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@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 and the second explicitly in RFC8643 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@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@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
>>
>>
>> __________________________________________________________
>> Kamailio - Users Mailing List - Non Commercial Discussions
>> * sr-users@lists.kamailio.org
>> Important: keep the mailing list in the recipients, do not reply only to the sender!
>> Edit mailing list options or unsubscribe:
>> * https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users

__________________________________________________________
Kamailio - Users Mailing List - Non Commercial Discussions
* sr-users@lists.kamailio.org
Important: keep the mailing list in the recipients, do not reply only to the sender!
Edit mailing list options or unsubscribe:
* https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users

__________________________________________________________
Kamailio - Users Mailing List - Non Commercial Discussions
* sr-users@lists.kamailio.org
Important: keep the mailing list in the recipients, do not reply only to the sender!
Edit mailing list options or unsubscribe:
* https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users