[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))
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
> 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
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.
Sr. Network Engineer, USAF TSAT Space Segment
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