[e2e] TCP implementations in various OS's

Detlef Bosau detlef.bosau at web.de
Wed May 12 11:01:49 PDT 2010

On 05/12/2010 05:11 PM, sthaug at nethelp.no wrote:
> We clearly have different goals. I work for a service provider. If I
> have a customer who wants to transmit more than 15 Mbps between end
> points with a 35 ms RTT, one of my goals is to make it *possible* for
> the customer to do this (assuming the customer has sufficient access
> capacity at both ends). To do this with normal TCP, the customer needs
> window scaling.

To do this with normal TCP, you need to know the reason for the 35 ms RTT.

I remember a situation in a lab, where the RTT between to 802.11 nodes 
was 90 SECONDS (sic!).

This was obviously due to noise/interference and 802.11 retransmissions. 
Window scaling does not solve this problem - window scaling worsens 
problems like these.

It was broadly discussed in this list, that some 802.11 nodes do up to 
254 retransmissions. Hence, the reason for 35 ms is not a large 
propagation delay or a large pipeline but simply retransmissions in the 
presence of noise. It is of no help to scale up queues and windows here 
and to make thousands of packets wait for a service they will never get.

> Real life networks tend to have a range of RTTs and capacities. We need
> to accomodate this range. If vendors supplied TCP implementations which
> always performed well with window scaling turned on, I would be happy
> with that. Unfortunately, I don't believe we're there today...

I well remember a discussion with a colleague who enjoyed a fine 
goodput, I think I talked about this before.

Lucky him.

Poor others...

That's what my criticism is all about. In order to achieve reasonable 
fairness, either _all_ competing users use window scaling - or none.

Up to now, I'm convinced that in the latter case hardly any user will 
benefit from window scaling in the majority of scenarios ;-)

>> With sufficient queueing memory, you can even grow a 1 meter Ethernet
>> link to a link capacity of 5000 Terabytes.
>> I only don't see any valid reason for doing so.
> Here we agree.


Detlef Bosau             Galileistraße 30                70565 Stuttgart
Tel.: +49 711 5208031    mobile: +49 172 6819937    skype:  detlef.bosau
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