The document discusses sliding window protocols for computer networks. It describes stop and wait protocol and how sliding window protocol improves efficiency over stop and wait by allowing data pipelining. It discusses concepts like sender window size, receiver window size, sequence numbers, timing diagrams for go-back-N, and examples of how sliding window protocol works.

1. Computer Networks Prof. Hema A Murthy
Sliding Window Protocol
• Sliding window protocol:
• Stop & Wait: inefficient if a is large.
• Data: - stream of bulk data
• - data can be pipelined
• - transmit window of date
• - donot worry about getting ack
immediately
Indian Institute of Technology Madras

2. Computer Networks Prof. Hema A Murthy
Sliding Window Protocol
• What should be the size of pipeline?
• How do we handle errors:
– Sender and receiver maintain – buffer space
– Receiver window = 1,
– Sender window = n
Indian Institute of Technology Madras

3. Computer Networks Prof. Hema A Murthy
Timing Diagram: Go back-N
Timeout
S 1 2 3 4 5 3 4 5
E D E
R
Indian Institute of Technology Madras

4. Computer Networks Prof. Hema A Murthy
Go-Back N
• Discard if correct frame not received
• Use same circuit for both directions
– Intermix data frames from both S R with ack frames
from R S
• Use kind field in header:
– decide whether data or ack
– piggy back ack on outgoing frame for R S
– Ack field in frame
– If frame not available for piggybacking Timeout
Indian Institute of Technology Madras

5. Computer Networks Prof. Hema A Murthy
Sliding Window Protocol
• Outbound frame sequence number
• Range - 0 – 2n-1
• n bit field
• Stop & Wait is Sliding window with n = 1
• Sender – maintain sequence number of frames it
is permitted to send
– sending window
• Receiver – maintain sequence number of frames
it is expected to accept
– Receiver window
Indian Institute of Technology Madras

6. Computer Networks Prof. Hema A Murthy
Sliding Window Protocol –
An example (Tanenbaum)
Example: SWP: sequence number: Sender 0 - 7
seqno – 3 bit
Sender
7 0 7 0
6 1 6 1
2 2
5 5
4 3 4 3
Indian Institute of Technology Madras

7. Computer Networks Prof. Hema A Murthy
Receiver
7 0 7 0
6 1 6 1
2 2
5 5
Sender 4 3 4 3
7 0 7 0
6 1 6 1
2 2
5 5
4 3 4 3
Receiver
7 0 7 0
6 1 6 1
2 2
5 5
4
Indian Institute of Technology Madras 3 4 3

8. Computer Networks Prof. Hema A Murthy
SWP -- Example
• Larger Sender Window Size
7 0
6 1
2
5
4 3
Indian Institute of Technology Madras

9. Computer Networks Prof. Hema A Murthy
Different Window Sizes:
Receiver, Sender (Peterson et al.)
If Sender Window is n
How large can the Receiver Window be?
LAR LFS
Sender window size
Last Acked Frame Last Frame Sent
Number of unacked frames
LFS – LAR ≤ SWS
Indian Institute of Technology Madras

10. Computer Networks Prof. Hema A Murthy
Receive Window Size (RWS)
• number of out order frames receiver is
willing accept
– LAF – Last acceptable frame (sequence
number)
≤
– LFR – Last frame received
≤ – LAF – LFR RWS
– When SeqNumber frame arrives:
– If SeqNumber LFR or Sequence Number ≤
> LAF – discard
– If LFR < Sequence Number LAF – accept
frame.
Indian Institute of Technology Madras

11. Computer Networks Prof. Hema A Murthy
Example: Larger RWS
• Example: LFS = 5, RWS = 4, LAF = 9
• If frame 7 & 8 arrive
– buffered
– but ack not sent since 6 not arrived.
– 7 & 8 out of order.
• If frame 6 delayed –
– Retransmitted, received later
• - Notice no NAK for 6.
• primarily timeout on 6 – retransmit 6.
Indian Institute of Technology Madras

12. Computer Networks Prof. Hema A Murthy
SWP – Go back-N – a variation
• largest Sequence Number not yet acked.
• receiver only acks SequenceNumberAck
even if higher numbered frames are
received.
• set LFR = SequenceNumberToAck
• LAF = LFR + RWS
Indian Institute of Technology Madras

13. Computer Networks Prof. Hema A Murthy
Selective Repeat Protocol
• Variation SWP:
– selective ack for frame
– sender knows what to send
– problem – complicated
– can RWS > SWS ?
Indian Institute of Technology Madras