Re: [Tsvwg] A coment on "draft-briscoe-tsvwg-relax-fairness-00" re file-sharing

["Robb Topolski" <[e-mail]>]

> ... How many active (unchoked) connections
> you get initially depends on configured upload slots, which is indeed
> about 3-4 per swarm (gets linearly larger for faster access lines -
> see graph in presentation below).

Bob, this may be quite valid, and I'm not sure whether the BitTorrent
development community has put much thought into this.  As residential upload
pipes grow larger, it is possible under BitTorrent's current configuration
recommendations to have many more parallel tasks going at once.  The 3-4
upload slots could become 30-40 (assuming 512 Kbps upload becomes 5 Mbps).
It's still not the scenario of 100 or more, but 30-40 is a sizable departure
from 3-4 and BitTorrent developers might need to consider how today's TCP
stacks would treat that in a congestion situation.  Do you think this is
worthy of discussing?  

Still, I don't think the goal is "fairness."  I can't think of anyone who
wants P2P uploads to compete "fairly" with online games, telephone calls, or
active web surfing.  

What I honestly don't understand is why we can't mark these packets using
any of the Internet Standard, vetted, approved methods so that applications
give ISPs instruction and permission to handle them on a "weather
permitting" basis.  Currently, we have ISPs that ignore these flags, inspect
the packets, and pass their own brand of judgment upon them.

If the network operators enable that on their networks, I think the apps
will follow.  And yes, there will be some end-user abuse of these flags --
but that's true of almost any technical change.  The great majority won't
abuse it.

> * But if peers stays on line after they have finished (seeders),
> other peers still downloading will activate (unthrottle) connections
> to nearly as many seeders as they can find, irrespective of the
> upload slots limit.
> 
> This is because upload capacity is the bottleneck (as you say). But
> once you're no longer downloading, BitTorrent no longer needs to
> preserve your upload capacity for the ack stream that was keeping
> your download going.

Your understanding has some errors.  

First, the downloaders don't activate the seeders, the downloaders only
indicate "Interest."  The seeder client is in positive control.   Before any
transfer can take place, the seeder must unchoke that peer so that peer can
make a request.  The seeder does not unchoke all of the downloaders, it only
unchokes when it has a slot available.  (Sometimes, your uploads are going
to people who can't download fast -- such as dial-up users.  Many clients
have a feature that will automatically open an additional slot if this is
detected.)

You are correct that a downloader will indicate "Interest" to all seeders,
and that many seeders might unchoke that downloader and allow it to make
requests.  But in that case, those flows are downward in direction --
currently not the big concern of residential ISP congestion.  Meanwhile the
seeding clients that are transferring to him still maintain control over the
number of uploading flows from their nodes so that they can respond to signs
of congestion.  

Since congestion is detected and responded to by the sender (the seeder),
there is nothing inappropriate with this model since the seeder is
controlling the number of outgoing flows it has.

A BitTorrent client will still respect the upload limit specified either at
install time or modified by the user, even if it's done downloading.
However, in many clients there is the option to specify both conditions --
an upload limit while downloading and upload limit while only seeding.  It's
against the users interests to turn this up higher -- for reasons similar to
the ones you mentioned.  The user still wants to surf and use other services
on the 'net.

> My presentation of that draft to the Transport Area in Vancouver
> <http://www.cs.ucl.ac.uk/staff/B.Briscoe/present.html#0712relax-fairness>
> shows a screen shot of a colleague's Azureus client (taken randomly
> at the time I asked him for it). He had 3 swarms running (fairly
> typical) and 105-114 active TCP data transfer connections, with only
> about 10-15 choked. One of the swarms is shown. You can see ~40
> connections are active in this one swarm. This merely implies there
> were a lot of nice friendly seeders in his swarm.

I think you misinterpreted that display, perhaps.  Notice that the upload
speed of all of these was only a few bytes a second.  That's overhead from
downloading.  More evidence is the fact that they're all seeders -- so they
aren't getting any data from you (they already have all of the pieces).
Client wide, it doesn't appear that he was uploading anything data all,
since he limited his upload to 11 KB/s (lower right corner) yet was only
sending out 2.2 KB/s -- that might be one upload slot or might all be
overhead chatter.

