Subject: Re: [p2pi] After-BoF charter

From: Bob Briscoe <[email-masked]>
Date: Thu, 04 Sep 2008 13:54:42 +0100

Phil,

[reposted with p2pi list added, after mistakenly dropping it]

At 23:25 02/09/2008, Philip Levis wrote:
>On Aug 29, 2008, at 5:20 AM, Bob Briscoe wrote:
>
>>Phil,
>>
>>Continuing after long intermission due to holiday season...
>>
>>You are right, but the explanation you give below uses a model of
>>how p2p networks work that is too restrictive to make it believable.
>>Reciprocity requirements don't mean 2 peers have to upload &
>>download from /each other/. However, you did help me realise I was
>>wrong.
>
>>I believe we can make this (carefully worded) statement:
>>
>>Localisation of file-sharing peers /only/ harms no-one if the app
>>enforces upload-download reciprocity, at least approximately.
>
>I'm not sure this condition is true.
>
>Stanislav said it clearly: in a peer-to-peer system, the number of
>downloaded bytes is the same as the number of uploaded bytes.

This is a correct statement, but insufficient to help understanding, 
ie it doesn't shed light on the discussion we're having. Thinking 
only like this led me to my previous incorrect statements that you 
rightly corrected. All this says is that unicast flows have two ends. 
The point you raised is the important one: a downloading node tends 
to need to also be an uploading node (reciprocity).

>Your
>argument is based on the case where a node could chose a distant node
>which would happily provide data with no reciprocation.

Eh? Quite the opposite. I'm agreeing that you are correct and I was 
wrong because there /is/ typically reciprocity. Or do I misunderstand?

>In this case,
>it's better to choose a distant node, as it will not consume your
>uplink capacity.
>
>Of course, it will consume *someone*'s uplink capacity. That node not
>providing uploads means some other node needs to. It's a zero sum
>game. Besides trying to force all of its customers to be leeches,
>there's not much an ISP can do.

I'm merely pointing out that the incentives for an ISP to provide an 
ALTO service depend on there being reciprocity. Reciprocity typically 
exists, so this is fine.

I'm just saying we should be clear what scenario we are assuming in 
the charter, ie. we should state reciprocity as an assumption.

-------------------
Warning: the further discussion below is subtle and optional for 
those who just want to get the charter sorted.

I agree reciprocity is likely to be stable, as it succumbs to the 
dismal view of humans as primarily self-interested.

BUT... there /are/ significant p2p communities with many altruistic 
members (seeders). And there are p2p systems that encourage altruism. 
An ISP will not want to offer ALTO services to these communities, as 
although they may be altruistic within their community, it is too 
much to expect them to /also/ be altruistic to all the ISP's other 
customers (because then peers would have to prevent their mates doing 
excessive uploading from them, which is a contradiction).

See an early research project on this <www.mmapps.info> (ended 2004) 
- pls ignore the project name which became an embarrassment once we'd 
done the research. In retrospect it would better have been called 
"motivation management of p2p services".

Finally, returning to your quote of Stas: the issue isn't about byte 
balance, it's about cost balance and bytes don't all cost the same. 
THeir cost is orders of magnitude different depending on which link 
and at which time they are sent. Each upload-download pair can happen 
at different times and the two ends are on different links with 
different levels of congestion at different times.

Bob

>Phil

____________________________________________________________________________
Bob Briscoe, <[email-masked]>      Networks Research Centre, BT Research
B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.    +44 1473 645196  

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---

From: "Vijay K. Gurbani" <[email-masked]>
Date: Thu, 04 Sep 2008 09:51:52 -0500

> the charter, ie. we should state reciprocity as an assumption.

That may inordinately hamstring us, no?  After all, an ISP-
operated ALTO server may know of a caching peer who does not
care about reciprocity.

- vijay
-- 
Vijay K. Gurbani, Bell Laboratories, Alcatel-Lucent
1960 Lucent Lane, Rm. 9C-533, Naperville, Illinois 60566 (USA)
Email: vkg@{alcatel-lucent.com,bell-labs.com,acm.org}
WWW:   http://www.alcatel-lucent.com/bell-labs
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