Fixing the Unfairness of TCP Congestion Control

duncan99 writes "George Ou, Technical Director of ZDNet, has an analysis today of an engineering proposal to address congestion issues on the internet. It's an interesting read, with sections such as "The politicization of an engineering problem" and "Dismantling the dogma of flow rate fairness". Short and long term answers are suggested, along with some examples of what incentives it might take to get this to work. Whichever side of the neutrality debate you're on, this is worth consideration."

Not all sessions experience the same congestion

The author of this analysis seems to have missed the fact that each TCP session in a P2P application is communicating with a different network user and may not be experiencing the same congestion as other sessions. In most cases (those where the congestion is not on the first hop) It doesn't make sense to throttle all connections when one is effected by congestion.

Re:

Well, I don't know about your Internet connection but the only place I notice congestion is on the first few hops (and possibly the last few hops if we're talking a single host and not P2P). Beyond that on the big backbone lines I at least don't notice it,

Re:Not all sessions experience the same congestion

Even if that is true, the congestion won't be correlated between between your streams, if it occurred on the final hops (and hence different final networks). There is a more basic problem than the lack of correlation between congestion on separate streams - the ZDnet editor, and the author of the proposal have no grasp of reality.

Here's an alternative (but equally effective) way of reducing congestion - ask p2p users to download less. Because that is what this proposal amounts to. A voluntary measure to hammer your own bandwidth for the greater good of the network will not succeed. The idea that applications should have "fair" slices of the available bandwidth is ludicrous. What is fair about squeezing email and p2p into the same bandwidth profile?

This seems to be a highly political issue in the US. Every ISP that I've used in the UK has used the same approach - traffic shaping using QoS on the routers. Web, Email, VoIP and almost everything else are "high priority". p2p is low priority. This doesn't break p2p connections, or reset them in the way that Verizon has done. But it means that streams belonging to p2p traffic will back off more because there is a higher rate of failure. It "solves" the problem without a crappy user-applied bandaid.

It doesn't stop the problem that people will use as much bandwidth for p2p apps as they can get away with. This is not a technological problem and there will never be a technological solution. The article has an implicit bias when it talks about users "exploiting congestion control" and "hogging network resources". Well duh! That's why they have have network connections in the first place. Why is the assumption that a good network is an empty network?

All ISPs should be forced to sell their connections based on target utilisations. Ie here is a 10Mb/s connection, at 100:1 contention, we expect you to use 0.1Mb/s on average, or 240GB a month. If you are below that then fine, if you go above it then you get hit with per/GB charges. The final point is the numbers, 10Mb/s is slow for the next-gen connections now being sold (24Mb/s in the UK in some areas), and 100:1 is a large contention ratio. So why shouldn't someone use 240GB of traffic on that connection every month?

Re:Not all sessions experience the same congestion

All ISPs should be forced to sell their connections based on target utilisations. Ie here is a 10Mb/s connection, at 100:1 contention, we expect you to use 0.1Mb/s on average, or 240GB a month. If you are below that then fine, if you go above it then you get hit with per/GB charges.

The author of the article, George Ou, explains why he thinks you are a stupid and evil for suggesting such a thing. [zdnet.com] Well, he doesn't actually use the word "stupid" and I don't think he actually uses the word "evil", but yeah that is pretty much says.

You see in Australia they have a variety of internet plans like that. And the one thing that all of the plans have in common is that they are crazy expensive. Obscenely expensive.

So George Ou is right any you are wrong and stupid and evil, and the EFF is wrong and stupid and evil, and all network neutrality advocates are all wrong and stupid and evil, you are all going to screw everyone over force everyone to pay obscene ISP bills. If people don't side with George Ou, the enemy is going to make you get hit with a huge ISP bill.

Ahhhh... except the reason Australian ISP bills are obscene might have something to do with the fact that there are a fairly small number of Australians spread out across an entire continent on the bumfuck other side of the planet from everyone else.

Which might, just possibly MIGHT, mean that the crazy high Australian ISP rates kinda sorta have absolutely no valid connection to those sorts of usage-relevant ISP offerings.

