The document describes application layer protocols and services including: - Application layer protocols provide end-user applications with network services and include HTTP, DNS, DHCP, SMTP/POP, and Telnet. - Peer-to-peer applications allow devices to act as both clients and servers to share resources without a dedicated server. - Common application layer functions are establishing communication rules, structuring message formats, and defining interactions with lower layers.

1. Ch 3 - Chapter 3 Application Layer Functionality and Protocols

2. Objectives Describe how the functions of the three upper OSI model layers provide network services to end-user applications Describe how the TCP/IP application layer protocols provide the services specified by the upper layers of the OSI model Define how people use the application layer to communicate across the information network Describe the functions of the well-know TCP/IP applications , such as the World Wide Web and e-mail, and other related services (HTTP, DNS, DHCP, SMTP/POP and Telnet) Describe the file sharing processes that use peer-to-peer applications and the Gnutella protocol Explain how protocols ensure that services running on one kind of device can send to and receive from many different network devices Use network analysis tools to examine and explain how common user applications work Ch 3 -

3. Application Layer Provides the interface between human and data networks people create the communication software and hardware convert communication to a digital format Application layer services initiate the data transfer Ch 3 -

4. Presentation Layer The Presentation layer has three primary functions coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device compression of the data in a manner that can be decompressed by the destination device encryption of the data for transmission and the decryption of data upon receipt by the destination Ch 3 -

5. Session Layer Creates and maintains dialogs between source and destination applications Handles the exchange of information to initiate dialogs and keeps them active Restarts sessions that are disrupted or idle for a long period of time Ch 3 -

6. Application Layer Protocols Domain Name Service (DNS) used to resolve Internet names to IP addresses Hypertext Transfer Protocol (HTTP) used to transfer files that make up the web pages of the world wide web Simple Mail Transfer Protocol used for the transfer of mail messages and attachments Telnet (terminal emulation protocol) used to provide remote access to servers and networking devices File Transfer Protocol (FTP) used for interactive file transfers between systems Ch 3 -

7. Application Layer Software Network-aware applications implement the application layer protocols and are able to communicate directly with the lower layers of the protocol stack e-mail clients and web browsers Application layer services are programs that interface with the network and prepare the data for transfer different types of data (text, graphics or video) require different network services to ensure that it is properly prepared for processing by the functions occurring at the lower layers of the OSI model Ch 3 -

8. Software Processes Processes are individual software programs running together at the same time Ch 3 -

9. User Applications, Services and Protocols User applications provide the user with a way to create messages Services establish an interface to the network Protocols provide the rules and formats that govern how data is treated Ch 3 -

10. Application Layer Protocol Functions Establish consistent rules for exchanging data between applications and services Specify how data inside the message is structured and the types of messages that are sent between the source and destination Define the message dialogues and the expected response Define the interaction with the next lower layer Ch 3 -

11. Client-Server Model The device requesting the information is called a client The device responding to the request is called a server Application layer protocols describe the format of the requests and responses between the clients and servers Ch 3 -

12. Servers A server is usually a computer that contains information to be shared with many client systems Some servers may require authentication of user control information to verify the user has access rights rely on a central list of user accounts and authorization The server runs a service, or process, called a daemon Ch 3 - daemon typically runs in the background the daemon “listens” for a request from a client the daemon exchanges appropriate information with the client when it “hears” a request

13. Application Layer Services and Protocols A single application may employ many different supporting Application layer services one request may contain several individual processes Servers can support multiple clients requesting information at the same time Ch 3 - the individual client requests must be handled simultaneously and separately Application layer processes and services rely on support from lower layer functions to successfully manage the multiple conversations

14. Peer-to-Peer Networks Two or more computers are connected via a network and can share resources without having a dedicated server every connected end device, known as a peer , can function either as a client or server information can be located anywhere on any connected device Ch 3 - user accounts and access rights are set individually on each peer device difficult to enforce security and access policies most current operating systems support file and print sharing without requiring additional server software

