Re: [PATCH] tcp: Socket option to set congestion window

[Hagen Paul Pfeifer <hagen@jauu.net>]

* David Miller | 2010-05-26 15:10:14 [-0700]:

>From: Andi Kleen <andi@firstfloor.org>
>Date: Wed, 26 May 2010 23:27:45 +0200
>
>> As I understand the idea was that the application knows
>> what flows belong to a single peer and wants to have
>> a single cwnd for all of those. Perhaps there would
>> be a way to generalize that to tell it to the kernel.
>> 
>> e.g. have a "peer id"  that is known by applications
>> and the kernel could manage cwnds shared between connections
>> associated with the same peer id?
>> 
>> Just an idea, I admit I haven't thought very deeply
>> about this. Feel free to poke holes into it.
>
>Yes, a CWND "domain" that can include multiple sockets is
>something that might gain some traction.
>
>The "domain" could just simply be the tuple {process,peer-IP}

This discussion - as once a month - is about fairness. But if we define a
domain as a tuple of {process,peer-IP} the fairness is applied only for the
last link before "peer-IP".

But fairness applies to *all* links in between! For example: consider a
dumpbell scenario:


+------+                                   +------+ 
|      |                                   |      |  
|  H1  |                                   |  H3  | 
|      |                                   |      |  
+------+                                   +------+  
  10MB  \   +------+            +------+  / 10MB
         \  |      |   1MB/s    |      | / 
          > |  R1  |------------|  R2  |<    
         /  |      |            |      | \      
  10MB  /   +------+            +------+  \ 10MB 
+------+                                   +------+  
|      |                                   |      |        
|  H2  |                                   |  H4  | 
|      |                                   |      | 
+------+                                   +------+


How can a domain defined as {process,peer-IP} fair to the 1MB bottleneck link?
It is not fair! And it is also not fair to open n simultaneous streams and so
on. This problem is discussed in several RFC's.

.02


Best regards, Hagen


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