![]() The end of a session also involves handshakes with FIN flagged packets and ACKs relating to them. The first is actually closer to reality, but all the data packets after the SYN do have to include an ACK, and also an acknowledgement number field which identifies the number of the next packet expected. So neither of the scenarios you describe is quite correct. Receiver will then ACK for 101 and set the window size back up to a higher number.īoth are actually fields in the TCP header and can be sent with data, though the SYN and the first ACK typically are data-less. Sender will resend 1 segment with sequence number 50. ![]() Receiver will then ACK for 50 (next segment it expects) and set window size to 1. E.g If sender sends 100 segments but segment number 50 gets lost, then receiver will get 1-49 & 51 -100. If the window size is 100 for example, the sender can send 100 segments before it expects to receive an ACK. But some application-layer protocols built on top of UDP will have some protocol-specific way of acknowledging data sent and received.ĪCK is on subsequent segments in either direction. Your acknowledgement will usually just be the server's response. | (send) | - data -> | (wait a bit) |Īs for UDP, there's no built-in concept of SYN and ACK - UDP is by nature "unreliable", and not connection-oriented, so the concepts don't apply as much. Most TCP/IP stacks try to reduce the number of naked ACKs without unduly risking retransmission or a connection reset. But data segments already include all that.so if you're sending data, you can send an ACK at the same time for free. Sending one requires at least a header's worth of bandwidth, plus whatever the lower layers tack on. An ACK is simply a flag and field in a TCP header.That means you can acknowledge two or more sends with one ACK. ACKs can build up, so one ACK can acknowledge everything received up to that point.Every byte sent must be accounted for, or it will be retransmitted (or the connection reset (closed), in severe cases).Īctual connections aren't usually exactly like the diagram above, though, for two reasons: But all data being sent via TCP requires an ACK. SYN starts a connection you'll usually only see it when the connection's being established. and so on, til the connection is shut down or reset
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