Subject: Re: [p2prg] New version of p2prg-mythbustering

From: [email-masked]
Date: Mon, 13 Jul 2009 17:04:29 +0100

Have you considered the case where the ISP might prefer that their users 
*not* localise the traffic? 

The dominant wholesale xDSL technology in the UK is BT's IPStream product. 
All customer traffic is backhauled via L2TP to the ISP's network (almost 
always to London telehouses) and no geographic localisation is possible. 

This means that two neighbouring customers of the same ISP are no closer 
in network terms than two customers at opposite ends of the country. 

Why this is particularly significant for UK ISPs is that BT's IPStream 
backhaul product ("BT Central") is typically 10 times more expensive than 
global IP transit. Currently BT charge at least £100 per Mbps pcm, whereas 
volume IP transit can be had for £10 per Mbps or even less. 

If two customers mutually exchange traffic at 1 Mbps continuously it'll 
cost the ISP £200 per month.  If one of the parties is off-net it'll only 
cost them £110.

Hence IMHO for most UK ISPs it's better for their leeching customers to be 
getting their data from a neighbour that's *not* on the same network. 

Ray 

-- 
Ray Bellis, MA(Oxon) MIET
Senior Researcher in Advanced Projects, Nominet
e: [email-masked], t: +44 1865 332211

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From: Enrico Marocco <[email-masked]>
Date: Mon, 13 Jul 2009 18:26:51 +0200

[email-masked] wrote:
> Have you considered the case where the ISP might prefer *that their
> users *not** localise the traffic?
> 
> The dominant wholesale xDSL technology in the UK is BT's IPStream
> product. All customer traffic is backhauled via L2TP to the ISP's
> network (almost always to London telehouses) and no geographic
> localisation is possible.
> 
> backhaul product ("BT Central") is typically 10 times more expensive
> than global IP transit. Currently BT charge at least £100 per Mbps pcm,
> whereas volume IP transit can be had for £10 per Mbps or even less.

The case of backhaul per-bit cost being higher than transit is actually
not that uncommon. However, even without localization, peers (seeders in
particular) would keep generating traffic and the resulting total cost
would consist of both transit AND backhaul. Localization in such
scenarios could help at least reduce the transit component. IOW,
localization does not produce more seeders in the local network; if it
did, it's use in such scenarios I agree would be harmful.

> cost the ISP £200 per month.  If one of the parties is off-net it'll
> only cost them £110.

Well, no, if the content was locally available (i.e. one seeder
existed), but the local leecher downloaded it from outside, the total
cost of the traffic generated by the two peers would amount to £220 as
the seeder would just be serving some other off-net leecher (and thus
costing another £110).

-- 
Ciao,
Enrico

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From: [email-masked]
Date: Mon, 13 Jul 2009 17:54:05 +0100

> the seeder would just be serving some other off-net leecher (and thus

I was thinking more of a trivial example of two leechers on-net, and one 
seeder off-net.

Using a 10:1 cost ratio between backhaul and transit, and assuming a cost 
of "1 unit" for the cost of transferring that data over the transit.

If both leechers download everything from the seeder:

  = 2 clients * (backhaul + transit)
  = 2 * (10 + 1)
  = 22 units

If each leacher downloads exactly half of the data from the seeder, and 
the remaining half from the other leacher (assuming non-overlapping 
fragments):

  =   2 clients * 0.5 * (backhaul + transit)
    + 2 clients * (up+down) + 0.5 * backhaul      [*]
  =  (2 * 5.5) + (2 * 2 * 5)
  =  33 units

so it appears that the on-net leeching costs 50% more than two downloads 
from off-net.

Ray

[*] I think I've done this right - each leecher downloads half of the file 
from the other leecher, but the ISP has to pay for it in both directions.

---

From: Matthew Kaufman <[email-masked]>
Date: Mon, 13 Jul 2009 09:58:30 -0700

[email-masked] wrote:
> This means that two neighbouring customers of the same ISP are no 
> closer in network terms than two customers at opposite ends of the 
> country.
>
> than global IP transit. Currently BT charge at least £100 per Mbps 
> pcm, whereas volume IP transit can be had for £10 per Mbps or even less.
>
> If two customers mutually exchange traffic at 1 Mbps continuously 
> it'll cost the ISP £200 per month.  If one of the parties is off-net 
> it'll only cost them £110.
>
> Hence IMHO for most UK ISPs it's better for their leeching customers 
> to be getting their data from a neighbour that's **not** on the same 
> network.
This is the extreme case of the much more common "I would prefer to 
export this traffic via free peering than to transport it to one of my 
own customers on the other side of the country" case.

Matthew Kaufman

ps. As someone who's designed nationwide ISP networks several times, 
I've often wondered why consumer ISPs care so much about uploaders. Once 
you've got the traffic out of the end-link multiplexing (which of course 
you can trivially manage) you are either handing it to another ISP you 
peer with, or sending it to a transit provider over a circuit that 
you've bought as symmetric full-duplex bandwidth but which (as you're a 
consumer ISP) you mostly use the downstream side of, both of which have 
zero incremental cost. Unless of course you have the misfortune of a 
user who manages to find someone on your network to send the data to.

---

From: Enrico Marocco <[email-masked]>
Date: Tue, 14 Jul 2009 11:38:20 +0200

A little slip notwithstanding -- (2 * 5.5) + (2 * 2 * 5) = 31 ;-) -- the
proportion sounds reasonable, but the two cases are fundamentally
different. In fact, while in the former the two on-net peers are
literally leeches, in the latter they upload at least half of the file.
With the same share ratio (0.5, that is still half the average), the
cost without localization would sum up to:

    2 clients * (backhaul down + transit)
  + 2 clients * 0.5 (backhaul up + transit)
  = 2 * (10 + 1) + 2 * 0.5 * (10 + 1)
  = 33 units

This makes sense, as in the case with localization (cost: 31 units) you
have spent the same amount for the backhaul, but you have paid transit
for 1 file only (the 2 halves the peers downloaded from the off-net
seeder) instead of 3 (the 2 entire downloads plus the two half uploads).

The bottom line here is that if p2p traffic localization does not
increase uplink bandwidth usage (this could be argued, we took a first
stab in section 5 and 5.2 but more thought is definitely needed), the
savings are proportional to the impact of transit on the overall per-bit
cost.

Enrico

[email-masked] wrote:
> Using a 10:1 cost ratio between backhaul and transit, and assuming a
> cost of "1 unit" for the cost of transferring that data over the transit.
> 
> [*] I think I've done this right - each leecher downloads half of the
> file from the other leecher, but the ISP has to pay for it in both
> directions.