[asterisk-users] Understanding NAT Traversal

[hugolivude]

Quick question re. NAT traversal.

I understand how sitting behind a NAT could cause problems for a SIP UA.
The SIP UA would create SIP mesages using IP addresses from inside the
network (i.e. 192.#.#.# or 10.#.#.#) and these IP addresses are of course
unnavigable for the recipient.

What I don't get is why don't web browsers suffer the same problem?

A web brower behind a NAT sends an HTTP request much the same way as a SIP
UA might send an INVITE.  However it seems to me that in the case of the Web
Browser, the HTTP request includes the IP address of the NAT and the NAT
takes care of routing the server's response back to the web browser.

So what am I missing here?  Why doesn't it work the same way for SIP?  Where
does the HTTP / SIP analogy breakdown?  Perhaps NATs just take care of HTTP
better than SIP because it's an older protocol (i.e.  perhaps NATs replace
the unnavigable IP address with a navigable one for HTTP requests)?  Curious
anyway.

Similarly, why do we need a timeout on a SIP registration?  Does this work
the same way as a "heartbeat" enabling disconnected UA to be unregistered?

Thanks,
H
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