I'm struck by this sentence: "And for legitimate content, you don't
have all the legal headaches you do on BitTorrent."
I had thought this entire conversation was centered on distribution of
legitimate content.
Am I wrong? If so, why?
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Nicholas Weaver
Sent: Wednesday, December 03, 2008 2:27 PM
To: Y. R. Yang
Cc: Le Blond, Stevens ; Nicholas Weaver; Arnaud Legout; [email protected]
Subject: Re: [alto] Paper on "Pushing BitTorrent Locality to the Limit"
On Dec 3, 2008, at 9:18 AM, Y. R. Yang wrote:
I found this discussion on simulation, controlled experiments, and
real
trials quite interesting. I found the paper Pushing BitTorrent
Locality to
the Limit quite interesting. Here are a few observations from our
experiences:
The other factor is that you can also push the mental dial up to 100%
localization:
With HTTP/server without caches:
N copies inbound on the transit link
0 copies outbound on the transit link
N copies through the ISP's internal cloud
N copies inbound on the last hop
0 copies outbound on the last hop
Performance is limited by the web server's upload bandwidth, the
transit link, and the end user's download bandwidth
ISP cost is the transit link and the last hop
Content provider's cost is their transit cost (often much less than
ISP transit cost)
With HTTP/server with distributed caches (akamai, CoralCache, inline
HTTP cache) with perfection
1 copies inbound on the transit link
0 copies outbound on the transit link
N+1 copies through the ISP's internal cloud
N copies inbound on the last hop
0 copies outbound on the last hop
Performance is limited by the cache's bandwidth and the end user's
download bandwidth
ISP's cost is just the last hop
Content provider's cost is the distributed cache's cost (which can be
low or can be VERY expensive, I'm looking at you Akamai!)
With P2P, some localization, 1 < X < N
~X copies inbound on the transit link
~X copies outbound on the transit link
N+X copies through the ISP's internal cloud
N copies inbound on the last hop
N copies outbound on the last hop
Performance is limited by the end user's download bandwidth, the
transit link, and the end user's upload bandwidth + ability to freeride
ISP's cost is both transit in and out, and the last hop in AND out.
Content provider's cost is very low (can go to near 0 if disallows
most freeriding once the swarm is seeded)
With 100% localization and without caches:
1 copies inbound on the transit link
1 copies outbound on the transit link
N+1 copies through the ISP's internal cloud
N copies inbound on the last hop
N copies outbound on the last hop
Performance is limited by the end user's download bandwidth, the
transit link, and the end user's upload bandwidth + ability to freeride
ISP's cost is the last hop in AND out
Content provider's cost is very low.
The interesting thing about localization is, even in the limit, it
does not eliminate the transit on the last hop uplinks, and it does
not eliminate the performance bottleneck of end user's upload
bandwidth, or the dependence on freeriding if you want to download
faster than the upload allows.
This is actually a big problem, as for many technologies (DSL, cable),
the last mile is highly asymmetric and, for some (eg, Cable), the last
hop uplink can often be a serious point of shared congestion and cost
(adding bandwidth means killing TV channels!). Thus for some
significant networks, P2P is a huge magnification in aggregate cost
even with perfect localization.
OTOH, if you add caches, then you get really GOOD behavior because,
unlike HTTP caches, a failure in the cache doesn't cause a failure of
the system (the cache is out of path), adding in such caches can be
done without client changes (a cache is just another client node which
allows freeriding ONLY to local nodes), while removing the uplink as a
performance bottleneck (the cache can be placed in the ISP's internal
cloud). And for legitimate content, you don't have all the legal
headaches you do on BitTorrent.
Thus:
With 100% localization and with caches:
1 copies inbound on the transit link
1 copies outbound on the transit link
N+1 copies through the ISP's internal clound
N copies inbound on the last hop
X < N copies outbound on the last hop
This suggests the following:
ANY localization discovery protocol must also include cache discovery,
treating caches as special nodes which will only serve a set of
subranges.
Any commercial P2P content distribution scheme should include caches.
Such caches can be cheap & cheerful (a low power 1u server with 2x1TB
disks and a GigE and a boot-from-CD to boot from net bootstrap
startup, if you can't get this down to $1.5K, you're doing something
wrong) but should be accounted for right from the start.
Simply because without caches, P2P costs ISPs a lot of money to save
the content provider money, and you get everyone working at cross
purposes. And the customers suffer too, if you overload their
outbound link.
But with caches, the costs radically shift the other way, so that P2P
can save both the ISP AND content providers a lot of money. At $.10/
GB, you only have to shift 15 TB out of the cache to pay for the cost
of the cache. If your cache is serving 100 Mbps, thats only 13 days
of operation. Even at a paltry 10 Mbps duty (so ~10 1Mbps customers)
its still 130 days payback...
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