the tcp-estats-mib matt mathis john heffner raghu reddy pittsburgh supercomputing center rajiv...
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The TCP-ESTATS-MIB
Matt Mathis
John Heffner
Raghu ReddyPittsburgh Supercomputing Center
Rajiv RaghunarayanCisco Systems
J. SaperiaJDS Consulting, Inc
IETF 62, March 2005
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The TCP Extended Statistics MIB• Use TCP’s ideal diagnostic vantage point
– Observe what the path is doing to segments
– Observe what the application is doing to TCP
• TCP already measures many path properties– RTT, RTT variance, MTU, window size
• Easily instrumented to measure other properties– Reordering, loss rate, congestion signals
• Instrument why tcp_output() stops sending– Receiver window, congestion window or the sender?
• Per-connection controls to support workarounds
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Example: a hard diagnostic problem
• Most symptoms scale with RTT– TCP Buffer Space, Network loss and reordering, etc– On a short path TCP compensates for the flaw
• Local Client to Server: all applications work– Including all standard diagnostics
• Remote Client to Server: all applications fail – Leading to faulty implication of other components
• This is the essence of the “End-to-end Problem”
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How extended TCP statistics can help• Without TCP instrumentation
– Symptoms are reduced on short sections of long paths
– Nearly all diagnostics yield a false pass on short paths
• With TCP ESTATS– Measure key properties of a short section of the path
– Extrapolate to the full path to pass judgment
– Tools get more sensitive as you test shorter sections
– Example uses Web100.org instrumented TCP• Target is a simple TCP discard service
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Example diagnostic tool outputEnd-to-end goal: 4 Mb/s over a 200 ms path including this sectionTester at IP address: xxx.xxx.115.170 Target at IP address: xxx.xxx.247.109Warning: TCP connection is not using SACKFail: Received window scale is 0, it should be 2.Diagnosis: TCP on the test target is not properly configured for this path.> See TCP tuning instructions at http://www.psc.edu/networking/perf_tune.htmlPass data rate check: maximum data rate was 4.784178 Mb/sFail: loss event rate: 0.025248% (3960 pkts between loss events)Diagnosis: there is too much background (non-congested) packet loss. The events averaged 1.750000 losses each, for a total loss rate of 0.0441836%FYI: To get 4 Mb/s with a 1448 byte MSS on a 200 ms path the total end-to-end loss budget is 0.010274% (9733 pkts between losses). Warning: could not measure queue length due to previously reported bottlenecks Diagnosis: there is a bottleneck in the tester itself or test target (e.g insufficient buffer space or too much CPU load)> Correct previously identified TCP configuration problems> Localize all path problems by testing progressively smaller sections of the full path.FYI: This path may pass with a less strenuous application: Try rate=4 Mb/s, rtt=106 ms Or if you can raise the MTU: Try rate=4 Mb/s, rtt=662 ms, mtu=9000Some events in this run were not completely diagnosed.
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Changes with -06 draft• Overhauled listen table
– Designed to instrument generic SYN flood defenses• Restructured per connection tables
– Required “perf” table• Expose TCP state variables (no memory footprint)• Basic performance instrumentation• First tier diagnostic instrumentation
– 3 optional tables for more detailed diagnosis• Path (loss, reordering, duplication, etc)• Stack (impact and state of control algorithms)• Application (Is the data motion timely?)
– 1 table of writeable controls for workarounds• LimCwnd, LinRwnd, and LimSsthresh
• Cleanup descriptions, references
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Open Technical Issues• Too much required stuff?
– There may be further juggling between tables
• The proper SMI type for “Duration”, we want:– microsecond resolution for short flows
– days (or months?) scale for exit stats
– meaningful deltas at all scales
• What have we forgotten?
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Next steps• Last call for input from implementers, researchers• MIB doctor review• WG last call sometime this summer