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02-application-layer.md

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Application Layer

  • Applications run on different end systems and communicate over a network;
  • Application Layer Protocols define the rules for the communication between applications - the top layer of the TCP/IP protocol stack;
  • There are two types of applications:
    • Client-Server Applications - one side initiates the communication and the other side responds;
      • Server - provides services to clients;
        • Always-on - always available to provide services;
        • Static IP Address - clients know the server's address;
        • Data Centers - large-scale servers;
      • Client - requests services from servers;
        • Dynamic IP Address - assigned by the network when the client connects;
        • Intermittently Connected - not always connected to the network;
    • Peer-to-Peer (P2P) Applications - both sides can initiate the communication;
      • No Always-on Server - peers can be clients and servers;
      • Self-Scaling - the more peers, the more resources;
      • Dynamic IP Address - assigned by the network when the peer connects;
      • Intermittently Connected - not always connected to the network;
      • Decentralized - no central server;
  • A process is a program running within a host;
    • Within the same host, processes communicate using inter-process communication, defined by the operating system;
    • Processes in different hosts communicate using message passing;
    • Client processes initiate the communication, while server processes wait to be contacted;
    • A process sends/receives messages to/from its socket - the interface between the application layer and the transport layer;
    • To identify a process, we use the IP address and the port number.

Web and HTTP

  • Web - a client-server application;

    • Web Clients - browsers;
    • Web Servers - store web pages and send them to clients;

  • Uniform Resource Locator (URL) - identifies the web page;

    • Scheme - protocol used to access the resource;
    • Authority - the server's domain name or IP address;
      • Host - the server's domain name or IP address;
      • Port - the server's port number;
    • Path - the resource's location on the server;
    • Query - parameters to be passed to the server;
    • Fragment - a reference to a specific part of the resource;

  • HyperText Transfer Protocol (HTTP) - the protocol used to transfer web pages;

    • Stateless Protocol - each request/response pair is independent;
    • Persistent Connections - multiple requests/responses can be sent over the same connection;
    • Uses TCP as the transport layer protocol - creates a connection (creates a socket), sends the request, receives the response, and closes the connection;
    • Round-Trip Time (RTT) - the time it takes for a small packet to travel from the client to the server and back;
    • HTTP Request:
      • Request Line - method, URL, and HTTP version;
      • Headers - additional information about the request;
      • Body - data sent to the server;
      • Example: 
        GET /index.html HTTP/1.1
Host: www.example.com
Connection: close
    • HTTP Response:
      • Status Line - HTTP version, status code, and status message;
      • Headers - additional information about the response;
      • Body - data sent by the server;
      • Example: 
        HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Content-Type: text/html
Content-Length: 155
<html>
<body>
<h1>Hello, World!</h1>
</body>
</html>
    • HTTP Methods:
      • GET - requests a resource;
      • POST - sends data to the server;
      • PUT - uploads a resource;
      • DELETE - deletes a resource;
      • HEAD - requests the headers of a resource;
      • OPTIONS - requests the server's capabilities;
      • TRACE - echoes the request back to the client;
    • HTTP Status Codes:
      • 1xx - informational;
      • 2xx - success - e.g., 200 OK, 201 Created;
      • 3xx - redirection - e.g., 301 Moved Permanently, 302 Found;
      • 4xx - client error - e.g., 400 Bad Request, 404 Not Found;
      • 5xx - server error - e.g., 500 Internal Server Error, 503 Service Unavailable.

