http://www.rfc-editor.org/rfc/rfc5596.txt
RFC 5596: Datagram Congestion Control Protocol (DCCP) Simultaneous-Open Technique to Facilitate NAT/Middlebox Traversal, G. Fairhurst2009-09-01T23:00:00-00:00This document specifies an update to the Datagram Congestion Control Protocol (DCCP), a connection-oriented and datagram-based transport protocol. The update adds support for the DCCP-Listen packet. This assists DCCP applications to communicate through middleboxes (e.g., a Network Address Port Translator or a DCCP server behind a firewall), where peering endpoints need to initiate communication in a near- simultaneous manner to establish necessary middlebox state. [STANDARDS-TRACK]