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SIP Headers

In the following sample SIP request message, NAT replaces the IP addresses in the header fields—shown in bold font—to hide them from the outside network.



INVITE bob@10.150.20.5 SIP/2.0
Via: SIP/2.0/UDP 10.150.20.3:5434
From: alice@10.150.20.3

To: bob@10.150.20.5

Call-ID: a12abcde@10.150.20.3

Contact: alice@10.150.20.3:5434
Route: <sip:netscreen@10.150.20.3:5060>
Record-Route: <sip:netscreen@10.150.20.3:5060>

How IP address translation is performed depends on the type and direction of the message. A message can be any of the following:

Inbound request
Outbound response
Outbound request
Inbound response

Table 75 shows how NAT is performed in each of these cases. Note that for several of the header fields the ALG must know more than just whether the messages comes from inside or outside the network. It must also know what client initiated the call, and whether the message is a request or response.

Table 75: Requesting Messages with NAT Table continued on next page

Inbound Request (from public to private)	To:	Replace domain with local address
	From:	None
	Call-ID:	None
	Via:	None
	Request-URI:	Replace ALG address with local address
	Contact:	None
	Record-Route:	None
	Route:	None
Outbound Response (from private to public)	To:	Replace ALG address with local address
	From:	None
	Call-ID:	None
	Via:	None
	Request-URI:	N/A
	Contact:	Replace local address with ALG address
	Record-Route:	Replace local address with ALG address
	Route:	None
Outbound Request (from private to public)	To:	None
	From:	Replace local address with ALG address
	Call-ID:	Replace local address with ALG address
	Via:	Replace local address with ALG address
	Request-URI:	None
	Contact:	Replace local address with ALG address
	Record-Route:	Replace local address with ALG address
	Route:	Replace ALG address with local address
Outbound Response (from public to private)	To:	None
	From:	Replace ALG address with local address
	Call-ID:	Replace ALG address with local address
	Via:	Replace ALG address with local address
	Request-URI:	N/A
	Contact:	None
	Record-Route:	Replace ALG address with local address
	Route:	Replace ALG address with local address

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