- Operational parameters:
- Sender and receiver use
2 bits sequence numbers
- Sequence numbers = 0, 1, 2, 3
The data frames are identified as follows:
Frame: #1 #2 #3 #4 #5 #6 #7 #8 #9 ..... Seq. No.: 0 1 2 3 0 1 2 3 0 .....
Notice that:
- Frame #1 and frame #5 are both identified by sequence number 0 !!!
- Suppose:
- Send window size = 2 (in previous example, it was 3)
- Receive window size = 2 (in previous example, it was 3)
- Sender and receiver use
2 bits sequence numbers
- Consider the following
scenario:
- Sender sends
2 frames (#1, #2)
(due to its window size = 2)
identified with sequence numbers
0 and
1 to the receiver
- All frames has been received (correctly)
- Receiver sends
ACK 0 and
ACK 1
- The fate of the ACK frames are unknown
Graphically:
Notice that:
- Receiver window =
[2,3]
Notice:
- The seq. no. 0 is not part of the receiver window !!!
-
Suppose the receiver received
a data frame
with
sequence number = 0:
Note:
- The receive window =
[2,3]:
The receive window has not wrapped around yet !!!
- Therefore:
- The sequence number 0 (to the receiver) identifies the old frame #1 !!!
- Sender sends
2 frames (#1, #2)
(due to its window size = 2)
identified with sequence numbers
0 and
1 to the receiver
- $64,000 question:
- Is the second frame
with sequence number 0:
- always (i.e., for sure) the frame #1 from the sender ??????
(Or can it be frame #5 under some circumstances ????)
- Is the second frame
with sequence number 0:
- Answer to the
$64,000 question:
- The second frame with sequence number 0 is always the frame #1 !!!
- Recall that:
- The receive window is
always ahead
of the send window
I.e.:
- The send window can never get ahead of the receive window !!!
- The receive window is
always ahead
of the send window
- The receive window is
[2,3]:
- Therefore:
- If the
sender
is
using
the sequence number 0 to
identify
the frame #5,
then
we have this situation:
This situation is not possible because:
- The send window is ahead of the receive window !!!
- If the
sender
is
using
the sequence number 0 to
identify
the frame #5,
then
we have this situation:
- Conclusion:
- The
only possible
frame
that
can be identified by
seq no 0
is
frame #1:
There is no ambiguity possible !!!!
- The
only possible
frame
that
can be identified by
seq no 0
is
frame #1: