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BitTorrent Calls UDP Report "Utter Nonsense"
Posted by
kdawson
on Mon Dec 01, 2008 07:03 PM
from the internet-still-alive-film-at-11 dept.
from the internet-still-alive-film-at-11 dept.
Ian Lamont writes "BitTorrent has responded to a report in the Register that suggested uTorrent's switch to UDP could cause an Internet meltdown. Marketing manager Simon Morris described the Register report as 'utter nonsense,' and said that the switch to uTP — a UDP-based implementation of the BitTorrent protocol — was intended to reduce network congestion. The original Register report was discussed enthusiastically on Slashdot this morning."
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Bittorrent To Cause Internet Meltdown 872 comments
Gimble writes "Richard Bennett has an article at the Register claiming that a recent uTorrent decision to use UDP for file transfers to avoid ISP 'traffic management' restrictions will cause a meltdown of the internet reducing everybody's bandwidth to a quarter of their current value. Other folks have also expressed concern that this may not be the best thing for the internet."
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Best of intentions (Score:5, Interesting)
BT may have the best of intentions here in developing this experimental protocol, but this quote leads me to believe that their understanding of the problem is terribly naive:
It so happens that the congestion control mechanism inside TCP is quite crude and problematic. It only detects congestion on the internet once "packet loss" has occurred - i.e. once the user has lost data and (probably) noticed there is a problem.
Packet loss is a normal and deliberate mechanism by which TCP detects the maximum thoughput of a path. Periodically it increases the number of packets in flight until the limit is reached, then it backs off. You have to test again from time to time, in order to increase throughput if more capacity becomes available. This in no way incurs "loss of data" or a noticeable problem. Packets lost due to congestion window growth are handled by the fast retransmit algorithm, which means that there is no timeout or drop in throughput (that would be pretty stupid if the whole purpose of growing the congestion window is to _maximize_ throughput).
I wonder if Simon Morris was merely oversimplifying for the benefit of the layman, but I still find that statement disturbing. As I sugggested in the other thread, it really sounds like they're going to reinvent TCP (poorly). That's not to say you couldn't design a better protocol specifically for point-to-multipoint transfer, but I question if they're on the right track here.
Re: (Score:3, Informative)
Re:Best of intentions (Score:4, Insightful)
This reminds me of grc.com's dire warnings that Windows XP's "raw sockets" would cause chaos on the Internet. Total and utter bunk. But I guess that's what you should expect from the Register.
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Re:Best of intentions (Score:4, Insightful)
Detecting maximum throughput via packet dropping is really bad in high-latency links and in applications that need low latency. It is also apparently easy to implement TCP in such a way that overall transfer speed takes a nosedive when latency gets high, as evinced by Microsoft having done just that.
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Re:Best of intentions (Score:5, Informative)
Detecting maximum throughput via packet dropping is really bad in high-latency links and in applications that need low latency.
I disagree. Please be more specific. First, what exactly do you believe is the problem? Secondly, how else would you do it (on an IP network)?
It is also apparently easy to implement TCP in such a way that overall transfer speed takes a nosedive when latency gets high, as evinced by Microsoft having done just that.
So you're saying it's possible to implement it with a bug? I've recently found a heinous bug in a recent Redhat kernel which would result in _deadlocked_ TCP connections. It happens to the best of us.
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Re:Best of intentions (Score:5, Informative)
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Re:Best of intentions (Score:5, Informative)
I agree with you Seanadams, but I just finished an interview with Simon Morris [networkper...edaily.com] and it's not that he's saying that the way TCP handles packet loss is a particular problem, he just thinks he can do better.
BitTorrent essentially already has it's own methods to deal with dropped packets of information - it gets the information from elsewhere. Moving to UDP eliminates the triple handshake, and it eliminates throttling down packet sizes in response to a dropped packet.
The only problem is that it also eliminates the Layer 4 [transport] traffic congestion safeguards, which is why BitTorrent is looking to establish new and better ones at layer 7 [application].
