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The Transport Layer
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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A Review
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  • Application Layer:
    • Involves user requests
  • Transport Layer:
    • Responsible for between processes/applications on different hosts
  • Network/Internetwork Layer:
    • Responsible for routing and delivering frames/datagrams
The Abstraction
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Processes/applications use the services provided by the transport layer without worrying about the details of the lower levels.

Implementation of Transport Layer Protocols
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  • Where is the code?
    • It resides on the host computers, not on routers
  • The Sender:
    • Process/application sends messages to the transport layer
    • Transport-layer software creates segments (each with a header)
    • Transport-layer gives the segments to the network-layer software which transmits them
  • The Receiver:
    • The network-layer software receives the datagrams
    • The network-layer software gives the segments contained in the datagrams to the transport-layer software
    • The transport-layer software gives the segments to the appropriate process/application
An Analogy
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  • The Situation:
    • You want to send a letter to a friend at another university
    • The address includes a postal code that is specific to the University and your friend's box number
  • The Network Layer:
    • The U.S. postal service delivers the letter from your university to your friend's university (through a variety of intermediate points)
  • The Transport Layer:
    • Your university's mail system gave the letter to the U.S. postal service
    • Your friend's university's mail system got the letter from the U.S. postal service and delivered it to your friend
There's Always More to Learn
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