Server Redundancy; Naptr (Naming Authority Pointer) - Yealink SIP-T2xP Administration Manual

Many SIP servers are deployed in redundant pairs, designated as primary and
secondary servers. The IP phone must always contact the primary server except in
failover conditions. Server redundancy is often required in VoIP deployments to ensure
continuity of phone service, for events where the server needs to be taken offline for
maintenance, the server fails, or the connection from the IP phone to the server fails. The
IP phone is able to route to a secondary (or alternate) server in a failure situation, which
requires the use of DNS SRV query for the resolution of proxy address as specified by
RFC 3263.
When the IP phone accesses the internet, it sends out a DNS SRV query to the server to
look up the IP address and port, and then waits for a response from the server. The DNS
SRV query involves NAPTR, SRV and A queries, which allows the IP phone to adapt to
various deployment environments. The DNS SRV query is configurable on a per-account
basis.

NAPTR (Naming Authority Pointer)

First, the IP phone sends the NAPTR query to get the SRV pointer and service type. As an
example, consider the IP phone wishes to resolve ―sip:user@example.com‖. The IP
phone performs a NAPTR query for that domain, and the following NAPTR records may
be returned:
order pref
IN NAPTR 90 50
IN NAPTR 100 50
Parameters are explained in the following table:
Parameter
Specifies preferential treatment for the specific record.
order The order is from lowest to highest, lower order is MORE
preferred.
Specifies the preference to process multiple NAPTR
pref records with the same order value. Lower value is MORE
preferred.
flags The flag ―s‖ means to do an SRV lookup.
Specifies the service available for SIP by the following
rules:
SIP+D2U: SIP over UDP
service
SIP+D2T: SIP over TCP
SIP+D2S: SIP over SCTP
SIPS+D2T: SIPS over TCP
flags service regexp
"s" "SIP+D2T"
"s" "SIP+D2U"
Description
Configuring Advanced Features
replacement
"" _sip._tcp.example.com
"" _sip._udp.example.com
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