[asterisk-dev] SIP replies to via or to ip/port (according toRFC)

[Nick Lewis]

>nat=yes:
>
>1) Force an rport parameter in the first Via header. The (default)
procedures
>described in paragraph A above will do the rest.
>
>2) Force symmetric RTP.

I think that strictly speaking it is down to the UAC behind the firewall
to 
request RFC3581 behaviour by adding an empty rport rather than the
asterisk 
UAS adding it unilaterally. However I agree with this approach so that
if 
the setting is nat=yes (rather than nat=route or nat=never) asterisk 
will operate "as if" the UAC had added the rport parameter. 

The situation with nat=no is more tricky - perhaps the default could be 
changed to nat=never and nat=no depecated. 

It may be worth having another setting nat=compliant that just does as
rfc3581 
requires i.e. if rport is received then use both symmetric rtp and
symmetric 
sip.

Also we need to consider an option nat=auto that operates as per nat=yes
but 
only for contact addresses 192.168.0.0/16 172.32.0.0/12 and 10.0.0.0/8

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