Transport Layer PART II

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Transport LayerPART II
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Principle of Reliable Data Transfer
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Building a Reliable Data Transfer Protocol

• Finite-State Machine (FSM) Protocol
• Simple protocol
• Sender and receiver have just one state

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Building a Reliable Data Transfer Protocol

• Finite-State Machine (FSM) Protocol
• The sender and receiver have one state
• The dash arrow indicates an initial state
• An event causes a transition
• Transition is from one state back to itself
• No error is assumed

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Building a Reliable Data Transfer Protocol

• Automatic Repeat reQuest Protocol (ARQ)
• Error is assumed
• Error may occur during transmission, propagation,
  or in a buffer
• The bits may be corrupt
• Use message-dictation protocol
• Positive Acknowledgement and Negative
  Acknowledgement

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Building a Reliable Data Transfer Protocol

• Automatic Repeat reQuest Protocol (ARQ) is based
  on retransmission
• ARQ requires three additional protocol
  capabilities
• Error Detection receiver detect when bit errors
  have occurred
• Receiver feedback Use ACK (positive ACK) and
  NAK (negative acknowledgement)
• Retransmission A packet that is received in
  error will be transmitted by the sender

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Building a Reliable Data Transfer Protocol

• What if ACK and NAK are corrupted
• How to handle this problem?
• Use message-dictation protocol cause garble of
  ACK and NAK
• Resend the current packet

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Building a Reliable Data Transfer Protocol

• Resend packet may cause
• Packets duplication in a sender-to-receiver
  channel
• The receiver does not know whether the ACK or NAK
  it last sent was received correctly at the sender
• Solution add a new field to the data packet
  that is a sequence number
• Receiver checks the sequence number to determine
  the packet
• Assumption the channel does not lose any packet