[74830] in North American Network Operators' Group
Re: why upload with adsl is faster than 100M ethernet ?
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Fri Oct 15 07:34:25 2004
In-Reply-To: <20041015100401.6848.qmail@web53609.mail.yahoo.com>
Cc: NANGO <nanog@merit.edu>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Fri, 15 Oct 2004 13:33:44 +0200
To: Joe Shen <joe_hznm@yahoo.com.sg>
Errors-To: owner-nanog-outgoing@merit.edu
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.
