CCNA 1 chapter 10 Application Layer

Application and Presentation (Protocols and Standards)

OSI Model Application DCHP POP TFTP HTTP DNS IMAP presentation jpg MPEG GIF TCP/IP Application DCHP POP DNS IMAP HTTP GIF MPEG TFTP JPG

TCP/IP Application Layer Protocols

System name = DNS Host config - BOOTP DHCP Email SMTP POP IMAP File Transfer FTP TFTP Web HTTP HTTPS

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. Note: DNS is scalable because hostname resolution is spread across multiple servers. 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

peer-to-peer applications (P2P)

computing application where client hosts provide services directly to other hosts. Music, gaming, etc. A P2P application allows a device to act as both a client and a server within the same communication, as shown in the figure. In this model, every client is a server and every server a client. P2P applications require that each end device provide a user interface and run a background service. 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.

File Transfer Protocol (FTP)

A communications method for transferring data between computers on the Internet FTP is another commonly used application layer protocol. FTP was developed to allow for data transfers between a client and a server. An FTP client is an application that runs on a computer that is used to push and pull data from an FTP server. FTP requires two connections between the client and the server, one for commands and replies, the other for the actual file transfer: 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.

Hypertext Transfer Protocol and Hypertext Markup Language

When a web address or uniform resource locator (URL) is typed into a web browser, the web browser establishes a connection to the web service running on the server using the HTTP protocol. URLs and Uniform Resource Identifier (URIs) are the names most people associate with web addresses. To better understand how the web browser and web server interact, we can examine how a web page is opened in a browser. For this example, use the http://www.cisco.com/index.html URL. First, as shown in Figure 1, the browser interprets the three parts of the URL: 1. http (the protocol or scheme) 2. www.cisco.com (the server name) 3. index.html (the specific filename requested) the browser then checks with a name server to convert www.cisco.com into a numeric IP address, which it uses to connect to the server. Using HTTP requirements, the browser sends a GET request to the server and asks for the index.html file. The server, sends the HTML code for this web page to the browser, the browser deciphers the HTML code and formats the page for the browser window.

nslookup command

When configuring a network device, one or more DNS Server addresses are provided that the DNS client can use for name resolution. Usually the Internet service provider (ISP) provides the addresses to use for the DNS servers. When a user's application requests to connect to a remote device by name, the requesting DNS client queries the name server to resolve the name to a numeric address. 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. In Figure 1, 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.

common P2P applications

eDonkey, G2, BitTorrent, Bitcoin, Gnutella 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 networks include: eDonkey G2 BitTorrent Bitcoin Some P2P applications are based on the Gnutella protocol, where each user shares whole files with other users. As shown in the figure, Gnutella-compatible client software allows users to connect to Gnutella services over the Internet and to locate and access resources shared by other Gnutella peers. Many Gnutella client applications are available, including gtk-gnutella, WireShare, Shareaza, and Bearshare. Many P2P applications allow users to share pieces of many files with each other at the same time. Clients use a small file called a torrent file to locate other users who have pieces that they need so that they can connect directly to them. This file also contains information about tracker computers that keep track of which users have what files. Clients ask for pieces from multiple users at the same time, known as a swarm. This technology is called BitTorrent. There are many BitTorrent clients including BitTorrent, uTorrent, Frostwire, and qBittorrent. Note: Any type of file can be shared between users. Many of these files are copyrighted, meaning that only the creator has the right to use and distribute them. It is against the law to download or distribute copyrighted files without permission from the copyright holder. Copyright violation can result in criminal charges and civil lawsuits. Complete the lab on the following page to find out more about these legal issues.

Server Message Block (SMB)

A protocol used by Windows to share files and printers on a network. The Server Message Block (SMB) is a client/server file sharing protocol that describes the structure of shared network resources, such as directories, files, printers, and serial ports. It is a request-response protocol. All SMB messages share a common format. This format uses a fixed-sized header, followed by a variable-sized parameter and data component. SMB messages can: Start, authenticate, and terminate sessions Control file and printer access Allow an application to send or receive messages to or from another device SMB file-sharing and print services have become the mainstay of Microsoft networking. With the introduction of the Windows 2000 software series, Microsoft changed the underlying structure for using SMB. In previous versions of Microsoft products, the SMB services used a non-TCP/IP protocol to implement name resolution. Beginning with Windows2000, all subsequent Microsoft products use DNS naming, which allows TCP/IP protocols to directly support SMB resource sharing, as shown in Figure 1. The SMB file exchange process between Windows PCs . 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.

DHCP Operation steps

DORA the Explorer 1. client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers. 2. DHCP server replies with a DHCP offer (DHCPOFFER) message, which includes an IP address, subnet mask, DNS server IP, and the IP address for the default gateway. 3. if there are multiple DHCP offers, the client my choose one, and sends the DHCP request (DHCPREQUEST) to identify the server that it chose. 4. The chosen server then sends a DHCP acknowledge (DHCPACK) to finalize the request.

HTTP specifies the message types used for that communication.

HTTP specifies the message types used for that communication. The three common message types are GET, POST, and PUT (see the figure): 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.

