The SIP code 503 is tricky in the sense that i can indicate either server maintenance or server overload. In both cases it can send Retry-After header and any subsequent requests from same source are ignored for the duration of Retry-After interval. [1].

Additionally RFC3261 and RFC3263 define that transport failures (generally due to fatal ICMP errors in UDP and connection failures in TCP) should be treated as 503 response. [2].

So in all above cases, it is most likely that dialog does not establishes at all and 503 response is treated similar to stateless response. Therefore, a to-tag can be added/replaced before sending it to UAC.

Theoretically, kamailio should check and use to-tag from 503 response when converting it to 500 response and only create new to-tag if it is absent.

References:

[1] https://tools.ietf.org/html/rfc3261#section-21.5.4

[2] https://tools.ietf.org/html/draft-hilt-sip-correction-503-01#section-4

Hope this helps.

On Wed, 22 Jul 2020 at 21:08, Henning Westerholt hw@skalatan.de wrote:

Hello,

Apparently, this is the way the code works:

t_reply.c:

                    if (relayed_code==503 && tm_remap_503_500){

                            /* replace a final 503 with a 500:

                             * generate a "FAKE" reply and a new

to_tag (for easier

                             *  debugging)*/

Lets see if maybe others can comment as well. Otherwise you could just open an issue on our tracker, it is probably not that hard to change this.

Cheers,

Henning

--

Henning Westerholt – https://skalatan.de/blog/

Kamailio services – https://gilawa.com

*From:* sr-users sr-users-bounces@lists.kamailio.org *On Behalf Of *Gerry | Rigatta *Sent:* Wednesday, July 22, 2020 8:58 PM *To:* Kamailio (SER) - Users Mailing List sr-users@lists.kamailio.org *Subject:* [SR-Users] bug ? remap_503_500 breaks dialogs

Hi,

I am using Kamailio 5.2.

Apparently the remapping of 503 to 500 codes in the tm module does also change the to-tag. This behaviour breaks dialogs with yate and therefore calls hang and the 503 remains unacknowledged. After disabling the 503 to 500 remapping with modparam("tm", "remap_503_500", 0) all works fine again.

Changing the to-tag in a dialog seems to contradict RFC3261, or do I see this wrongly? 12 https://tools.ietf.org/html/rfc3261#section-12 Dialogs A dialog is identified at each UA with a dialog ID, which consists of a Call-ID value, a local tag and a remote tag…"

Thanks for looking into this.

Gerry

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