[SR-Users] ACK and DNS failover

Fri Feb 25 14:09:23 CET 2022

Hello,

Thanks for pointing out this topic from sip implementors. It is very clear now, thanks for your answers.

Julien

De : Henning Westerholt <hw at gilawa.com>
Date : jeudi 24 février 2022 à 03:14
À : "Kamailio (SER) - Users Mailing List" <sr-users at lists.kamailio.org>
Cc : Julien Klingenmeyer <julien.klingenmeyer at ovhcloud.com>
Objet : RE: [SR-Users] ACK and DNS failover

Hello,

as Daniel pointed out:

> If the ACK is not received, the target is supposed to retransmit the 200ok, to force retransmission of the ACK

So the ACK is not supposed to be retransmitted, only the 200OK from the UAS side.

The ACK is handled from the UAC side, and it needs send at least one ACK for every 200 OK that it receives. Have a look e.g. here :

https://lists.cs.columbia.edu/pipermail/sip-implementors/2008-December/020969.html

But there are of course differences regarding implementation in user agents.

Cheers,

Henning

--
Henning Westerholt – https://skalatan.de/blog/
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From: sr-users <sr-users-bounces at lists.kamailio.org> On Behalf Of Julien Klingenmeyer
Sent: Wednesday, February 23, 2022 5:34 PM
To: miconda at gmail.com; Kamailio (SER) - Users Mailing List <sr-users at lists.kamailio.org>
Subject: Re: [SR-Users] ACK and DNS failover

Thanks, Daniel, for this complete explanation.

It is clear now why ACKs are not being processed by the DNS failover feature.
Actually, I expect the primary node to be restored quickly. So considering this it is OK if there is ACK retransmission to this primary node only, until it comes up again.

But from what I could see, Kamailio A does not receive multiple ACK (because of retransmission) all the time, although multiple 200 OK are generated by the peer.
When Kamailio A receives multiple ACK retransmission, it does relay them to Kamailio B1 each time: great!
But some previous hops (not Kamailio ones) can send to it one unique ACK although the multiple 200 OK => is it something RFC-compliant? Could it be due to a TLS connection between these previous hops and Kamailio A?
I wonder if some previous hops do not relay the retransmission packets because they know that the initial ACK was already correctly forwarded thanks to the TLS connection. So maybe this is why they only relay the initial ACK and this does not trigger ACK retransmission.
If so, would it be possible in the routing script to do a “manual” ACK retransmission (in a failure_route or something) when I detect a broken TLS connection?
In the logs I do see a failure:
ERROR: <core> [core/tcp_main.c:4457]: handle_tcpconn_ev(): connect <IP_Kamailio_B1>:5061 failed
Could I catch it to trigger a retransmission of the ACK request ?

Thanks

Julien

De : Daniel-Constantin Mierla <miconda at gmail.com<mailto:miconda at gmail.com>>
Répondre à : "miconda at gmail.com<mailto:miconda at gmail.com>" <miconda at gmail.com<mailto:miconda at gmail.com>>
Date : mercredi 23 février 2022 à 10:50
À : "Kamailio (SER) - Users Mailing List" <sr-users at lists.kamailio.org<mailto:sr-users at lists.kamailio.org>>, Julien Klingenmeyer <julien.klingenmeyer at ovhcloud.com<mailto:julien.klingenmeyer at ovhcloud.com>>
Objet : Re: [SR-Users] ACK and DNS failover

Hello,

well, the ACK is a request without a response, practically it is a stateless request, no sip transaction can be created for it because there is no response to wait for it. That's by design from SIP specs.

In other words, it is no way to know if the target received or not the ACK. If the ACK is not received, the target is supposed to retransmit the 200ok, to force retransmission of the ACK.

In the case of tcp/tls, one can leverage transport layer to know that it could not be transmitted, but for udp is no way, and even more, in sip transport layer is decoupled from sip singling layer (ie., same connection can carry traffic for many users, many transactions, etc...). Also, for tcp/tls, with asynchronous sending, the feedback of not able to send is not immediate. But can be coded somehow, it's about open source after all ...

Moreover, ACK does not belong to INVITE transaction and can have a different path than the INVITE, being a matter of record-route headers.

So lack (or limitations) of DNS failover for ACK comes from the above.

You can either consider this a corner case, if the target servers are supposed to run always and in case of one becoming unavailable for long time, the dns is updated accordingly, or, if you know that ACK has to be sent to the address where 200ok came from, then you can store that in htable and use it for sending out the ACK (but again, this may not be the case always according to the specs, but can be in your deployment).

Cheers,
Daniel
On 23.02.22 15:59, Julien Klingenmeyer wrote:
Hello,

I use DNS failover feature for routing some calls, and I wonder about its limitations.
I noticed that INVITE and ReINVITE requests are correctly routed based on SRV priority/weight in case of failure, but ACK requests do not use them.

Let’s say I have Kamailio A relaying requests to a pool of Kamailios B1 and B2 in TLS.
DNS records are the below ones.

kamailiob.net.            60 NAPTR 20   100  S SIPS+D2T _sips._tcp.kamailiob.net.
_sips._tcp.kamailiob.net.   60 SRV 1    10   5061       kamailiob-1.net.
_sips._tcp.kamailiob.net.   60 SRV 2    10   5061       kamailiob-2.net.

If B1 is down, I expect requests being relayed to B2 (because of priority 2 in the SRV record).

1.       For initial INVITEs: R-URI is sip:kamailiob.net

1.       If B1 is down, the request is retried to B2 => OK

2.       For in-dialog requests: Route header in incoming request is sip:kamailiob.net so it is set as next destination with loose_route function

1.       If B1 is down, the request is retried to B2 for ReINVITE => OK

2.       But once the ACK comes in, with the same Route header (sip:kamailiob.net), Kamailio tries to send it to B1 only (no retries to B2, and even no retransmissions to B1 either).

3.       When B1 is back UP again a few seconds later, Kamailio does not try to relay the ACK again. The ACK request is attempted to be retransmitted only once to B1. It fails and no more retries after that.

Is it the expected behavior? Or is it something misconfigured on Kamailio A? Or is it a TLS connection issue?

Here is the DNS configuration on Kamailio A (TM module is enabled):
dns_try_naptr=yes
use_dns_failover=yes
use_dns_cache=yes
dns_srv_lb=yes

enable_tls=yes
tcp_max_connections=4096
tls_max_connections=4096
tcp_reuse_port=yes
tcp_connect_timeout=10
tcp_send_timeout=10
tcp_connection_lifetime=3600

I had a look into the DNS tutorial (https://github.com/kamailio/kamailio/blob/master/doc/tutorials/dns.txt) without finding any hint about this.

If anyone has already played with SIP DNS failover in Kamailio, your help would be appreciated, thanks!

Julien

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