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# System prepended metadata

title: 50.012 Lecture 9

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# 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).