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Re:Best of intentions (Score:5, Insightful)
Because application-specific knowledge allows easier and often better optimizations than application-generic protocols, which have to be good enough for all applications at the expense of top end performance for specific applications. Isn't it obvious?
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Re:Best of intentions (Score:5, Interesting)
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Re:Best of intentions (Score:5, Interesting)
Uhh, TCP Vegas [wikipedia.org], TCP New Reno [faqs.org], BIC [wikipedia.org] and CUBIC [wikipedia.org]? All of which have been implemented in the Linux kernel?
TCP has only been standing still since the 80's if you're using an OS from the 80's... or a Microsoft OS.
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Re:Best of intentions (Score:5, Informative)
Uhh, TCP Vegas [wikipedia.org], TCP New Reno [faqs.org], BIC [wikipedia.org] and CUBIC [wikipedia.org]? All of which have been implemented in the Linux kernel? TCP has only been standing still since the 80's if you're using an OS from the 80's... or a Microsoft OS.
Note that the only one of those which made it into an RFC is New Reno, aka RFC 2582 [faqs.org], which has been implemented in the Windows TCP stack since Vista [microsoft.com], along with a number of other recent RFCs.
The others are basically different suggestions for implementing TCP congestion control. Microsoft has its own variant of those (Compound TCP [wikipedia.org], which is quite similar to TCP Vegas and has also been ported to Linux [caltech.edu]).
Your 1980s comment is not quite up to date, of course. Microsoft has been sticking to their BSD-based implementation of the TCP stack for quite a long time (too long in fact), but with Vista it's been undergoing quite a bit of change. I know it's unpopular to say something in favour of MS and/or Vista here and I'm far from being a MS apologetic, but it's worth actually reading their Cable Guy columns [microsoft.com] every now and then to be up to date with regards to what the Windows network stack actually does and doesn't do - especially if you are a sysadmin or interested in developments in the TCP arena.
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Re:Best of intentions (Score:5, Insightful)
Maybe he has spent years working with the technology he is trying to one up?
This relates directly to a reply I just finished on another thread regarding whether a degree is required for success.
This Morris character may be right, he may be wrong, but you citing the education level of his rivals blows your point out of the water IMO.
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Re:Best of intentions (Score:5, Funny)
I know, how dare anyone try to improve things without being old and having a PHD.
Fucking punk ass kids. Next thing we know we will have guys with only master degrees trying to write operating systems.
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Re: (Score:3, Insightful)
Wait, so does that also mean that Vista was written by Copyright Lawyers?
As it happens, yes it was. And when you think about, that explains a lot.
Re:Best of intentions (Score:4, Funny)
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Re:Best of intentions (Score:5, Insightful)
I wonder if you know what you're talking about. TCP is a great general-use protocol, as is UDP. But for specific cases, like this one, developers will tend to roll their own as an extension to UDP to fit their needs.
Take any modern networked game. It will not use straight TCP, as that's stupid, nor will it simply use UDP because that doesn't work, either. TCP is fine if you're writing apps that require data to get to the other end, without regards to time taken, but if you need a happy middle-ground you need to do it yourself.
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Re:Best of intentions (Score:5, Insightful)
does Simon Morris think that he, single-handedly, can do better than the folks at the IETF, most of whom have PhDs in computer science?
You really have a very strange over-estimation of the value of a PhD. It doesn't mean you're a super-genius, you're smarter than everyone (or even anyone) else, or that you're always right. It simply means you've been willing to go through some schooling. That's it. Hopefully you've learned something from that education.
There's plenty of examples of non-PhD's making major contributions. The WWW was largely invented by someone with a degree in physics (undergrad I believe), with no degree in computer science. Linus Torvalds only attained a mere masters degree in Computer Science, but yet his OS seems to have become a bit more successful than quite a few other OS's written by people with more education.
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Re:Best of intentions (Score:5, Insightful)
And don't forget that those IETF PhD's couldn't design a better way to upgrade IPV4 but the incompatible and essentially non-interoperable IPV6 (please don't argue about dual stacks or something similar.)
