[SIPForum-discussion] rport and received parameter in Via header

[Paul Kyzivat]

On 9/25/17 11:56 AM, Gnana Guru Natarajan wrote:
> Hi,
> 
>         What is the purpose of rport and received parameter in sip via 
> header? Will it be needed if sip client is behind NAT router? If so, how 
> router can process this sip header as router only handle the IP header 
> but cannot application layer header?

The router *doesn't* do anything special. When the request passes 
through the NAT the sender address/port in the IP packet it updated. The 
next *SIP* proxy that receives this message then takes the observed IP 
address/port and writes them into the topmost Via. That then makes it to 
the UAS, and eventually is copied into the response message(s). When the 
response message reaches that same SIP proxy it uses the info from the 
Via, including the received and rport parameters, to determine where 
next to send the response. That will take it to the NAT device, which in 
turn will rewrite the IP packet with address/port inside the private 
network.

	Thanks,
	Paul