IP is a connectionless protocol. It's job is just to look at the source and destination IP Addresses and forward packets. It works on best effort transport. If a packet goes missing this is not the responsibility of the IP/Network Layer to send this same packet again.
This TCP connection from source to destination is a virtual connection over the medium that is IP.
TCP Header
TCP Header
Payload
Within the TCP header we have certain fields within it as per below:
| Port Number | Used to inform how to access this application. This is comprised of two parts: Source Port - This is a random Ephemeral port. Ephemeral means if we open a connection we may get a specific source port, but if we close it and reinitiate another connection it may be a different port number. This is always a random number when opening a connection. Destination Port - This is a Well-Known Port Number. This is always fixed when opening a connection. Note that when a reply happens the Source/Destination ports become reversed. |
| Flags | A flag is just a mechanism of conveying information. In TCP there are 5/6 flags. These can only have two values, 1 or 0. 1 is up 0 is down. |
| Sequence Number | If we have to send multiple packets we mark them with a particular number. This is not a Flag but a specific field within the TCP header. The value is 2^32 = 4294967296 possible sequence number values. |
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Note
TCP Header + Payload = TCP Segment.
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