jbvb@VAX.FTP.COM (James Van Bokkelen) (04/22/88)
Our PC/DOS TCP/IP will only assemble packets smaller than the MTU of the attached networks. We decided that anything fancier than that would waste too much memory for essentially no benefit. It helps that the smallest MTU any of our interfaces use is about 1K bytes, well above the IP requirement of 576. Mobygrams fail miserably in the face of *any* packet loss, because none of the IPs I know anything about allow retransmissions with the same IP identification value. All the fragments of a particular retransmission of a mobygram must survive the net before it can be re-assembled. In PCs in particular, you can't get below a certain level of packet loss due to cheap network interfaces, so you're stuck. NFS is the only conspicuous user of mobygrams that I know of, and I think that Van Jacobsen and Mike Karels have demonstrated that Sun's choice of fragmented UDP mobygrams, unchecksummed, was a costly way to achieve high throughput. James VanBokkelen FTP Software Inc.