- Recall:
Receive/Receiver window
- Receive Window =
the list of
sequence numbers
that the receiver will
consider (= accept) as
new (data) frames
Example:
- Receive Window =
the list of
sequence numbers
that the receiver will
consider (= accept) as
new (data) frames
- Recall:
Receive/Receiver buffer
- Receiver buffer = buffer in the receiver to store undeliverable data frames
Example:
- Receiver window =
[5,6,7,8]
Receiver has 4 (frame) buffers
Then:
Receiver will buffer frames 5,6,7,8
(Typically: receiver window size = receive buffer size)
- Update algorithm for the
receiver window:
Let Receiver Window = [ A .. B ] Receiver receives a data frame; Let X = send sequence number in received data frame;
if ( A ≤ X ≤ B ) { Save the received data frame in the receiver buffer; Send ACK X; /* -------------------------------------- Try to deliver data frames in sequence -------------------------------------- */ if ( X == A ) { /* ----------------------------------------------------- First new frame is received, we can deliver some data frames ----------------------------------------------------- */ for ( frame i = A; i ≤ B; i++ ) do { if ( frame i was received (previously) ) { Deliver frame i } else { break; } } Receiver Window = [i .. i+(B-A)+1] // Advance receiver window !!! } } else { /* ========================================== Frame X has bee received previusly !!! ========================================== */ Discard frame !!! Send ACK X; // The ACK for frame X may have been lost !!! // That may be why the sender re-transmits X !!! }
- Example 1:
- Suppose:
- Sender window = [1,2,3,4]
- Receiver window = [1,2,3,4]
Events:
- The sender transmits frames 1,2,3,4
- But: Frame 1 is lost
Graphically:
Explanation: how the receiver process the received frames:
Receiver window = [1 .. 4] Frame 1: lost Frame 2: 1 ≤ 2 ≤ 4 ? Yes ===> buffer frame 2 Send ACK 2 2 == 1 ? No ===> Cannot deliver frame Frame 3: 1 ≤ 3 ≤ 4 ? Yes ===> buffer frame 3 Send ACK 3 3 == 1 ? No ===> Cannot deliver frame Same result for frame 4Note:
- The receiver window does
not slide forward !!!
(because no frame has been delivered !!!).
- Suppose:
- Example 2: example continues
- Since the receiver
did not
transmit any ACK frames
back:
- the sender will time out
Suppose after the time out, the sender retransmits frame 1 and we received correctly
Graphically:
How the receiver process the received frame (#1):
Receiver window = [1 .. 4] Frame 1: 1 ≤ 1 ≤ 4 ? Yes ===> buffer frame 2 Send ACK 2 1 == 1 ? Yes ===> deliver frames for ( i = 1; i ≤ 4 ; i++ ) do: { frame i = 1 received: Yes ===> deliver frame 1 frame i = 2 received: Yes ===> deliver frame 2 frame i = 3 received: Yes ===> deliver frame 3 frame i = 4 received: Yes ===> deliver frame 4 } Receiver window = [5.6.7.8]
- Note:
- When the sender
receives the ACK 1 frame,
the sender window will
move as
follows:
The sender window and receiver window are "lined up" (= starts with the same seq. no.)
- When the sender
receives the ACK 1 frame,
the sender window will
move as
follows:
- Since the receiver
did not
transmit any ACK frames
back:
- Example Program:
(Demo above code)
- Prog file: /home/cs455000/demo/SelAck/run
How to run the program:
- Send 2 new frames
- Pause
- Click on frame 0 and kill it
- Resume
Observe that:
- The Receiver window will slide only when the retransmission of frame 1 was received