[Blink] On re-invite ACK not reaching callee

[Robert Dyck]

Using latest flatpak for Linux.

Caller on sip2sip.info calls subscriber at a different domain. Callee answers and dialog is 
established. So far all good.

Caller does a re-invite ( hold ). The callee's UA sends 20OK. Caller's UA sends ACK. ACK 
never arrives. Callee's UA continues sending 200OK.

Callee is registered as GRUU. Incoming requests are normally translated by the local 
registrar and the request is forwarded to the callee's IP address. This also applies to re-
INVITE and its ACK. The caller learns the registrar's address from the Contact it previously 
received in the initial 200OK.

Summary of in-dialog requests
Initiial ACK - OK
re-InVITE - OK
second ACK - doesn't arrive
Bye - doesn't arrive

An important detail - Registrar/Proxy is behind NAT. Router use DMZ forwarding.

The request URI of the ACK following re-INVITE is pointing at the external address of the 
registrar but the ACK was received at a private address. The ACK is forwarded to the 
external address. It never reaches the registrar because the router cannot hairpin the 
forwarded message.

*ACK sip:8 at external-address:5060;gr=urn:uuid:38ee02db-f267-4dd9-9077-1b9d4fcfbf00 
SIP/2.0* 
                                                           *Record-Route: 
<sip:212.95.45.179;lr;ftag=803fbe29-6196-4af7-9771-db7d25007e6c>* 
                                                           *Record-Route: 
<sip:174.142.205.46;r2=on;lr;ftag=803fbe29-6196-4af7-9771-db7d25007e6c>* 
                                                           *Record-Route: 
<sip:174.142.205.46;transport=tcp;r2=on;lr;ftag=803fbe29-6196-4af7-9771-
db7d25007e6c>* 
                                                           *Via: SIP/2.0/UDP 
212.95.45.179:5060;branch=z9hG4bK45c1.804d44d2.2* 
                                                           *Via: SIP/2.0/UDP 
174.142.205.46:5060;branch=z9hG4bK45c1.ab795fa4.2;i=d0969e07* 
                                                           *Via: SIP/2.0/TCP 192.168.1.3:58491;received=external-
address;rport=58491;branch=z9hG4bKPj62eb9faf-6eba-4f91-9539-8b2954cad94a;a*
*lias*

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