Computer Networks Lecture 02: Application Layer

Computer Networks Lecture 02: Application Layer

• Lecture focused on the Application Layer in computer networks.
• Presented by Dr. Sahar M. Ghanem.
• Course is offered by the Computer and Systems Engineering Department, Faculty of Engineering, Alexandria University.

Outline

• Principles of Network Applications
• The Web and HTTP
• Electronic Mail in the Internet
• DNS—The Internet's Directory Service
• Peer-to-Peer File Distribution
• Video Streaming and Content Distribution Networks
• Socket Programming: Creating Network Applications

Applications (1/2)

• 1970s/1980s: text email, remote computer access, file transfers, newsgroups.
• Mid-1990s: World Wide Web, web surfing, search, e-commerce.
• New Millennium: voice over IP (VoIP) video conferencing (Skype, FaceTime, Google Hangouts), user generated video (YouTube), movies on demand (Netflix), multiplayer online games (Second Life, World of Warcraft), social networking (Facebook, Instagram, Twitter).
• Smartphone and 4G/5G Internet access: location-based apps (Yelp, Tinder, Waze), mobile payment apps (WeChat, Apple Pay), messaging apps (WeChat, WhatsApp).

Client-Server Architecture

• Always-on host (server) serving requests from multiple hosts (clients).
• Clients do not directly communicate.
• Servers have fixed, well-known IP addresses.
• Data centers often house servers, creating powerful virtual servers (hundreds of thousands of servers).

P2P Architecture

• Minimal reliance on dedicated servers in data centers.
• Direct communication between intermittently connected hosts (peers).
• Example: file-sharing application BitTorrent.
• Strong features: self-scalability, cost-effectiveness.
• Key challenges: security, performance, and reliability due to decentralized structure.

Process Communication

• Processes on different hosts (potentially different operating systems) communicate via messages.
• Processes communicate across the computer network, exchanging messages.
• A process can be both a client and a server (e.g., peer-to-peer file sharing).
• Network communication happens through a software interface called a socket.

Transport Layer Services

• Applications are assigned specific port numbers.
• Internet standard protocols have well-known port numbers (found at www.iana.org).
• Transport layer protocols provide services like reliable data transfer, throughput, timing, and security.
• TCP and UDP are common transport protocols.

HTTP (HyperText Transfer Protocol)

• Web's application-layer protocol.
• Client-server program exchange HTTP messages.
• HTTP/1.0 (RFC 1945), HTTP/1.1 (RFC 7230), and HTTP/2 (RFC 7540) are common versions.
• Web pages consist of base HTML files and referenced objects (e.g., images, JavaScript, CSS).
• Each object has a unique URL.
• HTTP uses TCP as its underlying transport protocol.
• The server sends requested files without storing client state (stateless protocol).

TCP Connections

• Request/response pairs can be sent over separate TCP connections (non-persistent) or a single TCP connection (persistent).
• HTTP defaults to persistent connections.
• Non-persistent connections impose additional delay per object.

HTTP Request Message

• Written in ASCII text.
• Components: request line (method, URL, HTTP version); header lines (Host, Connection, User-Agent, Accept-Language); entity body (often empty for GET requests).
• Common methods: GET, POST, HEAD, PUT, DELETE.
• Most HTTP requests use the GET method.

HTTP Response Message

• Three sections: status line (protocol version, status code, status message), header lines (Date, Last-Modified, Content-Type), entity body (requested object).
• Status codes like 200 OK, 301 Moved Permanently, 404 Not Found, indicate successful or failed requests.

User-Server Interaction: Cookies

• HTTP is stateless, making user identification challenging.
• Cookies are used to maintain user session state.
• Four components: cookie header in HTTP response, cookie header in HTTP request, browser-managed cookie file, database on the Web site.

Web Caching

• Proxy servers that act as intermediaries between clients and origin servers.
• Caching reduces response time for frequently accessed objects.
• Reduces traffic on the access links and improves overall Internet performance.
• Uses conditional GET requests to avoid unnecessary data transfers (when objects in the cache are up-to-date).

HTTP/2

• Aims to reduce latency by enabling request/response multiplexing over a single TCP connection, prioritizing requests, and compressing HTTP headers.
• Solves HEAD of Line (HOL) blocking by breaking messages into frames and interleaving them.
• Improves efficiency and reduces perceived latency.

DNS (Domain Name System)

• Translates hostnames to IP addresses.
• Hierarchical distributed database with root servers, top-level domain (TLD) servers, and authoritative servers.
• Local DNS servers act as proxies for resolving queries.
• Uses UDP (port 53) for communication.
• Offers services like host aliasing, mail server aliasing, load distribution.

DNS Caching

• Improves DNS performance and reduces message delays.
• DNS servers cache mappings in local memory for faster lookups.
• Cached information is discarded after a period.

DNS Records and Messages

• Resource records (RRs) contain information to map hostnames to IP addresses.
• Records have fields like name, value, type, and time to live (TTL).

DNS Message Format

• Query and reply messages share the same format with a header section and data sections.
• Types of data fields (question, answer, authority, additional).

nslookup

• A program for querying DNS servers to translate hostnames.

ICANN

• Accredits domain registrars, verifying uniqueness of domain names.

Socket Programming

• Programming network applications with sockets to communicate between processes.
• Two main types: client-side program, server-side program.
• Socket programs can use UDP (connectionless) or TCP (connection-oriented) protocol.