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Dns srv, Dns srv and failover – Siemens HIPATH 8000 User Manual

Page 42

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Technical Overview

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DNS SRV

The phone supports the use of DNS SRV record lookups to allow SIP serv-

ers to be located. This mechanism is described in detail in RFC 3263 - Lo-

cating 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. An ex-

ample query being:

_sip._tcp.example.com.

This indicates a query for a SIP server supporting the TCP transport proto-

col. The transport used in this query is determined by the SIP transport

menu setting Æ page 217. 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 ad-

dress. The response to a DNS SRV query will also contain information re-

garding 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.

DNS SRV and failover

Lists of candidate SIP server names are often returned in response to DNS

SRV queries to allow failover mechanisms to be implemented which in-

crease overall system availability.

If the phone sends a request to the first address in the list but fails to re-

ceive a response (currently the failover time is 6 seconds, this is not con-

figurable) the address is placed in a "penalty box" which means that it will

not be tried again until a specific time interval has past (currently pre-set to

1 minute). The request is sent to the next SIP server in the list and the pro-

cess continues. The penalty box mechanism ensures that the responsive-

ness of the phone is maintained by not continually retrying SIP servers that

are failing to respond.

For example the request to the first SIP server in the list fails, a call to an-

other user hosted on the SIP server will result in the user experiencing a 6

second delay before the failover to the secondary occurs. This means the

"Calling.." status will remain on the display until the second SIP server is

contacted. All subsequent messages for this call will go to the second SIP

server until the first SIP server is removed from the penalty box and it can

be tried once again to see if it is back in service. Note this mechanism is

independent of call setup. The first SIP server will not be retried necessar-

ily when the next call is established, only when it is removed from the pen-

alty box.