Some time ago I described my problems I have with the German Telekom ALL-IP product, where apparently proxy servers abruptly change in the background leading to all kinds of connection problems for a couple of minutes until the next (re)registration. Please note that I no longer say “does change”, but “apparently changes.”
I’d like to put my current observations up for discussion, which could point to a different root of the problem. The following came from evaluations of pcap traces.
The given registrar/SIP proxy, tel.t-online.de, can be evaluated using service requests with NAPTR, SRV and then A/AAAA requests.
At the top the following DNS servers are offered:
where one of them is used to evaluate the next level to retrieve the offered services like _sip._tcp.tel.t-online.de.
The request to get the A record, of say a currently preferred server m-epp-110.edns.t-ipnet.de, is then evaluted using servers like dns02.dns.t-ipnet.de (18.104.22.168). It seems as if the nsX servers and the more general servers dnsXX are not synchronized.
I evaluated the chain manually and never found any difference among the nsX servers.
I picked ns1.edns.t-ipnet.de and evaluated the chain for more than a day constantly to monitor the returned servers for the service _sip._tcp.tel.t-online.de. There was not a single change.
My current conclusion is that the way the addresses are resolved may play a role.
German Telekom has a different product “German Telkom SIP-Trunk” that does not show these problems. From the outside one difference is that one can resolve the proxy reg.sip-trunk.telekom.de only using NAPTR and SRV requests, whereas tel.t-online.de additionally returns an A record, which might or might not confuse the DNS resolution.
Could stricter specifications for which DNS servers to use solve the problem?