Re: [ledbat] Note well on the BitTorrent problem on the example...

On Nov 20, 2008, at 2:33 PM, Robb Topolski wrote:

> Can you say more about this?

In the aggregate, a bulk-data P2P system must upload as much as it  
downloads, and your performance becomes limited by uplink bandwidth.

When you have a significant group of hosts with a near-zero uplink,  
these hosts contribute very little to the overall performance of the  
system.

EG, if 1/2 of the systems are cable modems with 1 Mbps uplink, and 1/2  
are behind 128 kb uplink, the aggregate system bandwidth is 8-1: for  
every single bit of uplink from the DSL lines, you get 8x the uplink  
from the cable modem lines.

At such a point, you might as well, from an aggregate system  
viewpoint, reduce the upload of the DSL hosts to 0: they are  
contributing so little, but experiencing a huge degredation of  
performance that MUST occur because of the minimum buffer size of a  
few packets.

You can NEVER size a buffer for a DSL modem for normal TCP such that  
you will get good interactive behavior while a full rate upload occurs.

In contrast, you can use an effectivev 100-150ms buffer on the cable  
modems, so they can get full rate TCP but at the same time, not step  
on their interactive traffic (subequent message coming on that).

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