40 • Confi guration
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flag is again that it makes the messages bigger and this increases the
probability that you will have problems with UDP fragmentation.
4.3.12
Removing Headers
As stated before, you may want to remove some headers to
make messages shorter. The “
Remove the following headers”
setting
lists the headers (separated by space) that you want to strip from a SIP
packet. This setting does not only help you in making the packets shorter,
it can also help you to keep some parts of the SIP message secret. For
example, you might want to remove P-Asserted-Identity headers from all
SIP messages, because you don’t want others to see which identities you
already checked.
4.3.13
Codec Control
In many environments, you want to exclude codecs from being
used, although both communication partners could agree on them. The
“
Allow only the following codecs”
setting lists the codecs (separated
by space) that you will allow. If you don’t set anything here, all codecs will
be allowed. The codecs must be written in their SDP name, for example
“ulaw”, “alaw”, “gsm”, “g729”, “g723”, etc.
This feature can be used, for example, to make sure that only
low-rate codecs are being used. The user agents might otherwise agree
on ulaw, which might lead to breaking voice if the bandwidth is not suf-
ficient for a stream using ulaw.
4.3.14
Web Server Integration
The filter supports the integration of a web server. Before a call
is established the filter may change the to- and from-header content and
the request-URI.
Changing the to- and from header has several advantages. First,
it allows inserting the display name to a plain telephone number. If you
have a database where you can retrieve the information, you can make
your web server look this information up. Secondly, you can change the
outgoing identity and hide the true identity of the caller. This is sometimes
helpful when you want a certain number to be called back, e.g., in cases
when you have several alias names for an account.
4.