15. Peer-to-Peer Applications Allows a device to act as both a client and server within the same communication requires each end device to provide a user interface and run a background service Can be used on peer-to-peer networks, client/server networks and across the Internet Ch 3 -

16. DNS Service and Protocol DNS uses TCP/UDP port 53 Domain names were created to convert the numeric IP address into a simple, recognizable name Ch 3 - domain names are easier to remember than actual numeric addresses any change to the address is transparent to the user since the domain name is the same

17. DNS Resolver DNS is a client/server service the DNS client runs a service (and not an application) the DNS client, called the DNS resolver , supports name resolution for other network applications and services DNS client uses one or more DNS server addresses for name resolution Ch 3 - DNS client queries the name servers to resolve the name to a numeric address nslookup utility allows the user to manually query the name servers to resolve a given host name

18. DNS Message Format A DNS server uses the name resolution with the name daemon, named (pronounced name-dee) Uses the same message format all types of client queries and server responses error messages the transfer of resource record information between servers Ch 3 -

19. DNS Resource Records Ch 3 - A an end device address NS an authoritative name server , i.e. a server has resource records that corresponds to its level in the domain hierarchy CNAME the canonical name – an alias name for a host MX mail exchange record

20. DNS Directory Structure Uses a hierarchical structure to create a name database hierarchy is an inverted tree with the root at the top and branches below The different top-level domains (TLD) represent either the country origin or the type of organization Ch 3 - After TLD are second-level domain names, and below them are other lower level domains

21. WWW Service and Protocol The web browser establishes a connection to the web service running on the server using the Hypertext Transfer Protocol (HTTP) request a page using an URL (uniform resource locator) or web address a web browser is a client application running on a client device HTTP uses TCP port 80 An URL consists of three parts the protocol – hypertext transfer protocol ( http ) the server name the specific file name requested Browsers can interpret and present many data types plain text and Hypertext Markup Language (HTML) other data types require another service or program such as plug-ins or add-ons Ch 3 -

22. WWW Service and HTTP (cont’d) The web client makes a connection to the HTTP server and requests a page In response to the request, the HTTP server returns the code for a web page The browser interprets the HTML code and displays a web page Ch 3 -

23. HTTP Protocol HTTP specifies a request/response protocol HTTP protocol uses three common messages – GET , POST and PUT Ch 3 - HTTP is not a secure protocol POST messages in plain text can be intercepted and read HTML pages are not encrypted HTTP Secure (HTTPS) protocol can use authentication and encryption to secure the data

24. E-mail Services and SMTP/POP3 User composes an e-mail using an application called a mail user agent (MUA) or e-mail client Client sends e-mails to a server using Simple Mail Transfer Protocol ( SMTP ) and receives e-mails using Post Office Protocol version 3 ( POP3 ) Ch 3 - SMTP uses TCP port 25 POP uses UDP port 110

25. Mail Transfer Agent The MTA process is used to forward e-mail the MTA receives messages from a MUA or another MTA based on the message header, it determines how a message has to be forwarded to reach its destination Ch 3 - if the recipient resides on the local server, the mail is passed to the mail delivery agent (MDA) if the recipient is not on the local server, the MTA routes the e-mail to the MTA on the appropriate server

26. Mail Delivery Agent The MDA receives the inbound mail from the MTA and delivers the mail to the appropriate users’ mailboxes It can also resolve final delivery issues, such as virus scanning , spam filtering and return-receipt handling Ch 3 -

27. SMTP and POP POP and POP3 are inbound mail delivery protocols MDA listens for a client connection to the server Ch 3 - SMTP governs the transfer of outbound e-mail from the sending client to the MDA, as well as the transport of e-mail between MTA enables e-mail to be transported across data networks between different types of server and client software

28. SMTP Commands SMTP uses a rigid set of commands and replies commands support the procedures for session initiation , mail transaction, forwarding mail, verifying mailbox names, expanding mailing lists, and the opening an closing exchanges Ch 3 - HELO identifies the SMTP client process to the SMTP server process EHLO a new version of HELO , which includes service extensions MAIL FROM identifies the sender RCPT TO identifies the recipient DATA identifies the body of the message

