Yealink SIP-T38G Administrator's Manual

Also see for SIP-T32G: User guide Auto provisioning guide User guide User guide User guide

Contents

Yealink SIP-T38G Administrator's Manual Manual pdf 167 page image

Administrator’s Guide for SIP-T3xG IP Phones154Many SIP servers are deployed in redundant pairs, designated as primary andsecondary servers. The IP phone must always contact the primary server except infailover conditions. Server redundancy is often required in VoIP deployments to ensurecontinuity of phone service, for events where the server needs to be taken offline formaintenance, the server fails, or the connection between the IP phone and the serverfails. The IP phone is able to route to a secondary (or alternate) server in a failuresituation, which requires the use of DNS SRV query for the resolution of proxy address asspecified by RFC 3263.Before connecting a network through the domain name of the server, the IP phoneperforms a DNS SRV query. It sends out a DNS SRV query to the server to look up the IPaddress and port, and then waits for a response from the server. The DNS SRV queryinvolves NAPTR, SRV and A queries, which allows the IP phone to adapt to variousdeployment 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 anexample, consider the IP phone wishes to resolve ―sip:user@example.com‖. The IPphone performs a NAPTR query for the domain name. The sample of the NAPTR recordsfor reference:order pref flags service regexp replacementIN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.example.comIN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.comParameters are explained in the following table:Parameter Descriptionorder Specify preferential treatment for the specific record. The orderis from lowest to highest, lower order is MORE preferred.pref Specify the preference to process multiple NAPTR records withthe same order value. Lower value is MORE preferred.flags The flag ―s‖ means to do an SRV lookup.serviceSpecify the service available for SIP by the following rules:SIP+D2U: SIP over UDPSIP+D2T: SIP over TCPSIP+D2S: SIP over SCTPSIPS+D2T: SIPS over TCPregexp Always empty for SIP services.replacement Specify a DNS name to be used for the next query.