Re: [rtcweb] JSEP: Why always offer actpass?

Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com> Wed, 03 December 2014 15:48 UTC

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From: =?iso-8859-1?Q?Stefan_H=E5kansson_LK?= <stefan.lk.hakansson@ericsson.com>
To: Justin Uberti <juberti@google.com>
Thread-Topic: [rtcweb] JSEP: Why always offer actpass?
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Date: Wed, 3 Dec 2014 15:48:45 +0000
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Subject: Re: [rtcweb] JSEP: Why always offer actpass?
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On 01/12/14 18:26, Justin Uberti wrote:
>
>
> On Sun, Nov 30, 2014 at 7:26 AM, Stefan Håkansson LK
> <stefan.lk.hakansson@ericsson.com
> <mailto:stefan.lk.hakansson@ericsson.com>> wrote:
>
>     On 30/11/14 16:06, Makaraju, Maridi Raju (Raju) wrote:
>      >> On 30/11/14 14:51, Makaraju, Maridi Raju (Raju) wrote:
>      >>>> Hi,
>      >>>>
>      >>>>>> RFC 7345 (UDPTL-DTLS) says the following:
>      >>>>>>
>      >>>>>> "Once an offer/answer exchange has been completed, either
>      >>>>>> endpoint
>      >>>> MAY
>      >>>>>> send a new offer in order to modify the session.  The
>      >>>>>> endpoints
>      >>>> can
>      >>>>>> reuse the existing DTLS association if the key fingerprint
>      >>>>>> values
>      >>>> and
>      >>>>>> transport parameters indicated by each endpoint are unchanged.
>      >>>>>> Otherwise, following the rules for the initial offer/answer
>      >>>> exchange,
>      >>>>>> the endpoints can negotiate and create a new DTLS association
>      >>>>>> and, once created, delete the previous DTLS association,
>      >>>>>> following the same rules for the initial offer/answer exchange.
>      >>>>>> Each endpoint needs to be prepared to receive data on both the
>      >>>>>> new and old DTLS associations as long as both are alive."
>      >>>>>>
>      >>>>>> So, I guess that can be read in a way that the setup attribute
>      >>>>>> as such
>      >>>> does not impact previously
>      >>>>>> negotiated TLS roles - unless the key fingerprint values and/or
>      >>>>>> transport
>      >>>> parameters are modified.
>      >>>>>>
>      >>>>>> The SCTP-SDP draft currently says that a subsequent offer must
>      >>>>>> not change
>      >>>> the previously negotiated roles. But, I guess
>      >>>>>> we could say something similar as in RFC 7345.
>      >>>>>
>      >>>>> I have struggled with this language quite a bit from the
>      >>>>> implementation
>      >>>> perspective. I think we need to explicitly state
>      >>>>> that endpoints MUST reuse the existing association if the key
>      >>>>> fingerprint
>      >>>> values and transport parameters indicated
>      >>>>> by each endpoint are unchanged.
>      >>>>
>      >>>> We could take such general approach.
>      >>>>
>      >>>> However, for 'SCTP/DTLS' (DTLS over SCTP), I assume that the DTLS
>      >>>> connection will also be re-established if the underlying SCTP
>      >>>> association is re- established - even if no transport parameters
>      >>>> etc are changed.
>      >>>>
>      >>>> Also, RFC 6083 says:
>      >>>>
>      >>>> "Each DTLS connection MUST be established and terminated
>     within the
>      >>>> same SCTP association."
>      >>>>
>      >>>>
>      >>>>> The way this language reads to me is that endpoints can reuse the
>      >>>>> existing
>      >>>> association if they want to, but they can create a new one if they
>      >>>> don't. Also, when this new
>      >>>>> association is created, it is unclear if old setup roles MUST be
>      >>>>> preserved
>      >>>> or if they MUST be selected based on the current offer/answer
>      >>>> exchange. It seems the current
>      >>>>> specification language is not strong or clear enough on this
>      >>>>> topic.
>      >>>>
>      >>>> In my opinion, IF a new DTLS connection needs to be established
>      >>>> (for whatever reasons), the current roles should be used.
>      >>>
>      >>> <Raju> When ICE is NOT used, how does the offerer know that the
>      >>> answerer does not change the fingerprint and/or transport
>     parameters?
>      >>> I guess it does not know. So, offerer has to be prepared for
>     new DTLS
>      >>> association by offering actpass. When ICE is used, the answerer
>     can't
>      >>> change transport parameter unless offerer does ICE restart (which
>      >>> changes offerer transport parameters); Ref [1] is very clear on
>     this
>      >>> indicating "DTLS handshake procedure is repeated". However,
>     even when
>      >>> ICE is used, I do not find any restriction about answerer not
>      >>> changing fingerprint. So, even without ICE restart answerer can
>      >>> trigger a DTLS renegotiation by changing its fingerprint.
>      >>>
>      >>> To conclude all this, IMO whether ICE is used or not, sending
>     actpass
>      >>> for all new offers may be cover all possible scenarios.
>      >>
>      >> That is also my conclusion based on the discussion so far.
>      >>
>      >> I also think the JSEP draft as far as detailed out is correct,
>     but info
>      >> about how the implementation should behave for Subsequent answers is
>      >> missing. Text saying that the role must be maintained (unless
>     there is
>      >> an ICE restart) should be put in there.
>      >
>      > <Raju>
>      > Hi Stefan,
>      > Looks like, there is some misunderstanding here.
>
>     Probably my fault in that case :)
>
>     > My conclusion is to include
>     > actpass, but not the previously negotiated role, in all subsequent offers,
>     > not just during ICE-restarts.
>
>     I think that is what the JSEP draft says - and my conclusion is that it
>     _is_ correct.
>
>     My point was that the _answer_ should (when it is a subsequent answer)
>     should say the same role as in previous answers (unless there is an ICE
>     restart).
>
> I agree the JSEP text should indicate that roles should stay the same.
> We have had this as a TODO for a while:
> https://github.com/rtcweb-wg/jsep/issues/72

Great. I should have checked that out before.