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The phone supports the use of DNS SRV record lookups to allow SIP servers to be located. This mechanism is described in detail in RFC 3263 - Locating SIP Servers. If the location to which a SIP message is to be sent is defined as a name as opposed to an IP address a DNS-SRV lookup will be performed.
The DNS server may return an IP address for the requested SIP server or may return a single name or list of names which require further A or AAAA record lookups to determine an IP address. The response to a DNS-SRV query will also contain information regarding the Time To Live for the information returned, the port address to which requests should be sent and weighting information relating to load balancing of requests.
Lists of candidate SIP server names are often returned in response to DNS-SRV queries to allow failover mechanisms to be implemented which increase overall system availability.