Application Layer

LAYER PROTOCOLS

Application Layer

TCP/IP
  • TCP/IP Application Layer Protocols
    Domain Name Service Protocol (DNS)
    used to resolve Internet names to IP addresses

    Telnet
    a terminal emulation protocol used to provide remote access to servers and networking devices

    Bootstrap Protocol (BOOTP)
    a precursor to the DHCP protocol, a network protocol used to obtain IP address information during bootup

    Dynamic Host Control Protocol (DHCP)
    used to assign an IP address, subnet mask, default gateway and DNS server to a host

    Hypertext Transfer Protocol (HTTP)
    used to transfer files that make up the Web pages of the World Wide Web

    File Transfer Protocol (FTP)
    used for interactive file transfer between systems

    Trivial File Transfer Protocol (TFTP)
    used for connectionless active file transfer

    Simple Mail Transfer Protocol (SMTP)
    used for the transfer of mail messages and attachments

    Post Office Protocol (POP)
    used by email clients to retrieve email from a remote server

    Internet Message Access Protocol (IMAP)
    another protocol for email retrieval

Presentation and Session Layers

TCP/IP

  • Presentation layer
    • Coding and conversion of application layer data
    • Data compression
    • Data encryption for the transmission and decryption of data upon receipt by the destination
  • Session layer
    • Functions, creates, and maintains dialogs between source and destination applications
    • Handles the exchange of information to initiate dialogs, keep them active, and to restart sessions

Peer to Peer

Client and server in the same communication
TCP/IP
Both devices are considered equal in the communication.
The roles of client and server are set on a per request basis.

Peer-to-Peer Applications
TCP/IP
Both can initiate a communication and are considered equal in the
communication process.

Common P2P Applications
  • With P2P applications, each computer in the network running the
    application can act as a client or a server for the other computers in the
    network running the application.

  • Common P2P applications include:
    • eDonkey
    • eMule
    • Shareaza
    • BitTorrent
    • Bitcoin
    • LionShare

  • Some P2P applications are based on the Gnutella protocol which
    enables people to share files on their hard disks with others

Client-Server Model


Download
Files are downloaded from server to client

Upload
Files are uploaded from the client to the server

POPULAR PROTOCOLS & SERVICES

Common Application Layer Protocols

  • Hypertext Transfer Protocol and Hypertext Markup Language
    Fully Qualified Domain Name (FQDN)
    host name and the domain name
    links to an IP address of a server

    FQDN Components
    top level domain - TLD (e.g .com/.net)
    the domain name (e.g cisco.com/nmmu.ac.za)
    the server hosting the site (e.g www/localhost)

    First, the browser interprets the three parts of the URL:
    the protocol or scheme (http)
    the server FQDN name (e.g learn.nmmu.ac.za)
    the specific file name requested (e.g view.php)
    id=526 (e.g id=526)

    ...Also
    Browser checks the path on the server to link to the actual landing page

    HTTP and HTTPS
    HTTP Protocol using GET
    Developed to publish and retrieve HTML pagesHTTP GET
    Used for data transfer
    Specifies a request/response protocol
    Three common message types are GET, POST, and PUT
    GET is a client request for data
    POST and PUT are used to send messages that upload data to theweb server

    SMTP, POP, and IMAP
    Typically use an application called a Mail User Agent (email client)SMTP
    Allows messages to be sent
    Places received messages into the client's mailbox
    SMTP - Send email from either a client or a server
    POP - Receive email messages from an email server
    IMAP - Internet Message Access Protocol
    Email client provides the functionality of both protocols within one application

    • Simple Mail Transfer Protocol (SMTP)
      • transfers mail
      • message must be formatted properly
      • SMTP processes must be running on both the client and server
      • message header must have a properly formatted recipient email address and a sender
      • uses port 25
      • LionShare
    • Post Office Protocol (POP)
      • enables a workstation to retrieve mail from a mail server
      • mail is downloaded from the server to the client and then deleted on the server
      • uses port 110
      • POP does not store messages
      • POP3 is desirable for an ISP, because it alleviates their responsibility for managing large amounts of storage for their email servers
    		HTTP GET

    MTAMDA accepts a piece of email from MTA and performs the actual delivery.
    MDA receives all the inbound mail from the MTA and places it into mailboxes
    MDA can also resolve final delivery issues, such as virus scanning, spam filtering, and return-receipt handling


    MDA
    • Simple Mail Transfer Protocol (SMTP)
      • Transfers mail reliably and efficiently
    • Post Office Protocol (POP)
      • Enables a workstation to retrieve mail from a mail server
      • With POP, mail is downloaded from the server to the client and then deleted on the server
    • Internet Message Access Protocol (IMAP)
      • Another protocol that to retrieves email messages
      • Unlike POP, when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application
      • Original messages are kept on the server until manually deleted

Providing IP Addressing Services

A human legible
name is resolved
to its numeric
network device
address by the
DNS protocol.		DNS
DNSResolving DNS Address Step2
A human legible
name is resolved
to its numeric
network device
address by the
DNS protocol.		DNS
DNSResolving DNS Address Step4

The number returned back
to the client for use in
requests of the server


Resolving DNS Addresses Step 5
DNS
A domain name is resolved to its numeric network device address by the DNS protocol.

DNS Message Format

  • DNS server stores different types of resource records used to resolve names
  • Contains the name, address, and type of record
  • Record types are:
    • A – An end device address
    • NS – An authoritative name server
    • CNAME – The canonical 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
  • Unable to resolve the name using its stored records, contacts other servers
  • Server temporarily stores the numbered address that matches the name in cache memory
  • Windows ipconfig /displaydns displays all cached DNS

THE GLOBAL MESSAGE