Email

There are three main components of an email system:

  • User Agents - email clients (e.g., Outlook, Thunderbird);
    • Compose, read, reply, forward, save, and delete emails;
  • Mail Servers - store and forward emails;
    • Mailbox - stores incoming emails;
    • Message queue - stores outgoing emails;
  • Simple Mail Transfer Protocol (SMTP) - the protocol used to send emails between mail servers;
    • Uses TCP as the transport layer protocol, port 25;
    • Persistent Connections - multiple emails can be sent over the same connection;
    • 7-bit ASCII encoding;
    • Interaction with the server:
      • Handshake - client connects to the server;
      • Mail Transfer - client sends the email;
      • Closure - client disconnects from the server;
    • Commands and Responses, similar to HTTP;
    • Messages are sent in MIME format: headers and body;
  • Mail Access Protocols - used by the user agent to retrieve emails from the server;
    • Post Office Protocol (POP) - authorization and download of emails;
      • POP3 - version 3;
    • Internet Message Access Protocol (IMAP) - keeps emails on the server and synchronizes them with the client;
    • HTTP - web-based email clients.

DNS

  • Domain Name System (DNS) - translates domain names into IP addresses;
  • DNS is a distributed database implemented in a hierarchy of servers;
  • Hosts query the DNS servers to resolve domain names;
  • DNS Services:
    • Hostname to IP Address Translation - the most common service;
    • Mail Server Alias - maps domain names to mail servers;
    • Load Balancing - maps domain names to multiple IP addresses;
  • Its distributed nature allows for scalability and reliability;
  • DNS Hierarchy:
    • Root Servers - the top-level servers;
    • Top-Level Domain (TLD) Servers - servers for each TLD - .com, .org, .net, etc.;
    • Authoritative Servers - servers for each domain - example.com, example.org, etc.;
    • Local DNS Servers - the user's ISP DNS server;
  • Example:
    • Client wants to resolve www.example.com;
    • First, checks its cache, but the entry is not there;
    • Then, asks the local DNS server to resolve the domain;
    • The local DNS server asks the root server for the com TLD server;
    • The com TLD server asks the authoritative server for the example.com domain;
    • The authoritative server responds with the IP address;
    • The local DNS server caches the response and sends it to the client.

DNS

  • Once a DNS server resolves a domain name, it caches the result for a certain amount of time - the Time to Live (TTL);
  • DNS record - a mapping between a domain name and an IP address, with the format: (name, value, type, TTL);
    • Types:
      • A - name is hostname, value is IP address;
      • NS - name is domain, value is authoritative server;
      • MX - name is domain, value is mail server;
      • CNAME - name is alias, value is canonical name;
      • PTR - name is IP address, value is hostname;
  • DNS protocol - uses UDP as the transport layer protocol, port 53;
    • Query - client sends a query to the server;
    • Reply - server sends a response to the client;
    • Message Format for both queries and replies:
      • Header - identification, flags, number of questions, answers, authority, and additional records;
      • Question - domain name and type;
      • Answer - domain name, type, class, TTL, and value;
      • Authority - domain name, type, class, TTL, and value;
      • Additional - domain name, type, class, TTL, and value;
  • To insert a new domain into the DNS system, the domain owner must:
    • Register the domain with a registrar;
    • Provide the registrar with the IP address of the authoritative server;
    • The registrar notifies the TLD server of the new domain;
  • There are several attacks that can be performed on the DNS system:
    • DDoS - Distributed Denial of Service - bombard the DNS servers with requests, not very effective in root servers, but is more dangerous on TLD servers;
    • Redirect - redirect the DNS queries to malicious servers - DNS poisoning.

P2P

  • Decentralized - no central server;
  • Arbitrary end systems can directly communicate;
  • Examples:
    • File Sharing - e.g., BitTorrent;
    • Streaming - e.g., Skype, Zoom;
    • Collaboration - e.g., Google Docs;
  • Torrent - a group of peers exchanging chunks of a file;
    • Seeder - peer with the complete file;
    • Tracker - server that coordinates the peers;
    • Peers download and upload chunks of the file;
    • Churn - peers join and leave the network;
    • Tit-for-Tat - peers prefer to upload to other peers that upload to them.

CDN

  • Content Distribution Networks (CDNs) are geographically distributed network of proxy servers and their data centers;
  • The goal is to provide high availability and performance by distributing the service spatially relative to end users.