North American Network Operators Group

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TCP congestion

  • From: Philip Lavine
  • Date: Thu Jul 12 14:09:08 2007
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Can someone explain how a TCP conversation could degenerate into congestion avoi
dance on a long fat pipe if there is no packet/segment loss or out of order segm

Here is the situation:
WAN = 9 Mbps ATM connection between NY and LA (70 ms delay)
LAN = Gig Ethernet
Receiver: LA server = Win2k3
Sender: NY server = Linux 2.4
Data transmission typical = bursty but never more that 50% of CIR
Segment sizes =  64k to 1460k but mostly less than 100k

Typical Problem Scenario: Data transmission is humming along consistently at 2 M
bps, all of a sudden transmission rates drop to nothing then pickup again after
15-20 seconds. Prior to the drop off (based on packet capture) there is usually
a DUP ACK/SACK coming from the receiver followed by the Retransmits and congesti
on avoidence. What is strange is there is nothing prior to the drop off that wou
ld be an impetus for congestion (no high BW utilization or packet loss).

Also is there any known TCP issues between linux 2.4 kernel and windows 2003 SP1
? Mainly are there issues regarding the handling of SACK, DUP ACK's and Fast Ret

Of course we all know that this is not a application issue since developers make
 flawless socket code, but if it is network issue how is caused?


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