He is also misconfigured.  With a global upload limit of 11 KB/s, one might
suggest he try eMule instead, or only run one torrent at a time and let the
others queue.  In the long run, it is more efficient.

HTH

Robert M. "Robb" Topolski <[e-mail]>
Hillsboro, Oregon USA

Variables won't; constants aren't.
Osborn 

> -----Original Message-----
> From: Bob Briscoe [mailto:[e-mail]]
> Sent: Monday, April 14, 2008 5:07 AM
> To: Robb Topolski
> Cc: 'IETF Transport Area working group mailing list'
> Subject: Re: [Tsvwg] A coment on "draft-briscoe-tsvwg-relax-fairness-00"
> re file-sharing
> 
> Robb,
> 
> I've split out this more detailed response to your mail - see my
> previous posting for these two responses:
> a) Could my scenario become common, even if it's not currently prevalent?
> b) The discussion is over what the correct fairness /goal/ should be
> 
> c) Who is actually correct for today's Internet?
> ---------------------------
> The 3-4 active connections statement is just as much a fallacy as the
> hundreds of active connections statement. Which you get depends on
> the evolution of the situation over the lifetime of a swarm:
> * If the swarm is immature (ie few peers have finished) your peer
> will limit open connections. How many active (unchoked) connections
> you get initially depends on configured upload slots, which is indeed
> about 3-4 per swarm (gets linearly larger for faster access lines -
> see graph in presentation below).
> * But if peers stays on line after they have finished (seeders),
> other peers still downloading will activate (unthrottle) connections
> to nearly as many seeders as they can find, irrespective of the
> upload slots limit.
> 
> This is because upload capacity is the bottleneck (as you say). But
> once you're no longer downloading, BitTorrent no longer needs to
> preserve your upload capacity for the ack stream that was keeping
> your download going.
> 
> In some communities you get a lot of seeders staying online. In
> others, they're all mingey gits who disconnect once they've got what
> they want.
> 
> Also, if you're going for niche ('long-tail') content, of course you
> won't get many active connections, as it's hard to find peers.
> 
> My presentation of that draft to the Transport Area in Vancouver
> <http://www.cs.ucl.ac.uk/staff/B.Briscoe/present.html#0712relax-fairness>
> shows a screen shot of a colleague's Azureus client (taken randomly
> at the time I asked him for it). He had 3 swarms running (fairly
> typical) and 105-114 active TCP data transfer connections, with only
> about 10-15 choked. One of the swarms is shown. You can see ~40
> connections are active in this one swarm. This merely implies there
> were a lot of nice friendly seeders in his swarm.
> 
> 
> Bob
> 
> At 05:30 12/04/2008, Robb Topolski wrote:
> >Comment on
> >
> >Problem Statement: We Don't Have To Do Fairness Ourselves
> >draft-briscoe-tsvwg-relax-fairness-00
> >
> >
> >Dear Working Group members,
> >
> >Thank you for your time working on this committee and issues such as
> these.
> >
> >The above quoted paragraph requires a number of corrections.  This
> paragraph
> >is a basis for the problem statement that follows it -- so getting the
> basis
> >down right is key to analysing the issue and sizing up the problem.  The
> >corrections that I hope to effectively communicate seem to me to reduce
> the
> >stated size of the problem.
> >
> >--Quote--
> >Even though file-sharing generally uses TCP, it uses the
> >well-known trick of opening multiple connections--currently around
> >100 actively transferring over different paths is not uncommon. A
> >competing Web application might open a couple of flows at a time, but
> >perhaps only actively transfer data 1-10% of the time (its activity
> >factor). Combining 50x less flows and 10-100x lower activity factor
> >means the traffic intensity from the Web app can be 500-5,000x less.
> >However, despite being so much lighter on the network, it gets 50x
> >less bit rate through the bottleneck.
> >--EndQuote--
> >
> >I am relatively certain that the draft is talking about BitTorrent above,
