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Implementing DCCP: Insights and Challenges in Early Stage Development

This document outlines the ongoing experience of implementing DCCP (Datagram Congestion Control Protocol) with a focus on the DCCP-TP variant. The current release (R0.00) supports core DCCP features but does not encompass all functionalities. Key points include an original approach to the implementation, challenges faced due to the complexities of the specifications, and future steps, including work on CCID 3 and contributions from the community. Observations highlight the intricacies and interactions of DCCP features, particularly regarding memory and complexity.

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Implementing DCCP: Insights and Challenges in Early Stage Development

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  1. Implementing DCCP Experience so far… Tom Phelan tphelan@sonusnet.com

  2. DCCP-TP • Fresh start implementation optimized for portability • Thinking embedded systems • No code from Linux implementation – all original • Current release R0.00 – early stage • Supports core DCCP (RFC 4340, but with DCCP-NAT encap) and CCID 2 (RFC 4341). • Doesn’t support every feature of DCCP, but a lot • Good effort made to ensure supported features are responsibly implemented • Corners cut mostly in API at this stage • Includes Linux user-space port • DCCP service and apps run as single Linux process (with Pthreads)

  3. DCCP-TP (more) • Near-term next steps: • I’m working on CCID 3 • Jawad Shafi working on Windows user-space port • Documentation (wiki), source code and discussion forum at: • http://www.phelan-4.com/dccp-tp/ • Anyone who wants to contribute, see http://www.phelan-4.com/dccp-tp/tiki-index.php?page=Todo+List and jump in 

  4. Observations • The specs present a number of difficulties for implementers • Gerrit Renker’s recent post on rfc3448bis hits the nail on the head • Almost all features have relevant text scattered about • Often need to read multiple sections of two or three different RFCs to get full sense of a feature • Many features interact with other features • To implement a single feature you need to first understand several other features • Difficult to tell normative from informative • Conflicting text about optionality/desirability of features • Reducing oscillations in rfc3448bis good example

  5. Observations (2) • Much of the suggested implementation text didn’t work for me • Issues with fuzziness, memory use, complexity, efficiency often led me to pursue other solutions • Coming to an understanding of the suggested implementation helped • Pseudo-code was great • One, shows you how to do it, all in one place • Two, getting to understand it teaches that you need to look all over for relevant text • But does it really work? • I haven’t found any problems yet

  6. Observations (3) • Ack Vectors are difficult • Biggest problem is unbounded memory needs • Second is complexity • One of the features where relevant text is most scattered about • And what does all this cost buy you? • Knowledge of which packets were lost • What good is that?

  7. Conclusions • DCCP is feature-rich • Some features are highly complex • Easy to go down wrong path if you aren’t careful • Interop between independent implementations will be work

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