SER load balancing

From the SER administrator's manual

The SIP standard's use of DNS SRV records has been explicitly constructed to handle with server failures. There may be multiple servers responsible for a domain and referred to by DNS. If it is impossible to communicate with a primary server, a client can proceed to another one. Backup servers may be located in a different geographic area to minimize risk caused by areal operational disasters: lack of power, flooding, earthquake, etc.

Unfortunately, at the moment of writing this documentation (end of December 2002) only very few SIP products actually implement the DNS fail-over mechanism. Unless networks with SIP devices supporting this mechanism are built, alternative mechanisms must be used to force clients to use backup servers. Such a mechanism is disconnecting primary server and replacing it with a backup server locally. It unfortunately precludes geographic dispersion and requires network multihoming to avoid dependency on single IP access. Another method is to update DNS when failure of the primary server is detected. The primary drawback of this method is its latency: it may take long time until all clients learn to use the new server.

This means that
  • You can set SRV records of a domain to point to a SIP proxy in another domain. Like e-mail, where a mail server handles mail to many domains, a SIP proxy can handle users in many domains. The key is to set the DNS SRV records in the user domain (like zone file to point to a SIP Proxy somewhere else (like With a configuration like this, calls to will be automatically routed to the SIP proxy
  • In a DNS zone, you can specify multiple SIP proxies. If the proxy with the highest priority is not reachable, the SIP ua or proxy that tries to reach a user within the domain tries the next proxy defined in the zone with a SRV record.

No one really mails user@mailserver any more. We're mailing user@domain and the DNS MX records helps the mail client to send the mail to the correct mail server. Why should we call user@sip-proxy instead of using user@domain?

Remember that the later construction in addition to being user friendly, also adds redundancy, and with a proper DNS configuration also may add load balancing.

Created by: oej, Last modification: Fri 14 of Jul, 2017 (17:56 UTC) by ocgltd
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