[SR-Users] bug ? remap_503_500 breaks dialogs
Gerry | Rigatta
gjacobsen at rigatta.com
Thu Jul 23 09:41:24 CEST 2020
> Indeed, at this stage there is no dialog established and there can be many To-tags in 1xx provisional responses (eg, a parallel forking scenario) -- the to-tag of the dialog has to be taken from 200ok.
>
As far as I read this is not correct. Also a provisional dialog is a dialog according to RFC3261. Only in the case that the request did not contain a to-tag the provisional messages may insert their own to-tags:
"1xx and 2xx responses may be involved in the establishment of
dialogs. When a request does not contain a To tag, the To tag
in the response is used by the UAC to distinguish multiple
responses to a dialog creating request. A proxy MUST NOT
insert a tag into the To header field of a 1xx or 2xx response
if the request did not contain one. A proxy MUST NOT modify
the tag in the To header field of a 1xx or 2xx response.”
https://tools.ietf.org/html/rfc3261#page-111
In any case, this bug is not a about provisional messages. The 500 message terminates the dialog for the UAC (yate) and the UAC needs to be able to identify it. An UAC identifies the dialog by the call-id, local tag and remote tag.
>> 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…"
>>
> On 23 Jul 2020, at 10:07, Daniel-Constantin Mierla <miconda at gmail.com> wrote:
>
> Indeed, at this stage there is no dialog established and there can be many To-tags in 1xx provisional responses (eg, a parallel forking scenario) -- the to-tag of the dialog has to be taken from 200ok.
>
> This parameter is probably to have a shortcut of doing:
>
> failure_route[REMAP503] {
>
> if(t_check_status("503")) {
>
> t_reply("500", "Server error");
> exit;
>
> }
>
> Being like the server application is generating the 500 (so using own tag), instead of forwarding the 503. Not a bug, but if anyone is willing to add an option to allow re-using the to-tag from received reply, I am fine with it.
>
> Anyhow, even if this would be fixed, I am wondering how yate is going to work in parallel/serial forking scenarios where different to-tags flow for a while and the final failure response can have any to-tag, including a new one (e.g., from a device not sending any 1xx or again from kamailio (e.g., when last target doesn't reply at all)).
>
> Cheers,
> Daniel
>
> On 23.07.20 06:08, M S wrote:
>> 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 <https://tools.ietf.org/html/rfc3261#section-21.5.4>
>>
>> [2] https://tools.ietf.org/html/draft-hilt-sip-correction-503-01#section-4 <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 at skalatan.de <mailto:hw at 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/ <https://skalatan.de/blog/>
>> Kamailio services – https://gilawa.com <https://gilawa.com/>
>>
>> From: sr-users <sr-users-bounces at lists.kamailio.org <mailto:sr-users-bounces at lists.kamailio.org>> On Behalf Of Gerry | Rigatta
>> Sent: Wednesday, July 22, 2020 8:58 PM
>> To: Kamailio (SER) - Users Mailing List <sr-users at lists.kamailio.org <mailto:sr-users at 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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>>
>>
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> --
> Daniel-Constantin Mierla -- www.asipto.com <http://www.asipto.com/>
> www.twitter.com/miconda <http://www.twitter.com/miconda> -- www.linkedin.com/in/miconda <http://www.linkedin.com/in/miconda>
> Funding: https://www.paypal.me/dcmierla <https://www.paypal.me/dcmierla>_______________________________________________
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