16 • SIP Architecture
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S N O M
4 S P
R O X Y
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E G I S T R A R
M
A N U A L
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snom technology AG • 17
[
S N O M
4 S P
R O X Y
/ R
E G I S T R A R
M
A N U A L
]
As you can see, in contrast to the traditional PBX model, SIP user
agents fulfill most of the tasks that are needed in telephony. This follows
the model of the intelligent client which made the PC so popular as it is
today.
A user agent may be client or server in a SIP transaction. The
user agent client (UAC) is the user agent that requests something, the user
agent server (UAS) answers this request. The role may change during a
call, depending on who wants to initiate something. For example, A may
initiate a call to B. Then A will be the user agent client while B the is the
user agent server. When B decides to hang up, B will be the user agent
client while A is the user agent server.
Proxies
Proxies forward requests and help the user agent carry out its
tasks. Stateless proxies just forward messages and serve as a “hop” on
the path from a user agent client to a user agent server. The rules for
hopping may depend on all kinds of rules, e.g. traversing NAT using a
stateless proxy.
Stateful proxies keep a list of pending requests. This way proxies
can forward requests to different destinations at the same time. When
the responses come back from the destinations, the proxy merges the
responses, determines the best result and passes it down to the user
agent that sent the request (UAC). The snom 4S proxy is a stateful proxy
with some B2BUA extensions.
Location Server
From RFC3261 we read: “A location service is used by a SIP
redirect or proxy server to obtain information about a callee‘s possible
location(s). It contains a list of bindings of address-of-record keys to zero
or more contact addresses. The bindings can be created and removed in
many ways; this specification defines a REGISTER method that updates
the bindings.”
In other words, the location server finds out where a request to a
domain the proxy feels responsible for should go.
2.
Summary of Contents for 4S Media Server
Page 1: ...Administrator s Manual snom 4S SIP Proxy Registrar Version 2 30 ...
Page 14: ...14 VoIP Protocol S N O M 4 S P R O X Y R E G I S T R A R M A N U A L 1 ...
Page 84: ...84 Routing S N O M 4 S P R O X Y R E G I S T R A R M A N U A L 10 ...
Page 98: ...98 Maintenance S N O M 4 S P R O X Y R E G I S T R A R M A N U A L 13 ...