29. File Transfer Protocol FTP uses TCP ports 20 (data) and 21 (commands and replies) An FTP client is used to push and pull files from a server running the FTP daemon (FTPd) Ch 3 - Client establishes the first connection to the server on TCP port 21 for control traffic such as client commands and server replies Client establishes the second connection to the server over TCP port 20 for file transfer in both direction TCP port 21 TCP port 20

30. Dynamic Host Configuration Protocol DHCP uses UDP ports 67 (server) and 68 (client) Allows a host to obtain an IP address automatically from a DHCP server when it connects to the network server leases an IP address from a pool to the host for a set period can pose a security risk Ch 3 - DHCP is preferred on large LANs or where the user population changes frequently when accessing the Internet using wireless hotspots at airport or coffees shops

31. DHCP Process When a DHCP-configured device boots or connects to the network, the client broadcasts a DHCP Discover packet The DHCP server replies with a DHCP Offer a lease offer message with an assigned IP address, subnet mask , default gateway, DNS server and duration of lease client may receive multiple DHCP Offer packets Client broadcasts a DHCP Request that identifies the explicit server and lease offer that it is accepting Ch 3 - The DCHP would return a DHCP Ack that acknowledges the lease is finalized provided IP address is still valid

32. File Sharing Services Server Message Block (SMB) is a client/server file sharing protocol(Used in Microsoft Operating System) sharing directories , files , printers and serial ports clients establish a long term connection to the servers SMB resource sharing is supported by TCP/IP protocols Ch 3 -

33. SMB Protocol It describes the file system access and how clients can request for files All SMB messages share a common format a fixed size header followed by a variable-sized parameter and data component Ch 3 - 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

34. Peer-to-Peer (P2P) Services Users can make files on their hard disks available to others for downloading with P2P applications based on the Gnutella protocol Gnutella-compatible client software allows users to connect to Gnutella services over the Internet to locate and access resources shared by other Gnutella peers BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX Gnutella Developer Forum maintains the basic protocol application vendors develop extensions to make their protocol work better on their applications Ch 3 -

35. Gnutella Protocol Many P2P applications do not use a central database to record all files available on the peers Gnutella allows P2P applications to search for shared resources on peers (or nodes) Ch 3 - When a user connects to a Gnutella service, the client application will search for nodes to connect to nodes handle queries for resource location and replies

36. Telnet Telnet uses TCP port 23 Provides a method of emulating text-based terminals over the network allows a local device to access a remote device as if the keyboard and monitor are connected to the remote device directly A connection using Telnet is called a virtual terminal (VTY) session Ch 3 - The Telnet server runs a service called the Telnet daemon

37. Questions What application layer protocol is commonly used to support for file transfers between a client and a server? HTML HTTP FTP Telnet What is the purpose of resource records in DNS? temporarily holds resolved entries used by the server to resolve names sent by the client to the server during a query passes authentication information between the server and client Ch 3 -

38. Questions What application layer protocol is commonly used to support for file transfers between a client and a server? HTML HTTP FTP Telnet What is the purpose of resource records in DNS? temporarily holds resolved entries used by the server to resolve names sent by the client to the server during a query passes authentication information between the server and client Ch 3 -

39. What three protocols operate at the Application layer of the OSI model? (Choose three.) ARP DNS PPP SMTP POP ICMP What are two characteristics of peer-to-peer networks? (Choose two.) scalable one way data flow decentralized resources centralized user accounts resource sharing without a dedicated server Ch 3 -

40. What three protocols operate at the Application layer of the OSI model? (Choose three.) ARP DNS PPP SMTP POP ICMP What are two characteristics of peer-to-peer networks? (Choose two.) scalable one way data flow decentralized resources centralized user accounts resource sharing without a dedicated server Ch 3 -