North American Network Operators Group

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Re: TCP congestion

  • From: Jay Hennigan
  • Date: Thu Jul 12 22:44:34 2007


Philip Lavine wrote:
Can someone explain how a TCP conversation could degenerate into congestion av
oidance on a long fat pipe if there is no packet/segment loss or out of order se
gments?

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
Mbps, all of a sudden transmission rates drop to nothing then pickup again afte
r 15-20 seconds. Prior to the drop off (based on packet capture) there is usuall
y a DUP ACK/SACK coming from the receiver followed by the Retransmits and conges
tion avoidence. What is strange is there is nothing prior to the drop off that w
ould 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 S
P1? Mainly are there issues regarding the handling of SACK, DUP ACK's and Fast R
etransmits.

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

Duplex mismatch on an intermediate ethernet segment?

--
Jay Hennigan - CCIE #7880 - Network Engineering - [email protected]
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