1. March 21, 2001
Recall we have talked about the Sliding Window Protocol for Flow Control
Review this protocol
A window of size I can contain at most I frames that are to be transmitted
The window expands in size as new frames are added to be transmitted, but the window will not grow to size larger than I
See figure of sliding window on handout

2. Sliding Window Protocol
All frames before the window must have been acknowledged
Frames inside the window are either unsent or have been sent and have not been acknowledged (outstanding frames)
Frames beyond the widow are not yet sent

3. Sliding Window Protocol – Cont
If window size is equal to 1, then we have the Stop and Wait protocol
If window size is greater than the number of frames, then we have the unrestricted protocol

4. Typical Frame Format Source address – address of station sending frame
Destination address – address where frame is to be sent
Frame number – The sequence number starting with 0 and numbered sequentially
Ack – An integer value that corresponds to the frame being acknowledged. This avoids sending a separate acknowledgement. The Ack is piggybacked on the data frame
Type of frame
Data
CRC

5. Two implementations of the Sliding Window Protocol
Go-Back-n
Selective Repeat

6. Go-Back-n Protocol
Frame numbers are numbered between 0 and (2 to the power k) –1, where k = number of bits in frame number field
Suppose k = 6
Frames are numbered 0, 1, …,63 and then repeat these numbers
Receiver always expects to receive frames in order (modulo )
If one frame is received out of order, it ignores the frame and sends a NAK for the frame it expected
It then waits for the correct frame to arrive

7. Go-Back-n Protocol – Cont
If a frame arrives and it is damaged, the receiving station ignores it and sends a NAK for it
The receiving station does not acknowledge each received station explicitely
If a sending station receives an ACK for frame j and later receives one for frame k (k>j), it assumes all frames between j and k have been received correctly
A station uses the piggyback approach whenever possible to acknowledge the most recently arrived frame

8. Go-Back-n Protocol – Cont
The sending station buffers all the frames in the window in case it has to resend one or more
Frames are removed from the buffer as they are acknowledged
If a station does not receive an ACK over a period of time, it assumes something went wrong. It uses a frame timer, one for each frame, which is set when the data goes out
The frame timer counts down and stops when an ACK is received for that frame
If the frame timer expires then every frame in the window is resent (Explain this!)
The name of the protocol comes from the fact that all n frames in the window are sent again

9. Go-Back-n Protocol – Cont
What is the maximum window size?
The window size can’t be larger than 2 to the power k, where k = # of bits of frame number field
Why? Otherwise you could have two frames with the same number outstanding and the sender would have no way of knowing which frame is being acknowledged
If the window size = 2 to the power k, the protocol can still fail
See handout that shows failure in this case

10. Go-Back-n Protocol – Cont
Works well especially over reliable channels
When frames are rarely lost, the assumption that they arrive in the order they were sent is usually valid
In the unusual case that a frame is lost, little time will be spent in resending all the outstanding frames
As reliability decreases, the overhead of this approach increases

11. Selective Repeat Protocol
Another implementation of the sliding window protocol
Similar to Go-Back-n
Frames are numbered using a k-bit field
The sender has a window defining the maximum number of outstanding frames
Piggyback ACKs are used whenever possible
If a frame is acknowledged, the sending station assumes all prior ones have been received as well
NAKs are sent for damaged frames and frames sent out of order
Timer is used to resend frames that have not been acknowledged for a while

12. Differences in Selective Repeat Protocol
Two sliding windows are defined, one for the sender and one for the receiver
Thus each station has both a sending and a receiving window
The sending window is the same as the Go-Back-n protocol window- it defines which frames may be outstanding
The receiving window defines what frames can be received
Frames in the receiving window are numbered consecutively
The receiving station is not required to receive the frames in order
A frame arriving out of order can be received as long as its number is in the receiving window
Out of order frames are buffered until their predecessors arrive

13. Homework Questions Which type of Sliding Window Protocol does TCP use?
Examine the code of the Go-Back-n protocol to answer the following questions:
How many events are in the main while loop?
What are these events?
Where in the code does it indicate that the receiver B will send a second ACK should the first one be lost?