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.