NAT traversal for SIP-based presence services with symmetric dialog routing

SIP (Session Initiation Protocol) uses UDP and TCP as a transport protocol as well as SCTP, TLS. In the case of using UDP as a transport protocol, it is required to guarantee symmetric response routing for NAT or FW traversal. By the way, in the case of RFC3265-based presence services using SIP, there is some situation that requires symmetric dialog request/response routing for NAT. In this paper, we describe the way of guaranteeing symmetric SIP message routing within same IP-dialog.