[e2e] TCP implementations in various OS's

[Detlef Bosau]

sthaug at nethelp.no wrote:
>> To do this with normal TCP, you need to know the reason for the 35 ms RTT.
>>     
>
> Please assume that I am aware of this. The 35 ms I am talking about is
> mostly due to speed of light in optical fiber. 35 ms RTT corresponds 
> to a fiber path length of 3000+ km, and a few ms of forwarding delay,
> which is perfectly reasonable in my country (Norway).
>   

O.k.

Another consideration is the number of flows. Are those long distance 
fibers common for backbone connections or for connections with only some 
few flows?


>> That's what my criticism is all about. In order to achieve reasonable 
>> fairness, either _all_ competing users use window scaling - or none.
>>     
>
> Here again we have different goals. For my *core* network links, one
> of my primary goals is to ensure sufficient capacity. In a normal
> situation there is *no* congestion (throw bandwidth at the problem) -
> and as far as I can see fairness doesn't enter the picture at all.
>   

I don't see a conflict here. However, when we enable window scaling for 
_all_ competing flows, there may a moderate level of congestion here, 
because the flows' windows must achieve an equilibrium somehow.


> For my *customer* network links, the customer is paying for a certain
> capacity. How the customer shares the capacity between different flows
> is none of my business. Fairness is none of my concerns here either.
>   

And it's not your business whether or not the customer utilizes or 
underutilizes the line, I see.

> However, I *do* care about the customer having the possibility of using
> the capacity he's paying for - and that means the customer needs to be
> told about window scaling if expects decent TCP throughput on high RTT
> paths.
>
>   
I admit that I missed the situation of long distance _customer_ lines. 
That's an actual shortcoming in my rationale.

However, can we agree that a good measure to prevent misbehaviour (which 
_can_ result from a single flow using window scaling while the 
competitors don't) is to enable window scaling actually on _all_ flows 
or on _no_ flows? Although this might lead to some moderate level of 
congestions even in lines with comparably moderate load?

-- 
Detlef Bosau            Galileistraße 30        70565 Stuttgart
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