- Fact:
- The
discussion on
the correctness of the
sliding window algorithm
does not depend on
the ACK scheme used
- So,
for simplicity, I will
use
selective acknowledge
I.e.:
ACK n ===> ACK data frame n only
- The
discussion on
the correctness of the
sliding window algorithm
does not depend on
the ACK scheme used
- Fact:
- (Send/Receive) Sequence numbers
are found
in the
- Header portion
of data/ACK frames
- (Send/Receive) Sequence numbers
are found
in the
- Example: HDLC I-frame
- Sequence numbers used in
data communication:
- Sequence numbers is
always a
binary field of
n bits
- The number of (different) sequence numbers available = 2n
- Sequence numbers is
always a
binary field of
n bits
- Example:
- Suppose a
frame header uses
2 bits
sequence numbers:
Frame header Body (data) CRC +--------------+--------------------+-----+ |x|x| .... | ..... | ... | +--------------+--------------------+-----+ Seq. number
- The total number of
sequence numbers =
4
Namely:
- 00, 01, 10, 11
Or:
- 0, 1, 2, 3 (in decimal base notation)
- Suppose a
frame header uses
2 bits
sequence numbers:
- Effect:
- Different data frames will be identified using the same sequence number
- Example:
- Suppose the
available sequence numbers are:
- 00, 01, 10, 11
Or: 0, 1, 2, 3
- 00, 01, 10, 11
- Frames will be
identified with
sequence numbers as
follows:
Frames: #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 Infinite seq. no.: 0 1 2 3 4 5 6 7 8 9 4 seq no: 00 01 10 11 00 01 10 11 00 01 ... or: 0 1 2 3 0 1 2 3 0 1 ... ^ ^ | | Frame #2 and #6 are both identified with the seq. no. 01
Notice that the sequence numbers will repeat
- Suppose the
available sequence numbers are:
- Comment:
- I will use decimal sequence numbers
for convenience....
(Remember they are binary numbers in the frame header)
- I will use decimal sequence numbers
for convenience....
- Problem created by a
finite number of
sequence numbers:
Explanation:
- The sender and receiver must
re-use the
sequence numbers to
number different frames
(Sooner or later, the sender and receiver will run out of sequence numbers and must re-use (= repeat) them again.)
-
Result:
- The same sequence number is used to identified different frames
Consequently: ambiguity
- The receiver can not tell frame #2 from frame #6 by the sequence number 1 !!!
- The sender and receiver must
re-use the
sequence numbers to
number different frames
- Potential
problem scenario:
- The sender transmits
data frame 0 using
send seq no 0:
But:
- The ACK for data frame 0 is lost !!!
- Later, the
sender
times out and
re-transmits
the data frame 0
(with send seq no 0):
It is possible that:
- The receiver thinks it was (new) data frame #5 that was transmitted using send seq no 0 !!!
Result:
- The receiver will deliver a re-transmitted (old) frame more than once !!!
- The sender transmits
data frame 0 using
send seq no 0:
- 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 = 3
- Receive window size = 3
- Sender and receiver use
2 bits sequence numbers
- Consider the following
scenario:
- Sender sends
3 frames (#1, #2, #3)
(due to send window size = 3,
the sender
cannot
send more !!!)
The frames are identified with sequence numbers 0, 1 and 2 to the receiver
- All frames has been received (correctly)
Graphically:
- Receiver sends
ACK0,
ACK1 and
ACK2
- The fate of the ACK frames are unknown
Graphically:
Notice that:
- Receiver window =
[3,0,1]
Furthermore:
- The seq. no. 0 is used to identify the data frame #5 !!!
- Sender sends
3 frames (#1, #2, #3)
(due to send window size = 3,
the sender
cannot
send more !!!)
-
Suppose the receiver received
a data frame
with
sequence number = 0:
$64,000 question:
- Is the frame
identified by
sequence number #0:
- Always correspond to the frame #1 ?? or
- Always correspond to the frame #5 ??
- Is the frame
identified by
sequence number #0:
- Comment:
- Because
the receive window is
equal to [3,0,1]:
- The receiver is using seq no "0" to identify the data frame #5
Therefore:
- The receiver will always interpret that the second frame identified by the sequence number 0 is the frame #5
- Because
the receive window is
equal to [3,0,1]:
- The $64,000 question
re-phrased:
- Is the second frame 0:
- always (i.e., for sure) the frame #5 from the sender ??????
- Is the second frame 0:
- Answer:
- NO !!!!
I will show you 2 scenarios that that shows that both cases are possible !!!
-
Scenario 1:
- Initial state:
- Suppose
all ACK frames were
received
correctly
And: frame 3 (= #4) is lost:
Then:
- The second frame identified by sequence number 0 is frame #5
- Initial state:
-
Scenario 2:
- Initial state:
- Suppose
all ACK frames were
lost:
Then:
- The second frame identified by sequence number 0 is frame #0 !!!
- Initial state:
- Conclusion:
- The receiver
does not have
sufficient information to
distinguish these
2 case !!!
- The sliding window algorithm will fail to ensure reliable communication !!!
- The receiver
does not have
sufficient information to
distinguish these
2 case !!!
- Answer:
- Yes !!!
See next webpage !!!