The client-server model and application protocols

1. Order of server socket calls?
   Stuck? Peek at 4.4 The Socket API: system calls, client-server order & blocking calls

   • TCP Server order: socket() → bind() → listen() → accept() → recv()/send() → close().
   • TCP Client order: socket() → connect() → send()/recv() → close() (OS auto-binds ephemeral port).
   • UDP Server / Client: no listen() or accept(); uses recvfrom() / sendto() on a single bound socket.
   • Blocking calls: connect(), accept(), and recv() block by default; listen() does NOT block.

   Read lesson 4.4

2. How many RTTs does the TCP 3-way handshake take before the client may send data?
   Stuck? Peek at 4.3.2 TCP 3-way handshake, connection termination & state machine

   • 3-way handshake: SYN(seq=x) → SYN+ACK(seq=y, ack=x+1) → ACK(seq=x+1, ack=y+1).
   • Seq consumption: SYN = +1, FIN = +1, Pure ACK = 0, Data(L) = +L (full connection advances by D + 2).
   • Teardown states: Active closer goes FIN-WAIT-1 → FIN-WAIT-2 → TIME-WAIT (2·MSL) → CLOSED; Passive closer goes CLOSE-WAIT (half-close) → LAST-ACK → CLOSED.

   Read lesson 4.3.2

3. Which OSI layers does TCP/IP merge into its application layer?
   Stuck? Peek at 1.1.2 The OSI 7 layers and the TCP/IP model

   • OSI bottom-up: Physical, Data link, Network, Transport, Session, Presentation, Application.
   • Scope: link → hop → host-to-host → process-to-process.
   • Routers: layers 1–3; hosts: 1–7.
   • TCP/IP (5): application (= OSI 5–7), transport, network, data link, physical.
   • Encryption/compression = presentation; dialogue control = session.

   Read lesson 1.1.2

Every layer below has been a delivery service. Now, at the top of the stack, our story finally asks why the message is sent: you want a web page. Your browser (a client program) asks a server program on another machine. The rules for what they say to each other — the format of the questions and answers — form an application protocol. This phase covers the two you use before every page loads: the name lookup (5.2) and the page fetch (5.3).

A popular server has 1000 users connected at the same time, each over TCP. How many sockets does the server process hold open?

1001 or more: one listening (welcome) socket plus one connected socket per client connection (Lesson 4.4). If each client opens two connections, 2001.

Client and server

What an application protocol defines

• The message types (request, response, …).
• The syntax: fields and how they are written (text lines or binary).
• The semantics: what each field means.
• The rules: when and how to send and respond.
• Which transport it uses (TCP for reliability, UDP for small fast exchanges) and which well-known port the server listens on.

Peer-to-peer (brief)

In peer-to-peer (P2P) systems every machine is both client and server: peers download pieces from each other. The server's upload capacity stops being a bottleneck because every new peer also uploads.

Putting it together

• Clients ask, servers answer at well-known ports.
• An application protocol fixes message types, syntax, meaning and timing.
• P2P makes every user also a server, removing the upload bottleneck.

• A server listens on a well-known port; the client usually uses an ephemeral port chosen by its OS.
• "Server" is a program role, not a kind of hardware.
• Application protocols choose TCP or UDP — the choice is part of the protocol.

"The client knocks, the server answers — and the protocol is the language at the door."

Before your browser can knock, it needs the server's IP address — but you typed a name. Next (5.2): how is a name turned into an address, quickly and for billions of users?

1. MCQ

   Which statement about a server process is correct?

   1. It starts the conversation.
   2. It listens at a well-known port and answers requests.
   3. It must run on special hardware.
   4. It always uses UDP.
2. NAT

   A server has 300 clients with one TCP connection each. Sockets open at the server?

   Write your answer before opening the key.

3. MSQ

   An application protocol defines:

   1. message types
   2. message syntax and meaning
   3. the routing algorithm
   4. when to send which message
4. NAT

   Client-server: 10 copies of an 8 × 10⁸-bit file, server upload 200 Mbps. Minimum seconds for the server to upload all copies?

   Write your answer before opening the key.

• Client initiates; server listens at a well-known port and answers.
• Application protocol = message types + syntax + semantics + rules + transport/port choice.
• Server sockets = 1 listening + 1 per connection.
• P2P: peers serve each other; client-server distribution ≥ N·F/u_s.

• client-server model
• server, client
• application protocol (types, syntax, semantics, rules)
• peer-to-peer (P2P, brief)

1. Who initiates in client-server?
   Reveal answer

   The client.
2. What does an application protocol define?
   Reveal answer

   Message types, syntax, semantics, rules (and transport/port).
3. Sockets at a server with n connections?
   Reveal answer

   n + 1.

• Server sockets = 1 listening + 1 per connection. — TCP servers using the standard socket API.
• Client-server distribution ≥ N·F / u_s. — server upload is the bottleneck (many clients).