This might seem weird, but I’m a little confused, and I’d like an explanation, and perhaps an of the whole process of TCP data transfer from when there is a stream of data that needs to be sent.
I’m currently reading TCP documentation (RFC 9293), and in [Section 3.4](https://www.rfc-editor.org/rfc/rfc9293.html#name-sequence-numbers) it says:
>A fundamental notion in the design is that **every octet of data sent over a TCP connection has a sequence number**
So far, so good. In [Section 3.7](https://www.rfc-editor.org/rfc/rfc9293.html#name-segmentation) it says:
>The term “segmentation” refers to the activity TCP performs when ingesting a stream of bytes from a sending application and packetizing that stream of bytes into TCP segments.
The section later states that the segment’s size varies, but cannot exceed the [maximum segment size](https://www.rfc-editor.org/rfc/rfc9293.html#name-maximum-segment-size-option). I was confused and wondered if the segments are the octets, so I googled which is which. [RFC 879](https://www.ietf.org/rfc/rfc0879.txt) states that:
>The rule must match the default case. If the TCP Maximum Segment Size option is not transmitted then the data sender is allowed to send IP datagrams of maximum size (576) with a minimum IP header (20) and a minimum TCP header (20) and thereby be able to stuff **536 octets** of data into **each TCP segment**.
Meaning that octets go into segments, and given that octets have sequence numbers, then each segment would have multiple sequence numbers for each octet they send. However, googling “do TCP segments have sequential numbers” gives me:
>**At offset 32 into the TCP header is the sequence number**. The sequence number is a counter used to keep track of every byte sent outward by a host. If a TCP packet contains 1400 bytes of data, then the sequence number will be increased by 1400 after the packet is transmitted.
By [IBM](https://www.ibm.com/docs/en/zos-basic-skills?topic=4-transmission-control-protocol-tcp), meaning that the segments themselves also have their own sequence numbers?
My current understanding of the whole process is this:
1. A stream of data needs to be sent
2. TCP divides them into octets
3. TCP then packages them into segments
4. Segments get sequence numbers
5. Segments are sent sequentially.
Is my understanding correct? Please write an if possible.
In: 1
The segments/packets are the lowest level for TCP. They are the only part that has a sequence number in it. TCP header+data. So if sending 1000 bytes and max segment size is 512, it gets sent as two segments/packets. Each one has a sequence number.
The sequence number is used by the receiving TCP system to help reassemble packets in-order. The don’t necessarily have to be sent out sequentially (and may not arrive sequentially).
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