Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 10.1 Unsuccessful session setup through NAT

Of these three problems identified, only this first one has a solution in SIP. A

proxy or user agent (UA) receiving this request would compare the IP address

in the Viaheader to the IP address from which the packet was received. If the

two are different (as they would be if a NAT is present), the correct IP address

is added to the Viaheader with a received=parameter listing the actual IP

address. This IP address would be used to route the response successfully back

to user A, provided the NAT maintains the same binding between the private

IP address and public IP address. (This is not a problem if TCP is used as the

transport. When a TCP connection is opened, the NAT creates the binding

between the private IP address and port number and the assigned public IP

address and port number. When the connection closes, the NAT removes the

binding.) However, no easy solution exists for the other two problems.

The second problem could be solved by a persistent TCP connection for the

duration of the session. This would mean that the Contactheader would

never be used to route future requests (such as a re-INVITEor BYE), since

there would always be an open TCP connection.

A possible solution to the third problem has been proposed [5] that involves

making RTP flows symmetric. For the case where only one endpoint is behind

a NAT, RTP packet flow will be possible in at least one direction. This is so

because the SDP of the endpoint outside the NAT will contain a correct glob-

ally routable IP address and port number. The use of symmetric RTP would

SIP User Agent A NAT Network A

5. RTP Packets to 100.101.102.103


6. INVITE
   Via: 10.1.2.3
   Contact: [email protected]
   SDP 10.1.2.3


7. ACK

SIP can negotiate

NAT but SDP A

contains private IP

address – RTP fails.

Requests sent by B

to A’s Contact URI

Solution isSTUN and also fail.

TURN

protocols!

UCP

Scenario

(TCP also

works)

SIP User Agent B

2. 180 Ringing
   Via: 10.1.2.3; received= 1.2.3.4
   Contact: [email protected]


3. RTP Packets to 10.1.2.3 ??
   7. BYE [email protected] ??


4. 200 OK
   Via: 10.1.2.3; received= 1.2.3.4
   Contact: [email protected]
   SDP 100.101.102.103c

176 Chapter 10