Chapter 10: Application Layer CCENT Routing and Switching Introduction to Networks v6.0 CCNET v6 10
Chapter 10 - Sections & Objectives 10.1 Application Layer Protocols Explain the operation of the application layer in providing support to end-user applications. Explain how the functions of the application layer, session layer, and presentation layer work together to provide network services to end user applications Explain how common application layer protocols interact with end user applications. 10.2 Well-Known Application Protocols and Services Explain how well-known TCP/IP application layer protocols operate. Explain how web and email protocols operate. Explain how DNS and DHCP operate. Explain how file transfer protocols operate. CCENT v6 11
10.1 APPLICATION LAYER PROTOCOLS CCENT v6 12
OSI and TCP/IP Models The key parallels are in the transport and network layer Important to know the comparisons CCENT v6 13
Application Layer The TCP/IP Application layer performs the functions of the upper three layers of the OSI model Provides the interface to the network Closest to the end user Protocols help exchange data between programs running on the source and destination hosts Creates and maintains of dialogue between source and destination applications CCENT v6 14
Presentation and Session Layers Presentation layer has three primary functions: Formatting data at the source device into a compatible form for the receiving device - Coding and conversion of application layer data (Syntax) Compression of the data Encryption of the data for transmission and the decryption of data upon receipt by the destination Common standards for video include QuickTime and Motion Picture Experts Group (MPEG). Common graphic image formats are: GIF, JPEG and PNG 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 that are disrupted or idle CCENT v6 15
TCP/IP Application Layer Protocols TCP/IP application protocols specify the format and control information necessary for common Internet functions (the requests and responses between clients and servers) Application layer protocols must be implemented in both the source and destination devices Application layer protocols implemented on the source and destination host must be compatible to allow communication Know the main applications associated with the Application layer and what they are used for (10.2) CCENT v6 16
Client-Server Model Client and server processes are considered to be in the application layer Clients request information while servers provide it A dedicated server handles all requests Examples: A workstation initiates a DNS request when the user types www.cisco.com in the address bar of a web browser A client must login to a domain to gain access to resources and security credentials Email is an example of a Client-Server interaction CCENT v6 17
Common P2P Applications Peer-to-Peer Applications Some P2P applications use a hybrid system, where resource sharing is decentralized 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 Common P2P Applications Common P2P networks include: Bitcoin, edonkey, G2, BitTorrent. Some P2P applications are based on the Gnutella protocol, where each user shares whole files with other users BitTorrent: - Many P2P applications allow users to share pieces of many files with each other at the same time - A small torrent file contains information about the location of other users and tracker computers - Trackers are computers keeping track of the files hosted by users - BitTorrent clients include: BitTorrent, utorrent, Frostwire, and qbittorrent CCENT v6 18
Peer-to-Peer Networks Two or more computers can be connected to a P2P network to share resources Every connected end device (a peer) can function as both a server and a client The roles of client and server are set on a per request basis Both can initiate a communication Both devices are considered equal in the communication Decentralized resources Resource sharing without a dedicated server Because they do not require formalized accounts or permissions, they are best used in limited situations Peer-to-peer networks do not require the use of a dedicated server, and devices can assume both client and server roles simultaneously on a per request basis Peer-to-peer applications require a specific user interface, background service to be running, and can be used in more diverse situations CCENT v6 19
10.2 WELL-KNOWN APPLICATION LAYER PROTOCOLS AND SERVICES CCENT v6 20
TCP/IP Application Layer Protocols 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 CCENT v6 21
Hypertext Transfer Protocol / Markup Language A Uniform Resource Locator (URL) is a reference to a web server URLs and URIs are the names most people associate with web addresses The web browser establishes a connection to the web service running on the server, using the HTTP protocol URLs contain the protocol, the server name and the requested filename - http://www.cisco.com/index.html First, the browser interprets the three parts of the URL: http (the protocol or scheme) www (a folder on the server) cisco (the server name).com (top-level DNS domain) index.html (the specific file name requested) Using DNS, the server name portion of the URL is then translated to the associated IP address before the server can be contacted CCENT v6 22
