Chakchai So-In, Ph.D.

Transcription

1 Application Layer Functionality and Protocols Chakchai So-In, Ph.D. Khon Kaen University Department of Computer Science Faculty of Science, Khon Kaen University 123 Mitaparb Rd., Naimaung, Maung, Khon Kaen, Thailand CS Chakchai So-In

2 Agenda Define the application layer as the source and destination of data for communication across networks. Explain the role of protocols in supporting communication between server and client processes. Describe the features, operation, and use of well-known TCP/IP application layer services (HTTP, DNS, SMTP). Khon Kaen University CS Chakchai So-In

3 Applications - The Interface Between Human and Data Networks Explain that applications provide the means for generating and receiving data that can be transported on the network Khon Kaen University CS Chakchai So-In

4 Applications - The Interface Between Human and Data Networks (Cont.) Explore the role of applications, services and protocols in converting communication to data that can be transferred across the data network Khon Kaen University CS Chakchai So-In

5 Applications - The Interface Between Human and Data Networks (Cont.) Define the separate roles applications, services and protocols play in transporting data through networks Khon Kaen University CS Chakchai So-In

6 Applications - The Interface Between Human and Data Networks (Cont.) Application layer is the layer that provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. Application layer protocols specify the format and control information necessary for many of the common Internet communication functions e.g. DNS, HTTP, SMTP.

7 Applications - The Interface Between Human and Data Networks (Cont.) Example of application layer protocols The protocols in the TCP/IP suite are generally defined by Requests for Comments (RFCs). The Internet Engineering Task Force maintains the RFCs as the standards for the TCP/IP suite. Khon Kaen University CS Chakchai So-In

8 Example of Application Layer Protocols

9 The 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. E,g., Quicktime, MPEG, JPEG, GIF, TIFF

10 The Session Layer The Session layer creates and maintains 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. E.g. Network File System (NFC), RPC (Remote Procedure Call), SIP (Session Initiation Protocol), Real Time Transport Protocol (RTP)

11 Applications - The Interface Between Human and Data Networks (Cont.) Applications provide people a way to create messages. Network-Aware Applications: they implement the application layer protocols and are able to communicate directly with the lower layers of the protocol stack. clients and web browsers. Application Layer Services: other programs may need the assistance of Application layer services to use network resources, like file transfer or network print spooling. Application Layer Services establish an interface to the network. Protocols provide the rules and formats that govern how data is treated. Protocols specify what messages are exchanged between the source and destination hosts, the syntax of the control commands, the type and format of the data being transmitted, and the appropriate methods for error notification and recovery.

12 Applications - The Interface Between Human and Data Networks (Cont.)

13 User Applications/Services/Application Layer Protocol [Animation ] Khon Kaen University CS Chakchai So-In

14 Application Layer Protocol Function

15 The Role of Protocols in Supporting Communication (Client/Server Models) Khon Kaen University CS Chakchai So-In

16 Servers A server is usually a computer that contains information to be shared with many client systems e.g. web pages, documents, databases, pictures, video, and audio files Different types of server applications may have different requirements for client access e.g. some servers may require authentication The server runs a service, or process, sometimes called a server daemon. Daemons listen for a request from a client When a daemon "hears" a request from a client, it exchanges appropriate messages with the client

17 The Role of Protocols in Supporting Communication Khon Kaen University CS Chakchai So-In

18 The Role of Protocols in Supporting Communication (Cont.) Compare and contrast client server networking with peer-topeer networks and peer-to-peer applications

19 The Role of Protocols in Supporting Communication (Cont.) Compare and contrast client server networking with peer-to-peer networks and peer-to-peer applications (Centralized vs. Distributed) Difficulty: User Management/Policy Allow: Hybrid (index; resource) A peer-to-peer application (P2P), unlike a peer-to-peer network, allows a device to act as both a client and a server within the same communication.

20 Application Layer Protocols: Examples Domain Name System (DNS) - TCP/UDP Port 53 Hypertext Transfer Protocol (HTTP) - TCP Port 80 Simple Mail Transfer Protocol (SMTP) - TCP Port 25 Post Office Protocol (POP) - TCP Port 110 Telnet - TCP Port 23 Dynamic Host Configuration Protocol - UDP Ports 67 and 68 File Transfer Protocol (FTP) - TCP Ports 20 and 21 Khon Kaen University CS Chakchai So-In

21 Features, Operation, and Use of TCP/IP Application Layer Services Describe the features of the DNS protocol and how this protocol supports DNS services [Animation ] Khon Kaen University CS Chakchai So-In

22 DNS Services and Protocol DNS is a client/server service. DNS differs from other services - While other services use a client that is an application (such as web browser, client), the DNS client runs as a service itself. The DNS server, sometimes called the DNS resolver, supports name resolution for our other network applications and other services that need it. DNS Verification with nslookup

23 DNS Services and Protocol (Cont.) A DNS server provides the name resolution using the name daemon, which is often called named. The DNS server stores different types of resource records used to resolve names. These records contain the name, address, and type of record. A - an end device address NS - an authoritative name server CNAME - the canonical name (or Fully Qualified Domain 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 for that domain

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25 DNS Message Format

26 DNS Services and Protocol DNS uses a hierarchical system to create a name database to provide name resolution. At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the second-level domain servers and so on. The resource records list domain names that the server can resolve and alternative servers that can also process requests. If a given server has resource records that correspond to its level in the domain hierarchy, it is said to be authoritative for those records.

