On Thu, 15 Jun 2000 00:14:43 EDT, Andrew Brown said:
i think you have two things confused. raising the mtu will "speed up the modem" since you get more data for less overhead, however *lowering* the mtu will increase interactivity, since each packet is smaller, the transmit time is shorter, so the next one can get in/out sooner. you stand a better chance of getting a "word" in edgewise if the other guy is using smaller "phrases".
No, he's got it right. The user *percieves* the modem is "speeded up" if he's doing interactive work and fighting with a file download. For instance, if you're using BIG packets on a slower modem (yes, there's still 14.4 and 33.8 users out htere), the network latency on a telnet fighting with an FTP can be up to a major fraction of a second - if your MTU is 1/3 the size, then your character echo is 3 times as fast. The fact that if you lower the MTU from 1500 to 500 you're taking a 10% performance hit (2 extra IP/TCP headers amortized over 1500 bytes) pales in comparison... Yes, it's actually 10% lower for throughput. But for the "boot it, reinstall AGAIN, call tech support and listen when they tell you to wave a dead chicken over the CPU while re-re-installing drivers" crowd, it feels 3 times faster... So it *is* 3 times faster. Valdis Kletnieks Operating Systems Analyst Virginia Tech