# 50.012 Lecture 9 ###### tags: TCP-segment ## TCP Segment Structure ![](https://i.imgur.com/Z4E2AEF.png) ## TCP Sequence Numbers, ACKs ![](https://i.imgur.com/UZgU4rS.png) Sequence numbers: Byte stream "number" of first byte in segment's data. ACKs: * Sequence number of next byte expected from the other side. * Cumulative ACK Receiver will buffer out-of-order segments. It is up to the implementer to decide how out-of-order segments are handled (not specified by TCP).