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by ww520 3801 days ago
Fragmented IP datagram is re-assembled at the IP layer before it is handed up to UDP or TCP layer. If it can't be re-assembled, the datagram is considered lost.

UDP is unreliable and has small packet size. The TCP code is emulating that simple requirement. Why expand the requirement? Looping to read fully would block on one connection. One rogue client would hold up the whole server.
1 comments
im2w1l 3800 days ago
I looked it up, and it seems that while IP will reassemble packets, there is no 1-1 mapping between send calls and IP packets, when using TCP.

You are only guaranteed to get the bytes in the correct order but you have to find the boundaries between messages yourself.

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ww520 3800 days ago
That's correct. A send call could call with a 1GB buffer and the IP layer would have to break it up into multiple datagram packets. That's the nature of TCP. Again sending large message is expanding the requirement beyond what is capable in UDP, while we were striking to emulate UDP in sending short unreliable message.
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