TCP implementations are an abstraction that work 99.99% of the time, but are still vulnerable to two generals when you look close. TCP is implemented in the kernel with a buffer, the kernel responds with ACKs before an application reads the data.
There is no guarantee that the application reads from that buffer (e.g. the process could crash), so the client on the other end believes that the application has received the message even though it hasn't.
The kernel is handling at-least-once delivery with the network boundary and turning it into at-most-once with the process boundary.
There is no guarantee that the application reads from that buffer (e.g. the process could crash), so the client on the other end believes that the application has received the message even though it hasn't.
The kernel is handling at-least-once delivery with the network boundary and turning it into at-most-once with the process boundary.