JBVB@AI.AI.MIT.EDU ("James B. VanBokkelen") (11/30/87)
>It's hard to believe that in this age of utterly cheap dense RAM, >otherwise sane people are proposing inserting artificial delays between >Ethernet packets because a lowball vendor wouldn't put, say, TWO >buffers on their card! > >[I admit I'm prejudiced, since I worked on Suns, which currently seem >to have the highest Ethernet thruput, but they were built out of >standard Ethernet chips and DRAMs available to everyone. You too can >handle infinite back to back packets, if you just design with that in >mind as Sun did.] > But whhhat about all those old, old Suns, the ones without back to back capacity. What about the tens of thousands of NI5010's or 3C501's. People bought them, have them, are working with them daily even though no double buffering is available. Its not just a querstion of being a lowball vendor, reality is the fact that old outmode hardware is out there and used! ... Larry Backman One point that may not be well known is that it is not just the single- buffered PC interfaces that drop packets. I have observed a DEQNA dropping packets that arrived too close together (or when it had a resource crunch, or something), sent from a PC. There is also considerable variation in the performance of different device drivers for the same interface. I've observed a VMS TCP/IP sending packets (with DEC's driver) much closer together than Ultrix manages to. Another point is that even the newest PC interface cards apparently can't always handle back-to-back packets (I haven't yet met the LAN controller chip that is as good as its salesmen imply). Presumably other controllers based on similar chips will have some susceptibility to overrun, also. So, don't just dismiss the problem: The closer two IP fragments are, the more likely it is that any arbitrary receiver will drop the second, requiring a high-level retransmit. Fragments are expensive to deal with in most environments, and almost everyone does their best to avoid them entirely. The exceptions I know of (NFS, routing protocols on big networks) that send fragments deliberately are very much special cases. I wasn't asking that the gateway people go to enormous lengths to insert delays, just to do so when convenient, in order to make the best of an already bad situation. jbvb