- Send/Sender window:
- Send Window =
the list of
sequence numbers
that the sender can
use to
label (= identify)
an
outstanding frames
Example:
- Send Window =
the list of
sequence numbers
that the sender can
use to
label (= identify)
an
outstanding frames
- Note:
- # outstanding frame of the sender ≤ send window size !!!
- Reminder:
- ACK n acknowledges only the data frame n
-
Update algorithm for
the sender window:
Let Sender Window = [ A .. B ] Sender receives an ACK X frame (I.e.: X = recv seq no in received ACK frame)
/* ============================================== X = the number in the ACK X frame ============================================== */ if ( X == A ) { /* ============================================== "Slide" the window forward ============================================== */ i = A; while ( i ≤ B ) do { if ( frame i has been acknowledged ) { Release buffer for frame i i++; // Check next frame in send window } else { break; // Stops at the first unacknowledge frame } } Sender Window = [i, i+1, .., i+(B-A)+1)] // Update (slide) sender window to start at [i .....] }
- Example 1:
- Suppose the current
sender window is:
Sender window = 1, 2, 3, 4
- The sender transmits
the next data frame:
The next data frame will use the first available number in the sender window is its send seq. no.!!!
- When the sender
receives the ACK frame,
the send window will
be updated to
[2,3,4,5]:
- Suppose the current
sender window is:
- Example Program:
(Demo above code)
- Prog file: (cd /home/cs455001/demo/SelAck; run)
How to run the program:
- Send 1 new frames
- Example 2:
- Suppose:
- Sender window = [1,2,3,4]
Events:
- The sender transmits frames 1,2,3,4
- But: Frame 1 is
lost
- The receiver transmits back ACK frames 2,3,4
How the sender process the received ACK frames:
Sender window = [1 .. 3] Received ACK 2: Test 2 == 1 ? No - Done (do not update sender window) Received ACK 3: Test 3 == 1 ? No - Done (do not update sender window) Received ACK 4: Test 4 == 1 ? No - Done (do not update sender window)Graphically:
Note:
- The sender window does
not slide forward !!!
(because no send buffer has been released !!!).
- Suppose:
- Example 3: previous example continues
- The sender
times out and
retransmits
frame 1
and
frame 1
was received correctly
-
Current sender/receiver windows:
- Sender window = [1,2,3,4]
Events:
- The sender retransmits frame 1
- Frame 1 is
received
- The receiver transmits ACK frame 1
How the sender process the received ACK frame (#1):
Send window = [1 .. 3] ACK 1: 1 == 1 ? Yes for each frame Y = 1, 2, 3, 4 do: Y = 1: Y is ACKed ? Yes ===> Release buffer for frame 1 Sender Window = [2 .. 5] Y = 2: Y is ACKed ? Yes ===> Release buffer for frame 2 Sender Window = [3 .. 6] Y = 3: Y is ACKed ? Yes ===> Release buffer for frame 3 Sender Window = [4 .. 7] Y = 4: Y is ACKed ? Yes ===> Release buffer for frame 4 Sender Window = [5 .. 8]Graphically:
Result:
- The sender will release
all 4 buffered frames
(1,2,3,4)
- Now the sender window will slide forward by 4 positions
The sender will now buffer the data frames 5,6,7,8
- The sender
times out and
retransmits
frame 1
and
frame 1
was received correctly
- 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 Sender window will slide only when the retransmission was received and ACKed
- The movement of the
sender window is
called:
- "Sliding" window:
0 1 2 3 4 5 6 7 8 9 | | window 1 +----------+ | | window 2 +----------+ -------> sliding movement
- "Sliding" window:
- The reliability protocol
that uses
sender/receiver windows are called:
- Sliding window schemes