Designing DCCP: Congestion Control Without Reliability

The future of Internet transport is unreliable: fast-growing applications like streaming media and telephony prefer timeliness to reliability, and thus prefer UDP to TCP. Unfortunately, UDP lacks congestion control, so these applications are unsafe for use on underprovisioned best-effort networks unless they implement congestion control themselves—a difficult task. They might, however, happily use congestioncontrolled unreliable transport. DCCP, the Datagram Congestion Control Protocol, is a new transport protocol in the TCP/UDP family that provides a congestion-controlled flow of unreliable datagrams. It aimed to add to a UDP-like foundation the minimum TCP mechanisms necessary to support congestion control. A simple task, we thought; but removing reliability, and especially cumulative acknowledgements, forced us to reconsider almost every aspect of TCP’s design. The resulting protocol sheds light on how congestion control interacts with unreliable transport, how modern network constraints impact protocol design, and, particularly, how TCP’s reliable bytestream semantics intertwine with its other mechanisms, including congestion control.