State - Snom 4S Administrator's Manual

[
S N O M
"Session Timer". Depending on the refresh rate of the timer, the timing,
for billing purposes for example, can be made very accurate.
Transport Layer
SIP today uses UDP as main transport layer. Unfortunately, it is
perfectly legal that UDP packets get lost on their way to the destination.
Because of this, the SIP programs must retransmit their UDP messages
until they receive a feedback that the message has been received by the
other party.
This can cause problems when large packets are being
transmitted. In most networks, large UDP packets are split up into several
smaller packets, which are transmitted independently and which have
their own individual chance of getting lost. To avoid this, the latest SIP
standard recommends using a different transport layer for this, TCP. The
snom 4S proxy supports both transport layers and automatically detects
whether an old SIP device cannot handle TCP packets.

State

The snom 4S SIP proxy handles "stateful" information. This
is information that needs to be stored somewhere until another event
happens. This information primarily affects registrations, as they might be
refreshed only after hours or even days. It is therefore important to store
this information in a safe place which will be stable even after a crash or
reboot. The use of redundancy to make file systems more resilient is a
proven solution in these cases.
There are several solutions available here, and the snom 4S
builds upon these mechanisms. Registrations may be stored in a safe
place, and stateful information may be recovered by reading it from the
file system.
Other stateful information is less critical. When forking a request,
there needs to be some state information about the fork. However,
because this information usually has a scope of only a few seconds, this
information is not stored in a separate database. In a case of a reboot, the
complete system will recover fast enough.
4 S P / R
R O X Y E G I S T R A R
M ]
A N U A L
snom technology AG • 29
5