[IP] Re: BitTorrent net meltdown delayed

[David Farber <dave@farber.net>]

Begin forwarded message:

From: Richard Bennett <richard@bennett.com>
Date: December 8, 2008 4:03:35 PM EST
To: dave@farber.net
Subject: Re: [IP] Re:   WORTH READING   BitTorrent net meltdown delayed

It turns out that Nick's discussion of my alleged factual errors also  
has factual errors. Compound TCP, like Vegas TCP and Cubic TCP, does  
have a congesion-avoidance state in which it reduces its sending rate  
in response to increased latency. The point is that delay-based  
congestion mitigation is not a new technique in transport protocols.  
The larger issue is the extent to which a transport may think it's  
found lightly-loaded path because it has actually found one where  
traffic shaping doesn't detect it as a file-sharing protocol.

The traffic shaping wars have escalated to the point that high-end  
shapers use traffic signatures rather than header inspection to  
determine stream type, and the obvious reaction is to cloak high  
volume transactions to fool signature analysis. The use of protocols  
that look like gaming or VoIP traffic would accomplish that.

The goals of this working group are noble, the intentions of the  
BitTorrent company are pure, but the effects on traffic shapers are an  
important part of the system that needs to be understood. And if  
anyone still doubts that traffic shaping is necessary on the belief  
that TCP does the job of congestion control, please consider that the  
LEDBAT group is based on the premise that TCP does not actually do the  
job completely or correctly.

RB

David Farber wrote:
> Begin forwarded message:
>
> From: Lars Eggert <lars.eggert@nokia.com>
> Date: December 8, 2008 3:44:56 AM EST
> To: David Farber <dave@farber.net>
> Cc: dewayne@warpspeed.com
> Subject: Re: [IP] WORTH READING   BitTorrent net meltdown delayed
>
> Hi,
>
> this second article by Richard Bennett is much less alarmist than  
> his first one, but still has some factual errors. Nicholas Weaver  
> summed them up in a recent post to the IETF's LEDBAT list: http://www.ietf.org/mail-archive/web/ledbat/current/msg00080.html
>
> LEDBAT (http://www.ietf.org/html.charters/ledbat-charter.html) is a  
> new IETF working group on "low extra delay background transport".  
> Its main work item is to develop an experimental congestion control  
> algorithm for less-than-best-effort "background" transmissions,  
> i.e., an algorithm that attempts to scavenge otherwise idle  
> bandwidth for its transmissions in a way that minimizes interference  
> with regular best-effort traffic. BitTorrent has indicated that they  
> will submit their DNA scheme as input to this working group.
>
> Lars
>
> PS: For IP, if you think it's relevant.
>
> On 2008-12-7, at 16:37, David Farber wrote:
>
> > Begin forwarded message:
> >
> > From: dewayne@warpspeed.com (Dewayne Hendricks)
> > Date: December 5, 2008 3:25:07 PM EST
> > To: Dewayne-Net Technology List <xyzzy@warpspeed.com>
> > Subject: [Dewayne-Net] BitTorrent net meltdown delayed
> >
> > Original URL: <http://www.theregister.co.uk/2008/12/05/richard_bennett_bittorrent_udp/
> > >
> >
> > BitTorrent net meltdown delayed
> >
> > But is UTP the best approach?
> >
> > By Richard Bennett
> >
> > Posted in Telecoms, 5th December 2008 13:48 GMT
> >
> > The internet's TCP/IP protocol doesn't work very well. As the
> > internet's traffic cop, it's supposed to prevent applications from
> > overloading the network, but it's at a loss when it comes to managing
> > P2P applications. This deficiency, generally known to network
> > engineers but denied by net neutrality advocates, has been a central
> > issue in the net neutrality debate. BitTorrent Inc has now weighed in
> > on the side of the TCP/IP critics.
> >
> > The next official release of the uTorrent client – currently in alpha
> > test – replaces TCP with a custom-built transport protocol called  
> > uTP,
> > layered over the same UDP protocol used by VoIP and gaming. According
> > to BitTorrent marketing manager Simon Morris, the motivation for this
> > switch (which I incorrectly characterized in The Register earlier  
> > this
> > week as merely another attempt to escape traffic shaping) is to  
> > better
> > detect and avoid network congestion.
> >
> > Morris also told the media this week that TCP only reduces its  
> > sending
> > rate in response to packet loss, a common but erroneous belief. Like
> > uTP, Microsoft's Compound TCP (http://research.microsoft.com/~kuntan/index_files/Page315.htm
> > ) begins to slow down when it detects latency increases. Even though
> > TCP is capable of being just as polite as BitTorrent wants uTP to be,
> > the fact that it hides its delay measurements from applications makes
> > it troublesome for P2P clients with many paths to choose from. But
> > it's sensible to explore alternatives to TCP, as we've said on these
> > pages many times, and we're glad BitTorrent finally agrees.
> >
> > It remains to be seen whether uTP is the best approach. The company
> > has touted its close relationships with ISPs and the IETF's LEDBAT
> > task group, but has so far shared none of the specifics of uTP
> > operation in public fora. The PowerPoints they've shared with the  
> > IETF
> > are encouraging, but they're a long way from the source code,
> > simulations, and hard data from impartial sources that are
> > prerequisite to any new protocol standard.
> >
> > One thing that is certain is that uTP will not reduce the volume of
> > traffic that P2P moves across the internet, something that would be
> > commercial suicide for a company that depends heavily on aggressive
> > file sharers, and pirates, for its popularity. But it does try to  
> > find
> > the nooks and crannies of the internet where its content, legal and
> > otherwise, can be gathered with the least impact on other users. If
> > successful, this will make the internet more stable.
> >
> > The stakes in the competition to replace TCP are considerable for a
> > struggling company that recently had to lay off half its staff.
> > BitTorrent lives at the intersection of three groups with sharply
> > opposing interests: P2P users, most of them pirates; internet service
> > providers with traffic and legacy equipment issues; and major
> > television and movie studios who want wide distribution but no  
> > piracy.
> > These interests have clashed in the past, particularly when a
> > licensing deal with the MPAA raised the shackles of private P2P
> > trackers, causing them to temporarily ban uTorrent clients until they
> > could be satisfied that the privacy of those trafficking in stolen
> > content wouldn't be compromised.
> >
> > [snip]RSS Feed: <http://www.warpspeed.com/wordpress>
> >
> >
> >
> >
> > -------------------------------------------
>
>
>
>
> -------------------------------------------

--
Richard Bennett
Network Architect





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