27 DNS Services and Protocol (Cont.)

28 Features, Operation, and Use of TCP/IP Application Layer Services Describe the features of the HTTP protocol and how this protocol supports the delivery of web pages to the client

29 Features, Operation, and Use of TCP/IP Application Layer Services (Cont.) Describe the features of the HTTP protocol and how this protocol supports the delivery of web pages to the client

30 WWW Service and HTTP HTTP is used across the WWW for data transfer and is one of the most used application protocols. HTTP specifies a request/response protocol. Three common message types GET is a client request for data. POST and PUT are used to send messages that upload data to the web server. POST includes the data in the message sent to the server. PUT uploads resources or content to the web server.

31 Services and POP/SMTP Protocol The MUA allows messages to be sent and places received messages into the client's mailbox. In order to receive messages from an server, the e- mail client can use POP. Sending from either a client or a server uses message formats and command strings defined by the SMTP protocol.

32 Services Describe the features of the POP and SMTP protocols, and how these protocols support services Mail Transfer Agent (MTA) - forwards to MDA Mail Delivery Agent (MDA) - receives all the inbound mail from the MTA and places it into the appropriate users' mailboxes Mail User Agent (MUA)

33 Services and POP/SMTP Protocol (Cont.) Some of the commands specified in the SMTP protocol are: HELO - identifies the SMTP client process to the SMTP server process EHLO - Is a newer version of HELO, which includes services extensions MAIL FROM - Identifies the sender RCPT TO - Identifies the recipient DATA - Identifies the body of the message

34 FTP Services Describe the features of the FTP protocol and the role it plays in supporting file sharing (FTPd) TCP Port 21 (Control) and 20 (Data)

35 DHCP Services Describe the features of the DHCP protocol and the role it plays in allocating IP addresses to network nodes (IP, Netmask, Gateway, etc.) Come and Go (Dynamic Allocation) IP Pool and Lease Security = Mixed Static and Dynamic

36 DHCP Process

37 SMB Services Describe the features of the SMB protocol and the role it plays in supporting file sharing in Microsoft-based networks

38 File Sharing Service/ SMB Protocol (Cont.) The Server Message Block (SMB) (IBM/Microsoft) describes the structure of shared network resources, such as directories, files, printers, and serial ports. (SAMBA = Linux/Unix) Unlike the file sharing supported by FTP, clients establish a long term connection to servers. (Map Network Drive) Once 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.

39 File Sharing Service and SMB Protocol The SMB protocol describes file system access and how clients can make requests for files All SMB messages share a common format. This format uses a fixed-sized header followed by a variable-sized parameter and data component. Start, authenticate, and terminate sessions Control file and printer access Allow an application to send or receive messages to or from another device

40 P2P P Services and Gnutella Protocol Similar to FTP/SMB, P2P = File Sharing (Internet) based on Gnutella protocol, e.g., BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX Do not use a central database instead = Peer/Node

41 P2P P Services/ Gnutella Protocol (Cont.) Connected Gnutella service, the client applications will search for other Gnutella nodes to connect to. These nodes handle queries for resource locations and replies to those requests. Govern control messages, which help the service discover other nodes. Actual file transfers = HTTP 5 Gnutella Messages ping - for device discovery pong - as a reply to a ping query - for file location query hit - as a reply to a query push - as a download request

42 Telnet Services and Protocol Before = Text-based system (display terminal) physical Telnet = virtual terminal (software) based on TCP/IP to create session (VTY Virtual Terminal); command line interface (CLI) Telnet Client E.g., telnet (cmd), hyper terminal, TeraTerm

43 Telnet Services and Protocol (Cont.) Telnet is a client/server protocol; how a VTY session is established and terminated; provides the syntax and commands used to initiate the Telnet session Command = at least two bytes. The first byte is a special character called the Interpret as Command (IAC) character. As its name implies, the IAC defines the next byte as a command rather than text.

44 Telnet Services and Protocol (Cont.) Some sample Telnet protocol commands include: Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active. Erase Line (EL) - Deletes all text from the current line. Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program. Drawback: Not secure (Plaintext) only authentication Secure Shell (SSH) = encryption

45 Telnet/SSH Client (putty)

46 Summary Khon Kaen University CS Chakchai So-In

47 Thank you and Questions? Khon Kaen University CS Chakchai So-In