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

Figure 10.2 Unsuccessful call through firewall using UDP

Figure 10.3 Unsuccessful call through firewall using TCP

If the a UA outside the firewall attempts to establish a session with the UA

inside the firewall, all SIP and RTP packets will be blocked, regardless of trans-

port, resulting in no session.

Note that it is possible to configure a firewall to allow SIP. However, doing

so opens so many holes and weakens the protection provided by a firewall to

such a degree that few network administrators would allow it. This is in con-

trast to NATs, which currently cannot be reconfigured to pass SIP and media.

Solutions to the firewall and NAT traversal problem will now be discussed.

SIP User Agent A Firewall A

8 200 OK/TCP

11 RTP Packets/UDP

5 Open TCP Connection

Using TCP for SIP

enables the session

to be established,

but B’s RTP packets

are blocked by A’s

Firewall.

TCP

Scenario

SIP User Agent B

6 INVITE/TCP

7 180 Ringing/TCP

9 ACK/TCP

10 RTP Packets/UDP

SIP User Agent A Firewall A

4 RTP Packets/UDP

1. INVITE/UPD

Using UDP, all of B’s

responses and

packets are blocked

by A’s Firewall.

A is calling

from inside

the firewall

UDP

Scenario

SIP User Agent B

2. 180 Ringing/UDP
   3. 200 OK/UDP

178 Chapter 10