Hypertext Transfer Protocol (HTTP) and Secure Hypertext Transfer Protocol (HTTPS) Web page process: Browser checks with a name server (DNS) to convert www.cisco.com into a numeric address Using the HTTP protocol requirements, the browser sends a GET request to the server and asks for the file index.html The server sends the requested file (HTML code for this web page) to the client The index.html was specified in the URL and contains the HTML code for this web page The browser processes/deciphers the HTML code and formats the page for the browser window based on the code in the file Hypertext Transfer Protocol (HTTP) TCP 80, 8080 - Set of rules for exchanging text, graphic images, etc. on the World Wide Web HTTP Is not secure. Messages can be intercepted Hypertext Transfer Protocol Secure (HTTPS) TCP, UDP 443 Uses encryption and authentication to secure communication CCENT v6 23
Hypertext Transfer Protocol (HTTP) and Secure Hypertext Transfer Protocol (HTTPS) Developed to publish, transfer, and retrieve HTML pages Specifies a request/response protocol Three common message types: GET a client request for data from the web server POST Uploads data files to the web server PUT Uploads resources or content to the web server HTTPS is used to securely communicate with a web server Uses encryption and authentication to secure data http protocol using GET CCENT v6 24
Email Protocols Email is a store-and-forward method of sending, storing, and retrieving electronic messages Email messages are stored 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 Email relies on three separate protocols for operation: SMTP, POP, and IMAP CCENT v6 25
Simple Mail Transfer Protocol (SMTP) Used for the transfer of mail messages and attachments to and between servers Allows messages to be sent SMTP message formats require a message header and body The header must have a properly formatted recipient email address and a sender address An SMTP client sends an email by connecting to a SMTP server on port 25 The server receives the message and stores it in a local mailbox or relays the message to another mail server Users use email clients to retrieve messages stored on the server The mail transfer agent process governs e-mail handling between servers and clients SMTP processes must be running on both the client and server Message header must have a properly formatted recipient email address and a sender If the mail server is busy, SMTP will try to send the messages at a later time SMTP periodically checks the queue for messages and attempts to send them again CCENT v6 26
Post Office Protocol version 3 (POP3) Used by email clients to retrieve email from a remote server Mail is downloaded from the server to the client and then deleted on the server - Does not store messages A downloaded message resides on the device that triggered the download. Email clients direct their POP requests to mail servers on port TCP 110 Is desirable for an ISP, because it alleviates their responsibility for managing large amounts of storage for their email servers CCENT v6 27
Internet Message Access Protocol (IMAP) A newer protocol for email retrieval Unlike POP, when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application Allows for easy, centralized storage and backup of emails Allows for messages to be displayed to the user rather than downloaded Original messages are kept on the server until manually deleted Users view copies of the messages in their email client software Uses port TCP 143 Support folder hierarchy to organize and store mail When a user decides to delete a message, the server synchronizes that action and deletes the message from the server CCENT v6 28
SMTP, POP, and IMAP Typically use an application called a Mail User Agent (email client) Email client provides the functionality of both protocols within one application Clients send e-mails to a server using SMTP and receive e-mails using POP3 or IMAP Web mail typically uses IMAP CCENT v6 29
Domain Name Service Protocol (DNS) IP addresses are not easy to memorize Domain names make server addresses more user-friendly Computers still need the actual numeric address before they can communicate The DNS protocol allows for the dynamic translation of a domain name into the associated IP address Uses port TCP and UDP 53 Used to resolve Internet names to IP addresses DNS servers store different types of resource records used to resolve names Contains the name, IP address, and type of record CCENT v6 30
DNS Message Format Record types are: A - an IPv4 end device address AAAA an IPv6 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 When a client makes a query, the server s DNS process first looks at its own records to resolve the name If unable to resolve, it contacts other servers to resolve the name The response is then forwarded to the client Server temporarily stores the numbered address that matches the name in cache memory Windows ipconfig /displaydns displays all cached DNS CCENT v6 31
