- A window (in
computer networks) is:
- Window =
a consecutive sequence
of integer values
used as
sequence numbers for
frames
Examples:
- 0, 1, 2, 3, 4, 5
- 3, 4, 5, 6
- Window size = the number of integers in the window
- Window =
a consecutive sequence
of integer values
used as
sequence numbers for
frames
- Observation:
- The sequence numbers of
the serie of
outstanding frames
is a
window !!!
Example:
- The sequence numbers of
the serie of
outstanding frames
is a
window !!!
- Sender window:
- Sender window = the serie of sequence numbers that the sender can use to number a outstanding frame
Example:
- Suppose the next sequence number
that the sender will use is 1
The sender has allocated send buffers for 4 data frames
- Then the sender window is:
1, 2, 3, 4because the sender can send at most 4 frames without waiting for acknowledgements
- Graphically:
Note: the sender can send less than 4 frames if it does not have that many frames.
- Now you will understand why they
call this method:
"Sliding Window" method.
- Movement of the sender window:
- Suppose the sender has
4 frame buffers and
the
next sequence number
= 1
Then:
Sender window = 1, 2, 3, 4
- Receiving an ACK message
will make the sender window
slide over
one position
Example:
Explanation:
- Initially, the sender
can tag the
outstanding frames
using the numbers
1, 2, 3, 4
- After receiving the
ACK 1 frame,
the sender
must tag its
outstanding frames
using the numbers
2, 3, 4, 5
New sender window = 2, 3, 4, 5
- Initially, the sender
can tag the
outstanding frames
using the numbers
1, 2, 3, 4
- Suppose the sender has
4 frame buffers and
the
next sequence number
= 1
- "Sliding" window:
- Because of the way
the window moves,
the reliability protocol
that we will study next is called:
- Sliding window schemes
- Because of the way
the window moves,
the reliability protocol
that we will study next is called:
- Recall that:
- The receiver has some receive buffer space that it can use to buffer out-of-order data frames
- Receiver window:
- Receiver window =
the
sequence numbers
of the data frames that the
receiver will
store in its
receive buffers
(when these frames are received)
- Furthermore:
- If the frame
corresponding to the
first number in
the receiver window was received,
the receiver will:
- deliver some frames
- Slide the receiver window forward by the number of delivered frames
- If the frame
corresponding to the
first number in
the receiver window was received,
the receiver will:
- Receiver window =
the
sequence numbers
of the data frames that the
receiver will
store in its
receive buffers
(when these frames are received)
-
Example 1:
- Suppose the receiver window =
[1, 2, 3] and
frame 1 was
lost:
The receiver will buffer the frames 2 and 3 (these frames are inside the receiver window)
- The receiver window does not slide forward because no frame could be delivered....
- Suppose the receiver window =
[1, 2, 3] and
frame 1 was
lost:
- Example 2:
- Suppose the sender
times out after:
And after the time out, the sender retransmits frame 1:
Result:
- The receiver will deliver
all 3 buffered frames to the
user program
- Now the receiver window will slide forward by 3 positions
- The receiver will also
send ACKs for the
correct frames !
- The ACK frames will cause the sender window to slide forward when they are received.
- The receiver will deliver
all 3 buffered frames to the
user program
- Suppose the sender
times out after:
- So:
- The sliding window protocols use a "sliding window" to determine the acceptability of of each data frame.
- A very important ARQ protocol is the Go-Back-N method which we will study next.
- A window is a
consecutive range of sequence number
- Sender winder:
- Sender window =
numbers that the sender
can use to
label outstanding frames
- I.e., the sender must use a number in the sender window to label a frame that it wants to transmit
- Sender Window Size (SWS) = Size of the range of numbers in the sender window
- Sender window =
numbers that the sender
can use to
label outstanding frames
- Receiver window:
- Receiver window
=
labels of frames
that the receiver will
buffer when they are received
- The receiver will only buffer a data frame inside the receiver window
- If the first frame
in the receiver window was received:
- The receiver will also deliver some frames (from its receive buffer).
- And slide the receive window forward by the number of frames delivered
- Receiver Window Size (RWS) = Size of the range of numbers in the receiver window
- Receiver window
=
labels of frames
that the receiver will
buffer when they are received
- Further properties/requirements:
- Initially, the
sender and
receiver window
must
have the
same starting
number (i.e., "lined up")
- Sender Window Size
and Receiver Window Size
may be different
- Sender window will
slide (forward)
when:
- the first frame in the sender window is acknowledged
- Receiver window will
slide (forward)
when:
- the first frame
in the receiver window is
received
(then the receiver can deliver some frames !)
- the first frame
in the receiver window is
received
- Initially, the
sender and
receiver window
must
have the
same starting
number (i.e., "lined up")
- Important property:
- The receiver window
will always be:
- lined up with the sender winder, or:
- ahead of the sender window
(I.e.: the receiver window can never fall behind the sender window)
Reason:
- The receiver window will
always slide
before
the sender window !!!
- The receiver window
will slide when
the first frame
in the receiver window is
received
- This event
will trigger the
receiver to
send the ACK frame
for the first frame
in the sender window
- The sender window will only slide (later) when this ACK frame is received
- The receiver window
will slide when
the first frame
in the receiver window is
received
Graphically illustrated:
- The receiver window
will always be: