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The TCP rfc 9293 says this:

A 3WHS is necessary because sequence numbers are not tied to a global clock in the network, and TCP implementations may have different mechanisms for picking the ISNs. The receiver of the first SYN has no way of knowing whether the segment was an old one or not, unless it remembers the last sequence number used on the connection (which is not always possible), and so it must ask the sender to verify this SYN. The three-way handshake and the advantages of a clock-driven scheme for ISN selection are discussed in [69].

Im very confused by this. Does it say that the receiver (server) of the SYN, must ask the sender to verify this SYN? That cannot be true, how does the client verify its own SYN?

user86942
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    Read again: each side verifies/acknowledges the other side's ISN. – Zac67 Jan 13 '23 at 21:52
  • i read it, but im still confused by this: "and so it must ask the sender to verify this SYN." So the receiver gets the SYN, but it doesnt know if the sequence number is part of a previous connection? – user86942 Jan 13 '23 at 22:02

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