CCNA-1. Chapter3. Application layer functionality and protocols

Executable program components:

• application (provide people with a way to create messages)
• service (establish an interface to the network, can support  multiple protocols, may use different protocols in 1 program)
• protocol (provide the rules and formats/types of messages that govern how data is directed, treated, structured)

Application layer protocols specify:

• The processes that occur at end of the communication: what has to happen to the data and how the PDU is to be structured.
• The types of messages: requests, acknowledgements, data messages, status messages and error messages.
• The syntax of the message: this gives the expected order of information (fields) in a message.
• The meaning of the fields within specific message types has to constant so the services can correctly act.
• The message dialogs: which messages elicit which responses so the correct services are invoked and the data transfer occurs.

Types of applications:

• 
   network aware (implement the application layer protocols and are able to communicate directly with the lower layers)
  
  • Example: Web browsers, e-mail client
• not network aware (need the assistance of application layer services (alg.exe and etc.) to use network resources)
  
  • Example: File transfer, network printer spooling

Network architecture models:

• Client/Server model
• Peer-to-Peer Model  (computers in network  share resources  without dedicated server. In large P2P hard to enforce security)

DNS server record types:

• A - an end device address
• NS - an authoritative name server
• CNAME - the canonical name (or Fully Qualified Domain Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS
• MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain

Authoritative DNS server - server has resource records that correspond to its level in the domain hierarchy. DNS resolve process: Client-LocalDNS-RootDNS-regionalDNS(.com;.ru)-lowerlevelDNS Common HTTP message types:

• GET (request for data, respond - HTTP/1.1 200 OK, and a requested file, an error message, or some other information)
• POST (includes the data in the message sent to the server)
• PUT (uploads resources or content to the web server)

E-mail processes:

• Mail User Agent (MUA) - allows messages to be sent and places received messages into the client's mailbox
• Mail Transfer Agent (MTA) - forward e-mail. If a recipient mailbox on the local server -to the MDA if not - to another MTA
• Mail Delivery Agent (MDA) - receives mail, places to mailboxes, resolve virus scanning, spam filtering, return-receipt handling.

E-mail protocol types,, functions, commands:

• Simple Mail Transfer Protocol (SMTP) TCP 25. Message formats, command strings used to send e-mail from client or server.
  
  • Functions:
    
    • session initiation
    • mail transaction
    • forwarding mail
    • verifying mailbox names
    • expanding mailing lists
    • opening and closing exchanges.
  • Commands:
    
    • HELO - identifies the SMTP client process to the SMTP server process
    • EHLO - Is a newer version of HELO, which includes services extensions
    • MAIL FROM - Identifies the sender
    • RCPT TO - Identifies the recipient
    • DATA - Identifies the body of the message
• Post Office Protocol (POP) TCP 110. Used to receive e-mail messages from an e-mail server.
• Internet Message Access Protocol (IMAP). TCP 143. Allows a central server to provide remote access to e-mail messages
• Propriatary protocols (IBM's Lotus Notes, Novell's Groupwise, or Microsoft's Exchange have their own internal e-mail format)

File Transfer Protocol (FTP) - allow download (pull) or upload (push) file between a client and a server. FTP requires two connections:

• Connection to the server on TCP port 21. Control traffic, consisting of client commands and server replies.
• Connection to the server over TCP port 20. Actual file transfer, created every time there is a file transferred.

DHCP messages:

• DHCP DISCOVER (broadcast to identify any available DHCP servers on the network)
• DHCP OFFER, (server reply  with an assigned IP address, subnet mask, DNS server, default gateway, duration of the lease)
• DHCP REQUEST (broadcast packet that identifies DHCP server if >1, can request an address that previously been allocated)
• DHCP ACK (acknowledges IP address client requested or offered by the server, is still valid and the lease is finalized)
• DHCP NAK (Offer is no longer valid if a time-out or another client allocating the lease. Selection process must begin again)

Server Message Block (SMB)-client/server file sharing request-response protocol. Provide access to server resources as if they local. SMB messages functions:

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

Telnet provides a standard method of emulating text-based terminal devices over the data network. provides access to the server command line interface (CLI).Connection using Telnet is called a Virtual Terminal (VTY) session. Telnet command first byte - Interpret as Command (IAC) character. IAC defines the next byte as a command rather than text. Telnet protocol commands include:

• Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active.
• Erase Line (EL) - Deletes all text from the current line.
• Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program.

Gnutella P2P protocol defines five different packet types:

• ping - for device discovery
• pong - as a reply to a ping
• query - for file location
• query hit - as a reply to a query
• push - as a download request