So that is why George Ou is right and why you are wrong and stupid and evil and why no one should listen to your stupid evil alternative. Listen to George Ou and vote No on network neutrality or else the Network Neutrality Nazis are gonna make you pay crazy high for internet access.

-

Re:Not all sessions experience the same congestion

And I bet you pay a lot more for those "unlimited minutes" than the $5 a month I pay.

Which is my point in a nutshell: - people who want unlimited gigabytes, should be paying a lot more than what I'm paying for my limited service ($15 a month). That's entirely and completely fair. Take more; pay more.

Just like electrical service, cell service, water service, et cetera, et cetera.

Re:Not all sessions experience the same congestion

P.S.

There are some persons who think they should be able to download 1000 or even 10,000 times more data than what I download, and yet still pay the exact same amount of money.

That's greed.

If you want more, than you should pay more than what other people pay.

Re:Not all sessions experience the same congestion

No, its expecting the ISP to live up to its side of the contract... either way is fine, but they have to follow their agreement.

Are you saying that your ISP isn't living up to its contract with you? You don't need anything fancy to fix that -- just file a lawsuit. If they truly promised you unlimited bandwidth (as you interpret it), then you should easily win.

On the other hand, you might not completely understand your contract, and thus would take a serious beating in court. Either way, you need to accept the harsh reality that any ISP that offers broadband service (1+ Mbps) without transfer caps will go out of business within 2 years.

A single slow connection changes your TCP window

If you're using Linux, which TCP Congestion algorithm are you using? Reno isn't very fair; if a single connection is congested beyond the first hop, you'll slow down the rest of your connections when the window slides to smaller units. Have you tried Bicubic, Veno, or any of the other 9 or 10 congestion algorithms?

You can change them on the fly by echoing the name into your procfs, IIRC. Also, if you have the stomache for it, and two connections to the internet, you can load balance and/or stripe them using Linux advanced Routing & Traffic Control [lartc.org] (mostly the ip(1) command). Very cool stuff if you want to route around a slow node or two (check out the multiple path stuff) at your ISP(s).

Re:Not all sessions experience the same congestion

Right. The article seems to be written on the assumption that the bandwidth bottleneck is always in the first few hops, within the ISP. And in many cases for home users this is probably reasonably true; ISPs have been selling cheap packages with 'unlimited' and fast connections on the assumption that people would use a fraction of the possible bandwidth. More fool the ISPs that people found a use [plus.net] for all that bandwidth they were promised.

Obviously AIMD isn't going to fix this situation - it's not designed to. Similarly, expecting all computers to be updated in any reasonable timeframe won't happen (especially as a P2P user may have less little motivation to 'upgrade' to receive slower downloads). Still, since we're assuming the bottleneck is in the first hops, it follows that the congestion is in the ISPs managed network. I don't see why the ISP can't therefore tag and shape traffic so that their routers equally divide available bandwidth between each user, not TCP stream. Infact, most ISPs will give each home subscriber only 1 IP address at any point in time, so it should be easy to relate a TCP stream (or and IP packet type) to a subscriber. While elements of the physical network are always shared [plus.net], each user can still be given logical connection with guaranteed bandwidth dimensions. This isn't a new concept either, it's just multiplexing using a suitable scheduler, such as rate-monotonic (you get some predefined amount) or round-robin (you get some fraction of the available amount).

Such 'technology' could be rolled by ISPs according their roadmaps (although here in the UK it may require convincing BT Wholesale to update some of their infrastructure) and without requiring all users to upgrade their software or make any changes. However, I suspect here the "The politicization of an engineering problem" occurs because ISPs would rather do anything but admit they made a mistake in previous marketing of their services, raise subscriber prices, or make the investment to correctly prioritise traffic on a per user basis, basically knocking contention rates right down to 1:1. It's much easier to simply ban or throttle P2P applications wholesale and blame high bandwidth applications.

I have little sympathy for ISPs right now; the solution should be within their grasp.

Weighted TCP solution

Under a weighted TCP implementation, both users get the same amount of bandwidth regardless of how many TCP streams each user opens...Background P2P applications like BitTorrent will experience a more drastic but shorter-duration cut in throughput but the overall time it takes to complete the transfer is unchanged.

