Re: [MMUSIC] WGLC comments on draft-ietf-mmusic-sdp-bundle-negotiation-12 - Paul's comments

[Paul Kyzivat <[e-mail]>]

On 10/22/14 5:56 AM, Christer Holmberg wrote:
> Hi,
>
>>>>>>> What would you like to mention about SCTP?
>>>>>>>
>>>>>>>> IIUC, DTLS/SRTP uses DTLS packets to do key exchange, but doesn't
>>>>>>>> use DTLS payload packets. So *one* m-line with a protocol that
>>>>>>>> uses DTLS payload packets (e.g., >DTLS/SCTP) can coexist with
>>>>>>>> STUN and SRTP. If there is more than one such m-line then some other specification is required to associate those packets with m-lines.
>>>>>>>
>>>>>>> I think that is covered, as SRTP is distinguished from DTLS.
>>>>>>>
>>>>>>> But, if you think that needs to be more clear, maybe the following modified sentence:
>>>>>>>
>>>>>>> 	"[RFC5764] does not describe how to identify different protocols transported on DTLS (i.e. using DTLS payload packets), only how to..."
>>>>>>>
>>>>>> I think this document should bite the bullet and define how SCTP coexists with SRTP in a bundle.
>>>>>
>>>>> Personally, I would not want to do that at this stage. We decided
>>>>> earlier that the draft will cover DTLS, SRTP and STUN, and that any other protocols/combinations have to be defined elsewhere.
>>>>
>>>> If it isn't here, then it needs to be somewhere else. Where?
>>>
>>> In a separate draft.
>>>
>>>> It shouldn't be hard.
>>>
>>> Who is asking for it? RTCWEB will not support SCTP transport (they will support SCTP over DTLS).
>>
>> I'll be satisfied if it covers SCTP over DTLS. That is the case where it needs to be able to coexist with RTP.
>
> The draft describes how you distinguish between DTLS and SRTP, so that also covers SCTP over DTLS (SCTP is the "application protocol" carried on DTLS).
>
> How you differentiate different protocols carried over DTLS need to be described elsewhere, but if you only have SCTP carried over DTLS nothing else is needed.
>
> In addition, if you want to define how to separate SCTP from something else, you would need to know what that something else is.

The draft describes how to distinguish DTLS and SRTP packets. It doesn't 
describe how to associate the DTLS packets with m-lines.

Rather than single SCTP over DTLS out as special, this draft could 
simply say that:

- if the bundle contains *no* m-lines for protocols that utilize DTLS
   payload packets then DTLS payload packets are erroneous.

- if the bundle contains *one* m-line for a protocol that utilizes
   DTLS payload packets then all the payload packets are to be
   associated with it.

- if the bundle contains *more than one* m-line for protocols that
   utilize payload packets then some other specification is needed
   to define how the packets are associated with m-lines.

	Thanks,
	Paul