Table of Contents Link to heading

• OSI Application Layer
• OSI Presentation Layer
• OSI Session Layer
• TCP/IP Application Layer Protocols
  • Name System
    • Domain Name System (DNS)
    • Windows Internet Naming Service (WINS)
  • Host Configuration
    • Dynamic Host Control Protocol (DHCP)
    • Bootstrap Protocol (BOOTP)
  • Email
    • Simple Mail Transfer Protocol (SMTP)
    • Post Office Protocol (POP3)
    • Internet Message Access Protocol (IMAP) 
  • File Transfer
    • File Transfer Protocol (FTP)
    • Trivial File Transfer Protocol (TFTP)
    • Server Message Block (SMB)
  • Web
    • Hypertext Transfer Protocol (HTTP)
    • HTTP Secure (HTTPS)
  • Remote Access
    • Telnet
    • Secure Shell (SSH)
  • Network Management
    • Simple Network Management Protocol (SNMP)

OSI Application Layer Link to heading

This layer provides services to application processes (such as electronic mail, file transfer, and terminal emulation) that are outside the OSI model. The application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronises cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity.

OSI Presentation Layer Link to heading

OSI Session Layer Link to heading

TCP/IP Application Layer Protocols Link to heading

• 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.

Name System Link to heading

Domain Name System (DNS) Link to heading

• While IP addresses are crucial for network communication, they are not easy to memorise.
• Domain names are created to make server addresses more user-friendly.
• Domain names such as http://www.cisco.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.
• The DNS protocol allows for the dynamic translation of a domain name into the correct IP address.

Windows Internet Naming Service (WINS) Link to heading

Host Configuration Link to heading

Dynamic Host Control Protocol (DHCP) Link to heading

• Computers need network addresses to communicate over a network.
• Additional crucial information includes gateway address, subnet mask, and DNS server.
• Manually configuring end devices is not scalable. DHCP allows for automated distribution of network information.
• DHCP-distributed addresses are leased for a set period of time.
• Addresses are returned to the pool for reuse when no longer in use.
• DHCP supports IPv4 and DHCPv6 supports IPv6.

Bootstrap Protocol (BOOTP) Link to heading

Email Link to heading

• 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.

Simple Mail Transfer Protocol (SMTP) Link to heading

• SMTP message formats require a message header and body.
• The body can contain any amount of text.
• The header must have a properly formatted recipient email address and a sender address.
• An SMTP client sends an email by connecting to a SMTP server on port 25.
• The server receives the message and stores it message in a local mailbox or relays the message to another mail server.
• Users use email clients to retrieve messages stored on the server.
• IMAP and POP are two protocols commonly used by email clients to retrieve messages.

Post Office Protocol (POP3) Link to heading

• Messages are downloaded from the server to the client.
• The server listens on port 110 TCP for client requests.
• Email clients direct their POP requests to mail servers on port TCP 110.
• The POP client and server exchange commands and responses until the connection is closed or aborted.
• POP allows for email messages to be downloaded to the client’s device (computer or phone) and removed from the server.
• There is no centralized location where email messages are kept. Recipient

Internet Message Access Protocol (IMAP)  Link to heading

• Allows for messages to be displayed to the user rather than downloaded.
• The original messages reside on the server until manually deleted by the user.
• Users view copies of the messages in their email client software.
• Users can create a folder hierarchy on the server to organise and store mail.
• That file structure is displayed on the email client.
• When a user decides to delete a message the server synchronises that action and deletes the message from the server.

File Transfer Link to heading

File Transfer Protocol (FTP) Link to heading

• FTP was developed to allow the transfer of files over the network.
• An FTP client is an application that runs on a client computer used to push and pull data from an FTP server.
• FTP requires two connections between the client and the server: one connection for commands and replies and another connection for the actual file transfer.
• The client initiates and establishes the first connection to the server for control traffic on TCP port 21.
• The client then establishes the second connection to the server for the actual data transfer on TCP port 20.
• The client can download (pull) data from the server or upload (push) data to the server.

Trivial File Transfer Protocol (TFTP) Link to heading

One use case of TFTP is in the creation of ACLs where you can use a PC text editor to create and modify ACLs and then send them to the router via TFTP.

Server Message Block (SMB) Link to heading

• All SMB messages share a common format.
• SMB file-sharing and print services have become the mainstay of Windows networking.
• Microsoft products now support TCP/IP protocols to directly support SMB resource sharing.
• After the connection is established, the user of the client can access the resources on the server as if the resource is local to the client host.
• The Linux and UNIX operating systems also provide a method of sharing resources with Microsoft networks using a version of SMB called SAMBA. The Apple Macintosh operating systems also support resource sharing using the SMB protocol.

SMB messages can perform the following tasks:

1. Start, authenticate, and terminate sessions
2. Control file and printer access
3. Allow an application to send or receive messages to or from another device

Web Link to heading

Hypertext Transfer Protocol (HTTP) Link to heading

HTTP:

• Is a request/response protocol.
  • 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.
• Has three common message types: GET, POST, PUT.
• Is not secure. Messages can be intercepted.

HTTP Secure (HTTPS) Link to heading

Remote Access Link to heading

Telnet Link to heading

Both the protocol itself and the client software that implements the protocol are commonly referred to as Telnet.

Telnet client is a software program that implements the telnet protocol and allows the user to interact with a remote host. Telnet clients are available for various operating systems, such as Windows, Linux, and Mac OS X. Some examples of telnet clients are PuTTY, SecureCRT, and Terminal.

Telnet server is a software program that listens for incoming telnet connections and provides access to the host system. Telnet servers are usually configured to require authentication, such as a username and password, before granting access. Some examples of telnet servers are Telnetd, Inetd, and Xinetd.

A connection using Telnet is called a VTY (Virtual Terminal) session, or connection. Telnet specifies how a VTY session is established and terminated.

Each Telnet command consists of at least 2 bytes. The first byte is a special character called the Interpret as Command (IAC) character. As its name implies, the IAC character defines the next byte as a command rather than text.

Although the Telnet protocol supports user authentication, it does not support the transport of encrypted data. All data exchanged during a Telnet session is transported as plain text across the network. This means that the data can be intercepted and easily understood.

Secure Shell (SSH) Link to heading

SSH provides the structure for secure remote login and other secure network services. It also provides stronger authentication than Telnet and supports the transport of session data using encryption. As a best practice, network professionals should use SSH in place of Telnet, whenever possible.

Network Management Link to heading

Simple Network Management Protocol (SNMP) Link to heading