HTTP and HTTPS

Hypertext Transfer Protocol. A protocol for controlling how Web browsers and servers pass information back and forth over the Internet. HTTPS uses encryption. 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 (see the figure): 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. Although HTTP is remarkably flexible, it is not a secure protocol. The request messages send information to the server in plain text that can be intercepted and read. The server responses, typically HTML pages, are also unencrypted. 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.

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.

client/server model

In the client-server model, the device requesting the information is called a client and the device responding to the request is called a server. Client and server processes are considered to be in the application layer. The client begins the exchange by requesting data from the server, which responds by sending one or more streams of data to the client. Application layer protocols describe the format of the requests and responses between clients and servers. In addition to the actual data transfer, this exchange may also require user authentication and the identification of a data file to be transferred. One example of a client-server network is using an ISP's email service to send, receive and store email. The email client on a home computer issues a request to the ISP's email server for any unread mail. The server responds by sending the requested email to the client. As shown in the figure, 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.

Peer-to-Peer Networks

Networks that enable any computer or device on the network to provide and request services. peer-to-peer (P2P) networking model, the data is accessed from a peer device without the use of a dedicated server. The P2P network model involves two parts: P2P networks and P2P applications. Both parts have similar features, but in practice work quite differently. In a P2P network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server. Every connected end device (known as a peer) can function as both a server and a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another. The roles of client and server are set on a per request basis. A simple example of P2P networking is shown in the figure. In addition to sharing files, a network such as this one would allow users to enable networked games, or share an Internet connection.

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.

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, as shown in the figure. 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.

Email Protocols

SMTP, POP, IMAP One of the primary services offered by an ISP is email hosting. To run on a computer or other end device, email requires several applications and services, as shown in the figure. Email is a store-and-forward method of sending, storing, and retrieving electronic messages across a network. 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. 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. Email supports three separate protocols for operation: Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and IMAP. The application layer process that sends mail uses SMTP. A client retrieves email, however, using one of the two application layer protocols: POP or IMAP.

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.

Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) for IPv4 service automates the assignment of IPv4 addresses, subnet masks, gateways, and other IPv4 networking parameters. This is referred to as dynamic addressing. The alternative to dynamic addressing is static addressing. When using static addressing, the network administrator manually enters IP address information on hosts. When a host connects to the network, the DHCP server is contacted, and an address is requested. The DHCP server chooses an address from a configured range of addresses called a pool and assigns (leases) it to the host. On larger networks, or where the user population changes frequently, DHCP is preferred for address assignment. New users may arrive and need connections; others may have new computers that must be connected. Rather than use static addressing for each connection, it is more efficient to have IPv4 addresses assigned automatically using DHCP. 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. As the figure shows, various types of devices can be DHCP servers. The DHCP server in most medium-to-large networks is usually a local, dedicated PC-based server. With home networks, the DHCP server is usually located on the local router that connects the home network to the ISP. Many networks use both DHCP and static addressing. DHCP is used for general purpose hosts, such as end user devices. Static addressing is used for network devices, such as gateways, switches, servers, and printers. 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.

Domain Name Service (DNS)

The Internet service that manages access to Internet domain names and the naming system it uses for computers and resources connected to the Internet or in a private network. (13) In data networks, devices are labeled with numeric IP addresses to send and receive data over networks. Domain names were created to convert the numeric address into a simple, recognizable name. On the Internet, these domain names, such as http://www.cisco.com, are much easier for people to remember than 198.133.219.25, which is the actual numeric address for this server. If Cisco decides to change the numeric address of www.cisco.com, it is transparent to the user because the domain name remains the same. The new address is simply linked to the existing domain name and connectivity is maintained. 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.

Presentation Layer

The Presentation Layer 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 presentation layer formats data for the application layer, and it sets standards for file formats. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). Some well-known graphic image formats that are used on networks are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Portable Network Graphics (PNG) format.

Session Layer

The fifth layer in the OSI model. This layer establishes and maintains communication between two nodes on the network. It can be considered the "traffic cop" for network communications. As the name implies, functions at the session layer create and maintain dialogs between source and destination applications. 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

The seventh layer of the OSI model. Application layer protocols enable software programs to negotiate formatting, procedural, security, synchronization, and other requirements with the network. The application layer is closest to the end user. As shown in the figure, it is the layer that provides the interface between the applications used to communicate and the underlying network over which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. The upper three layers of the OSI model (application, presentation, and session) define functions of the single TCP/IP application layer. 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.

Application Layer Protocols

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.

DHCP Operation

when 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. Assuming that the IPv4 address requested by the client, or offered by the server, is still available, the server returns a DHCP acknowledgment (DHCPACK) message that acknowledges to the client that the lease has been finalized. If the offer is no longer valid, then the selected server responds with a DHCP negative acknowledgment (DHCPNAK) message. If a DHCPNAK message is returned, then the selection process must begin again with a new DHCPDISCOVER message being transmitted. After the client has the lease, it must be renewed prior to the lease expiration through another DHCPREQUEST message. The DHCP server ensures that all IP addresses are unique (the same IP address cannot be assigned to two different network devices simultaneously). Most Internet providers use DHCP to allocate addresses to their customers. DHCPv6 has similar set of messages to those shown in the figure for DHCP for IPv4. The DHCPv6 messages are SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY