Hi,
CC'ing tsvwg, which would be a better venue for this discussion.
On 2015-2-4, at 20:22, Phillip Hallam-Baker
<phill(_at_)hallambaker(_dot_)com> 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
But looking at the problem from the perspective of the network it is really
hard to see why setting up five TCP/IP streams between the same endpoints
should provide any more bandwidth than one. If the narrow waist is
observed, then the only parts of the Internet that are taking note of the
TCP part of the packet are the end points. So having five streams should
not provide any more bandwidth than one unless the bandwidth bottleneck was
at one or other endpoint.
You don't get more bandwidth in stead state (well, with old Reno stacks, you
got a little more, but not much). The real win is in getting more bandwidth
during the first few RTTs of TCP slow-start, which is the crucial phase when
transmitting short web objects.
Now there are some parts of the deployed Internet that do actually perform
statefull inspection. But I would expect increasing the number of channels
to degrade performance at a firewall or any other middle boxen.
So we have a set of behavior that seems at odd with the theory. Has anyone
done any experiments recently that would show which is right?
I haven't seen any performance study, but another concern is that
middleboxes obviously need to maintain state per connection, and multiple
parallel connections eat that binding space up more quickly. (And for a NAT,
reduce the number of clients it can serve.)
The reason it makes a difference is that it is becoming clear that modern
applications are not best served by an application API that is limited to
one bi-directional stream. There are two possible ways to fix this
situation. The first is to build something on top of TCP/IP the second is
to replace single stream TCP with multi-stream.
SCTP has what you call multiple streams in your second option, and is
designed the same way.
My preference and gut instinct is that the first is the proper
architectural way to go regardless of the performance benefits. When
Thompson and co were arguing that all files are flat sequences of bits,
they were saying that was the right O/S abstraction because you could build
anything you like on top.
But then I started to ask what the performance benefits to a multi-stream
TCP might be and I am pretty sure there should not be any. But the actual
Internet does not always behave like it appears it should.
See above.
Also, one motivation for SPDY/HTTP2.0 is to reduce the number of parallel
connections, since web people have noticed that more is not always better
here.
I suspect that the preference for multiple streams probably comes from the
threading strategies it permits. But that is an argument about where the
boundary between the kernel and application is placed in the network stack
rather than where multiplex should live in the stack. Microsoft already
provides a network stack for .NET where the boundary is in the HTTP layer
after all.
So anyone got hard data they could share?
The TSVWG folks may have.
Lars