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Chapter 10: Application Layer, DIFFA MAHARANI NINGTYAS, 201710893, MINDMAP…

- - - - : Presentation layer has three primary functions
        
        Coding and conversion of application layer data
        
        Compression of the data
        
        Encryption of the data for transmission and the decryption of data upon receipt by the destination.
      - : Session Layer
        
        Functions create and maintain dialogs between source and destination applications
        
        Handles the exchange of information to initiate dialogs, keep them active, and to restart sessions
    - - 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
  - - - Both devices are considered equal in the communication
    - - Client and server in the same communication, Both can initiate a communication and are considered equal in the communication process.
    - - 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, and LionShare.
      - Some P2P applications are based on the Gnutella protocol which enables people to share files on their hard disks with others
- - - - Three application layer protocols that are involved in everyday work or play are:
        
        Hypertext Transfer Protocol (HTTP) : browse the web
        
        Simple Mail Transfer Protocol (SMTP) : used to enable users to send email
        
        Post Office Protocol (POP) : used to enable users to receive email
    - - Example URL: http : //www.cisco.com/index.html
      - First, the browser interprets the three parts of the URL:
        
        http (the protocol or scheme)
        
        www.cisco.com (the server name)
        
        index.html (the specific file name requested)
      - Browser checks with a name server to convert www.cisco.com into a numeric address
      - Using the HTTP protocol requirements sends a GET request to the server and asks for the file index.html
      - Server sends the HTML code for this web page
      - Browser deciphers the HTML code and formats the page
    - - Developed to publish and retrieve HTML pages
      - 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 the web server
    - - Typically use an application called a Mail User Agent (email client)
        
        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 reliably and efficiently
        
        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
      - Post Office Protocol (POP) : Enables a workstation to retrieve mail from a mail server and With POP, mail is downloaded from the server to the client and then deleted on the server
        
        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
      - 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, and Original messages are kept on the server until manually deleted
  - - - A human legible name is resolved to its numeric network device address by the DNS protocol
    - - DNS server stores different types of resource records used to resolve names
      - Contain 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
    - - Examples top-level domains:
        
        .au - Australia
        
        .co - Colombia
        
        .com - business or industry
        
        .jp - Japan
        
        .org - non-profit organization
    - - Operating system utility called nslookup allows the user to manually query the name servers to resolve a given host name
      - Utility can be used to troubleshoot name resolution issues and to verify the current status of the name servers
    - - DHCP allows a host to obtain an IP address dynamically
      - DHCP server is contacted and address requested - chooses address from a configured range of addresses called a pool and “leases” it to the host for a set period
      - DHCP used for general purpose hosts such as end user devices, and static addressing is used for network devices such as gateways, switches, servers and printers
    - - FTP allow data transfers between a client and a server
      - FTP client is an application that runs on a computer that is used to push and pull data from a server running an FTP daemon
      - To successfully transfer data, FTP requires two connections between the client and the server, one for commands and replies, the other for the actual file transfer
    - - Clients establish a long term connection to servers
      - After the connection is established, the user can access the resources on the server as if the resource is local to the client host