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Topic Transport Layer

Question

How does TCP session termination work?

Answer

A four-way exchange: FIN → ACK → FIN → ACK. Each direction is closed independently, so it needs four segments (vs. three for setup); either host can initiate, and the two middle steps can sometimes combine into one FIN+ACK.

TCP four-way connection teardown sequence

* FIN, ACK, FIN, ACK: each direction is closed independently, so teardown takes four segments. *

TCP Session Termination (Four-Way Handshake):

Host A                              Host B
  │                                    │
  │──────── FIN ─────────────────────→│  Step 1: A has no more data
  │                                    │
  │←─────── ACK ──────────────────────│  Step 2: B acknowledges FIN
  │                                    │
  │←─────── FIN ──────────────────────│  Step 3: B has no more data
  │                                    │
  │──────── ACK ─────────────────────→│  Step 4: A acknowledges FIN
  │                                    │
  │         Connection Closed          │

Four Steps:

Step	Action	Purpose
1	Client sends FIN	"I have no more data to send"
2	Server sends ACK	Acknowledges client's FIN
3	Server sends FIN	"I have no more data to send"
4	Client sends ACK	Acknowledges server's FIN

Note: Steps 2 and 3 can occur in the same segment (FIN+ACK combined).

Key insight: Either host can initiate termination. Both directions must be closed independently.

Why four segments instead of three? During setup, the server can combine SYN+ACK because both sides agree to open at once. During teardown, when one side sends FIN the other may still have data to send, so its ACK and its own FIN are often separate - giving four steps: "I'm done" → "OK" → "Me too" → "OK".

Go deeper:

TCP Connection Termination — Four-Way Handshake — walks through the FIN / ACK / FIN / ACK exchange that closes each direction independently.
RFC 9293 §3.6 — Closing a Connection — the authoritative teardown sequence, including the TIME-WAIT state.
TCP — Connection termination — Wikipedia's four-way handshake summary for the carousel.

TCP sequence numbers and receive windows behave very much like a clock. The receive window shifts each time the receiver receives and acknowledges a new segment of data. Once it runs out of sequence numbers, the sequence number loops back to 0.

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Topic Build a Small Network

Question

What are the common protocols used in a small network, and what does each do?

Answer

Remote access: Telnet/SSH; web: HTTP/HTTPS; email: SMTP (send), POP3/IMAP (retrieve); file transfer: FTP/SFTP; addressing: DHCP; name resolution: DNS.

Common Protocols of a Small Network:

Network protocols support the applications and services employees use.

Protocol	Purpose
Telnet / SSH	Remote access to network devices and servers (SSH is the secure option)
HTTP / HTTPS	Communication between web clients and web servers
SMTP	Sends email
POP3 / IMAP	Used by clients to retrieve email
FTP / SFTP	Downloads and uploads files between a client and an FTP server
DHCP	Lets clients acquire an IP configuration automatically from a DHCP server
DNS	Resolves domain names to IP addresses

Key insight: A single server can provide several of these services at once (e.g. an email, FTP, and SSH server on one box), so a small network often needs only a handful of physical servers.

Go deeper:

Wikipedia — DHCP — how the DORA exchange auto-configures clients, the addressing service in this table.
Wikipedia — Domain Name System (DNS) — the name-resolution service that maps hostnames to IP addresses.

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