North American Network Operators Group|
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Re: MTU of the Internet?
<lots of speculation about packet sizes and mtus and the occasional bash at pmtud and microsfot thrown in for good measure> recently i was doing a fair amount of image-intensive web browsing from home. the last-hop dialup line was 28.8. was on a win95 machine with ftp software's stack. i was getting real bad performance. aggregate receive 'goodput' on the order of 10s or 100s of bytes/second. but the modem lights were constantly on. so i fired up the ol packet monitor and watched the packets. i saw many tcp connections open all at once. i saw many many many retransmissions. i saw many duplicate acks going back. transmissions in both directions were very very bursty it was ugly. so, the line was running at full speed, but most of what came across was useless glop. my theory was that it was a classic case of connection fratricide. in short, i'd open a web page and get umpity-poo connections established, one for each image, etc, etc. they'd all be in slow start and not interfere with each other. they'd get a reasonable rtt and then start cranking out the data. they'd all do this at once. implosion of packets onto the poor dialup server. dialup server q-length grows. that reasonable rtt gets blown all to hell. the webserver misses some acks. it resends. even though all the connections start slowing up, there still are umpity-poo connections all (slowly) dumping packets into the dialup server. lots of retransmissions come across. which i constantly re-ack. then things go quiet a bit. then the cwnd opens up again. repeat. so, to test the theory, i crank down the number of connections that netscape will open up (an older version of navigator will let you do this. i haven't figured out how to do it in the newer browsers). i crank it down to 2. goodput goes _way_ up. i consistantly get over 3kbytes/sec(24+kb) on the 28.8 line (as opposed to maybe 500bytes/sec...). i look at the packet trace then. nice beautiful packet-in, ack-out, packet-in, ack-out, series. seq-nums going up nicely as they should. no retransmissions. note well that _none_ of this has to do with pmtu. none of this has to do with the oft-proclaimed poor quality of m.s. software. <soapbox> moral of the story theorizing is wonderful but a good packet trace beats a theory every time and understanding how the protocols are _supposed_ to work and then trying to figure out why they don't is a lot more productive than bashing mammoth-software-corporations, though a lot harder </soapbox> frank kastenholz