[SR-Users] Comunication between devices RTP/SRTP and viceversa

[Sergiu Pojoga]

You would achieve this in a failure route, detecting the 488|415 negative
replies then resuming the branch with a different SDP profile.

However, as others have said already, not all UAs will accept this "hack".
Some will consider the new branch with a CSeq NOT incrementing as
retransmission - and reject it with something like "482 Merged Request".

https://tools.ietf.org/html/rfc3261#section-8.2.2.2

8.2.2.2 Merged Requests

If the request has no tag in the To header field, the UAS core MUST
check the request against ongoing transactions. If the From tag,
Call-ID, and CSeq exactly match those associated with an ongoing
transaction, but the request does not match that transaction (based
on the matching rules in Section 17.2.3), the UAS core SHOULD
generate a 482 (Loop Detected) response and pass it to the server
transaction.

Regards,
--Sergiu

On Thu, Jan 27, 2022 at 6:26 PM Marrold <kamailio at marrold.co.uk> wrote:

> Hi,
>
> Unfortunately in my experience "Opportunistic SRTP" support is broken in
> many UAs. There are (at least) two ways of doing it covered indirectly in
> RFC3264 <https://tools.ietf.org/html/rfc3264> and the second explicitly
> in RFC8643 <https://www.rfc-editor.org/rfc/rfc8643.html> and devices seem
> to handle them differently. I've seen end devices that *can *support SRTP
> choose to use standard RTP when offered the wrong type of SDP body.
>
> One thing I considered (but haven't tested so far) is using a late offer
> to discover what format the end device expects before sending an offer, but
> that might cause interop problems of its own.
>
> Cheers
>
> On Thu, Jan 27, 2022 at 10:54 PM Social Boh <social at bohboh.info> wrote:
>
>> I can examine the request but how examine the answer if is a Error?
>> Using a failure route?
>>
>> Regards
>>
>> ---
>> I'm SoCIaL, MayBe
>>
>> El 27/01/2022 a las 5:39 p. m., Alex Balashov escribió:
>> > Does an examination of the SDP offer and answer respectively not divine
>> the issue?
>> >
>> >> On Jan 27, 2022, at 5:35 PM, Social Boh <social at bohboh.info> wrote:
>> >>
>> >> I'm trying to configure Kamailio to permit communication between
>> devices:
>> >>
>> >> some use TLS and SRTP
>> >>
>> >> some UDP and RTP
>> >>
>> >> with RTPEngine in the middle.
>> >>
>> >> A priori I don't know if the device support SRTP or RTP so in the
>> routing I have to detect on the fly how to act. The problem is this:
>> >>
>> >> Call from RTP to SRTP device: 488 Not Acceptable here
>> >>
>> >> Call from SRTP to RTP device: 415 Unsupported Media Type
>> >>
>> >> Any hint?
>> >>
>> >> Thank you
>> >>
>> >>   Regards
>> >>
>> >> --
>> >> ---
>> >> I'm SoCIaL, MayBe
>> >>
>> >>
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