Yealink SIP-T38G, Administrator'S Manual

Page: 167 / 402

Administrator’s Guide for SIP-T3xG IP Phones
154
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 between the IP phone and 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.
Before connecting a network through the domain name of the server, the IP phone
performs a DNS SRV query. 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 the domain name. The sample of the NAPTR records
for reference:
order pref flags service regexp replacement
IN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.example.com
IN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.com
Parameters are explained in the following table:
Parameter Description
order
Specify preferential treatment for the specific record. The order
is from lowest to highest, lower order is MORE preferred.
pref
Specify the preference to process multiple NAPTR records with
the same order value. Lower value is MORE preferred.
flags The flag ―s‖ means to do an SRV lookup.
service
Specify the service available for SIP by the following rules:
SIP+D2U: SIP over UDP
SIP+D2T: SIP over TCP
SIP+D2S: SIP over SCTP
SIPS+D2T: SIPS over TCP
regexp
Always empty for SIP services.
replacement Specify a DNS name to be used for the next query.