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Re:Best of intentions (Score:4, Insightful)
Further, the guys at IETF are not tasked with coming up with optimal protocols for specific applications. They're not necessarily even tasked with coming up with better versions of TCP. There are tons of things they have come up with that improvements on what we use now, for that matter, that haven't been picked up by the Internet at large.
The expertise of the IETF members really has no bearing on this matter. If you were trying to actually recreate TCP (i.e., a general-purpose protocol), and the IETF said that theirs is better -- they're probably right. That's not what's going on here.
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Re:Best of intentions (Score:5, Informative)
Please mod parent up. This place is so damn full of armchair wannabe network experts who've clearly no understanding of how TCP congestion avoidance works it's bordering on the physically painful.
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Re: (Score:3, Insightful)
Bittorrent doesnt need reliable communications, and congestion control can be tuned for bittorrent connections.
IMHO its a good move.
Re:Best of intentions (Score:5, Insightful)
TCP does two things at the cost of some overhead: it ensures packets arrive in order and it ensures they arrive. While doing that, it also has to ensure that in the case of network congestion, packets are resent within reasonable time while being fair and allowing each TCP connection an equal share of the speed and bandwidth. The problems TCP solves are at the root of the success of bittorent; bittorrent is extremely good at spreading traffic in such a way that links that have those problems are avoided. It can do this since the order in which the packets arrive does not matter and in fact it does not matter whether they arrive from a certain host at all; it can simply request a lost packet from another host minutes or hours later. In the case of TCP/IP there is no provision to handle such a case; it keeps trying to get the packets at their destination in the right order as quickly as possible.
So none of the problems that TCP solves affect bittorrent and all the overhead that TCP causes, however small, serves no purpose in this case. Instead of many small TCP ACKs, resends, negotiation, and what else TCP does, bittorrent will do just fine with one status update every now and then, which it can conveniently combine with the packets that are sent the other way anyway. Therefore, IMHO, using UDP for bittorrent is fine; it will help spread bittorrent traffic even better over the fastest links while using less bytes of network traffic and it might even end up making it easier on the Internet since now bittorrent traffic no longer has to fight with other TCP connections for a fair share of the bandwidth; it can be tuned to get just a little bit less.
Of course they can fuck it up completely and fill the poor pipes of the Internet with loads of packets that never arrive, but they don't have to; it is not inherent to this solution and therefore such rumours should be classified as... FUD.
Disclaimer: I did not RTFA and now practically nothing about TCP/IP :-)
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Re:Best of intentions (Score:4, Informative)
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Re:Best of intentions (Score:5, Insightful)
If you're going to argue UDP, you might as well bring out the major benefits:
* Going to a NAK-based or hybrid NAK/ACK-based protocol which can significantly improve performance over high latency or poor connections
* Multicast - assuming anyone implements IPv6 or multicast over the internet
* NAT to NAT transfers (you can do it with TCP, but it is just harder and you generally have to build a user-space TCP stack anyway).
* Faster start time since you no longer have to do a three-phase startup and all the annoying things Microsoft does to prevent people from starting too many per second
There are plenty of UDP-based protocols with TCP-friendly congestion control mechanisms out there and plenty of research into the subject.
The biggest problems I see happening here revolve around different BitTorrent clients all reimplementing uTP and doing a poor job at it. I'd like to see a spec for uTP and a public domain implementation to help minimize the problems that could pop up.
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Re: (Score:3, Informative)
He was simplifying, and you can see it by looking at what happens before packet loss starts. When traffic loads increase, routers don't immediately start dropping packets. First they start queueing packets up for handling, trying their best to avoid packet loss. Only when their queues fill up do they start discarding packets. TCP waits until that's happened before it starts backing off. A better way is what uTP sounds like it's designed to do: watch for the increasing latency that signals routers are starti
Japan (Score:5, Insightful)
I just got back from living in Japan not too long ago. Over there, we got 100 mbps up AND down for about ~$40/month. Near the end of my stay there, I got a letter from my ISP stating that they're going to start implementing a bandwidth cap, and that no user can upload 500gb/month, but downloading is still unlimited.
This year, I read on Slashdot that AU/KDDI is unrolling a 1gbps line for a similarly cheap price.