> >and it unforunately misinterprets what it does.  BitTorrent is the #1 P2P
> >File Sharing protocol in use today.  Furthermore, neither #2 P2P file
> >sharing protocol ED2K (aka eDonkey, eDonkey2000, eMule, or MFTP) nor #3
> >Gnutella (aka G1, G2, or Gnutella2) work like BitTorrent, nor as
> Briscoe's
> >draft described above.
> >
> >In the case of BitTorrent, between 35-70 TCP connections are generated.
> >However, only three or four of these connections are actively
> transferring
> >data.  Therefore, any benefit to the "trick" is erased.  The reason the
> >remainder sit idle is described in the specification
> >(http://wiki.theory.org/BitTorrentSpecification#Choking_and_Optimistic_Un
> cho
> >king), chief of which is the desire that BitTorrent NOT interfere with
> TCP
> >congestion control.
> >
> >It is important to remember that P2P users are largely residential users,
> >most using connections that are configured with asymmetric connections
> where
> >the maximum download bandwidth allowance is several times that of their
> >upload allowance.  In all of the discussions about "fairness" that I've
> been
> >involved in, it has to do with sharing the tiny upload pipe.  Filling the
> >generously-configured download pipe is seldom a user fairness concern (it
> is
> >also very difficult to control at the receiving end).  A trend is barely
> >beginning toward more symmetrical residential accounts, but it is
> probably
> >too small to accurately determine changes in usage patterns created by
> it.
> >
> >That BitTorrent uses "hundreds of connections" that are simultaneously
> >active is a common misperception.  While the draft likely did not mean to
> >inject emotionalism, the label of "trick" is both inaccurate and
> >inflammatory.  The reason that BitTorrent makes these idle connections is
> to
> >allow for one connection to "hunt" for the best reciprocating peer, which
> >allows two efficiently located peers to detect and reciprocate with each
> >other.  If not interfered with by network operators, this pretty much
> >ensures that the lowest-latency peers will eventually find one another
> and
> >engage in a long-term exchange of data.  This is not only fast for the
> user,
> >choosing the most efficient (lowest latency) pairings is friendliest to
> the
> >local ISP network and the Internet as a whole.
> >
> >All three of the top P2P protocols do open multiple upload connections.
> >ED2K seems happiest with 3-4 KB/s upload flows, Gnutella with 10 KB/s
> upload
> >flows, and BitTorrent with anywhere from 3 KB/s to 7 KB/s flows.
> Therefore,
> >the number of upload flows that a single peer might generate (think
> >different archives being sent by the same client simultaneously) is a
> >function of dividing the configured upload bandwidth by the amounts
> >decribed.
> >
> >An example:
> >
> >BitTorrent:  Upload limit set to 20 KB/s, 3-6 upload TCP connections
> >(remainder are idle or downloading only)
> >ED2K:  Upload limit set to 20 KB/s, 5-6 uploading TCP connections
> (remainder
> >of connections are downloading only or performing light-bandwidth
> >housekeeping, such as queue placement requests or updates)
> >Gnutella:  Upload limit set to 20 KB/s, 1-2 uploading TCP connections
> >(remainder, same as for ED2K).
> >
> >All three of the above have a UDP component, which is used for searching
> or
> >other light network housekeeping.  There is an emerging use of UDP for
> "NAT
> >hole punching" but these "connections" are still quite rare.
> >
> >I am happy to answer any questions or further clarify anything that I
> can.
> >
> >Sincerely,
> >
> >Robert M. "Robb" Topolski
> >[e-mail]
> 
> __________________________________________________________________________
> __
> Notice: This contribution is the personal view of the author and does
> not necessarily reflect the technical nor commercial direction of BT plc.
> __________________________________________________________________________
> __
> Bob Briscoe,                           Networks Research Centre, BT
> Research
> B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.    +44 1473
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>