As shown in the image below, Proxy 1 tries to do serial forking to Proxy 3 and then Proxy 4.

Upon reception of the 480 answer ( blue frame) from Proxy 2, the Proxy 1 generated an ACK (red frame) and sent it to Proxy 1. The problem I have is that the ACK generated by Proxy 1 has a wrong RURI. I expect the ACK to be sent to the URI of the CONTACT header of the 480 message (blue frame). However, its RURI is the same as the one in the first INVITE of the dialog.  Note that there is NO ROUTE header in the ACK message.

As there is serial forking here, the ACK is generated by the Proxy 1 itself.  

Any hint would be appreciated,

Best Regards,

Minh Phan