[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))

Joe Touch touch at ISI.EDU
Fri Jan 5 09:48:06 PST 2007

Detlef Bosau wrote:
> Joe Touch wrote:
>> In high BW*delay product nets, the same stalling happens - you
>> send data, get an ACK, send more, get ACKs of that, etc - and the data
>> keeps bunching up at the source.
>> I.e., ACK clocking works only when the data-ACK look experiences a
>> bottleneck. When it doesn't, things bunch up, and TCP doesn't 'match
>> rates' at all.
> This was a little bit too fast for me....
> Shouldn´t the ACKs be clocked by the TCP data packets, at least in
> symmetric paths? Thus, the ACK clocking should reflect the TCP rate
> which is achieved downstream?

It does - 'downstream' is really the splitter, i.e., the thing
generating the ACKs. Since the path to the splitter and back has no
bottleneck, there's no ACK pacing going on.

>> FWIW, the same thing happens when the receiver application doesn't drain
>> the incoming data fast enough. The receive buffers fill up, and the
>> sender is stalled. The same thing is happening here.
> Yes, absolutely. When a splitter is in use, the sending socket (directed
> to the final receiver) doesn´t drain its incomming data fast enogh.
> It´s an interesting question whether data of short term flows can be
> buffered entirely at the splitter and then sent to the receiver with a
> rate the link can handle.

Sure it can; that's what a true proxy does.

> It´s interesting what handles to the final CLOSE ACK here which is
> typically not spoofed in splitters to ensure poper ACK semantics.

I don't understand "proper ACK semantics". The splitter destroys those.
The semantics that may be kept are at the connection level
(open/closed), but the semantics of data ACKs are irrevocably destroyed.

>> Splitters are bad for other reasons, but as you said, let's ignore them
>> for this discussion..
> I just see that they are in use. And so I think one should weigh up the
> pro´s and con´s here.
> In the particular case of wide area mobile networks, I personally think
> splitters can be helpful because of the extremely irregular delivery
> times of datagrams.
> I had great difficulties to see a reason for this and found Thierry
> Kleins paper !Improved TCP Performance in Wireless IP Networks through
> Enhanced Opportunistic Scheduling Algorithms" (Globecom 2004) extremely
> interesting.
> Perhaps, the scheduling caused variations in packet delivery times are
> the most distinguishing mark for mobile wide area networks compared to
> other network technologies. (I would be glad to get comments on this
> claim!)

Variations in delivery times can be handled via PEPs that don't spoof
ACKs, e.g., ones that pace the data and/or ACK paths, but don't actively
participate in the communication.

Joe Touch
Sr. Network Engineer, USAF TSAT Space Segment

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