The work presented in this Master’s Thesis is an examination of how the SIP signaling, which occurs when a so called IP Telephony session is set up, will be able to traverse firewalls. It is necessary to solve the problems/issues that SIP brings about when the SIP messages traverse firewalls if this protocol ever will gain popularity. In order to set up those data streams needed for transporting the sound in an IP telephony session the client enters his IP address and a port number in the SDP part of the SIP message to tell the other party where he should sent his audio data. Here is where problems occurs with the firewall. It needs to understand and interpret what the SIP message says to be able to set up rules for allowing traffic to pass through the firewall to these addresses. The problem is extended by the fact that it is common today to use “private addresses” on the LAN. These addresses are not allowed to exist on the Internet and thus the firewall software must remove this address and replace it with an address that is allowed on the Internet. A Network Address Translator (NAT) in the firewall normally does this together with Application Level Gateways (ALGs).
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