1. Guide to TCP/IP Fourth Edition
Chapter 9:
TCP/IP Transport Layer Protocols

2. Objectives
Explain the key features and functions of the User Datagram Protocol and the Transmission Control Protocol
Explain, in detail, the header fields and functions of the UDP packet, as well as port numbers, processes, and how UDP behaves when used as a transport protocol by IPv6
Explain in detail, the mechanisms that drive segmentation, reassembly, and retransmission for TCP as well as how TCP behaves when used as a transport protocol by IPv6
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3. Objectives (cont’d.)
Describe how UDP and TCP pseudo-headers are organized with the IPv6 header and extension
headers
Explain the differences between connectionless and connection-oriented transport mechanisms
Choose between using User Datagram Protocol and Transmission Control Protocol
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4. Understanding UDP and TCP
UDP and TCP actually function as peers
IP operates at the Network layer of the OSI model
Responsible for transferring variable length data sequences from a source host to a destination host
TCP guarantees acknowledged delivery
At the cost of both time and bandwidth
UDP is used for applications that do not need verified delivery
Gain in speed and a reduced use of network bandwidth
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5. UDP with IPv4 and IPv6 Connectionless protocols UDP
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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6. UDP with IPv4 and IPv6 (cont’d.)
Connectionless protocols handle the following kinds of tasks
Message checksum
Higher-layer protocol identification
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7. TCP with IPv4 and IPv6 TCP (connection-oriented protocol)
Used by applications that rely on data reaching its destination
TCP offers connection-oriented services with:
Sequencing, error recovery, and a 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
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8. TCP with IPv4 and IPv6 (cont’d.)
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9. User Datagram Protocol
UDP limitations
No reliability mechanisms
No delivery guarantees
No connection handling
Identifies the Application layer protocol
Checksum for entire message carried in UDP header
No buffering services
No segmentation
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10. UDP Header Fields and Functions
UDP header’s 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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11. UDP Header Fields and Functions (cont’d.)
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12. UDP Header Fields and Functions (cont’d.)
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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13. UDP Header Fields and Functions (cont’d.)
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14. UDP Header Fields and Functions (cont’d.)
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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15. UDP Port Numbers and Processes
UDP and TCP
Use port numbers to define the source and destination processes or applications
By default
Windows Vista, Windows 7, and Windows Server 2008 support a port range of to 65535
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16. UDP Port Numbers and Processes (cont’d.)
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18. UDP and IPv6
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19. Transmission Control Protocol
Responsible for providing ordered delivery of a stream of bytes
From an application on one network node to an application on another network node
Communications between two computers that are being established through TCP
Use a three-way handshake procedure
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20. TCP and IPv4 Primary functions and features of TCP communications:
Start-up connection process (TCP handshake)
Keep-alive process
Connection termination
Sequence and acknowledgment process
Error-detection and error-recovery processes
Congestion control
Sliding window
Header fields and functions
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21. 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 initiator’s sequence number
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26. TCP Startup Connection Process (cont’d.)
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27. 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 >>>>>
<<<<< ACK SYN
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28. TCP Keep-Alive Process
Can maintain connection when there is no data sent across the wire
TCP keep-alives
Disabled by default on Windows XP, Windows Vista, Windows 7, Windows Server 2003, and Windows Server 2008
KeepAliveTime setting
Defines how long to wait before sending the first TCP keep-alive packet
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29. TCP Keep-Alive Process (cont’d.)
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30. 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
Returns ACK response
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33. 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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34. TCP Sequence and Acknowledgment Process (cont’d.)
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36. 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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37. TCP Congestion Control
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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38. TCP Congestion Control (cont’d.)
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39. TCP Congestion Control (cont’d.)
TCP has four defined congestion control mechanisms
Slow Start
Congestion Avoidance
Fast Retransmit
Fast Recovery
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40. TCP Congestion Control (cont’d.)
Table 9-11 GlobalMaxTcpWindowSize Registry setting
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41. TCP Congestion Control (cont’d.)
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42. 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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43. 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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44. TCP Header Fields and Functions (cont’d.)
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45. Table 9-13 TCP flag settings
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46. TCP Header Fields and Functions (cont’d.)
Window Size Field
TCP Checksum Field
Urgent Pointer Field
TCP Options Field(s)
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47. TCP Header Fields and Functions (cont’d.)
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48. TCP Header Fields and Functions (cont’d.)
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49. TCP and IPv6
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50. UDP, TCP, and IPv6 Extension Headers
IPv6 headers put all optional data in extension headers
Extension header
Contains optional Internet-layer data that is stored in separate headers
IPv6 header and each extension header, if present, contain a Next Header field
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51. UDP, TCP, and IPv6 Extension Headers (cont’d.)
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53. 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 is no longer as important as it once was
Long-haul and local area networks have significantly increased speed, capacity, and reliability
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54. 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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55. Summary (cont'd.) Transmission Control Protocol TCP header
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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56. Summary (cont'd.) 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
Although there are no updated versions for TCP and UDP that correspond to IPv6
The functions of these transport protocols are treated differently when used by IPv4 or IPv6
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