>     combination).
>
> The idea is that the endpoint(s) which does something the requires a new set of transport parameters needs to use a new set.
>
> So, if e.g. endpoint A wants to change the fingerprint, which requires a new DTLS association, endpoint A needs to use a new set of transport parameters.

That is certainly one approach that will work. See more on this below.

>> But there may be more to sort out about the o/a protocol for guaranteeing this. I can see a few:
>>
>> - if the offer has connection:new then the offer must have new
>>    transport parameters. Else, if the offer has connection:existing
>>    but the answer has connection:new then the answer must have new
>>    transport parameters.
>
> Yes.

This is the algorithm you suggested above.

>> - if the answer has connection:new and the offer didn't have new
>>    transport parameters, then the answer must have new ones.
>
> Yes.

This is a subtly different algorithm. It doesn't require the offerer to 
change transport parameters when specifying connection:new. It *can*, 
but it can also choose not to - forcing the answered to change them. (Or 
refuse the m-line, or refuse the offer.)

It gives the offerer a little more flexibility, but then allows the 
offerer to put more burden on the answerer. There may be some obscure 
cases where the extra flexibility might be useful, though I have not yet 
thought of any.

Either algorithm will work. This spec needs to choose one and explicitly 
state it.

>> Something about the above should probably go into section 6.
>>
>> * Section 5.1
>>
>>     A 'connection' attribute value of 'new' indicates that a new DTLS
>>     association MUST be established.  A 'connection' attribute value of
>>     'existing' indicates that a new DTLS association MUST NOT be
>>     established.
>>
>> I think this is wrong - that the answerer is permitted to answer with connection:new.
>
> The was meant to say that an endpoint sending 'new' needs to use a new set of transport parameters.
>
> However, if the offerer sends 'new', meaning it uses a new set of transport parameters, I am not sure we need to mandate the answerer to use a new set of transport parameters even if it sends 'new' in the answer.

So your intent was to specify the first of the algorithms I mentioned 
above. I did not get that out of this paragraph.

	Thanks,
	Paul

>> * Section 6.3
>>
>>     If an answerer receives an offer that contains an SDP 'connection'
>>     attribute with a 'new' value, the answerer MUST insert a 'new' value
>>     in the associated answer.  The same applies if the answerer receives
>>     an offer that contains an SDP 'connection' attribute with a 'new'
>>     value, but the answerer determines (based on the criteria for
>>     establishing a new DTLS association) that a new DTLS association is
>>    to be established.
>>
>> I think I previously commented on this: the 2nd sentence "The same applies if ... attribute with 'new' value ...". *That* 'new' should be 'existing' - it > is covering an alternative to the first sentence. And this aligns with my comment above about section 5.1.
>
> I'll have a look at that.
>
> Regards,
>
> Christer
>
>