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Chapter 10 Application Layer (Application, Presentation, and Session…

- - - - Application layer protocols are used to exchange data between programs running on the source and destination hosts
        
        There are many application layer protocols, and new protocols are always being developed. Some of the most widely known application layer protocols include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Trivial File Transfer Protocol (TFTP), Internet Message Access Protocol (IMAP), and Domain Name System (DNS) protocol.
  - - - The presentation layer has three primary functions:
        
        Formatting, or presenting, data at the source device into a compatible form for receipt by the destination device
        
        Compressing data in a way that can be decompressed by the destination device
        
        Encrypting data for transmission and decrypting data upon receipt
  - - - 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.
- - - - Application layer protocols describe the format of the requests and responses between clients and servers
        
        data transfer from a client to a server is referred to as an upload and data from a server to a client as a download.
- - - - Some P2P applications use a hybrid system where resource sharing is decentralized, but the 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.
- - - - the browser interprets the three parts of the URL:
        
        http (the protocol or scheme)
        
        www.cisco.com (the server name)
        
        index.html (the specific filename requested)
      - HTTP is a request/response protocol. When a client, typically a web browser, sends a request to a web server, HTTP specifies the message types used for that communication. The three common message types are GET, POST, and PUT
        
        GET - A client request for data. A client (web browser) sends the GET message to the web server to request HTML pages.
        
        POST - Uploads data files to the web server such as form data.
        
        PUT - Uploads resources or content to the web server such as an image.
    - - For secure communication across the Internet, the HTTP Secure (HTTPS) protocol is used. HTTPS uses authentication and encryption to secure data as it travels between the client and server. HTTPS uses the same client request-server response process as HTTP, but the data stream is encrypted with Secure Socket Layer (SSL) before being transported across the network.
  - - - An email client does not communicate directly with another email client when sending email. Instead, both clients rely on the mail server to transport messages.
    - - SMTP Operation
        
        SMTP message formats require a message header and a message body. While the message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address.
        
        When a client sends email, the client SMTP process connects with a server SMTP process on well-known port 25. After the connection is made, the client attempts to send the email to the server across the connection. When the server receives the message, it either places the message in a local account, if the recipient is local, or forwards the message to another mail server for delivery
        
        The destination email server may not be online or may be busy when email messages are sent. Therefore, SMTP spools messages to be sent at a later time. Periodically, the server checks the queue for messages and attempts to send them again. If the message is still not delivered after a predetermined expiration time, it is returned to the sender as undeliverable.
      - POP Operation
        
        POP is used by an application to retrieve mail from a mail server. With POP, mail is downloaded from the server to the client and then deleted on the server. This is how POP operates, by default.
        
        The server starts the POP service by passively listening on TCP port 110 for client connection requests. When a client wants to make use of the service, it sends a request to establish a TCP connection with the server. When the connection is established, the POP server sends a greeting. The client and POP server then exchange commands and responses until the connection is closed or aborted.
        
        With POP, email messages are downloaded to the client and removed from the server, so there is no centralized location where email messages are kept. Because POP does not store messages, it is undesirable for a small business that needs a centralized backup solution.
      - IMAP Operation
        
        IMAP is another protocol that describes a method to retrieve email messages. Unlike POP, when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application. The original messages are kept on the server until manually deleted. Users view copies of the messages in their email client software.
        
        Users can create a file hierarchy on the server to organize and store mail. That file structure is duplicated on the email client as well. When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.
- - - - The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data. The DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers.
        
        DNS Message Format
        
        The DNS server stores different types of resource records used to resolve names. These records contain the name, address, and type of record. 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
        
        When a client makes a query, the server’s DNS process first looks at its own records to resolve the name. If it is unable to resolve the name using its stored records, it contacts other servers to resolve the name. After a match is found and returned to the original requesting server, the server temporarily stores the numbered address in the event that the same name is requested again.
        
        The DNS Client service on Windows PCs also stores previously resolved names in memory. The ipconfig /displaydns command displays all of the cached DNS entries.
        
        DNS Hierarchy
        
        The DNS protocol uses a hierarchical system to create a database to provide name resolution. The hierarchy looks like an inverted tree with the root at the top and branches below (see the figure). DNS uses domain names to form the hierarchy.
        
        The naming structure is broken down into small, manageable zones. Each DNS server maintains a specific database file and is only responsible for managing name-to-IP mappings for that small portion of the entire DNS structure. When a DNS server receives a request for a name translation that is not within its DNS zone, the DNS server forwards the request to another DNS server within the proper zone for translation.
        
        The different top-level domains represent either the type of organization 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
        
        Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name. This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers.
        
        when the nslookup command is issued, the default DNS server configured for your host is displayed. The name of a host or domain can be entered at the nslookup prompt. The nslookup utility has many options available for extensive testing and verification of the DNS process.
- - - - DHCP-distributed addresses are leased for a set period of time. When the lease is expired, the address is returned to the pool for reuse if the host has been powered down or taken off the network. Users can freely move from location to location and easily re-establish network connections through DHCP.
        
        DHCPv6 (DHCP for IPv6) provides similar services for IPv6 clients. One important difference is that DHCPv6 does not provide a default gateway address. This can only be obtained dynamically from the router's Router Advertisement message.
        
        DHCP Operation
        
        hen an IPv4, DHCP-configured device boots up or connects to the network, the client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers on the network. A DHCP server replies with a DHCP offer (DHCPOFFER) message, which offers a lease to the client. The offer message contains the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway. The lease offer also includes the duration of the lease.
        
        The client may receive multiple DHCPOFFER messages if there is more than one DHCP server on the local network. Therefore, it must choose between them, and sends a DHCP request (DHCPREQUEST) message that identifies the explicit server and lease offer that the client is accepting. A client may also choose to request an address that it had previously been allocated by the server.
        
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- - - - The client establishes the first connection to the server for control traffic using TCP port 21, consisting of client commands and server replies.
      - The client establishes the second connection to the server for the actual data transfer using TCP port 20. This connection is created every time there is data to be transferred.
      - The data transfer can happen in either direction. The client can download (pull) data from the server, or the client can upload (push) data to the server.
- - - - Unlike the file sharing supported by FTP, clients establish a long-term connection to servers. 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.
    - - Start, authenticate, and terminate sessions
      - Control file and printer access
      - Allow an application to send or receive messages to or from another device