Guide to TCP/IP, Third Edition

Guide to TCP/IP, Third EditionChapter 6: Basic TCP/IP Services

Basic TCP/IP Services

ObjectivesUnderstand how TCP/IP Application layer protocols and services workExplain the capabilities, message types, and request/reply architectures for basic TCP/IP servicesDiscuss real-time streaming protocols (MSN Messenger and VoIP) for messaging and telephony applications


Objectives (continued)Understand the operations of other basic TCP/IP servicesExplain 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)


How Application Layer IP Protocols Work and BehaveFundamental behavior of TCP/IP Application layer protocols depends onSpecifications for the message structures that the protocol or service supportsDefinition of a well-known port address on which servers listen for service requestsAvailability of appropriate software components


How Application Layer IP Protocols Work and Behave (continued)Request/reply messagesTypes of messages that generally occur within TCP/IP Application layer servicesRequest messagesUsed by clients to request servicesReply messagesUsed by servers to reply to messagesServer-to-server trafficServer-specific information is replicated from one server to another


Understanding FTPFTP Offers a method for transferring files over a connection-oriented transport, or TCPKey components of an FTP communication User interface (UI)Protocol interpreter (PI)FTP commandsData transfer process (DTP)Files transferred TCP transport TCP-based command connectionTCP-based data transfer connection


FTP User InterfaceOffers the visual front end to the userThird-party productsOffer an FTP client application with a graphical, intuitive interfaceProtocol Interpreter (PI) Interprets commandsInitiates control connection from users dynamic port to the servers FTP portInitiates FTP commandsMonitors the data transfer process


FTP ElementsData Transfer Process (DTP)Moves data across the data connection onlyFile SystemFiles at either end of an FTP communication may consist of various formatsTCP TransportFTP relies on TCP to Establish the underlying connectionTrack the order of packetsDefine and adjust the data transfer window


FTP Elements (continued)TCP-based Command ConnectionRuns over TCP Created automatically when FTP connection is established in response to open command from FTP client TCP-based Data Transfer ConnectionRuns over TCPCreated by the server by default


Sample FTP CommunicationsFTPApplication layer protocol that relies on TCPCommands immediately follow the FTP headerRFC 959Has complete list of FTP protocol commands


Active and Passive FTPIn active modeClient connects from random TCP port with an address greater than 1023 to FTP servers command port, port 21Passive FTPDeveloped to sidestep issue of an apparent server connection initiationClient initiates connections for both the command port and data portNot supported by some FTP clients


Understanding TelnetTelnet Offers a bidirectional byte-oriented communicationUses well-known port 23 on the server sideUses dynamic port number on the client sideTelnet ElementsTelnet hosts exchange information about options that they support


Telnet Elements (continued)NVTConsidered a printer-keyboard device that receives bytes from the other host and prints the informationThe DO, DONT, WILL, WONT Structure253 (0xFD)DO254 (0xFE)DONT251 (0xFB)WILL252 (0xFC)WONT


Sample Telnet CommunicationsTelnet Information immediately follows the TCP headerRemains an important and widely used TCP/IP serviceStelnet Becoming increasingly availableSecure Sockets Layer (SSL) interfaceUsed to encrypt traffic between a Telnet client and a Telnet server


Understanding SMTPSimple Mail Transfer Protocol (SMTP) Most widely used protocol for sending electronic mail (e-mail) on the Internet Sends and receives e-mail messages throughSender-SMTP process Receiver-SMTP process that perform e-mail transfer services


SMTP ElementsSender-SMTP and Receiver-SMTPSMTP Commands and ExtensionSMTP Reply CodesSender-SMTP and Receiver-SMTP processes Client or server applications that support SMTP functionality


Sample SMTP CommunicationsSMTP commands and reply codesImmediately follow the TCP headerSMTP commandsOccur behind the scenes in some e-mail clients and some server-to-server mail transfers on the InternetMajority of e-mail clients todayUse SMTP to send outgoing mail through some nearby SMTP server


Understanding HTTPHypertext Transfer Protocol (HTTP) Application layer protocol Provides distributed information from various hypermedia systemsHas been in use on the Web since 1990Uses a request/response model


