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Chapter 2: Application Layer

Kurose & Ross, Computer Networking: A Top-Down Approach, 9th edition (2025) – Full lecture coverage

1. Principles of Network Applications

Network apps run on end systems (hosts) – not on core network devices. They communicate using the transport layer's services.

🔌 Processes, Sockets, and Addressing

A process is a program running within a host. Processes communicate via sockets (API to the network). To identify a socket, we need both the IP address (host) and a port number (e.g., HTTP:80, SMTP:25).

📡 What transport service does an app need?

• Data integrity: some apps (file transfer, web) need 100% reliable transfer; others (audio) tolerate some loss.
• Timing: low delay is critical for Internet telephony, interactive games.
• Throughput: multimedia apps require minimum throughput; elastic apps use whatever is available.
• Security: encryption, data integrity, authentication (e.g., TLS).

🚚 Internet transport protocols

• TCP: reliable, in-order delivery, flow control, congestion control, connection-oriented. No timing/minimum throughput guarantees.
• UDP: unreliable, unordered, no connection setup, no congestion control. Used for streaming, DNS, VoIP (where speed matters over reliability).
• TLS (Transport Layer Security): provides encrypted TCP connections, data integrity, and endpoint authentication (implemented in application layer).

2. Web and HTTP

HTTP (HyperText Transfer Protocol) – Web's application-layer protocol. Uses TCP port 80 (default).

• Non-persistent HTTP: one object per TCP connection → 2 RTTs per object (connection setup + request/response).
• Persistent HTTP (HTTP/1.1): multiple objects over single TCP connection → lower response time, less overhead.

📄 HTTP request message (ASCII)

GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-Agent: Mozilla/5.0
Accept: text/html,application/xhtml+xml
Accept-Language: en-us
(blank line)
    

📬 HTTP response message

HTTP/1.1 200 OK
Date: Mon, 10 Mar 2025 12:00:00 GMT
Server: Apache/2.4
Content-Type: text/html
Content-Length: 1024

(html data...)
    

Status codes: 200 OK, 301 Moved Permanently, 400 Bad Request, 404 Not Found, 505 HTTP Version Not Supported.

🍪 Cookies – maintaining user/server state

HTTP is stateless; cookies add state via:

1. Cookie header in HTTP response (set by server)
2. Cookie header in next HTTP request (sent by browser)
3. Cookie file stored on user's host
4. Back-end database at website

💾 Web Caching (Proxy Servers)

Web cache (proxy) stores copies of objects, reducing response time and access link traffic. Browser sends requests to cache; if object is fresh, cache returns it; otherwise cache fetches from origin server.

⚡ HTTP/2 and QUIC

• HTTP/2: multiplexed frames, server push, prioritized responses → reduces head-of-line (HOL) blocking.
• QUIC (Quick UDP Internet Connections): application-layer over UDP, integrates TLS + reliability + congestion control, reduces handshake to 1 RTT (or 0 RTT).

3. E‑mail: SMTP and IMAP

Three major components: user agents (mail readers), mail servers, and SMTP (Simple Mail Transfer Protocol).

• SMTP: uses TCP port 25, persistent connections, push protocol (sending server pushes message to receiving server). Requires 7-bit ASCII.
• Mail access protocols: IMAP (retrieves, stores, manages messages on server) or HTTP (web-based email like Gmail).

Sample SMTP interaction

S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr
C: MAIL FROM: <alice@crepes.fr>
S: 250 Sender ok
C: RCPT TO: <bob@hamburger.edu>
S: 250 Recipient ok
C: DATA
S: 354 Enter mail, end with "." on a line by itself
C: Do you like ketchup?
C: How about pickles?
C: .
S: 250 Message accepted for delivery
C: QUIT
S: 221 closing connection
    

4. Domain Name System (DNS)

DNS translates hostnames (e.g., www.example.com) to IP addresses. It is a distributed hierarchical database implemented by many name servers.

• Root name servers (13 logical, heavily replicated) – managed by ICANN.
• Top-Level Domain (TLD) servers – for .com, .org, .edu, country domains, etc.
• Authoritative DNS servers – provide mapping for specific organization’s hosts.
• Local DNS server – not part of hierarchy, caches results, acts as proxy.

DNS resolution: iterative vs. recursive

Iterated query: contacted server replies with name of next server to ask. Recursive query: puts burden on name server (rare in practice).

DNS records (Resource Records – RR)

• A – hostname to IP address
• NS – domain to authoritative name server hostname
• CNAME – alias to canonical name
• MX – mail server for domain

5. Video Streaming & Content Distribution Networks (CDNs)

Streaming video dominates Internet traffic. Challenge: deliver high-quality video to millions of heterogeneous users.

DASH (Dynamic Adaptive Streaming over HTTP)

• Server splits video into chunks, each encoded at multiple bitrates.
• Client dynamically requests the best chunk based on measured bandwidth and buffer level.
• Intelligence at client: avoids rebuffering, maximizes quality.

Content Distribution Networks (CDNs)

• Replicate content across many geographically distributed servers (e.g., Akamai, Cloudflare, Amazon CloudFront).
• Enter deep: many small clusters near users; Bring home: fewer large clusters in IXPs.
• Netflix uses its own CDN (OpenConnect) to store content close to ISPs.

6. Socket Programming (UDP & TCP)

Socket = door between application process and transport layer. Two main types: UDP socket (unreliable datagram) and TCP socket (reliable byte stream).

UDP socket programming (Python)

Client attaches destination IP and port to each packet; no connection setup.

# UDPClient.py
from socket import *
serverName = 'hostname'
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_DGRAM)
message = input('Input lowercase sentence:')
clientSocket.sendto(message.encode(), (serverName, serverPort))
modifiedMessage, serverAddress = clientSocket.recvfrom(2048)
print(modifiedMessage.decode())
clientSocket.close()
    

# UDPServer.py
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_DGRAM)
serverSocket.bind(('', serverPort))
print('The server is ready to receive')
while True:
    message, clientAddress = serverSocket.recvfrom(2048)
    modifiedMessage = message.decode().upper()
    serverSocket.sendto(modifiedMessage.encode(), clientAddress)
    

TCP socket programming (Python)

Connection-oriented; server must first create a welcoming socket, then accept connections.

# TCPClient.py
from socket import *
serverName = 'servername'
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_STREAM)
clientSocket.connect((serverName, serverPort))
sentence = input('Input lowercase sentence:')
clientSocket.send(sentence.encode())
modifiedSentence = clientSocket.recv(1024)
print('From Server:', modifiedSentence.decode())
clientSocket.close()
    

# TCPServer.py
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_STREAM)
serverSocket.bind(('', serverPort))
serverSocket.listen(1)
print('The server is ready to receive')
while True:
    connectionSocket, addr = serverSocket.accept()
    sentence = connectionSocket.recv(1024).decode()
    capitalizedSentence = sentence.upper()
    connectionSocket.send(capitalizedSentence.encode())
    connectionSocket.close()
    

Homework Assignment (Theoretical, Graded)

Chapter 2 Summary

• Application architectures: client-server, P2P, hybrid.
• Transport service requirements: reliability, timing, throughput, security.
• TCP vs. UDP: reliable connection-oriented vs. unreliable datagram.
• HTTP: stateless, persistent connections, cookies for state, web caching, HTTP/2, QUIC.
• Email: SMTP (push), IMAP (pull), mail access protocols.
• DNS: distributed hierarchical database, iterative/recursive queries, caching, resource records.
• Video streaming: DASH, CDNs for scalability and low latency.
• Socket programming: UDP (datagram) and TCP (stream) examples in Python.