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Guide to TCP/IP, Third Edition

Understand how FTP, Telnet, and SMTP protocols work and how to decode packets containing these protocols. Learn about their key components and message structures.

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Guide to TCP/IP, Third Edition

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  1. Guide to TCP/IP, Third Edition Chapter 6: Basic TCP/IP Services

  2. Objectives • Understand how TCP/IP Application layer protocols and services work • Explain the capabilities, message types, and request/reply architectures for basic TCP/IP services • Discuss real-time streaming protocols (MSN Messenger and VoIP) for messaging and telephony applications Basic TCP/IP Services

  3. Objectives (continued) • Understand the operations of other basic TCP/IP services • Explain how to decode packets that contain Application layer protocols, and how to relate message types or other similar information to the kinds of requests and replies moving between a client and a server (or between hosts in general) Basic TCP/IP Services

  4. How Application Layer IP Protocols Work and Behave • Fundamental behavior of TCP/IP Application layer protocols depends on • Specifications for the message structures that the protocol or service supports • Definition of a well-known port address on which servers listen for service requests • Availability of appropriate software components Basic TCP/IP Services

  5. How Application Layer IP Protocols Work and Behave (continued) • Request/reply messages • Types of messages that generally occur within TCP/IP Application layer services • Request messages • Used by clients to request services • Reply messages • Used by servers to reply to messages • Server-to-server traffic • Server-specific information is replicated from one server to another Basic TCP/IP Services

  6. Understanding FTP • FTP • Offers a method for transferring files over a connection-oriented transport, or TCP • Key components of an FTP communication • User interface (UI) • Protocol interpreter (PI) • FTP commands • Data transfer process (DTP) • Files transferred • TCP transport • TCP-based command connection • TCP-based data transfer connection Basic TCP/IP Services

  7. Basic TCP/IP Services

  8. FTP User Interface • Offers the visual front end to the user • Third-party products • Offer an FTP client application with a graphical, intuitive interface • Protocol Interpreter (PI) • Interprets commands • Initiates control connection from user’s dynamic port to the server’s FTP port • Initiates FTP commands • Monitors the data transfer process Basic TCP/IP Services

  9. Basic TCP/IP Services

  10. Basic TCP/IP Services

  11. Basic TCP/IP Services

  12. FTP Elements • Data Transfer Process (DTP) • Moves data across the data connection only • File System • Files at either end of an FTP communication may consist of various formats • TCP Transport • FTP relies on TCP to • Establish the underlying connection • Track the order of packets • Define and adjust the data transfer window Basic TCP/IP Services

  13. FTP Elements (continued) • TCP-based Command Connection • Runs over TCP • Created automatically when • FTP connection is established in response to open command from FTP client • TCP-based Data Transfer Connection • Runs over TCP • Created by the server by default Basic TCP/IP Services

  14. Sample FTP Communications • FTP • Application layer protocol that relies on TCP • Commands immediately follow the FTP header • RFC 959 • Has complete list of FTP protocol commands Basic TCP/IP Services

  15. Basic TCP/IP Services

  16. Basic TCP/IP Services

  17. Active and Passive FTP • In active mode • Client connects from random TCP port with an address greater than 1023 to FTP server’s command port, port 21 • Passive FTP • Developed to sidestep issue of an apparent server connection initiation • Client initiates connections for both the command port and data port • Not supported by some FTP clients Basic TCP/IP Services

  18. Understanding Telnet • Telnet • Offers a bidirectionalbyte-oriented communication • Uses well-known port 23 on the server side • Uses dynamic port number on the client side • Telnet Elements • Telnet hosts exchange information about options that they support Basic TCP/IP Services

  19. Telnet Elements (continued) • NVT • Considered a printer-keyboard device that receives bytes from the other host and prints the information • The DO, DON’T, WILL, WON’T Structure • 253 (0xFD)DO • 254 (0xFE)DON’T • 251 (0xFB)WILL • 252 (0xFC)WON’T Basic TCP/IP Services

  20. Sample Telnet Communications • Telnet • Information immediately follows the TCP header • Remains an important and widely used TCP/IP service • Stelnet • Becoming increasingly available • Secure Sockets Layer (SSL)interface • Used to encrypt traffic between a Telnet client and a Telnet server Basic TCP/IP Services

  21. Understanding SMTP • Simple Mail Transfer Protocol (SMTP) • Most widely used protocol for sending electronic mail (e-mail) on the Internet • Sends and receives e-mail messages through • Sender-SMTP process • Receiver-SMTP process that perform e-mail transfer services Basic TCP/IP Services

  22. Basic TCP/IP Services

  23. SMTP Elements • Sender-SMTP and Receiver-SMTP • SMTP Commands and Extension • SMTP Reply Codes • Sender-SMTP and Receiver-SMTP processes • Client or server applications that support SMTP functionality Basic TCP/IP Services

  24. Basic TCP/IP Services

  25. Sample SMTP Communications • SMTP commands and reply codes • Immediately follow the TCP header • SMTP commands • Occur behind the scenes in some e-mail clients and some server-to-server mail transfers on the Internet • Majority of e-mail clients today • Use SMTP to send outgoing mail through some nearby SMTP server Basic TCP/IP Services

  26. Basic TCP/IP Services

  27. Understanding HTTP • Hypertext Transfer Protocol (HTTP) • Application layer protocol • Provides distributed information from various hypermediasystems • Has been in use on the Web since 1990 • Uses a request/response model Basic TCP/IP Services

