[rwiggins]

Out of curiosity, how would you describe TCP in these terms? Does the TCP stack's handling of sequence numbers constitute processing (on the client and server both I assume)? Which part(s) of a TCP connection could be described as delivery?

[wmf]

From TCP's perspective it has delivered the data when read() returns successfully. This is also the point at which TCP frees the packet buffers. From the app's perspective, TCP is an at-most-once system because data can be lost in corner cases caused by failures.

(Of course plenty of people on the Internet use different definitions to arrive at the opposite conclusion but they are all wrong and I am right.)

[YZF]

I agree this is what you'd consider delivery. Also agree everyone else is wrong an I am right ;)

The similar question in TCP is what happens when the sender writes to their socket and then loses the connection. At this point the sender doesn't know whether the receiver has the data or not. Both are possible. For the sender to recover it needs to re-connect and re-send the data. Thus the data is potentially delivered more than once and the receiver can use various strategies to deal with that.

But sure, within the boundary of a single TCP connection data (by definition) is never delivered twice.