I am all for a change in the protocols as long as it helps everybody. The ISPs win, and so do the customers. As long as the ISPs don't continue to complain and forge packets to BT users I would see an upgrade to the TCP protocol as a solution to what is going on with neutrality issues, as well as an upgrade to fiber optic networks so the US is on par with everyone else.

Sadly, no, upgrading doesn't help...

There have been plenty of lessons, Japan most recently, that upping the avaible capacity simply ups the amount of bulk-data P2P, without helping the other flows nearly as much.

A New Way to Look at Networking

A New Way to Look at Networking [google.com] is a Google Tech Talk [google.com]. It's about an hour long, but there's a lot of very good and fascinating historical information, which sets the groundwork for this guy's proposal. Van Jacobson was around at the early days when TCP/IP were being invented. He's proposing a new protocol layered on top of TCP/IP that can turn the Internet into a true broadcast medium -- one which is even more proof against censorship than the current one!

Good luck with that..

For what it's worth, Net Neutrality IS a political fight, p2p is not the cause, but just the straw that broke the camels back. Fixing the fairness problem of tcp flow control will not make Net Neutrality go away. Nice fix though, too bad getting people to adopt it would be a nightmare. Where was this suggestion 15 years ago?

So right, yet so wrong

Weighted TCP is a great idea. That doesn't change the fact that net neutrality is a good thing, or that traffic shaping is a better fix for network congestion than forging RST packets.

The author of this article is clearly exploiting the novelty of a technological idea to promote his slightly related political agenda, and that's deplorable.

Re:So right, yet so wrong

Shaping only works as long as you can recognize and classify the data.

Not entirely true. It works better the more you know about your data, but even knowing nothing you can get good results with a simple rule of prioritizing small packets.

My original QoS setup was just a simple rule of anything small gets priority over anything large. This is enough to make (most) VoIP, games, SSH, and anything else that is lots of small real time packets all get through over lots of full queued packets (transfers).

Admittedly BitTorrent was what hurt my original setup, as you end up with a lot of slow peers each trickling transfers in slowly. You could get around this with a hard limit of overall packet rate, or with connection tracking and limiting the number of IPs you hold a connection with per second (and then block things like UDP and ICMP)

Yeah its an ugly solution, but we're all the ISP's bitch anyways, so they can do what they want.

Protocol filtering != Source/Destination filtering

This article is well fine and good, but it fails to recognize that there are two types of packet discrimination being kicked around. Protocol filtering/prioritization, and Source/Destination filtering/prioritization. There are certainly good and bad ways of doing the former, and some of the bad ways are really bad (for a "for instance", see Comcast). However, the basic concept, that network bandwidth is finite over a set period of time, and that finite resource must be utilized efficiently, is not one most geek types will disagree with you on. Smart treatment of packets is something few object to.

What brings a large objection is the Source/Destination filtering. I'm going to downgrade service on packets coming from Google video, because they haven't paid our "upgrade" tax, and coincidentally, we're invested in Youtube. This is an entirely different issue, and is not an engineering issue at all. It is entirely political. We know is technologically possible. People block sites today, for parental censorship reasons, among others. It would be little challenge, as an engineer, to set to a VERY low priority an arbitrary set of packets from a source address. This however violates what the internet is for, and in the end, if my ISP is doing this, am I really connected to the "Internet", or just a dispersed corporate net, similar to the old AOL.

This is, and will be, a political question, and if it goes the wrong way, will destroy what is fundamentally interesting about the net. The ability to, with one connection, talk to anyone else, anywhere in the world, no different then if they were in the next town over.

Confusing...

I need coffee before I'll really understand this, but here's a first attempt:

Despite the undeniable truth that Jacobsons TCP congestion avoidance algorithm is fundamentally broken, many academics and now Net Neutrality activists along with their lawyers cling to it as if it were somehow holy and sacred. Groups like the Free Press and Vuze (a company that relies on P2P) files FCC complaints against ISPs (Internet Service Providers) like Comcast that try to mitigate the damage caused by bandwidth hogging P2P applications by throttling P2P.

Ok, first of all, that isn't about TCP congestion avoidance, at least not directly. (Doesn't Skype use UDP, anyway?)

