Medias Mediatrix 1204, Справочное руководство

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Chapter 18 - SIP Protocol Features SIP Penalty Box
226 Mediatrix 1204
SIP Penalty Box
The penalty box feature is used to “quarantine” a given host which address times out. During that time, the
address is considered as “non-responding” for all requests.
This feature is most useful when using multiple servers and some of them are down. It ensures that users wait
a minimal period of time before trying a secondary host.
You can also set these parameters via the web interface, as described in “SIP Penalty Box” on page 68 .
Penalty Box vs Transport Types
Mediatrix recommends to use this feature with care when supporting multiple transports (see “SIP Transport
Type” on page 224 for more details) or you may experience unwanted behaviours.
When the Mediatrix 1204 must send a packet, it retrieves the destination from the packet. If the destination
address does not specify a transport to use and does not have a DNS SRV entry that configures which
transport to use, then the Mediatrix 1204 tries all transports it supports, starting with UDP. If this fails, it tries
with TCP. The unit begins with UDP because all SIP implementations must support this transport, while the
mandatory support of TCP was only introduced in RFC 3261.
Let’s say for instance that the Mediatrix 1204 supports both the UDP and TCP transports. It tries to reach
endpoint “B” for which the destination address does not specify a transport and there is no DNS SRV entry to
specify which transports to use in which order. It turns out that this endpoint “B” is also down. In this case, the
Mediatrix 1204 first tries to contact endpoint “B” via UDP. After a timeout period, UDP is placed in the penalty
box and the unit then tries to contact endpoint “B” via TCP. This fails as well and TCP is also placed in the
penalty box.
Now, let’s assume endpoint “B” comes back to life and the Mediatrix 1204 tries again to contact it before UDP
and TCP are released from the penalty box. First, the unit tries UDP, but it is currently in the penalty box and
there is another transport left to try. The Mediatrix 1204 skips over UDP and tries the next target, which is TCP.
Again, TCP is still in the penalty box, but this time, it is the last target the Mediatrix 1204 can try, so penalty
box or not, TCP is used all the same to try to contact endpoint “B”.
There is a problem if endpoint “B” only supports UDP (RFC 2543-based implementation). Endpoint “B” is up,
but the Mediatrix 1204 still cannot contact it: with UDP and TCP in the penalty box, the unit only tries to contact
endpoint “B” via its last choice, which is TCP.
The same scenario would not have any problem if the penalty box feature was disabled. Another option is to
disable TCP in the Mediatrix 1204, which makes UDP the only possible choice for the unit and forces to use
UDP even if it is in the penalty box.
You must fully understand the above problem before configuring this feature. Mixing endpoints that do not
support the same set of transports with this feature enabled can lead to the above problems, so it is suggested
to either properly configure SRV records for the hosts that can be reached or be sure that all hosts on the
network support the same transport set before enabling this feature.
Penalty Box Configuration
The following steps describe how to configure the penalty box feature.
X
To set the penalty box feature:
1. In the sipMIB , locate the sipPenaltyBox group.
2. Set the amount of time, in seconds, that a host spends in the penalty box in the sipPenaltyBoxTime
variable.
Changing the value does not affect IP addresses that are already in the penalty box. The
sipPenaltyBoxTime only affects new entries in the penalty box.
Note:
It is not the destination itself that is placed in the penalty box, but the combination of address, port
and transport. When a host is in the penalty box, it is never used to try to connect to a remote host unless it
is the last choice for the Mediatrix 1204 and there are no more options to try after this host.