| 12.1 |  | Advanced Protocols |
| 12.1.6 |  | SIP fixup configuration |

| The next protocol that will be examined is the Session Initiation Protocol (SIP). The SIP is a signaling protocol that is used in establishing telephony connections. These connections are referred to as call handling sessions, which is another way of saying two-party audio conferences, or calls.

SIP works with Session Description Protocol (SDP) for call signaling. SDP specifies the ports to be used for the media stream. The PIX Security Appliance also supports SIP proxies. Using SIP, the PIX Security Appliance can support any SIP Voice over IP (VoIP) gateways and VoIP proxy servers. SIP and SDP are defined in the following request for comments (RFCs):

- SIP: Session Initiation Protocol, RFC 2543
- SDP: Session Description Protocol, RFC 2327

To support SIP calls through the PIX Security Appliance, signaling messages for the media connection addresses, media ports, and embryonic connections for the media must be inspected. This inspection occurs because while the signaling is sent over a well-known destination port, UDP/TCP 5060, the media streams are dynamically allocated. The fixup protocol sip command can be used to enable or disable SIP support. SIP is a text-based protocol and contains IP addresses throughout the text. With the SIP fix up enabled, the PIX Security Appliance inspects the packets. Both NAT and PAT are supported.

Figure illustrates the SIP connection process. The important item to notice is that the media streams are being dynamically allocated. The Command Reference illustrates the command and its syntax.