THE TRANSPORT LAYER - LAYER 4



The four main functions of the Transport layer are:

  • Segment and assemble upper-layer applications
  • Transport segments from one host to another host
  • Establish and manage end-to-end operations
  • Error recovery

    • In carrying out its duties, the Transport layer performs a range of support activities, including:

  • Maintaining data integrity through flow control techniques
  • Multiplexing the data from upper layer applications
  • Setting up and tearing down any virtual circuits established to transport the data over the network
  • Hiding any network-dependent information from the upper layers (which will only confuse them)
  • Breaking down Session layer (layer 5) datagrams into segments
  • Monitoring the error-free delivery of the data to its destination
  • Providing for general connection management and data transfer services
  • Providing for the reliable (but, not guaranteed) delivery of data
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    Transporting Protocols

  • TCP (Transmission Control Protocol): The protocol primarily concerned with the reliable delivery of packets that requires an acknowledgement of a packet's arrival at its destination.
  • UDP (User Datagram Protocol): The TCP/IP best-effort protocol that isn't concerned with the reliable delivery of packets and doesn't bother with overhead such as acknowledgments.
  • SPX (Sequence Package Exchange): The Novell protocol most akin to TCP. It guarantees data delivery.
  • NWLink (NetWare Link): Microsoft's version of Novell's IPX/SPX.
  • ATP/NBP (AppleTalk Transaction Protocol/Name Binding Protocol): AppleTalk's data transport protocols.
  • NetBIOS/NetBEUI (Network Basic Input/Output System/NetBIOS extended User Interface): Microsoft's network protocols that work together to manage communications and provide data transport services.
    •

    TCP, UDP and SPX are Transport layer protocols. Network layer protocols include IP, ICMP, and IPX.

    SPX is connection-oriented and its packets are tracked through the use of a sequence number associated with each packet. A positive acknowledgment must be received from the destination device for each packet before another packet is sent. A print server is an example of an application that implements SPX.

    A protocol is considered connection-oriented if it meets one of two criteria:

  • Data is transmitted over a negotiated, established path, a virtual circuit, between two nodes.
  • The protocol includes a process for error-recovery.
    •

    The following protocols and link types are connection-oriented:

  • Frame Relay
  • TCP
  • SPX
  • X.25
    •

    Connectionless protocols:

  • IP
  • IPX
  • UDP

    • Note: IP and IPX are Network layer protocols.

    Layer 4 protocols that implement error recovery have the following characteristics:

  • They are connection-oriented, which means they establish a connection prior to the transmission of data.
  • Each PDU has header information used by the receiver to acknowledge the receipt of a packet and a system to check for errors in transmission.
  • The sender requires notification of packets that have been successfully received.
    •

    You use three primary methods for error-checking. They are parity bit, check-sum, and CRC.

    The three basic forms of flow control are:

  • Buffering
  • Congestion avoidance
  • Windowing
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