HTTP Response Status Codes Quiz

HTTP Response Status Codes

• Indicates the status of the HTTP request.
• 200 OK: The request succeeded, and the requested object is later in the response message.
• 301 Moved Permanently: The requested object has moved, the new location is specified in the Location header.
• 400 Bad Request: The request message is not understood by the server.
• 404 Not Found: The requested document is not found on the server.
• 505 HTTP Version Not Supported: The server does not support the HTTP version used in the request.

Trying out HTTP (Client-Side):

• Telnet to a web server: telnet cis.poly.edu 80 opens a TCP connection to port 80 (default HTTP server port).
• Send a GET HTTP request: GET /~ross/ HTTP/1.1\r\n Host: cis.poly.edu\r\n\r\n This sends a minimal but complete GET request to the HTTP server.

Non-Persistent HTTP: Response Time

• HTTP response time is the time taken for a response message to be sent back to the client.
• Response time= 2RTT + File Transmission Time:
  • One RTT to initiate the TCP connection.
  • One RTT for the HTTP request and the first few bytes of the HTTP response to return.
  • File transmission time from the server to the client.

Persistent HTTP

• Employs a single TCP connection for multiple HTTP requests and responses.
• Benefits:
  • Reduces Response time because only one TCP connection is needed.
  • Server leaves the connection open after sending the response, allowing for subsequent HTTP messages to be sent over the open connection.

HTTP Request Message

• A request message is sent from the client to the server containing the request method, URL, version, headers, and data.
• Common methods: GET, POST, HEAD, PUT, DELETE.
• GET: Requests a resource from the server, used for retrieving information, typically with data included in the URL.
• POST: Used to send data to the server, often for form submissions or uploading data, typically with data in the entity body.
• HEAD: Similar to GET, but returns only the headers of the response message, useful for retrieving metadata.
• PUT: Uploads a file in the entity body to a path specified in the URL field.
• DELETE: Deletes a file or resource specified in the URL field.

HTTP Response Message

• Sent by the server back to the client.
• Includes the status line: Protocol version, status code, status phrase.
• Headers: Metadata related to the response like Content-Type, Content-Length, Date, and Server.
• Data: The actual content of the requested resource.

HTTP Connections

• Non-Persistent HTTP:
  • One TCP connection for each request.
  • Each TCP connection is closed after one object transmission.
• Persistent HTTP:
  • One TCP connection can be shared for multiple requests.
  • Server maintains the connection open after sending the response, allowing several requests/responses over the same connection.

Application Layer

• Chapter 2 delves into the Application Layer, a crucial layer in the network architecture responsible for enabling communication between applications running on different end systems.
• It covers various aspects like the principles of network applications, the implementation details of popular application protocols, and the creation of network applications using socket programming.

Application Architecture

• There are two primary application architectures: client-server and peer-to-peer (P2P).
• In the client-server architecture, one or more clients communicate with a dedicated server, which provides services.
• The server typically has a permanent IP address and is always on, while clients can be intermittent and use dynamic IP addresses.
• P2P architecture lacks a centralized server; instead, peers directly communicate with each other.
• This model allows for self-scalability as new peers contribute to both service capacity and demand.

Processes and Sockets

• Processes running on different end systems communicate via sockets, which act like doors allowing processes to send and receive messages.
• Sockets are controlled by the application developer but rely on the transport layer (TCP/UDP) and network layer (IP) for data transmission.

Addressing Processes

• Processes need unique identifiers for communication, consisting of both an IP address and port number.
• Host aliasing allows multiple IP addresses to be associated with a single hostname.
• The Domain Name System (DNS) provides a hierarchical and distributed database for mapping hostnames to IP addresses.

DNS: A Distributed Database

• Local name servers query root DNS servers to find the authoritative name server for a specific hostname.
• Authoritative name servers store the records for a domain, including IP addresses, alias names (CNAME), and mail server names (MX).
• Caching mechanisms are used by DNS servers to improve performance but can introduce potential inconsistencies.
• DNS records (RRs) are stored as tuples of (name, value, type, ttl) and can be updated using mechanisms like RFC 2136.

