Understanding HTTP Status Codes

Application Layer

Data Integrity and Timing

• Some applications require 100% reliable data transfer, e.g., file transfer and web transactions.
• Other applications can tolerate some loss, e.g., audio and video streaming.
• Some applications require low delay to be effective, e.g., Internet telephony and interactive games.
• Some applications require a minimum amount of throughput to be effective, e.g., multimedia.
• Elastic applications make use of whatever throughput they get.

Transport Service Requirements

• File transfer: reliable, no loss, no delay.
• Email: reliable, no loss, no delay.
• Web documents: reliable, no loss, no delay.
• Real-time audio/video: loss-tolerant, low delay.
• Interactive games: low delay, throughput guarantee.
• Text messaging: loss-tolerant, no delay.

Internet Transport Protocols

• TCP (Transmission Control Protocol):
  • Reliable transport between sending and receiving processes.
  • Flow control: sender won't overwhelm receiver.
  • Congestion control: throttle sender when network overloaded.
  • Connection-oriented: setup required between client and server processes.
• UDP (User Datagram Protocol):
  • Unreliable data transfer between sending and receiving processes.
  • No flow control, congestion control, throughput guarantee, or connection setup.

Application Layer Protocols

• HTTP (Hypertext Transfer Protocol):
  • Client-server model.
  • Client initiates TCP connection to server, port 80.
  • HTTP messages (application-layer protocol messages) exchanged between browser and Web server.
  • TCP connection closed.
• FTP (File Transfer Protocol):
  • Used for file transfer.
• SMTP (Simple Mail Transfer Protocol):
  • Used for electronic mail.
• POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol):
  • Used for electronic mail.
• DNS (Domain Name System):
  • Distributed database implemented in a hierarchy of many name servers.
  • Application-layer protocol: hosts and name servers communicate to resolve names (address/name translation).

Web and HTTP

• HTTP overview:
  • Client-server model.
  • Web's application layer protocol.
  • Client initiates TCP connection to server, port 80.
• HTTP messages:
  • Request messages: GET, POST, etc.
  • Response messages: 200 OK, 301 Moved Permanently, 400 Bad Request, 404 Not Found, etc.
• HTTP connections:
  • Non-persistent HTTP: at most one object sent over a single TCP connection.
  • Persistent HTTP: multiple objects sent over a single TCP connection.

Sockets

• Process: a program running within a host.
• Socket: a door for sending and receiving messages.
• Addressing processes:
  • IP address (32-bit) used for addressing datagrams.
  • Hostname (e.g., www.yahoo.com) used by humans.
  • DNS (Domain Name System) maps between IP address and hostname.

Electronic Mail

• User agents:
  • Mail clients (e.g., Mail, Outlook).
  • Mail servers.
• Simple Mail Transfer Protocol (SMTP):
  • Used for sending and receiving email.
• Mailbox:
  • A storage location for email messages.

DNS (Domain Name System)

• DNS: a distributed database implemented in a hierarchy of many name servers.
• Application-layer protocol: hosts and name servers communicate to resolve names (address/name translation).
• DNS resolution:
  • User machine runs the client side of the DNS application.
  • Browser extracts the hostname from the URL and passes it to the DNS client.
  • DNS client sends a query to a DNS server.
  • DNS client receives a reply with the IP address for the hostname.
  • Browser uses the IP address to initiate a TCP connection to the HTTP server process.

Learn about HTTP status codes such as 200 OK, 301 Moved Permanently, 400 Bad Request, and 404 Not Found. Explore how these codes indicate the outcome of a client-server request and familiarize yourself with common responses.