Guide to TCP/IP, Third Edition

1
Guide to TCP/IP, Third Edition

• Chapter 6
• Basic TCP/IP Services

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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

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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)

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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

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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

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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

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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 users dynamic
  port to the servers FTP port
• Initiates FTP commands
• Monitors the data transfer process

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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

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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

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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

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Active and Passive FTP

• In active mode
• Client connects from random TCP port with an
  address greater than 1023 to FTP servers 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

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Understanding Telnet

• Telnet
• Offers a bidirectional byte-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

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Telnet Elements (continued)

• NVT
• Considered a printer-keyboard device that
  receives bytes from the other host and prints the
  information
• The DO, DONT, WILL, WONT Structure
• 253 (0xFD)DO
• 254 (0xFE)DONT
• 251 (0xFB)WILL
• 252 (0xFC)WONT

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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

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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

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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

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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

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Understanding HTTP

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

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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)

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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

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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

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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

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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

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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

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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

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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

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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) device for translating among
  mutually independent protocol stacks
• The Gatekeeper

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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.323 protocol suite
• Blanket specification for establishing real-time,
  streaming, qualitative multimedia communications
• H.245
• Defines control protocol for multimedia
  communication

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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

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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

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Other Common TCP/IP-Based Services

• Echo (TCP and UDP)
• Quote of the Day (QOD)
• Character Generator (Chargen)
• Whois
• Trivial File Transfer Protocol (TFTP)

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Other Common TCP/IP-Based Services (continued)

• Finger
• Remote Procedure Call (RPC)
• Simple Network Management Protocol (SNMP)
• NetBIOS over TCP/IP

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Decoding Application Layer Protocols

• Decoding TCP/IP Application layer traffic largely
  depends on two separate forms of analysis
• Understanding specific services 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

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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

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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

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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