But the problem here, I think, is that George Ou is assuming that Comcast is deliberately targeting P2P, and moreover, that they have no choice but to deliberately target P2P. I'd assumed that they were simply targeting any application that uses too many TCP connections -- thus, BitTorrent can still work, and still be reasonably fast, by decreasing the number of connections. Make too many connections and Comcast starts dropping them, no matter what the protocol.

They tell us that P2P isnt really a bandwidth hog and that P2P users are merely operating within their contracted peak bitrates. Never mind the fact that no network can ever support continuous peak throughput for anyone and that resources are always shared, they tell us to just throw more money and bandwidth at the problem.

Well, where is our money going each month?

But more importantly, the trick here is that no ISP guarantees any peak bitrate, or average bitrate. Very few ISPs even tell you how much bandwidth you are allowed to use, but most reserve the right to terminate service for any reason, including "too much" bandwidth. Comcast tells you how much bandwidth you may use, in units of songs, videos, etc, rather than bits or bytes -- kind of insulting, isn't it?

I would be much happier if ISPs were required to disclose, straight up, how much total bandwidth they have (up and down), distributed among how many customers. Or, at least, full disclosure of how much bandwidth I may use as a customer. Otherwise, I'm going to continue to assume that I may use as much bandwidth as I want.

But despite all the political rhetoric, the reality is that the ISPs are merely using the cheapest and most practical tools available to them to achieve a little more fairness and that this is really an engineering problem.

Yes, it is a tricky engineering problem. But it's also a political one, as any engineering solution would have to benefit everyone, and not single out individual users or protocols. Most solutions I've seen that accomplish this also create a central point of control, which makes them suspect -- who gets to choose what protocols and usage patterns are "fair"?

Under a weighted TCP implementation, both users get the same amount of bandwidth regardless of how many TCP streams each user opens. This is accomplished by the single-stream application tagging its TCP stream at a higher weight than a multi-stream application. TCP streams with higher weight values wont be slowed as much by the weighted TCP stack whereas TCP streams with smaller weight values will be slowed more drastically.

Alright. But as I understand it, this is a client-side implementation. How do you enforce it?

At first glance, one might wonder what might prompt a P2P user to unilaterally and voluntarily disarm his or her multi-stream and persistence cheat advantage by installing a newer TCP implementation.

Nope. What I wonder is why a P2P user might want to do that, rather than install a different TCP implementation -- one which tags every single TCP connection as "weighted".

Oh, and who gets to tag a connection -- the source, or the destination? Remember that on average, some half of th

FUD

The whole article is disingenuous. What he is describing are not "loopholes" being cynically exploited by those evil, and soon to be illegal, P2P applications. They are the intended behavior of the protocol stack. Are P2P applications gaming the system by opening multiple streams between each pair of endpoints? No. While we could have a legitimate debate on what is fair behavior, he poisons the whole issue by using it as a vehicle for his anti-P2P agenda.

Bandwidth still isn't free.

In the UK bandwidth out of BTs ADSL network costs ~ £70/Mbit/Month wholesale. Consumer DSL costs ~ £20/month.

You've got three options,

#1 Have an uncapped uncontended link for the £20/month you pay - you'll get about 250kbps.

#2 Have a fast link with a low bandwidth cap - think 8Mbits with a 50GB cap and chargeable bandwidth after that at around ~ 50p-£1/GB

#3 Deal with an ISP who's selling bandwidth they don't have and expect them to try as hard as possible to make #1 look like #2 with no overage charges.

If you want a reliable fast internet connection you want to go with a company that advertises #2. If you can't afford #2, you can spend your time working against the techs at ISP #3, but expect them to go our of their way to make your life shit until you take your service elsewhere because you cost them money.

Driving Miss Internet

WARNING ~! Core dump follows.

It occurred to me this morning that driving on public roadways and surfing the public networks were identical experiences for the vast majority of people. That experience being; "mine, mine, ALL MINE!....hahahaha!" AKA "screw you...it's all about me!"

Now, I have the joy of managing a global network with links to 150 countries AND a 30 mile one way commute. So, I get to see, in microcosm, how the average citizen behaves in both instances.

