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Implementation Report on Experiences with Various TCP RFCs

This report discusses the implementation and testing experiences of various TCP RFCs, including Window Scaling, ECN, DSACK, and F-RTO. It focuses on the issues encountered and the impact on Windows Vista.

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Implementation Report on Experiences with Various TCP RFCs

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  1. Implementation Report on Experiences with Various TCP RFCs Murari Sridharan muraris@microsoft.com Deepak Bansal dbansal@microsoft.com Dave Thaler dthaler@microsoft.com IETF 68 - TSVAREA

  2. We did a new implementation of a bunch of RFCs… Started new TCP/IP stack in 2001, shipped in Windows Vista. Including: • Window Scaling (RFC 1323, PS) • ECN (RFC 3168, PS) • DSACK (RFC 3708, EXPERIMENTAL) • F-RTO (RFC 4138, EXPERIMENTAL) Then we did lots of testing of the RFCs above… IETF 68 - TSVAREA

  3. Window Scaling (RFC 1323), 1/3 • Previously in Windows, but off by default • We ran into a number of problems testing it for Vista: • IGD problems: multiple popular vendors, popular models • Popular vendor #1, several models having SPI block TCP handshake with WS: automatic detection and recovery at end system • Popular vendor #2, models with SPI drop packets on connections after a large scaling factor (>2) successfully negotiated during initial handshake: no way to recover at TCP layer • Firewall problems: multiple vendors have almost identical bug in old versions • Multiple popular vendors, multiple old models (one only if a non-default feature enabled) drop packets if scaling factor > 2 has been negotiated • Core Issue: Broken firewall logic that does not scale window in TCP headers correctly while determining how much data to let through IETF 68 - TSVAREA

  4. Window Scaling, 2/3 • More problems… • Broken websites: websites running old versions of firewalls in front of their web servers are broken when scaling factor > 2 is negotiated • A more thorough testing (of over 10 million websites) has revealed 2-4% websites impacted by this issue • Proxy problems: • Popular web proxy TCP splicing option (has been disabled by default since 2006) has problems understanding the scale factor. IETF 68 - TSVAREA

  5. Window Scaling, 3/3 • Bottom Line: • Problems are fairly likely if used by a consumer (not behind a proxy) • Much less problematic if behind a proxy since proxy may not negotiate Window Scaling • Result: Limited to scale factor of 2 in Windows Vista for HTTP connections IETF 68 - TSVAREA

  6. ECN (RFC 3168), 1/2 • New for Windows Vista • Straightforward to implement • We ran into a number of problems: • IGD problem #1: one of the most popular versions from one of the most popular vendors • When a data packet arrives with either ECT(0) or ECT(1) (indicating successful ECN capability negotiation) indicated, router crashed • Cannot be recovered at TCP layer • IGD problem #2: multiple popular vendors with popular models • When the 3-way handshake completes, prior to any data being sent, router hit a bug causing further packet loss • Easily detected and recovered at TCP layer IETF 68 - TSVAREA

  7. ECN, 2/2 • Bottom Line: • Problems are fairly likely if used by a consumer (bad IGDs exist) • Much less problematic in campuses and enterprises (IGDs less likely to be present) • Result: Turned off by default in Windows Vista IETF 68 - TSVAREA

  8. DSACK (RFC 3708) • New for Windows Vista • Straightforward to implement • No issues yet • Now enabled by default • Actual benefits hard to quantify to a specific improvement in real world IETF 68 - TSVAREA

  9. F-RTO (RFC 4138) • New for Windows Vista • Straightforward to implement • No issues yet • Now enabled by default • We have seen improvements in wireless environments but we have not isolated the benefits to specific enhancements • We agree with advancing F-RTO to PS IETF 68 - TSVAREA

  10. Questions? • Details will be at http://support.microsoft.com/kb/934430(not published yet) IETF 68 - TSVAREA

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