DNS Attacks

• Denial-of-service (DDoS) attacks can target root servers, TLD servers, or even exploit DNS itself to amplify traffic.
• Redirect attacks, like DNS poisoning, exploit DNS to send bogus replies to DNS servers, which then cache incorrect mappings.

P2P Applications

• P2P applications benefit from the absence of a centralized server and allow direct communication between peers.
• Examples include file distribution (BitTorrent), streaming (KanKan), and VoIP (Skype).

Socket Programming (TCP/UDP)

• Sockets provide a programming interface for creating client/server applications that communicate using either UDP or TCP.
• UDP is a connectionless, unreliable datagram protocol suitable for applications with lower latency requirements.
• TCP is a connection-oriented, reliable protocol that guarantees delivery of data in the correct order.

Summary

• The chapter stresses the importance of understanding network protocols and their key features:
  • Control vs. data messages
  • In-band vs. out-of-band signaling
  • Centralized vs. decentralized architecture
  • Stateless vs. stateful communication
  • Reliable vs. unreliable message transfer
• The concept of "complexity at the network edge" highlights the importance of application layer considerations for network performance and security.

Application Layer Protocols

• Principles

  • Web and HTTP: Client/server model using TCP
  • Non-persistent HTTP
    • Requires 2 RTTs per object
    • Overhead for each TCP
  • Persistent HTTP
    • Server keeps connection open, subsequent requests travel over open connection
    • Can complete all requests with only 1 RTT

• HTTP Request Message

  • Message Format: Consists of request line, header lines, and entity body
  • Methods:
    • GET: Retrieves data from the server
    • POST: Sends data to the server
    • HEAD: Asks server for headers only

• HTTP Response Message

  • Message Format: Consists of status line, header lines, and entity body
  • Status Code:
    • 200 OK: Successful Request
    • 400 Bad Request
    • 404 Not Found

• Uploading Form Input

  • POST: Form input uploaded as entity body
  • GET: Input encoded in URL field

Electronic Mail: Mail Reader & User Agents

• User agents are applications that enable users to compose, edit, and read email messages.
• Examples of user agents include Outlook, Thunderbird, and iPhone mail clients.
• User agents interact with mail servers to send and receive messages.
• Users interact with user agents, user agents interact with mail servers, and mail servers interact with other mail servers using protocols like SMTP.

Electronic Mail: Mail Servers

• Mail servers are responsible for storing and managing email messages.
• They maintain a mailbox for each user, containing their incoming messages.
• They also manage a queue of outgoing messages waiting to be sent.

Electronic Mail: SMTP

• SMTP (Simple Mail Transfer Protocol) is a protocol used for sending email messages between mail servers.
• It relies on TCP (Transmission Control Protocol) for reliable message transfer over port 25.
• SMTP operates through a direct transfer between sending and receiving servers.
• The transfer process has three distinct phases: handshaking (greeting), message transfer, and closure.
• SMTP uses a command/response interaction system similar to HTTP and FTP.
• Commands are sent in ASCII text, while responses include a status code and a phrase.
• Email messages must be formatted in 7-bit ASCII.

Scenario: Alice Sends a Message to Bob

• Alice composes an email using her user agent and addresses it to [email protected].
• Alice's user agent sends the message to her mail server, where it is placed in the message queue.
• The SMTP client on Alice's mail server establishes a TCP connection with Bob's mail server.
• The message is transmitted over the TCP connection.
• Bob's mail server stores the message in Bob's mailbox.
• Bob uses his user agent to access and read the message.

Sample SMTP Interaction

• The server initiates the connection by sending a "220" status code, indicating readiness.
• The client sends a "HELO" command with the sender's domain name, in this case, crepes.fr.
• The server responds with "250" acknowledging the connection and greeting.
• The client uses the "MAIL FROM" command to specify the sender's email address.
• The server responds with "250" indicating successful acceptance of the sender address.
• The interaction continues with further commands and responses for recipient addresses, message contents, and final delivery.

Test your knowledge of HTTP response status codes and client-side HTTP requests. Learn about the meanings of different status codes like 200, 301, and 404, and how to send requests using Telnet. This quiz will help solidify your understanding of HTTP communication.