Go-Back-N%20(GBN)

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Go-Back-N (GBN)

• Yan Gao
• Feb. 17, 2006

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Go-Back-N protocol

• Concept
• Introduce a window of size n
• Can inject n packets into net before hearing an
  ACK
• Sliding window
• Label each packet with a sequence number
• A window is a collection of adjacent sequence
  numbers
• The size of the collection is the senders window
  size

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Example
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Example
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Example
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Receive Window Maintenance

• Receiver keeps a similar window
• Receiver has a finite buffer
• Let window edge is first packet receiver wants to
  see
• Right window edge is last packet it can hold
• Packets lt left edge or gt right edge dropped
• Other packets are queued, allowing for fixing up
  out-of-order packets

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Example
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Go-Back-N

• Sender has to buffer all unacknowledged packets,
  because they may require retransmission.
• Receiver may be able to accept out-of-order
  packets, but only up to its buffer limits.
• The sender needs to set timers in order to know
  when to retransmit a packet that may have been
  lost