[e2e] Question about propagation and queuing delays

Bill Sommerfeld sommerfeld at sun.com
Wed Aug 24 09:23:01 PDT 2005

On Wed, 2005-08-24 at 05:36, Fred Baker wrote:
> The peak rate sftp reported was about 141.3 KB/s, and the least rate  
> was 34.2 KB/s. The difference most likely relates to the effects of  
> packet loss (1.3% loss is non-negligible), delay variation (a  
> standard deviation in ping RTT of 442 ms and an absolute variation in  
> delay of 2034 ms are also non-negligible), the effects of slow-start  
> and fast-retransmit procedures, or the bandwidth remaining while  
> other users also made use of the link.

If by sftp you mean the protocol described in some version of
draft-ietf-secsh-filexfer running over ssh, i'd be cautious in using it
as a path benchmark.

ssh muxes multiple stream channels over a single tcp connection, and as
a result does its own flow control on a per-channel basis so that
implementations aren't forced to choose between unlimited buffer usage
within implementations or deadlock.  

There have been anecdotal reports to the secure shell wg that default
channel window sizes in commonly used implementations are far from
optimal but I haven't heard any updates in a while.  

But then, I'm just the cat-herder for that WG and not an implementor of
that protocol family...

							- Bill

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