From a network perspective, charge by usage...period. Fairness only works in FAIRy tales.

We do very good traffic shaping and management across the world. QoS policies are very well designed and work. The end user locations do get charged an allocation for their network costs. So, you'd think the WAN would run nicely and fairly. After all, if the POS systems are impacted, we don't make money and that affects everyone, right?

Hardly. While we block obvious stuff like YouTube and Myspace, we have "smart" users who abuse the privilege. So, when we get a ticket about "poor network performance", we go back to a point before the problem report and look at the flows. 99 out of 100 times, it's one or more users hogging the pipe with their own agenda. Now, the branch manager gets a detailed report of what the employees were doing and how much it cost them. Of course, porn surfers get fired immediately. Abusers of the privilege just get to wonder what year they'll see a merit increase, if at all.

So, even with very robust network tuning and traffic shaping, the "me, me" crowd will still screw everybody else...and be proud that they did. Die a miserable death in prison you ignorant pieces of shit.

Likewise the flaming assholes I compete with on the concrete and asphalt network link between home and office every day. This morning, some idiot in a subcompact stuck herself about 2 feet from my rear bumper...at 70mph. If I apply ANY braking for ANY reason, this woman will collide with me. So, I tapped the brakes so she'd back off. She backed off with the upraised hand that seemed to be "yeah, I know I was in the wrong and being unsafe" She then performed 9 lane changes, all without signaling once, and managed to gain....wait for it.... a whole SEVEN SECONDS of time over 10 miles of driving.

I see it every day. People driving with little regard for anyone else and raising the costs for the rest of us. On the network, or on the highway, same deal. And they feel like they did something worthwhile. I've talked to many users at work and the VAST majority are not only unapologetic, but actually SMUG. Many times, I'll get the "I do this at home, so it must be okay at work". To which I say, "well you cannot beat your wife and molest your kids at the office, now can you?"

My tolerance of, and faith in, my fellow man to "do the right thing" are at zero.

A technical solution (to TCP Congestion Control, etc.) is teaching the pig to sing; horrible results. Charge the thieving, spamming bastards through the nose AND constrain their traffic. That'll get better results than any pollyanna crap about "fair".

Doesn't stand a chance

Every year or so somebody else proposes to "fix TCP". It never happens. Why?

1) TCP works well.
2) TCP is in a lot of code and cannot easily be replaced
3) If you need something else, alternatives are there, e.g. UDP, RTSP and others.

Especially 3) is the killer. Applications that need it are already using other protocols. This article, like so many similar ones before it, is just hot air by somebody that did either not do their homework or want attention without deserving it.

Why this is an issue now

As the one who devised much of this congestion control strategy (see my RFC 896 and RFC 970, years before Van Jacobson), I suppose should say something.

The way this was supposed to work is that TCP needs to be well-behaved because it is to the advantage of the endpoint to be well-behaved. What makes this work is enforcement of fair queuing at the first router entering the network. Fair queuing balances load by IP address, not TCP connection, and "weighted fair queueing" allows quality of service controls to be imposed at the entry router.

The problem now is that the DOCSIS approach to cable modems, at least in its earlier versions, doesn't impose fair queuing at entry to the network from the subscriber side. So congestion occurs further upstream, near the cable headend, in the "middle" of the network. By then, there are too many flows through the routers to do anything intelligent on a per-flow basis.

We still don't know how to handle congestion in the middle of an IP network. The best we have is "random early drop", but that's a hack. The whole Internet depends on stopping congestion near the entry point of the network. The cable guys didn't get this right in the upstream direction, and now they're hurting.

I'd argue for weighted fair queuing and QOS in the cable box. Try hard to push the congestion control out to the first router. DOCSIS 3 is a step in the right direction, if configured properly. But DOCSIS 3 is a huge collection of tuning parameters in search of a policy, and is likely to be grossly misconfigured.

The trick with quality of service is to offer either high-bandwidth or low latency service, but not both together. If you request low latency, your packets go into a per-IP queue with a high priority but a low queue length. Send too much and you lose packets. Send a little, and they get through fast. If you request high bandwidth, you get lower priority but a longer queue length, so you can fill up the pipe and wait for an ACK.

