OSI7

Presented on 11th Oct 2021 by Taeim

Table of contents

1. Section 1 Application Layer Protocols
2. Topic 1.1: 
Application, Presentation, and Session
   1. Application Layer
   2. Presentation and Session Layer
   3. TCP/IP Application Layer Protocols
3. Topic 1.2: How Application Protocols Interact with End-User Applications
   1. Client-Server Model
   2. Peer-to-Peer Networks
   3. Peer-to-Peer Applications
   4. Common P2P Applications
4. Section 2 Well-Known Application Layer Protocols and Services
5. Topic 2.1:
Web and Email Protocols
   1. Hypertext Transfer Protocol and Hypertext Markup Language
   2. HTTP and HTTPS
   3. Email Protocols
6. Topic 2.2:
IP Addressing
   1. Domain Name Service
   2. DNS Message Format
   3. DNS Hierarchy
   4. The nslookup Command

Section 1 Application Layer Protocols

Topic 1.1: 
Application, Presentation, and Session

Application Layer

• The application layer is closest to the end user.
• Network applications enable users to send and receive data with ease.
• The application layer acts as interface between the applications and the underlying network.
• Application layer protocols help
exchange data between programs
running on the source and
destination hosts.
• The TCP/IP application layer
performs the functions of the upper
three layers of the OSI model.
• Common application layer protocols
include: HTTP, FTP, TFTP, DNS.

Presentation and Session Layer

• The presentation layer has three primary functions:
  • Format data
  • Compress data
  • Encrypt data
• Common standards for video include QuickTime and Motion Picture Experts Group (MPEG).
• Common graphic image formats are:
  • Graphics Interchange Format (GIF)
  • Joint Photographic Experts Group (JPEG)
  • Portable Network Graphics (PNG) format
• The session layer creates and maintains dialogs between source and destination applications.
• The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

TCP/IP Application Layer Protocols

• TCP/IP application protocols specify the format and control information necessary for common Internet functions.
• Application layer protocols must be implemented in both the source and destination devices.
• Application layer protocols implemented on the source and destination host must be compatible to allow communication.

Topic 1.2: How Application Protocols Interact with End-User Applications

Client-Server Model

• The device requesting the information is called a client.
• The device responding to the request is called a server.
• Client and server processes are considered to be in the application layer.
• The client initiates the exchange by requesting data from the server.
• The server responds by sending one
or more streams of data to the client.
• Application layer protocols describe
the format of the requests and
responses between clients and
servers.
• The contents of the data exchange
will depend of the application in use.
• Email is an example of a 
Client-Server interaction.

Peer-to-Peer Networks

• In the peer-to-peer (P2P) networking model, the data is accessed without the use of a dedicated server.
• Two or more computers can be connected to a P2P network to share resources.
• Every connected end device (a peer) can function as both a server and a client.
• The roles of client and
server are set on a per
request basis.

Peer-to-Peer Applications

• Some P2P applications use a hybrid system.
• In hybrid P2P, resource sharing is decentralized.
• Indexes that point to resource locations are stored in a centralized directory.
• In a hybrid system, each
peer accesses an index
server to get the location
of a resource stored on
another peer.

Common P2P Applications

• Common P2P networks include: eDonkey, G2, BitTorrent, Bitcoin.
• Many P2P applications allow users to share pieces of many files with each other at the same time.
• A small torrent file contains information about the location of other users and tracker computers.
• Trackers are computers keeping
track of the files hosted by users.
• This technology is called BitTorrent.
• There are many BitTorrent clients,
including BitTorrent, uTorrent,
Frostwire, and qBittorrent.

Section 2 Well-Known Application Layer Protocols and Services

Topic 2.1:
Web and Email Protocols

Hypertext Transfer Protocol and Hypertext Markup Language

• A URL allows a web browser to establish a connection to that web server.
• URLs and Uniform Resource Identifier (URIs) are the names most people associate with web addresses.
• The URL http://cisco.com/index.html has three basic parts:
  • http (the protocol or scheme)
  • www.google.com (the server name)
  • index.html (the specific filename requested)
• Using DNS, the server name portion of
the URL is then translated to the
associated IP address before the server
can be contacted.
• The browser sends a GET request to the server’s IP address and asks for the index.html file.
• The server sends the requested file to the client.
• The index.html was specified in the URL and contains the HTML code for this web page.
• The browser processes the HTML code and formats the page for the browser window based on the code in the file.

HTTP and HTTPS

• HTTP
  • Is a request/response protocol.
  • Has three common message types: GET, POST, PUT.
  • Is not secure. Messages can be intercepted.
• HTTPS uses authentication and
encryption to secure data.

Email Protocols

• Email is a store-and-forward method of sending, storing, and retrieving electronic messages.
• Email messages are stored in databases on mail servers.
• Email clients communicate with mail
servers to send and receive email.
• Mail servers communicate with other
mail servers to transport messages
from one domain to another.
• Email clients do not communicate
directly when sending email.
• Email relies on three separate protocols
for operation: SMTP (sending),POP (retrieving),
IMAP (retrieving).

Topic 2.2:
IP Addressing

Domain Name Service

• While IP addresses are crucial for network communication, they are not easy to memorize.
• Domain names are created to make server addresses more user-friendly.
• The DNS protocol allows for the dynamic translation of a domain name into the correct IP address.
• The DNS protocol communications using a single format called a message.
• Domain names such as http://www.google.com are user-friendly addresses associated with the IP address of a specific server.
• However, computers still need the actual numeric address before they can communicate.

DNS Message Format

• DNS supports different types of records. Some of these record types are:
  • A - An end device IPv4 address
  • NS - An authoritative name server
  • AAAA - An end device IPv6 address (pronounced quad-A)
  • MX - A mail exchange record
• DNS servers will first look at its own records to resolve the name. If the server is unable to resolve the name using its locally stored records, it relays the query to other servers.
• The response is then forwarded to
the requesting client.
• The DNS Client service on Windows
PCs also stores previously resolved
names in memory.
• ipconfig /displaydns displays all of
the cached DNS entries on Windows.

DNS Hierarchy

• The DNS protocol uses a hierarchical system, with the root at the top and branches below. The naming structure is broken down into small, manageable zones.
• Each DNS server is only responsible for managing name-to-IP mappings for that small portion of the DNS structure.
• Requests for zones not stored in a specific DNS server are forwarded to other servers for translation.
• Top-level domains represent either the type
of domain or the country of origin.
Examples of top-level domains are:
  • .com - a business or industry
  • .org - a non-profit organization
  • .au - Australia
  • .co - Colombia

The nslookup Command

• Allows the user to manually place DNS queries.
• It can also be used to troubleshoot name resolution issues.
• Has many options available for extensive testing and verification of the DNS process.