DNS Hierarchy The DNS protocol uses a hierarchical system The naming structure is broken down into small, manageable zones Server is responsible for managing/mapping name-to-ip mappings for a small portion of the DNS structure Requests for zones not stored in a specific DNS server are forwarded to other servers for translation Top-level domains represent either the type of domain or the country of origin Examples of top-level domains are:.com.org.au.co CCENT v6 32
The nslookup Command Operating system utility that allows the user to manually query the name servers to resolve a given host name Can be used to troubleshoot name resolution issues and to verify the current status of the name servers In the DNS hierarchy, Cisco.com is the authoritative DNS server for the domain name record CCENT v6 33
Dynamic Host Configuration Protocol (DHCP) Computers need network IP information to communicate over a network 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 Used to assign an IP address, subnet mask, default gateway and DNS server to a host Allows for automated and scalable distribution of IP information DHCP-distributed addresses are leased for a set period of time, then returned to pool for reuse A 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 Addresses are returned to the pool for reuse when no longer in use Uses port UDP 67 for server and 68 for client DHCP supports IPv4 and DHCPv6 supports IPv6 CCENT v6 34
DHCP Operation The DHCPDISCOVER message is used to locate and identify any DHCP servers on a network. The DHCPOFFER message is used by a server to offer or suggest a lease to a client. The DHCPREQUEST message is used to identify the explicit DHCP server and the lease that the client is accepting A client may also request an address previously been allocated by the server The DHCPACK message is used by a server to finalize a successful lease with a client The DHCPNAK message is used when an offered lease is no longer valid CCENT v6 35
DHCP Operation A PC uses DHCPDISCOVER and DHCPREQUEST to broadcast messages when communicating with a DHCP server A DHCP Discover message: The destination IP address is 255.255.255.255 The source MAC address is FF-FF-FF-FF-FF-FF The message comes from a server offering an IP address The message comes from a client seeking an IP address All hosts receive the message, but only a DHCP server replies CCENT v6 36
File Transfer Protocol (FTP) Used for interactive file transfer between systems Allow data transfers between a client and a server Both download and upload capabilities FTP client is an application that runs on a computer that is used to push and pull data from a FTP server running an FTP daemon To successfully transfer data, FTP requires two connections between the client and the server Port TCP 21 for control (commands and replies) Port TCP 20 for the actual file transfer Port 21 Port 20 CCENT v6 37
Trivial File Transfer Protocol (TFTP) Used for connectionless active file transfer Usually anonymous Allow data transfers between a client and a server Uses port UDP 69 CCENT v6 38
Server Message Block (SMB) A client/server file sharing protocol Clients establish a long-term connection to servers and can access the resources on the server as if the resource is local to the client host All messages share a common format SMB file-sharing and print services have become the mainstay of Windows networking Microsoft products now support TCP/IP protocols to directly support SMB resource sharing 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 Mac, LINUX, and UNIX operating systems have their own implementation of SMB SMB would be more efficient than FTP when uploading the same file to multiple remote servers CCENT v6 39
New Terms and Commands Hypertext Transfer Protocol (HTTP) File Transfer Protocol (FTP) Trivial File Transfer Protocol (TFTP) Internet Message Access Protocol (IMAP) Domain Name System (DNS) Simple Mail Transport Protocol (SMTP) Post Office Protocol (POP) Dynamic Host Configuration Protocol (DHCP) QuickTime and Motion Picture Experts Group (MPEG) Graphics Interchange Format (GIF) Joint Photographic Experts Group (JPEG) Portable Network Graphics (PNG) BOOTP HTTPS Client-server model Peer-to-peer network (P2P) P2P networks P2P applications Gnutella protocol BitTorrent Uniform Resource Locator (URL) Uniform Resource Identifier (URIs) Get Post Put HTTP Secure (HTTPS) Secure Socket Layer (SSL) Simple Mail Transfer Protocol (SMTP) Post Office Protocol (POP) CCENT v6 40
New Terms and Commands IMAP Port 25 (SMTP) TCP Port 110 (POP) Domain name DNS Protocol Record types: A, NS, AAAA, MX ipconfig /displaydns Top-level domains are:.com,.org,.au,.co Nslookup Dynamic Host Configuration Protocol (DHCP) for IPv4 DHCPv6 DHCP Discover DHCP Offer DHCP Request DHCP Acknowledgement DHCP Negative Acknowledgement DHCPv6: SOLICIT, ADVERTISE, INFORMATION REQUEST, and REPLY FTP daemon (FTPd) Server Message Block (SMB) CCENT v6 41
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