If you want my sympathy for ISPs in America, get back to me when I get even one tenth of the service I got in Japan. I'd like to extend a big middle finger of gratitude to all American ISPs. No one is spouting the gloom and doom over in Asia, and meanwhile, they're shooting ahead into the future of the internet. Asia's been rolling out fibre-optics, at great cost to them, but with spectacular results. Conversely, we sit on our asses and wonder how we can charge more money to more users to put them through the same amount of pipes without upgrading infrastructure.
**** you, American ISPs.
Signed,
Someone who's seen the other side of the world, and it was better.
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Re:Japan (Score:4, Informative)
I have 100MBit full-duplex for $30 a month, and this is standard pricing where I live (in Europe). There is a gigabit fiberlink going into our building (apartment building with 24 apartments) and from there it's cat 6 going to each apartment.
This was the cost of getting it...
About $1500 for getting the fiber into the building (one time cost including the router). In fact, I had a choice of two different providers and there is four separate fibercables in the road outside my apartment block. It was put there when the roads were dug up for changing some pipes. After the waterpipes were changed, any company that wanted to lay down fiber along the road was allowed to do so, four companies put it there, out of which two companies were selling fiber connections to consumers. The companies paid nothing for digging up the road and the work took two days longer due to the fibers being put down. This is standard for any roadworks and has been for quite some time, which is the main reason for the wide spread of fiber in the cities here.
Some equipment was installed in the basement, from there, a cable was pulled to each apartment. The cost for this was $200 / apartment. So, total cost per apartment was about $275.
Then, the company that we got the fiber from sells 100/100 subscriptions for just under $30/month. The only thing with this company, as opposed to the other company, was that the initial cost was lower but in return you have to get the internet connection via them (three year contract). After the three years have passed, you can "get out" of the contract with them but keep the fiber for an additional $1000. If this is done you can then freely choose you ISP, and the cost currently, for the cheapest provider, is $19 for a 100/100 connection. For this apartment block we were paying $150 a year as a base-fee to our cable provider (per apartment). Since TV channels can be gotten via the fiber, at a lower cost then what the cable company charges for the same channels, the $275 was weighted against the $150 a year we now don't have to pay (this is how I got even the "old" people to agree to the fiber installation). Two years, even if you don't have an internet subscription, and it's a good dael. Finally, you can also use this for VoIP, $5 / month if you want a subscription where you get a phone number and free calls to any phone in the entire country (mobiles excluded).
I can add that the other provider wanted $5000 for getting the fiber in to the building, so the difference was $3500. Over three years it was about the same (may even have worked out cheaper depending on the subscriptions you choose), but I felt it was easier to convince people in the apartment block to get the fiber in to the building if I could keep the initial cost to a minimum.
So, I can simply not understand why some countries are having such a hard time rolling out fiber connections. There are roadworks all the time, since roadworks are handled by some central government function (gen
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It is The Register... (Score:4, Insightful)
The Register (Score:5, Funny)
Nonsense? From The Register? Ya don't say.
In other news... (Score:5, Funny)
My ISP, Comcast, is already on top of this new bittorrent over UDP idea and has summarily blocked all UDP traffic.
So, I wonder if I'll be experNO CARRIER
unknown host slashdot.org
Re: (Score:3, Informative)
Sorry, SMTP (e-mail) uses TCP (almost always port 25). Perhaps you mean SNMP (network management)? A typical home router doesn't normally use SNMP, but more expensive ones (Cisco, etc.) do.
The last mile is the real problem (Score:4, Insightful)
I don't really see how this is going to kill VoIP and online gaming. Those two services are big users of UDP, no doubt, but its not like all of a sudden the explosion of UDP requests is going to sqeeze VoIP traffic out. If anything it should encourage ISPs and providers to increase the rate of their roll out of new tech.
This move should bring into focus the last mile problem that is the real source of most of the internet connection speed debate. I don't care how the solution ends up working, but I think there needs to be a plan given that most of the plans I have heard involve several years of lead time.
