20 • Architecture
[
S N O M
4 S N A T F
I L T E R
]
a=silenceSupp:off - - - -
The NAT Filter changes the private address to a globally routable
address and inserts the local port. It also inserts a hint that tells the other
user agent that it should not do silence suppression. This reduces the risk
that the connection is closed during a talk spurt of one of the parties.
2.4 Scaling and Redundancy
The NAT Filter product was designed to support distributed server
farms. That means that the servers act stateless in principle; user agents
may randomly pick a server in a server farm. This feature allows operators
to set up very large and robust networks.
The distribution of user agents to a server is performed using
DNS SRV (RFC 2782). This means that you need to list the available serv-
ers on DNS level; the user agents must perform DNS SRV look ups and
pick one of the servers (possible using the detection algorithms described
below).
The following table shows an example configuration for Linux
named(8):
_sip._udp IN SRV 3 5 5082 frankfurt1
_sip._udp IN SRV 3 5 5082 newyork1
_sip._udp IN SRV 3 5 5082 newyork2
_sip._udp IN SRV 3 5 5082 newyork3
_sip._udp IN SRV 3 5 5082 tokyo2
_sip._udp IN SRV 3 5 5082 tokyo1
If one of the servers should become unavailable, the SRV algo-
rithm will make sure that the other servers will be contacted. The user
agents that are refreshed by that particular server will become unreach-
able until the user agents initiate a new REGISTER request. Therefore,
you should make sure that your servers have a high uptime probability
and that the registration period is not too long. We think that registration
periods of thirty minutes up to one hour are a good balance between ser-
vice failure time and performance.
2.