  28. HTTP Elements • HTTP clients • Send HTTP requests that contain a method that indicates what the client wants • HTTP servers • Respond with a numeric code • URI • Combination of a Uniform Resource Locator (URL) and a Uniform Resource Name (URN) Basic TCP/IP Services

  29. Basic TCP/IP Services

  30. HTTP Elements (continued) • HTTP methods • Commands that the HTTP client issues to the HTTP server • Status codes • Sent by HTTP server to indicate success or failure of request Basic TCP/IP Services

  31. Sample HTTP Communications • HTTP commands and status codes • Immediately follow the TCP header • HTTPS protocol • Represents a secure implementation of HTTP that incorporates use of SSL • TCP port 443 • Assigned to HTTP protocol that uses TCP with Transport layer security over SSL • UDP port 443 • Assigned to HTTP protocol that uses UDP with Transport layer security over SSL Basic TCP/IP Services

  32. Understanding MSN Messenger • MSN Messenger • Proprietary protocol specification • Handles audio and video applications and can be embedded within other protocols • MSN Messenger Elements • Possesses one of the largest subscriber bases in the world • Serves as a presence information provider Basic TCP/IP Services

  33. Obtaining and Using MSN Messenger • MSN Messenger • Readily available at no charge online at messenger.msn.com • Four fundamental entities define the MSN Messenger protocol architecture • The principal (or client) application • Dispatch server • Notification server • Switchboard server Basic TCP/IP Services

  34. MSN Messenger Commands • To reach a broad audience of subscribers • MSN Messenger protocol encodes character values outside the printable ASCII range • Principal and server commands • Three-letter abbreviations, followed by any number of specified parameters • During initialization phase • Commands sent between principal and server, occur synchronously Basic TCP/IP Services

  35. Basic TCP/IP Services

  36. Sample MSN Messenger Communications • Authentication procedures for MSN Messenger Services begin with dispatcher • Dispatcher • Negotiates an appropriate protocol version and format for each connection • Protocol revision 13 • Provides full support for offline instant messaging Basic TCP/IP Services

  37. Basic TCP/IP Services

  38. Understanding VoIP • Voice over Internet Protocol (VoIP) • Describes the properties and procedures for routing voice conversations across the Internet • Responsible for transporting analog voice streams using digital signaling Basic TCP/IP Services

  39. VoIP Elements • Four primary entities involved in a H.323-basedVoIP communications network • The terminals, or voice/video/data clients • A Multipoint Control Unit (MCU)for bridging videoconference connections • The Gateway (GW) devicefor translating among mutually independent protocol stacks • The Gatekeeper Basic TCP/IP Services

  40. VoIP Elements (continued) • Session Initial Protocol (SIP) • Standard for initiating, modifying, or terminating interactive users sessions • Often used in tandem with H.323 for Voice over IP applications • H.323protocol suite • Blanket specification for establishing real-time, streaming, qualitative multimedia communications • H.245 • Defines control protocol for multimedia communication Basic TCP/IP Services

  41. VoIP Elements (continued) • H.235 • Provides coverage of security and encryption for H.323 and H.245 • H.450 • Describes the supplementary services for H.323 networks • H.239 • A recommendation for relaying communications across different network media • Q.931 • Documents the protocol and protocol state machine for ISDN connections Basic TCP/IP Services

  42. Sample VoIP Communications • Advantages of digitizing voice signals for transmission over TCP/IP • Better control can be exercised over the digital format • A digital signal is more forgiving of noise-bearing transmissions than an analog signal • VoIP communications avoid the costs associated with pay-per-usage Basic TCP/IP Services

  43. Other Common TCP/IP-Based Services • Echo (TCP and UDP) • Quote of the Day (QOD) • Character Generator (Chargen) • Whois • Trivial File Transfer Protocol (TFTP) Basic TCP/IP Services

  44. Other Common TCP/IP-Based Services (continued) • Finger • Remote Procedure Call (RPC) • Simple Network Management Protocol (SNMP) • NetBIOS over TCP/IP Basic TCP/IP Services

  45. Basic TCP/IP Services

  46. Basic TCP/IP Services

  47. Decoding Application Layer Protocols • Decoding TCP/IP Application layer traffic largely depends on two separate forms of analysis • Understanding specific service’s request/reply messages • Learning how to recognize related headers and payload information • Learning to assemble multiple lower-layer packets • To reconstitute Application layer messages when payloads exceed MTU for a single packet Basic TCP/IP Services

  48. Summary • Foundation upon which all TCP/IP Application layer services rest • A request/reply message architecture • Certain client/server Application layer services • Involve server-to-server traffic • FTP • A file transfer service • Telnet • Provides a method to log on and access the command line on a remote computer using TCP/IP Basic TCP/IP Services

  49. Summary (continued) • SMTP • Provides store and forward services for e-mail messages • HTTP • Provides the underpinnings for the World Wide Web • MSN Messenger and Voice over IP (VoIP) communications • Provide mechanisms to set up, manage, and tear down ongoing communications using TCP Basic TCP/IP Services

  50. Summary (continued) • Other common TCP/IP Application layer services • Echo, Chargen • QOD, TFTP, Finger • Remote Procedure Call • NetBIOS over TCP/IP • SNMP • Understanding how to decode Application layer protocols • Means learning how to identify and interpret request and reply messages Basic TCP/IP Services

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