Re: (Score:3, Interesting)
When someone says "the last mile problem," I think the last mile is short on bandwidth. The problem here is that the last mile has (and is using) more bandwidth than the upstream connections can handle.
Re:The last mile is the real problem (Score:4, Funny)
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The problem is... (Score:5, Informative)
----
The major problem I see is that UDP doesn't play as nicely as TCP. Not by a longshot.
As soon as TCP notices a single packet loss, the Jacobson Algorithm kicks in and it's throttled to maybe 50-60%, and raises the limit slowly. I highly doubt that uTorrent's reworked version of UDP will play this nicely.
As soon as TCP's throttling kicks in, space will be cleared in the tubes. uTorrent will be able to send more data through UDP without noticing any loss, so it'll quickly move to fill this space. Then, TCP gets hit with more data loss - and goes slower. It seems like a vicious cycle.
Re:The problem is... (Score:5, Informative)
The major problem I see is that UDP doesn't play as nicely as TCP. Not by a longshot.
That statement makes no sense whatsoever. It's the same as saying "IP doesn't play as nicely as TCP". They are not comparable.
You have to be talking about whatever transfer protocol might be implemented ON TOP OF UDP, because UDP is merely a datagram protocol. It's nothing more than an IP packet plus port numbers.
In other words, I could write a simple program which blindly fires UDP packets at the rate of 1GB/s. This would kill my internet connection. I could also write a program which transmits one UDP packet per hour. This would have no effect. See what I mean? It's entirely a function of the application.
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UDP and DHT are a nightmare (Score:5, Interesting)
everyone thought dht was great too, but I found every time I used it it caused massive headaches. I would jump on a popular torrent and for days afterward I would be having poor performance, checking logs etc would show several dozen connection attempts per second on the utorrent port, even 2-3 days after I was done with the torrent because the DHT tracker was still advertising my IP address. I'd have to release renew to bring my performance back up. This was with a fairly standard Linksys router. Any situation where the other party might not just get the message that I'm not there anymore is bound to lead to headaches on popular torrents.
Re: (Score:3, Informative)
Update your firmware and tweak your BT settings. I run the WRT54G and I get no problems whatsoever even with multiple torrents.
I run a WRT54G V4 with the Tomato firmware. Likewise I have no problems.
Remove TCP-like requirements, and it's a WIN. (Score:5, Informative)
TCP guarantees in-order, mildly error-corrected, delivery of transmitted *DATA*. Not packets. It is a streaming protocol where the data being transmitted is of unknown and indeterminate length, or open-ended length. Since BT already doesn't care that files arrive in pieces at the beginning, middle or end (well, it does a little, but not enough to matter) then you can relax and basically eliminate one of the TCP guarantees right there: in-order delivery of data.
You can eliminate sliding windows, ACK-based retransmits, fast-retransmission, and pretty much every other mechanism that TCP uses to guarantee in-order delivery of data. You can simplify it and the application itself beautifully, and provided it correctly throttles back based on detected packet loss (it MUST be exponential back-off,) you end up with a net win for those reasons. The application can set up its own optimised data structures that don't necessarily have (but likely will end up having anyway) the overhead of an OS-backed TCP stack.
I mean who cares if you miss pieces, until the end when you can re-request them?
Heck, there are already P2P apps that use a UDP-based transfer mechanism and they are WAY less impactful on systems that a typical BitTorrent stream is. They way the hell slower, too, but that's not the point.
I do think there is a point that bears repeating: BT *MUST* have exponential back-off. If it doesn't there is logic already built-in to core routers, ISPs, and firewalls that WILL drop the connection more severely when the endpoints don't respond properly to an initial packet-drop attempt at slowing them down.
There are some really nice academic papers about it, and there are lots of algorithms and choices that companies have. They all assume TCP-like back-off on the endpoints, and they ALL uniformly punish greedy floods.
What BitTorrent REALLY needs (Score:5, Interesting)
The probablem with BitTorrent is not that it uses a large amount of bandwidth, but that it's using the wrong bandwidth. In every country other than the United States, international bandwidth is substantially more expensive than local bandwidth, and often in short supply. Local bandwidth is cheap, or even free. Even in the US, inter-ISP bandwidth has the same cost issues, but is plentiful.
