On Oct 7, 2011, at 6:49 PM, Phil Mayers wrote: > On 10/07/2011 02:11 PM, Glyph wrote: > >>> It is quite unusual (though not unheard of) for something to >>> re-segment the TCP stream. IP level fragmentation might occur, but >>> it's relatively uncommon in todays IP networks, and is anyway >>> irrelevant to TCP - the TCP stack will only see a reassembled IP >>> packet. >> >> I am skeptical; my recollection (the last time I worked at this layer >> of the network) is that this happens all the time over the public >> internet between diverse endpoints. > > Are we talking about TCP re-segmentation, or IP fragmentation?
Pretty sure I'm talking about TCP re-segmentation. > If you mean TCP re-segmentation, then I have no data. The only places > I've personally seen it are in TCP terminating load-balancers (F5, > etc.), NAT64 devices, and as a side effect in higher-layer proxies > (Squid, etc.). Hrm. Now that I think of it, the packets I was examining were probably coming from environments that were heavily managed by "smart" networking hardware, although I could only see the edges of the network. So it was probably a combination of high-level transparent proxies and wacky load-balancing and caching hardware. > If you mean IP fragmentation, I claim that fragmentation by routers in > the internet is rare. Whether it's 1% or 0.001% I couldn't say, but the > vast majority of the time, routers do not end up fragmenting. For one > thing, an awful lot of OSes send TCP segments with "don't fragment" set, > to trigger ICMP error messages and path MTU discovery. For another, a > lot of low MTU networks employ TCP MSS clamping to prevent this ever > happening. That's interesting, and good to know :). > For what it's worth, this "layer of the network" is my day job, but I > don't claim oracle status. > >> But the sizes are also not important. The point is that TCP is about >> streams, not packets, and you have to deal with arbitrary chunking if >> you want your code to work right. What layer of the network this >> happens at is not important to your code :). > > Agreed. Since this is your day job - is there a formally-correct name for this phenomenon? I usually refer to it as "packet fragmentation" even though I know that's not really it - as you say, IP fragmentation doesn't really happen. It's not like it's really TCP re-segmenting either, because one send() is not necessarily one segment. Since it's such an important concept that every Twisted developer needs to know, I'd love to have a good word for it :). _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
