Chapter 7: Computer Networks

Application Layer

Application Layer

The layer where all the applications are found and provides transport services for users.

DNS (Domain Name System)

A support protocol that maps Internet names to IP addresses.

Electronic Mail

A real network application that allows users to send and receive messages over the Internet.

World Wide Web

A real network application commonly referred to as "the Web" that allows users to access and navigate web pages.

Multimedia

A real network application that includes modern video streaming and allows users to access and stream multimedia content.

Content Distribution

The process of distributing content over the Internet, including peer-to-peer networks and content delivery networks.

DNS Lookup Process

The process of mapping a name to an IP address using DNS, involving recursive and iterative queries.

DNS Name Space and Hierarchy

The hierarchical structure of the DNS naming system, managed by ICANN, with top-level domains and subdomains.

Top-level domain (TLD)

A domain name that appears at the highest level in the hierarchical Domain Name System (DNS) structure, such as .com, .edu, or .gov.

gTLD

Generic top-level domain (gTLD) refers to a category of TLDs that are not associated with a specific country or region, such as .com, .org, or .net.

FQDN

Fully Qualified Domain Name (FQDN) is a domain name that specifies the exact location of a resource within the DNS hierarchy, including all levels of the domain name, such as www.cs.uchicago.edu or cisco.com.

Registries

Companies appointed by ICANN to operate and manage the top-level domains (TLDs), such as Verisign for the .com registry.

Registrars

Companies that sell domain names directly to users, operating under the authority of the registries, such as Domain.com, GoDaddy, and NameCheap.

DNSBL

DNS-based blacklist (DNSBL) is a list used to track IP addresses associated with spammers and malware, commonly used for DNS queries to check if a domain is blacklisted.

EDNS0 CS

EDNS0 CS (Extended DNS Client Subnet) is an extension to DNS queries that allows a client's local recursive resolver to pass the IP address subnet of the stub resolver to the authoritative name server.

Resource Records

Information associated with a domain name in the DNS database, including the domain name, time to live, class, type, and value.

Time-to-Live (TTL)

The duration for which a DNS record can be cached before it needs to be refreshed.

Class

The field in a resource record that specifies the type of information it contains. For Internet information, it is always IN.

Type

The field in a resource record that indicates the kind of record it is. It can be A, AAAA, MX, NS, CNAME, PTR, SPF, SRV, or TXT.

Start of Authority (SOA)

A resource record that provides information about the primary source of information for a name server's zone.

A Record

A resource record that holds the IPv4 address of a host.

AAAA Record

A resource record that holds the IPv6 address of a host.

MX Record

A resource record that specifies the mail exchange server for a domain.

NS Record

A resource record that specifies the name server for a domain.

CNAME Record

A resource record that creates an alias for a domain name.

PTR Record

A resource record that points to another name and is used for reverse lookups.

SRV Record

A resource record that identifies a host providing a specific service in a domain.

SPF Record

A resource record that encodes information about a domain's mail sending policy.

TXT Record

A resource record that contains descriptive ASCII text.

Value Field

The field in a resource record that can hold a number, domain name, or ASCII string, depending on the record type.

DNSSEC

A set of DNS extensions that provide security by allowing DNS name servers to carry digital signatures and verify the integrity of DNS records.

Remote query

When a domain needs to find the IP address of another domain that is not locally cached, a remote query is initiated.

Name server

A server that stores and provides information about domain names and their corresponding IP addresses.

Root name server

The top-level name server in the DNS hierarchy that has information about each top-level domain.

Local name server

The name server that receives the query from the client and initiates the DNS resolution process.

DNS cache

A local storage that stores previously resolved DNS queries to improve performance and reduce network traffic.

Anycast routing

A routing technique where a packet is delivered to the nearest instance of a destination address, improving reliability and performance.

DNS resolver

A component of the local name server that sends queries to other name servers to resolve domain names.

Authoritative name server

The name server that has the final and accurate information about a specific domain.

DNS privacy

The concern and efforts to encrypt DNS queries and responses to protect user privacy and prevent eavesdropping.

DNS architecture

The distributed system of millions of name servers that work together to translate human-readable domain names into IP addresses.

Email conventions and styles

Email has developed its own conventions and styles, being informal and having a low threshold of use.

Content-focused debates

Email debates often focus on content rather than status, eliminating cues associated with rank, age, and gender.

Jargon and emoticons

Email is full of jargon such as BTW, ROTFL, and IMHO, as well as ASCII symbols called smileys or emoticons.

