Daily Learning: Computer Networks – Sliding Window Protocol

Go Back N

If there is a packet lost in the receiver side then it is going to discard the subsequent packets and re transmit the the entire window means it is going back N values and re transmit the the packets

  • Sender window size is go back N is N
  • Receiver Window size is always 1.
  • Acknowledgement –
    • Cumulative – One acknowledgement is used for many packets.
      • Adv – Traffic is low
      • Dis Adv – Reliability low
    • Independent – One acknowledgement is used for one packet.
      • Adv – Reliability is high
      • Dis Adv – Traffic is high

Relationship Between Window Sizes and Sequence Numbers

Minimum sequence numbers required in GBN = N + 1

Bits Required in GBN = log2 (N + 1)

If the sequence number is N then Sender window size is N – 1. Receiver window size is 1.

If the bit is K then sequence number 2^k, Sender Window size is 2^k – 1. Receiver window size is 1.


  • Maximum window size = 1 + 2 * a [a=Tp/Tt]
  • Minimum sequence numbers required = 1 + 2 * a
  • Number of Bits in Sequence Number Field = log2(1+2a)

Selective Repeat

Selective Repeat

This protocol(SRP) is mostly identical to GBN protocol, except that buffers are used and the receiver, and the sender, each maintains a window of size. Here we only re transmit the lost packet not the entire window again.

  • Sender window size is greater than 1 (Ws > 1)
  • Receiver window size = sender window size (Wr == Ws)
  • Acknowledgements are independent. If bits in the packets are corrupted then SR will send a negative acknowledgement.

Note – If N is the size of sequence number field in the header in bits, then we can have 2N sequence numbers.

Window Size = min(1+2*a, 2^N)

Stop & WaitGBNSR
Segment Number1+1N+1N+N

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