Re: [AVT] SSRC to DTLS-SRTP mapping

["John W. Floroiu" <john.floroiu at iptel.org>]

Hello,

regarding the first error case that you mention, which would translate 
for instance in both Bob and Charlie sending both of them media streams 
with SSRC=1, what would happen if the media stream from Bob arrives 
first (and triggers the SSRC mapping) but Alice's UA actually cancels 
the call with Bob (because for instance the 200 OK from Charlie arrived 
first)?

Regards,
John

 >    [...]
>    In some cases there will be multiple DTLS-SRTP associations for a
>    given SRTP endpoint.  For instance, if Alice makes a call which is
>    SIP forked to both Bob and Charlie, she will use the same local host/
>    port pair for both of them, as shown in Figure 8.  (The SSRCs shown
>    are the ones for data flowing to Alice).
> 
>                                           Bob (192.0.2.1:6666)
>                                          /
>                                         /
>                                        / SSRC=1
>                                       /  DTLS-SRTP=XXX
>                                      /
>                                     v
>                Alice (192.0.2.0:5555)
>                                     ^
>                                      \
>                                       \  SSRC=2
>                                        \ DTLS-SRTP=YYY
>                                         \
>                                          \
>                                           Charlie (192.0.2.1:6666)
> 
>                   Figure 8: RTP sessions with SIP forking
> 
>    Because DTLS operates on the host/port quartet, the DTLS association
>    will still complete correctly, with the foreign host/port pair being
>    used to distinguish the associations.  However, in RTP the source
>    host/port is not used and sessions are identified by the destination
>    host/port and the SSRC.  Thus, some mechanism is needed to determine
>    which SSRCs correspond to which DTLS associations.  The following
>    method SHOULD be used.
>    [...]  
>    There are two error cases that should be considered.  First, if an
>    SSRC collision occurs, then only the packets from the first source
>    will be processed.  When the packets from the second source arrive,
>    the DTLS association with the first source will be used for
>    decryption, which will fail, and the packet will be discarded.  This
>    is consistent with [3], which permits the receiver to keep the
>    packets from one source and discard those from the other.
>    [...] 

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