Medias Mediatrix 1204, Reference Manual

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Mediatrix 1204 145
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H A P T E R
10
DNS SRV Configuration
This chapter describes the configuration required for the Mediatrix 1204 to work with a DNS SRV.
What is a DNS SRV?
Currently, one must either know the exact address of a server to contact it, or broadcast a question.
DNS SRV is an extension of the standard DNS server. SRV (Service Record) is a type of entry a network
administrator may put into the DNS answers. A DNS SRV is used to get one or more IP addresses of servers,
each one having its own weight and priority.
Each server received when using DNS SRV, depending on its weight and priority, can be used as a primary
or backup server or can be part of a load balancing system.
For instance, the client requests the SRV for SIP servers in some domain. The DNS server may return the A,
B, and C addresses, which are all SIP servers. Each address has a weight and the client must choose one of
those three addresses by using a random algorithm that considers the weight.
To use DNS SRV, an administrator must set a service records (SRV) into the DNS servers available on the
network.
DNS SRV implementation should imply a shared database between servers since a REGISTER and an
INVITE can be sent to any server, not necessarily the same one.
DNS SRV applies to both TCP and UDP transport types.
Priority vs Weight
A DNS SRV uses the priority and weight concepts to distribute the requests.
Standards Supported • RFC 2782 – A DNS RR for specifying the location of services
(DNS SRV)
• RFC 3263 - Session Initiation Protocol (SIP): Locating SIP
Servers
Table 91: Priority vs. Weight
Parameter Description
Priority A client must attempt to contact the target host with the lowest-numbered priority it can
reach.
Weight Specifies a relative weight for entries with the same priority. Larger weights should be
given a proportionately higher probability of being selected.