HTTP ElementsHTTP clients Send HTTP requests that contain a method that indicates what the client wantsHTTP serversRespond with a numeric codeURICombination of a Uniform Resource Locator (URL) and a Uniform Resource Name (URN)


HTTP Elements (continued)HTTP methodsCommands that the HTTP client issues to the HTTP serverStatus codesSent by HTTP server to indicate success or failure of request


Sample HTTP CommunicationsHTTP commands and status codesImmediately follow the TCP headerHTTPS protocol Represents a secure implementation of HTTP that incorporates use of SSLTCP port 443 Assigned to HTTP protocol that uses TCP with Transport layer security over SSLUDP port 443 Assigned to HTTP protocol that uses UDP with Transport layer security over SSL


Understanding MSN MessengerMSN MessengerProprietary protocol specification Handles audio and video applications and can be embedded within other protocolsMSN Messenger ElementsPossesses one of the largest subscriber bases in the worldServes as a presence information provider


Obtaining and Using MSN MessengerMSN Messenger Readily available at no charge online at messenger.msn.comFour fundamental entities define the MSN Messenger protocol architectureThe principal (or client) applicationDispatch serverNotification serverSwitchboard server


MSN Messenger CommandsTo reach a broad audience of subscribersMSN Messenger protocol encodes character values outside the printable ASCII rangePrincipal and server commandsThree-letter abbreviations, followed by any number of specified parametersDuring initialization phaseCommands sent between principal and server, occur synchronously


Sample MSN Messenger CommunicationsAuthentication procedures for MSN Messenger Services begin with dispatcherDispatcherNegotiates an appropriate protocol version and format for each connectionProtocol revision 13Provides full support for offline instant messaging


Understanding VoIPVoice 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


VoIP ElementsFour primary entities involved in a H.323-basedVoIP communications networkThe terminals, or voice/video/data clientsA Multipoint Control Unit (MCU) for bridging videoconference connectionsThe Gateway (GW) device for translating among mutually independent protocol stacks The Gatekeeper


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 applicationsH.323 protocol suite Blanket specification for establishing real-time, streaming, qualitative multimedia communicationsH.245Defines control protocol for multimedia communication


VoIP Elements (continued)H.235Provides coverage of security and encryption for H.323 and H.245H.450Describes the supplementary services for H.323 networksH.239A recommendation for relaying communications across different network mediaQ.931Documents the protocol and protocol state machine for ISDN connections


Sample VoIP CommunicationsAdvantages of digitizing voice signals for transmission over TCP/IPBetter control can be exercised over the digital formatA digital signal is more forgiving of noise-bearing transmissions than an analog signalVoIP communications avoid the costs associated with pay-per-usage


Other Common TCP/IP-Based ServicesEcho (TCP and UDP)Quote of the Day (QOD)Character Generator (Chargen)WhoisTrivial File Transfer Protocol (TFTP)


Other Common TCP/IP-Based Services (continued)FingerRemote Procedure Call (RPC)Simple Network Management Protocol (SNMP)NetBIOS over TCP/IP


Decoding Application Layer ProtocolsDecoding TCP/IP Application layer traffic largely depends on two separate forms of analysisUnderstanding specific services request/reply messagesLearning how to recognize related headers and payload informationLearning to assemble multiple lower-layer packets To reconstitute Application layer messages when payloads exceed MTU for a single packet


SummaryFoundation upon which all TCP/IP Application layer services rest A request/reply message architectureCertain client/server Application layer services Involve server-to-server trafficFTP A file transfer service Telnet Provides a method to log on and access the command line on a remote computer using TCP/IP


Summary (continued)SMTP Provides store and forward services for e-mail messagesHTTP Provides the underpinnings for the World Wide WebMSN Messenger and Voice over IP (VoIP) communicationsProvide mechanisms to set up, manage, and tear down ongoing communications using TCP


Summary (continued)Other common TCP/IP Application layer services Echo, Chargen QOD, TFTP, FingerRemote Procedure Call NetBIOS over TCP/IPSNMPUnderstanding how to decode Application layer protocols Means learning how to identify and interpret request and reply messages
