ch5：OSI层次：传输层.ppt

Functions of OSI Layers(IV)
Software Institute, Nanjing University
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Layer 4: The Transport Layer
An Overview of Layer 4
TCP (Transmission Control Protocol)
UDP(User Datagram Protocol)
An application: NAT and PAT
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Layer 4 performs multiple functions to provide this “quality of service”:
segmenting upper-layer application data
establishing end-to-end operations
sending segments from one end host to another
ensuring data reliability
providing flow control
Layer 4: The Transport Layer
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Layer 4: The Transport Layer
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The TCP/IP Protocol Stack
IP is a Layer 3 protocol and TCP is a Layer 4 protocol. TCP is connection-oriented and ensures reliability but IP is connection-less with best effort attempts at delivery.
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Flow control and reliability of the transport layer can pared to a student who studies only one year of a foreign language, then visits the country that uses that language. Whenever the student tries to join a conversation, he or she has to ask everyone, frequently, to repeat his/her words (reliability) and to speak slowly (flow control).
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Two particularly important Layer 4 protocols:
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Layer 4: The Transport Layer
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TCP :
connection-oriented
reliable
divides outgoing messages into segments
reassembles messages at the destination station
re-sends anything not received
UDP:
connectionless
unreliable
provides no software checking for segment delivery (unreliable)
uses no acknowledgments
provides no flow control
Layer 4: The Transport Layer
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