CS 3413 Data Communications Data Link Layer Protocol Review: Fall 2004

Assume 3-bit sequence numbers
Maximum sender's window size is 7
Receiver's window size is 1

Sender:

next_frame_to_send	upper edge of sender's window + 1
ack_expected	1st frame in sender's window
nbuffered	size of sender's window

Receiver:

frame_expected

next frame expected on inbound stream

Suppose:

• next_frame_to_send = 6
• ack_expected = 2
• nbuffered = 4
• frame_expected = 5

Receiver's window (always empty, green=empty)

5

What happens if:
receive ack with sequence number: 0, 1, 2, 3, 4, 5, 6, 7
Note: unlike in the book, ack is actual frame acknowledged

What happens if receive frame with sequence number: 0, 1, 2, 3, 4, 5, 6, 7

Protocol 6 review:

Assume 3-bit sequence numbers
Maximum sender's window size is 4
Receiver's window size is 4

Sender:

next_frame_to_send	upper edge of sender's window + 1
ack_expected	1st frame in sender's window
nbuffered	size of sender's window

Receiver:

frame_expected	next frame expected on inbound stream
too_far	upper edge of receiver's window + 1
arrived	bit flags for each receiver buffer

Suppose:

• next_frame_to_send is 6
• ack_expected = 2
• nbuffered = 4
• frame_expected = 5
• too_far = 1
• receiver buffered: 6, 7

Receiver's window: (green = empty)

5

6

7

0

What happens if:
receive ack with sequence number: 0, 1, 2, 3, 4, 5, 6, 7
receive frame with sequence number: 0, 1, 2, 3, 4, 5, 6, 7