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<TITLE>SIP over TCP or TLS - Contact Header - NAT problematic</TITLE>
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<P><FONT SIZE=2 FACE="Arial">Hi,</FONT>
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<P><FONT SIZE=2 FACE="Arial">If a SIP device is using TCP, to accept incoming calls it must bind on port 5060.</FONT>
<BR><FONT SIZE=2 FACE="Arial">To call someone, this device must create a socket to reach the proxy on port 5060. The port source of this socket cannot be 5060 ! In TCP you cannot bind on a port and in same time using it in another socket. So the port source is generally chosen randomly (XYZ)</FONT></P>
<P><FONT SIZE=2 FACE="Arial">Now that all sockets and bind are done, what are the correct info used to create the Contact header ?</FONT>
<BR><FONT SIZE=2 FACE="Arial">For me, the port used in the contact header in this case must be 5060 since the device accepts incoming calls on this port and never XYZ.</FONT></P>
<P><FONT SIZE=2 FACE="Arial">But in a NAT context, if this SIP device is behind a NAT, the port 5060 cannot be used to reach it ! It's necessary to reuse the socket previously opensed to reach it.</FONT></P>
<P><FONT SIZE=2 FACE="Arial">So in conclusion, the Contact header must be set with 5060 by the device and on the proxy side, if the client is detected behind a NAT, this Contact header is modified to used instead XYZ</FONT></P>
<P><FONT SIZE=2 FACE="Arial">Am I right ? </FONT>
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<P><FONT SIZE=2 FACE="Arial">For TLS, the problem is the same but instead of 5060 it's 5061.</FONT>
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<P><FONT SIZE=2 FACE="Arial">Thanks,</FONT>
<BR><FONT SIZE=2 FACE="Arial">Christophe</FONT>
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