Silence Suppression, Connection Oriented Media

[ S N O M 4 S N A T F I L T E R ]

If you set this variable, the NAT ﬁlter will attempt to compress the message until it ﬁts into the size. By default, it will use the short names (e.g. “l” instead of “Content-Length”). If this should not be enough, it will start to remove headers. These headers are: “User-Agent”, “Accept-Lan- guage”, “P-Key-Flags”, “Allow”, and “Allow-Events”. If the packet is still too big, it will stop compressing the packet and send it as it is. If you want to remove other headers, please use the “remove the following headers” feature described below.

4.3.12Silence Suppression

Silence suppression is a little problematic for the ﬁlter. When a user agent does not send media, it might lead to closing of allocated NAT ports on the media. Therefore, it is usually safer to turn silence suppres- sion off. We recommend doing this by provisioning the respective setting to the user agents; however there is a way to indicate this in the SDP as well. If you turn the Add silence suppression ﬂag on, the ﬁlter will add this hint to the SDP. Usually it does not cause any additional problems; however it makes the packet a little bit bigger which could cause addi- tional problems with the UDP fragmentation problem.

4.3.13Connection Oriented Media

Typically, you want two-way communication between the same ports in a conversation. Unfortunately, the IETF speciﬁcations do not mandate this. For example, it is allowed to have different ports for send- ing and for receiving data. This causes big problems when trying to make phone calls trough NAT. The comedia approach tries to standardize the requirements on using the same port for sending and receiving and to indicate if two-way communication is really desired. By turning the Add comedia ﬂag feature on, you will make the ﬁlter add a suitable ﬂag to SDP to indicate that this behaviour is desired. The disadvantage of this ﬂag is again that it makes the messages bigger and this increases the probability that you will have problems with UDP fragmentation.

4.3.14Removing 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

38 • Conﬁguration