But I have no idea what to do about streaming video on demand, other than heavy buffering. Multicast works for broadcast (non-on-demand) video, but other than for sports fans who want to watch in real time, it doesn't help much. (I've previously suggested, sort of as a joke, that when a stream runs low on buffered content, the player should insert a pre-stored commercial while allowing the stream to catch up. Someone will probably try that.)

John Nagle

Re:Why this is an issue now

I'd argue for weighted fair queuing and QOS in the cable box.

Seem to me that for ADSL it would be ideally placed in the DSLAM, where there is already a per-subscriber connection (in any case, most home users will only get 1 IP address, hence making a 1:1 mapping for subscriber to IP -nothing need be per IP connection as the original article assumes). In fact, the wikipedia page on DSLAMs [wikipedia.org] says QoS is already an additional feature, mentioning priority queues.

So I'm left wondering why bandwidth hogs are still a problem for ADSL. You say that this is a "huge collection of tuning parameters", and I accept that correctly configuring this stuff maybe complex, but this is surely the job of the ISPs. Maybe I'm overestimating the capabilities of the installed DSLAMs, in which case I wonder if BTs 21CN [btplc.com] will help.

Certainly though, none of the ISPs seem to be talking about QoS per subscriber. Instead they prefer to differentiate services, ranking P2P and streaming lower than uses on the subscribers behalf. PlusNet (a prominent UK ISP) have a pizza analogy [plus.net] to illustrate how sharing works - using their analogy, PlusNet would give you lots of Margarita slices, but make you wait for a Hawaiian even if you aren't eating anything else. Quite why they think this is acceptable is unknown to me; they should be able to enforce how many slices I get at the DSLAM, but still allow me to select the flavours at my house (maybe I get my local router to apply QoS policies when it takes packets from the LAN to the slower ADSL, or mark streams using TOS bits in IPv4 or the much better IPv6 QoS features to assist the shaping deeper into the network).

If you can't support it don't sell it!

Back in 1994 to 1997 I was in many debates on just this subject.

We were buying T1 and T3 for use with video streaming and the ISP where getting upset that we were using 90% of the capacity they sold us. Apparently they specked out their cost based on office use doing web surfing. And based their models on older Telco traffic models where they needed 100 lines of outbound bandwidth for every 10000+ phone lines based on supporting 95% of the peak throughput.

But we concluded if you are selling us 1.5Mbps I dam well better be able to use 1.5Mbps, don't blame me when I use what was sold to me.

Well I see this as the same problem. If Comcast or Verizon sells me internet at at data rate, then I expect to be able to use all of it. There is nothing unfair about me using what I was sold. If they don't like it then they need to change their contractual agreements with me and change their hardware to match!

Same goes with the internal infrastructure, backbones and exchange point. If you can't support it don't sell it! Don't attack the P2P users, they are using what they PAID FOR and what was sold to them!!! If they are not getting it, they should file a class action suit.
No more then if you local cable company decided that 4 hr of TV was your limit and they would start to degrade your reception if you watched more, though this wasn't in the contract you signed up for.

On the other side, P2P should be given the means to hug the edges of the network. By this I mean communication between 2 cable modem or DSL users running off the same upstream routers (less hops) should be preferable and more efficient, not clogging up the more costly backbones. Currently P2P doesn't take any of that into consideration. Maybe ISP's could consider some technical solution to that rather then trying to deny customers the very access they are contractually bound to provide...

Re:This is a good proposal

But it's ridiculous that when I'm spending 30 seconds downloading CNN.com during a high-demand period, some asshat is using twenty times my bandwidth downloading some file that could just as easily be sent at any time of day.

1. Could that possibly be to the processor demand on the CNN servers at peak times?
2. Does not certain companies like Blizzard force P2P patches onto their customers?
3. Is your 30 second video file just as important as a technician using torrents to download a Linux Distro to put on a server used for business they need up and running ASAP?
4. And lastly... Someone using a torrent shouldn't soak up an ISPs entire bandwidth... Unless someone at CNN is using the web server to host torrents but thats nothing you or your ISP can control.