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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."
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.
A New Way to Look at Networking is a Google Tech Talk. 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!
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.
There's no failure quite as dissatisfying as a complete and total solution to the wrong problem.
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.
Mea navis aericumbens anguillis abundat
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.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
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