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RE: DSL and/or Routing Problems

  • From: John Renwick
  • Date: Tue Mar 30 11:26:51 2004

Jon,

I saw something like this once, and it was a router bug.  Your ping trace 
seems to be saying that after the first ping something gets stuck, and 
ten more pings accumulate in a queue somewhere until a ten-second timer 
expires, or maybe ten messages in a queue is a maximum that triggers some 
action.  Then the queued-up pings are all released at once and after that 
things run normally.

Very strange, but I'll bet it's some peculiarity deep in the guts of 
whatever box you're pinging.  Maybe a process stops running and gets 
restarted by an inactivity timer.  The ISP might be able to find 
something in a log file or system trace that would be a clue to someone 
familiar with the router's internals.  Did they just have an equipment
change or a software upgrade?  It doesn't feel like a link outage or
quality problem because it always starts after the first ping and lasts 
for exactly ten more.

I also wonder if the ping anomaly is even the same problem as the one 
that's causing the poor performance you're observing on your benchmarks.

Cheers, and good luck!

John Renwick


> -----Original Message-----
> From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu]On Behalf Of
> Jon.Kibler@aset.com
> Sent: Tuesday, March 30, 2004 6:08 AM
> To: nanog@merit.edu
> Subject: DSL and/or Routing Problems
> 
> 
> Greetings NANOGers,
> 
> Yesterday we starting noticing long delays on an ADSL connection. I spent most 
> of the day trying to track down the problem and getting no where. Telco says 
> they do not detect any problem on the line... so I am kind of lost. Anyone here 
> have any ideas? Here are the specifics:
> 
> This connection uses a Cisco 827 ADSL router and has several static IPs. All IPs 
> show identical delays. Using other circuits between the same two locations, we 
> do not see any delays. 
> 
> Normally on this DSL connection, local can ping remote with packet transit times 
> around 60-70ms. Here is what we are seeing now:
> 
> # ping -s SOMEHOST 68 25; sleep 1; ping -s SOMEHOST 68 25
> PING SOMEHOST: 68 data bytes
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=0. time=105. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=1. time=9132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=2. time=8132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=3. time=7132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=4. time=6132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=5. time=5133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=6. time=4133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=7. time=3133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=8. time=2133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=9. time=1133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=10. time=133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=11. time=104. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=12. time=110. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=13. time=109. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=14. time=112. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=15. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=16. time=114. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=17. time=107. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=18. time=109. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=19. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=20. time=112. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=21. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=22. time=108. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=23. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=24. time=110. ms
> 
> ----SOMEHOST PING Statistics----
> 25 packets transmitted, 25 packets received, 0% packet loss
> round-trip (ms)  min/avg/max = 104/1918/9132
> PING SOMEHOST: 68 data bytes
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=0. time=112. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=1. time=9131. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=2. time=8132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=3. time=7132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=4. time=6132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=5. time=5132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=6. time=4133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=7. time=3132. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=8. time=2133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=9. time=1133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=10. time=133. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=11. time=111. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=12. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=13. time=109. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=14. time=116. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=15. time=108. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=16. time=107. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=17. time=113. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=18. time=106. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=19. time=107. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=20. time=108. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=21. time=108. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=22. time=105. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=23. time=109. ms
> 76 bytes from SOMEHOST (w.x.y.z): icmp_seq=24. time=106. ms
> 
> ----SOMEHOST PING Statistics----
> 25 packets transmitted, 25 packets received, 0% packet loss
> round-trip (ms)  min/avg/max = 105/1918/9131
> 
> 
> What really has me bugged is the pattern shown by the first dozen packets... why 
> the relatively quick first time, followed by a long but decreasing delay which 
> repeats every time you restart the ping (that's why I provided 2 samples)? 
> 
> 
> Despite the fact that Telco says there are not any line problems, we are seeing 
> a change in DSL performance compared to our benchmark. When we first started 
> noticing the problem yesterday, both in and out connections were using the Fast 
> path, but compared to the benchmark, the inbound speed had dropped to 576 and 
> the Capacity had jumped to 99%, plus we had some RS and CRC errors on both in 
> and out connections. Later in the day, the connection switched from using the 
> Fast path to the Interleave path (we did nothing on our end to cause this to 
> change) and the performance settled down to what is shown below under "DSL NOW."
> 
> 
> DSL BENCHMARK:
> ==============
>                 ATU-R (DS)                      ATU-C (US)
> Capacity Used:   72%                             21%
> 
>                  Interleave             Fast    Interleave              Fast
> Speed (kbps):             0              960             0               256
> Reed-Solomon EC:          0                0             0                 0
> CRC Errors:               0                0             0                 0
> Header Errors:            0                0             0                 0
> Bit Errors:               0                0
> BER Valid sec:            0                0
> BER Invalid sec:          0                0
> 
> 
> 
> DSL NOW:
> ========
>                 ATU-R (DS)                      ATU-C (US)
> Capacity Used:   94%                             63%
> 
>                  Interleave             Fast    Interleave              Fast
> Speed (kbps):           736                0           256                 0
> Reed-Solomon EC:         99                0             4                 0
> CRC Errors:               4                0             1                 0
> Header Errors:            3                0             0                 0
> Bit Errors:               0                0
> BER Valid sec:            0                0
> BER Invalid sec:          0                0
> 
> 
> 
> This leaves me with a few questions...
>    1) What exactly is the difference between the Fast and Interleave paths?
>    2) At the same data rates, should both Fast and Interleave show similar 
> throughput?
>    3) What would cause a switch from Fast to Interleave to occur?
>    4) If the Telco cannot find any line problems, what can account for the 
> increased capacity use, the decreased data rates, and the data errors?
>    
> And the bottom line is: Why the bizarre ping times, and could the changes 
> observed in DSL performance account for that... plus the near doubling of 
> average ping transit times?
> 
> TIA for everyone's expert thoughts on this problem!
> 
> Jon R. Kibler
> A.S.E.T., Inc.
> Charleston, SC  USA
> (843) 849-8214
> 
> 
> 
> 
> 
> 
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