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

Christer Holmberg <> Sat, 29 November 2014 05:01 UTC

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From: Christer Holmberg <>
To: Roman Shpount <>
Thread-Topic: [rtcweb] JSEP: Why always offer actpass?
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Subject: Re: [rtcweb] JSEP: Why always offer actpass?
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>>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.

BTW, I think this whole discussion belongs to MMUSIC, as it is Offer/Answer related :)