Designing DCCP: Congestion Control Without Reliability
| View Paper |  | Public Review (by Steven Low) | |
Eddie Kohler, UCLA
Mark Handley, University College London
Sally Floyd, ICSI Center for Internet Research
Abstract
Fast-growing Internet applications like streaming media and telephony prefer timeliness to reliability, making TCP a poor fit. Unfortunately, UDP, the natural alternative, lacks congestion control. High-bandwidth UDP applications must implement congestion control themselves—a difficult task—or risk rendering congested networks unusable. We set out to ease the safe deployment of these applications by designing a congestion-controlled unreliable transport protocol. The outcome, the Datagram Congestion Control Protocol or DCCP, adds to a UDP-like foundation the minimum mechanisms necessary to support congestion control. We thought those mechanisms would resemble TCP’s, but without reliability and, especially, cumulative acknowledgements, we had 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 how TCP’s reliable bytestream semantics intertwine with its other mechanisms, including congestion control.
Regarding the public review: Readers interested in the authors' take on partially-reliable variants of TCP might check out our discussion of Time-lined TCP in the related work section, or ask more questions here. Readers interested in a more high-level discussion of design choices might enjoy reading RFC 4336, "Problem Statement for [DCCP]", which discusses such questions as "why not just implement congestion control at the application layer?" and "why not SCTP?". Links: http://www.ietf.org/rfc/rfc4336.txt or http://www.cs.ucla.edu/~kohler/pubs/rfc4336.pdf for a PDF. It will be really interesting to see whether DCCP ends up widely deployed -- and I at least don't know which way it will go!
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