Guide to TCP/IP, Third Edition

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Guide to TCP/IP, Third Edition

• Chapter 5
• Transport Layer TCP/IP Protocols

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Objectives

• Understand the key features and functions of the
  User Datagram Protocol
• Explain the mechanisms that drive segmentation,
  reassembly, and retransmission for the
  Transmission Control Protocol
• Choose between using User Datagram Protocol and
  Transmission Control Protocol

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UDP A Connectionless Transport Layer Protocol

• Connectionless protocols
• Provide the simplest kind of transport services
• UDP
• Used by applications that contain their own
  connection oriented timeout values and retry
  counters
• Runs up to 40 percent faster than TCP

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UDP A Connectionless Transport Layer Protocol
(continued)

• Connectionless protocols handle the following
  kinds of tasks
• Message checksum
• Higher-layer protocol identification

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Overview of UDP

• UDP limitations
• No reliability mechanisms
• No delivery guarantees
• No connection handling
• Identifies Application layer protocol conveyed
• Checksum for entire message carried in UDP header
• No buffering services
• No segmentation

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UDP Header Fields and Functions

• UDP headers main function
• To define the process or application that is
  using the IP and UDP Network and Transport layers
• UDP header fields
• Source Port Number field
• Destination Port Number field
• Length field
• Checksum field

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UDP Header Fields and Functions (continued)

• Source Port Number field
• Defines the application or process that sends the
  packet using the UDP header
• Well-known port numbers (0 Through 1023)
• Assigned to core services that systems offer
• Registered port numbers (1024 Through 49151)
• Assigned to industry applications and processes
• Dynamic ports
• Used as temporary ports for specific
  communications while they are underway

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UDP Header Fields and Functions (continued)

• Destination Port Number Field
• Defines destination application or process that
  uses the IP and UDP headers
• Length field
• Defines the length of the packet from the UDP
  header to the end of valid data
• Checksum field is optional

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UDP Port Numbers and Processes

• UDP and TCP
• Use port numbers to define the source and
  destination processes or applications
• By default
• Windows 2000 and Windows XP support up to 5,000
  ports

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TCP A Connection-Oriented Protocol

• Functions of connection-oriented protocols
• Create a logical connection directly between two
  peers on an internetwork
• Track the transfer of data and ensure it arrives
  successfully
• Use sequence number tracking
• Have a timeout mechanism
• Have a retry mechanism

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Overview of TCP

• TCP offers connection-oriented services with
• Sequencing, error recovery
• Sliding window mechanism
• TCP hosts
• Create a virtual connection with each other using
  a handshake process
• TCP
• Transfers data as a continuous stream of bytes
• Maximum TCP segment size is 65,495 bytes

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TCP Startup Connection Process

• Begins with handshake between two hosts
• One host initiates the handshake to another host
  to
• Ensure the destination host is available
• Ensure the destination host is listening on the
  destination port number
• Inform destination host of initiators sequence
  number

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TCP Half-Open Connections

• Occur when the handshake process does not end
  successfully with a final ACK
• Half-open connection communication sequence
  occurs in the following order
• SYN gtgtgtgtgt
• ltltltltlt ACK SYN
• ltltltltlt ACK SYN
• ltltltltlt ACK SYN

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TCP Keep-Alive Process

• Can maintain connection when there is no data
  sent across the wire
• TCP keep-alives
• Disabled by default on Windows 2000, Windows
  Server 2003, and Windows XP
• KeepAliveTime setting
• Defines how long to wait before sending the first
  TCP keep-alive packet

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TCP Connection Termination

• Requires four packets
• Host 1
• Sends a TCP packet with the FIN and ACK flags set
• Host 2
• Sends an ACK in response
• Then sends a TCP packet with FIN and ACK flags
  set
• Host 1
• Returns ACK response

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TCP Sequence and Acknowledgment Process

• Guarantees that packets are ordered properly and
  protects against missing segments
• During handshake process
• Each side of connection selects its own starting
  sequence number
• Each side increments its sequence number value by
  the amount of data included in the outbound packet

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TCP Error-Detection and Error-Recovery Process

• Retransmission timer
• First error-detection and error-recovery
  mechanism
• Retransmission timeout (RTO)
• Value specified by timer
• Retransmission operation increments
• 1st retransmit RTO seconds
• 2nd retransmit 2 x RTO seconds
• 3rd retransmit 4 x RTO seconds
• 4th retransmit 8 x RTO seconds
• 5th retransmit 16 x RTO seconds

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TCP Congestion Control

• Congestion
• The overloading of the network or a receiver
• Overloading of the network
• Occurs when there is too much data on the network
  medium
• Overloading a receiver
• Occurs when the number of data bytes is greater
  than the advertised window
• Current window
• Always the lesser of what the network and
  receiver can handle

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TCP Congestion Control (continued)

• TCP has four defined congestion control
  mechanisms
• Slow Start
• Congestion Avoidance
• Fast Retransmit
• Fast Recovery

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TCP Sliding Window

• Used to determine the amount of unacknowledged
  data that can go out on the wire from any sender
• Nagle algorithm
• When small data segments are being sent, but not
  acknowledged, no other small segments can be sent
• Silly Window Syndrome (SWS)
• Caused when enough data is sent to a TCP host to
  fill its receiver buffer
• Puts receiver in a zero-window state

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TCP Header Fields and Functions

• Source Port Number Field
• Destination Port Number Field
• Sequence Number Field
• Acknowledgment Number Field
• Header Length Field

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TCP Header Fields and Functions (continued)

• Window Size Field
• TCP Checksum Field
• Urgent Pointer Field
• TCP Options Field(s)

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Choosing Between TCP and UDP

• Because TCP is robust and reliable
• It carries a lot of baggage, including
• Additional header fields
• Explicit meta-messages in the form of TCP
  messages
• For some lightweight services, such as Microsoft
  Messenger Service
• TCP is overkill and UDP is used instead
• TCP
• No longer as important as it once was because
• Long-haul and local area networks have
  significantly increased speed, capacity, and
  reliability

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Summary

• Transport layer protocols come in two types
• Connectionless and connection-oriented
• User Datagram Protocol
• The connectionless protocol associated with
  TCP/IP protocol suite
• UDP header is short and simple, consisting of
• A protocol identifier in the IP header
• An optional checksum value
• Source and destination port addresses

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Summary (continued)

• Transmission Control Protocol
• Heavyweight, connection-oriented protocol that
  helps name the TCP/IP protocol suite
• TCP header
• Longer and more complex,
• Includes a variety of flags, values, and message
  types

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Summary (continued)

• Appropriate (and historical) uses for UDP
• Concentrate on Application layer services that
  manage their own reliability and connections
• Appropriate (and historical) uses for TCP
• Concentrate on providing reliable delivery of
  user services