What I've never understood is what's the excuse for not implementing a peer selection algorithm that prioritises nearby users. Even a naive algorithm is going to be vastly better than a purely random selecton. Simply selecting peers based on, say, the length of the common prefix of the IP address will often produce excellent results. Why in God's name should I transfer at 0.1 kbps from some guy in Peru, when a peer down the road could be uploading to me at 500 kbps?
The truth is that the BitTorrent folks are not playing ball with ISPs. In reality, I think most major ISP could care less about copyright violation, or excessive bandwidth - it makes people pay for more expensive monthly plans - but they DO care about international bandwidth costs.
If they just took 10 minutes to revamp the peer selection algorithm, they would reduce the impact in ISPs enormously, and then they woudldn't be villified and throttled.
I think you're making a bad assumption. (Score:4, Informative)
The truth is that the BitTorrent folks are not playing ball with ISPs. In reality, I think most major ISP could care less about copyright violation, or excessive bandwidth ...
Unfortunately, the major ISPs are components of conglomerates whose primary moneymaker is selling "content". As such they have a perverse incentive structure that can put "protecting against piracy" above the quality of the network's operation.
The networks also provided asymmetric transport and vastly oversold their bandwidth, assuming a central server / many small clients "broadcast media" model. The rise of peer-to-peer usage bit them mightily and Bit Torrent was the spearhead of that rise. So rather than spending the added billions to expand their backbones to meet their advertised service's requirements they chose to throttle it.
The ISPs were the ones to turn this into a war and fire the first shots. BitTorrent is just trying to engineer a solution on which to build peace - and is being vilified for the attempt.
Having said that, your suggestion for improving things by smarter selection of peers is good. Unfortunately the Internet doesn't have any easy mechanism to indicate which peers would be better. Good solutions would likely have to be built on additional knowledge - which implies a database to hold and serve it - which implies a new central infrastructure and queries of it - which both breaks the decentralized model and provides additional points of attack if the ISPs continue to treat this as a war and attempt to suppress "unauthorized"/"enemy" torrents.
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Re: (Score:3, Insightful)
Having said that, your suggestion for improving things by smarter selection of peers is good. Unfortunately the Internet doesn't have any easy mechanism to indicate which peers would be better. Good solutions would likely have to be built on additional knowledge - which implies a database to hold and serve it - which implies a new central infrastructure and queries of it - which both breaks the decentralized model and provides additional points of attack if the ISPs continue to treat this as a war and attempt to suppress "unauthorized"/"enemy" torrents.
I posted a while ago an idea I'd like to see for a Request For Comment (RFC), a new protocol ISPs could easily run in-house. See http://slashdot.org/comments.pl?sid=590741&cid=23883635 [slashdot.org]
Actually even Cisco could add such a protocol to their routers which merely look up the internal routing protocol to decide which IPs were local, and anything out a border gateway (routes advertised via BGP) could be regarded as "non-local". Anyone from Cisco here?
Re:I think you're making a bad assumption. (Score:4, Insightful)
Autonomous system numbers are the perfect metric.
http://en.wikipedia.org/wiki/Autonomous_system_(Internet) [wikipedia.org]
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The original Register report... (Score:3, Funny)
Reducing network congestion (Score:5, Interesting)
Where where? (Score:5, Informative)
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Re:Where where? (Score:5, Funny)
Their their now, let the poor unedjumicated bastard post in piece.
I mean, those in glass houses shouldn't through stones. ;)
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Re:Where where? (Score:5, Funny)
You misspelled bastige.
If you're glass house is made of bullet proof glass then it's OK to throw stones.
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Re:Where where? (Score:5, Informative)
I'm not sure, bouncing stones could be pretty painful.
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Re:Where where? (Score:5, Funny)
Of course, the whole enterprise ended in tears when he bought one too many 24-karat armchair and the building simply disintegrated. He was whining to me on the phone afterwards and I could only offer him the following simple advice: "People in grass houses shouldn't stow thrones".
Sorry.
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