Re: [ledbat] list of reasons for needing multiple TCP connections

[Bryan Ford <[e-mail]>]

On Nov 20, 2008, at 3:54 PM, Joe Touch wrote:
> Caitlin Bestler wrote:
>> Andrew G. Malis wrote:
>>> As was mentioned in the IAB plenary yesterday, a reason to not use
>>> multiple TCP connections is the possible exhaustion of middlebox  
>>> (read
>>> NAT) resources, such as state memory or port numbers. So if you are
>>> going to use multiple connections in parallel, you may wish to  
>>> include
>>> a method for the user to control that behavior, and a way to detect
>>> that you're behind a NAT and adapt appropriately.
>>>
>>
>> The gotcha here is that every item in the list Joe supplied is
>> already solved by SCTP, and about the only legitimate reason
>> for not using SCTP is the presence of middleboxes that do
>> not support it.
>
> SCTP doesn't address the need for different applications to have
> different connections (one variant of the demultiplexing issue).  
> I.e., a
> single socket used by more than one application.
>
> SCTP also doesn't address the need to support endpoints that don't  
> speak
> SCTP yet, or services that aren't defined over SCTP yet.

Just to fill this out, a third issue is that the multiple logical  
streams sharing an SCTP connection have no independent flow control:  
SCTP provides flow control only for the connection as a whole.  Thus,  
the receive side of any connection has to accept data on all the  
streams at whatever (relative) rates the sending side is sending data  
on those streams, and can't "push back" on one stream while allowing  
another to flow, unless the application implements some additional  
higher-level windowing and flow control mechanism.

If a web browser is downloading two HTTP objects at once using two  
SCTP streams sharing one connection, for example, and one is a file  
downloading to a fast disk but the other is an mp3 or avi file that  
the browser is streaming to a constant-rate audio/video frame, then  
the browser has no direct way to limit the download rate of the a/v  
stream to the output rate without also holding back the file  
download.  (Granted, this particular example doesn't typically happen  
with current browsers, since browser plug-ins typically handle media  
streams, using completely separate connections to the server.)

Bryan
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