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Re: why upload with adsl is faster than 100M ethernet ?

  • From: Iljitsch van Beijnum
  • Date: Fri Oct 15 07:35:58 2004

On 15-okt-04, at 12:04, Joe Shen wrote:

Your explanation on TCP behavior seems reasonable, but
why TCP over fast access line express so much packet
loss than slow access line ? Do WindowsXP/Win2k
determine its startup sending window according to
access speed or path MTU ?
I don't think there is much of a difference in the actual window size. But assuming 1500 byte packets, you can transmit a packet over 100 Mbps every 121 microseconds, while over 2 Mbps this is every 6 milliseconds. Now suppose that somewhere along the path there is a link that has some congestion. This means that most of the time, that link is busy so when a packet comes in, it must wait until it can be transmitted. This isn't much of an issue, unless so many packets come in that the buffers fill up. After that, any additional packets will be dropped (tail drop) until more packets have been transmitted and there is buffer space again.

In this situation, it's likely that the 100 Mbps link will deliver several packets while the buffer is full, so that several successive packets are dropped. But the ADSL 6 ms spacing makes sure that there is always time for the buffer to drain between two packets. Tail drops will still happen, but it's less likely that several successive packets are dropped. And as I wrote in my previous message, TCP will slow down slightly for each individual packet dropped, but it will slow down dramatically when three successive packets are dropped.

If congestion is indeed the cause, it would be helpful to increase the buffer space and turn on random early discard (or detect, RED) so packet drops increase gradually as the buffers fill up and tail drops are avoided.

However, the cause can also be rate limiting. Rate limiting is deadly for TCP performance so it shouldn't be used on TCP traffic.