Scaling And Redundancy - Snom 4S NAT Filter Admin Manual

a=fmtp:101 0-15
a=sendrecv
The NAT Filter will detect that the user agents needs help and
allocates local ports for relaying media. It will forward the request with
changed SDP:
v=0
o=root 19387 19387 IN IP4 217.115.141.99
s=call
c=IN IP4 217.115.141.99
t=0 0
m=audio 49170 RTP/AVP 0 8 3 18 2 101
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:3 gsm/8000
a=rtpmap:18 g729/8000
a=rtpmap:2 g726-32/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=sendrecv
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). That means, you need to list the available servers
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).
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