Why can it never happen? If I send a bunch of large messages with small last fragments, your modification will allow all messages to be sent, because it disables Nagle for large messages, right? If so, many small last fragments can be outstanding at any one time (one from each message). Technically, this violates Nagle, which aims to prevent more than one small fragment from ever being outstanding, but I'm not sure that it really violates the spirit of what Nagle is trying to accomplish. Nagle is really meant to prevent the case of an application like telnet from sending a whole lot of small packets containing only 1 or a few characters. If the receive window is, say, 10000 bytes, Nagle would allow 10000 packets to be outstanding, all clogging up the network.
I meant to say that *without* Nagle, you'd be able to have 10000 packets outstanding. This is what Nagle is trying to prevent.
But if the PMTU is, say, 1000 bytes and the user tries to send a bunch of 1001 byte messages, your method (if I understand it correctly) will allow 9 unacknowledged messages to be outstanding. Those 9 messages will be split into 9 full-sized packets and 9 packets carrying only 1 byte of data. 18 outstanding packets isn't all that bad. If the user were instead sending 1000 byte messages, Nagle would have nothing to say about it, and you'd be able to have 10 packets outstanding. The increase from 10 to 19 outstanding packets isn't likely to cause network collapse. --Doug
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