[bittorrent] Open letter to the BBC

Alon Rohter arohter at nolar.com
Sat Jun 25 03:36:00 EDT 2005

Correct, the percentage just counts the raw number of bytes written/read 
to/from the socket, not tcp headers.  However, Azureus makes a best-effort 
attempt at 1) sending "full" (1460 byte) packets out of the socket, 
minimizing ip header overhead, and 2) grouping/piggybacking Have messages. 
The Have logic is quite simple:  if the other peer is not interested in us 
yet, but would be, send the Have right away; otherwise delay the send until 
a) we have enough Haves for a full packet or b) we need to send some other 
message anyway (like unchoke, interested, request, keep-alive etc) and 
piggyback it on that packet.  This way it doesn't "waste" a full ip packet 
on a single 9 byte Have message unless absolutely necessary, and yet still 
retains all the completion information that would otherwise be lost if we 
just skip sending Haves for pieces the other peer already has.  It seems 
like a good compromise between efficiency and correctly "seeing" the swarm.

>> On 6/20/05, Alon Rohter <arohter at nolar.com> wrote:
>> > Azureus client, popular 350MB 1:1 share ratio at 20KB/s upload rate 200
>> > peers.
>> > 5.6% download overhead
>> > 5.7% upload overhead
>> And what exactly does the statistics view consider overhead? Is TCP
>> included? IP? Ethernet? Physical?
> Not being an expert on Azureus, but a user with the interface in front of
> me, it appears to use bytewise receives to do the calculation, which
> would likely be TCP packets - header. Including the header would probably
> double the overhead because the protocol messages are smaller. As for a
> running system performing a variety of uploads on a variety of torrents
> (a recipe for high protocol overhead), I am currently connected to 135
> peers (including seeds) and overhead in the last few seconds has been
> 4.9% download, 1.2%upload, with variances between about 2% and 7% for
> download and 0.3% and 4% for upload, spontaneous share ratio is 1:2 (half
> as much upload as download), across the entire session which is at about
> 2 days, 3% download overhead, 1% upload overhead, share ratio of 1.2:1
> (1.2 bytes upload for each one downloaded). These numbers indicate that
> there is a client or group of clients that is being actually quite
> chatty, but I don't feel like tracing it down, it's probably one of the
> older ones though..
>                 Joe

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