Evolution of email protocols

Email protocols have evolved over time, adding features like the ability to send messages to a list of recipients and multimedia capabilities.

User agents and message transfer agents

Email systems consist of user agents, which allow people to read and send email, and message transfer agents or mail servers, which move messages from source to destination.

Envelope and message format

Email messages have an envelope that contains information for transporting the message, and a message inside the envelope consisting of a header and a body.

User agent interface

User agents provide a graphical or text-based interface for composing, receiving, and replying to messages, as well as manipulating mailboxes.

Message disposition

After reading a message, users can choose to delete it, reply to it, forward it, or keep it for later reference.

Automatic message filtering

User agents can automatically filter messages based on fields and contents to determine if they are likely spam.

Popular spam content

Common types of spam include fake diplomas, cheap drugs, unclaimed Nigerian bank accounts, and pills for enlarging body parts.

Filing rules

User-defined rules that specify conditions and actions for organizing email messages into folders.

Inbox

Folder for incoming mail not filed elsewhere.

Junk Mail

Folder for messages that are thought to be spam.

Message Formats

The format of email messages, including the primitive envelope, header fields, blank line, and message body.

Header fields

Single lines of ASCII text in email messages that contain field names and values.

Envelope fields

Fields in email messages that are used to construct the actual envelope for message transfer.

Principal header fields

Header fields related to message transport, including To:, Cc:, Bcc:, From:, Sender:, Received:, and Return-Path:.

To

field:Email address(es) of primary recipient(s).

Cc

field:Email address(es) of secondary recipient(s).

Bcc

field:Email address(es) for blind carbon copies.

From

field:Person or people who created the message.

Sender

field:Email address of the actual sender.

Received

field:Line added by each transfer agent along the route.

Return-Path

field:Can be used to identify a path back to the sender.

Reply-To

field:Email address to which replies should be sent.

Message-Id

field:Unique number for referencing the message later.

Content-Description

field:Human-readable string telling what is in the message.

Content-Id

field:Unique identifier for the content.

Content-Transfer-Encoding

field:Specifies how the body is wrapped for transmission.

Content-Type

field:Specifies the type and format of the content.

MIME

Multipurpose Internet Mail Extensions, a standard for handling richer content in email messages.

MIME-Version

header:Identifies the MIME version used in the message.

Content-Transfer-Encoding

header:Specifies how the body is wrapped for transmission.

Content-Type

header:Specifies the type and format of the content.

Base64 encoding

ASCII encoding of binary data used to safely send arbitrary binary text in email messages.

MIME types

Categories that specify the nature of the message body, such as text, image, audio, video, font, model, application, and message.

Subtypes

Specific examples within each MIME type category, such as plain, html, xml, css, gif, jpeg, tiff, basic, mpeg, mp4, octet-stream, pdf, javascript, zip, etc.

MIME types

A way to identify the type of content in an email message, including attachments.

SMTP protocol

The protocol used for transferring email messages between mail transfer agents.

Mail submission

The process of sending email messages from user agents to the mail system for delivery.

Message transfer

The process of relaying email messages from the originator to the recipient using SMTP.

EHLO

A command used by clients to initiate an extended SMTP session.

SMTP extensions

Additional functionalities added to SMTP, such as client authentication and support for binary messages.

Telnet

A command used to establish a TCP connection to a mail server and interact with it using SMTP commands.

Mail server

A server that accepts incoming connections and delivers email messages.

MX record

A DNS record that specifies the mail servers responsible for accepting email messages for a domain.

Local delivery

The process of delivering email messages to the correct mailbox for the recipient to read.

Relaying

The process of forwarding mail from one address to another.

Mail transfer agent

A server responsible for sending and receiving email messages.

Final delivery

The process of transferring a copy of a mail message to the user agent for display.

User agent

A program or interface that allows users to access and manipulate their email.

IMAP

Internet Message Access Protocol, a protocol used for final delivery of email messages.

IMAP server

A server that listens to port 143 and allows users to access their email using the IMAP protocol.

IMAP client

A program or interface that connects to the IMAP server and issues commands to access and manipulate email.

Webmail

An interface for sending and receiving email using a web browser.

Webmail server

A server that accepts messages for users and provides a web interface for accessing and managing email.

Webmail client

A user interface provided via web pages that allows users to access and manipulate their email.

Hyperlink

A piece of text, icon, graphic image, photograph, or other page element that can be associated with another page.

Web page

A document or content that can be accessed through a browser.