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

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

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

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

Two implementations of the Sliding Window Protocol

• Go-Back-n • Selective Repeat

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

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

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

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

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

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

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

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?