Questions and Answers
What is the main function of the Domain Name System (DNS)?
Which protocol is commonly used for sending and receiving emails?
What is the purpose of the hierarchy of name servers in DNS?
In the DNS process, what does the DNS client do after receiving a reply from the server?
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Which application-layer protocol do hosts and name servers use to communicate for name resolution?
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What happens once the browser extracts a hostname from a URL in the DNS process?
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Which component of the Internet uses IP addresses for addressing datagrams?
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What role does SMTP play in electronic mail communication?
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Study Notes
Identifying Processes on a Host
- A process is a program running within a host and can communicate with other processes using inter-process communication (defined by OS) within the same host.
- Processes in different hosts communicate by exchanging messages.
Process Identifiers
- A process needs an identifier to receive messages, but an IP address of the host on which the process runs is not enough to identify the process.
- An identifier includes both the IP address and port numbers associated with a process on a host.
- Examples of port numbers: HTTP server (80), mail server (25).
Application Layer Protocols
- Application layer protocols define the types of messages exchanged, message syntax, and message semantics.
- Examples of application layer protocols: HTTP, SMTP, FTP.
- Open protocols are defined in RFCs, allowing for interoperability between different systems.
- Proprietary protocols, such as Skype, do not allow for interoperability.
HTTP
- An HTTP client initiates a TCP connection to an HTTP server (process) at a specific port number (usually 80).
- The HTTP client sends an HTTP request message into the TCP connection socket.
- The HTTP server receives the request message, forms a response message containing the requested object, and sends the message back into its socket.
Network Applications
- Network applications can be categorized into two main architectures: client-server and peer-to-peer (P2P).
- In a client-server architecture, the server is always-on and has a permanent IP address.
- In a P2P architecture, arbitrary end systems directly communicate with each other, and peers request service from other peers and provide service in return.
Sockets
- A socket is a door through which a process sends and receives messages.
- A process sends messages to its socket, and the socket relies on the transport infrastructure to deliver the message to the receiving process.
Electronic Mail
- Electronic mail (email) involves mail servers, mail clients, and the SMTP protocol.
- Mail servers have a mailbox containing incoming messages for users and a message queue of outgoing mail messages.
- SMTP is a protocol for exchanging email messages between mail servers using TCP, usually on port 25.
- The SMTP protocol involves three phases of transfer: handshaking, transfer of messages, and closure.
Mail Message Format
- An email message consists of header lines (e.g., To:, From:, Subject:), a blank line, and the message body.
- The message body consists of ASCII characters only.
Mail Access Protocols
- Mail access protocols, such as POP and IMAP, allow users to retrieve email messages from a mail server.
- POP and IMAP are mail access protocols, whereas HTTP is used for web-based email services like Gmail, Hotmail, and Yahoo!.
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Description
Test your knowledge on principles of network applications, Web and HTTP, FTP, electronic mail, P2P applications, peer-to-peer applications, and DNS as covered in Chapter 2 of the book 'Computer Networking: A Top Down Approach' by Jim Kurose and Keith Ross.
