Content

1. Go-Back-N
   1. Flow
2. Selective Repeat
   1. Flow
3. Window Size

Go-Back-N

• Sender

  • Sends up to N packets
  • Sets timer of oldest un-ACKed packet
  • Retransmit all un-ACKed packets on timeout

• Receiver

  • Discards out-of-order packets
  • Resend ACK of last received packets to inform sender

Flow

• Sender
  • rdt_send()
    • If window not exhausted
      • Transmit data
      • Increase seq #
      • Start timer if not previously done
    • If exhausted (nextseqnum == base+N)
      • Block & wait for window to open
  • rdt_rcv(ACK(n)): all packets before seq # = n have arrived (cumulative ACK)
    • Advance window until base + seq #
  • timeout()
    • Retransmit all packets after oldest unACKed seq #
• Receiver
  • rdt_rcv(data(n)): receive window of size 1
    • If error || not expectedseqnum
      • Discard
      • Re-ACK the highest-order seq #
    • Else
      • Increment expectedseqnum
      • Advance window by 1
      • Send ACK

Selective Repeat

• Sender
  • Sends up to N packets
  • Sets timer of every un-ACKed packet
  • Retransmit that un-ACKed packets on timeout
• Receiver
  • Buffers out-of-order packets
  • Send ACK for each received packet

Flow

• Sender
  • rdt_send()
  • rdt_rcv(ACK(n))
    • If in current window [base,nextseqnum-1]
      • Mark packet n as received
      • If n == base, advance window until next unACKed packet
  • timeout(n)
    • Resend packet n
    • Restart timer
• Receiver
  • rdt_rcv(data(n))
    • If in current window [base,base+N-1]
      • Send ACK(n)
      • If out-of-order
        • Buffer
      • If in-order
        • Deliver to upper layer
        • Advance window until next not-yet-received packet
    • If in previous window [base-N,base-1]
      • Resend ACK(n)
    • Else
      • Ignore

Window Size

• Go-Back-N

Window size must be less than 2^k.

• Selective Repeat

Window size must be at most 2^(k-1) (so that can differentiate between current & previous windows).