Re: Naive question on multiple TCP/IP channels

[Jim Gettys <jg@xxxxxxxxxxxxxxx>]

On Thu, Feb 5, 2015 at 9:30 AM, Phillip Hallam-Baker <phill@xxxxxxxxxxxxxxx> wrote:

On Wed, Feb 4, 2015 at 6:54 PM, Brian E Carpenter <brian.e.carpenter@xxxxxxxxx> wrote:

On 05/02/2015 08:49, Eggert, Lars wrote:
> Hi,
>
> CC'ing tsvwg, which would be a better venue for this discussion.
>
> On 2015-2-4, at 20:22, Phillip Hallam-Baker <phill@xxxxxxxxxxxxxxx> wrote:
>>
>> Today most Web browsers attempt to optimize download of images etc. by opening multiple TCP/IP streams at the same time. This is actually done for two reasons, first to reduce load times and second to allow the browser to optimize page layout by getting image sizes etc up front.
>>
>> This approach first appeared round about 1994. I am not sure whether anyone actually did a study to see if multiple TCP/IP streams are faster than one but the approach has certainly stuck.
>
> There have been many studies; for example, http://www.aqualab.cs.northwestern.edu/publications/106-modeling-and-taming-parallel-tcp-on-the-wide-area-network

GridFTP only exists because of lots of experience that several parallel FTP
streams achieve better throughput than a single stream, especially on paths
with a high bandwidth-delay product. I'm guessing that since buffer bloat
creates an artificially high BDP, that could apply pretty much anywhere.

SCTP is not the only one-acronym answer: try MPTCP. The interesting thing there
is that because there is explicit coupling between the streams, the throughput
increases sub-linearly with the number of streams.

Buffer bloat is a great example of the consequences of the co-operative nature 

of the Internet for one of the audiences I am writing for (Folk trying to

make/understand Title II regulations).

The Internet is actually a demonstration that the commons are not such a tragedy 

after all.

 If we look at the problem of buffer bloat there are several possible solutions

and the problem is picking one:

* Persuade manufacturers to reduce buffer sizes so the old congestion

algorithms work.

* Change the congestion algorithm.

* Break with the pure end to end principle and have the middleboxen with the 

huge buffers do some reporting back when they start to fill.

The first is difficult unless you get control of the benchmarking suites that are

going to be used to measure performance. Which would probably mean getting

the likes of nVidia or Steam or some of the gaming companies on board.

​The missing metric is "latency under load".  Right now, we only measure bps.

​

 

The second is certainly possible. There is no reason that we all have to use the

same congestion algorithm. In fact a mixture might be beneficial. Instead of looking 

for packets being dropped, the sender could look at the latency and the difference

between the rate packets are being sent and the rate at which acknowledgements

are being received.

​Doesn't matter.  TCP congestion control has effectively been defeated by web browsers/servers.  Most of the data is transmitted in the IW's of n connections, where n is large.  The incentives are such that applications can/will abuse the network and not be "cooperative". I expect there will continue to be such applications, even if we "fix" the web to be better behaved; incentives are not aligned properly for cooperation.

So you can argue all you want about your favorite congestion control algorithm, and it won't solve this fundamental issue.​

 

Whether folk are willing to consider the third depends on how serious the

buffer bloat problem is considered to be. If the stability of the Internet really

is at stake then everything should be on the table. But my gut instinct is that

a middlebox does not actually have any more useful information available

to it than can be derived from the acknowledgements.

​While you need to run a mark/drop algorithm (self tuning) to keep long lived TCP flows under control, you can't touch the transient problems we have short of some sort of flow queuing (fair or not...).  It's head of line blocking that is killing you.

​

 

​  Ergo fq_codel combining flow queuing (of a clever nature) with a mark/drop algorithm.​  The exact details (other than being self tuning) of the mark/drop algorithm is *much* less important than whether flow queuing is implemented. Once you isolate the flows, life gets much easier.

​So the days of single, huge bloated FIFO queues have to go.​

Jim

​