Flow Control and Buffering

1
Flow Control and Buffering

• In this section
• Buffer organization
• Flow control
• Go-back-N
• Selective repeat/reject
• Deadlock avoidance
• Examples

2
Flow Control and Buffering

• Flow control keep a fast transmitter from
  overrunning a slow receiver.
• Buffering by the sender, if network is
  unreliable
• Sender must store frames that are sent in a
  buffer until they are acknowledged by receiver.
• Frames that are lost (that is, they are not
  acknowledged within a timeout) are retrieved from
  buffer and resent.
• When the network is reliable
• If sender knows that receiver always has
  available buffer space, and can accept all
  incoming frames, the sender may not need a buffer.

3
Buffer Organization

• Chained, fixed-size buffers
• Best for protocols where frames are of similar
  size

Frame
Frame
Frame
Unused

• Single, large circular buffer
• Good for heavily loadedconnections, with
  variablesized frames
• For lightly loadedconnections, much ofbuffer
  may remainunused

Frame 1
Frame 2
Frame 3
Unused
4
Buffering at Sender versus Receiver

• Depends on the type of traffic carried by the
  connection.
• Low bandwidth, bursty traffic
• Example interactive terminal session with
  occasional key strokes.
• Avoid allocating buffers in advance.
• Dynamic allocation as needed
• Sender cannot be sure that receiver can always
  allocate a buffer, so sender must retain frame
  until acknowledged.
• Buffer at sender.
• High bandwidth, continuous traffic flow
• Example audio, video, large file transfer
• Receiver should allocate a full set of buffers to
  keep flow at maximum speed.

5
Adjusting Buffer Allocation

• Sender and receiver should be able to dynamically
  adjust to changes in traffic flow.
• Sender may be able to request receiver to
  allocate an amount of buffer space.
• Receiver can inform sender of how much buffer
  space is available.
• Buffers can be allocated separately per
  connection, or for all communication between the
  two hosts.
• General method for dynamic buffer allocation
• Use a sliding window protocol, but decouple
  buffer allocation from acknowledgments.

6
Flow control methods

• Simple Stop-and-wait
• Sender sends frame, and must receive positive
  acknowledgement from receiver before sending next
  frame.
• Automatic Repeat reQuest (ARQ)
• Frames sent with a sequence number to identify
  frames
• Receiver will inform sender if
• There is no more buffer space.
• A damaged frame is detected.
• On request, sender must
• Stop transmission.
• Re-send frames.

7
Go-back-N ARQ

• Frames transmitted continuously (when available)
  up to window size N.
• (Logically) different timer associated with each
  frame not yet acknowledged.
• Receiver
• Acknowledge (ACK) frame if correctly received and
  in order pass to higher layer.
• Negative acknowledge (NACK), or ignore (possibly
  discard) corrupt, or out-of-order frame.
• Sender
• If NACK received, or timer expires, for frame n,
  begin resending from frame n all over again
• ACK of frame n implicitly acknowledges up to n

8
Selective Repeat ARQ

• As in Go-Back-N
• Frames transmitted when available up to limit.
• Timer associated with each unacknowledged frame.
• Receiver NACKs or ignores corrupted frames.
• Differences
• Out of order, but otherwise correct, frame is
  ACKd.
• Receiver buffers out of order frames.
• Sender on timeout or NACK of frame n, resend
  only single frame n

sequence number
x 1
x
x 1
x 2
x 3
x 4
x 5
x 6
received and delivered to user
received and buffered
not yet received
9
Comparison

• Go-back-N ARQ
• Simplicity in buffering and protocol processing
  at sender and receiver.
• If receiver discards bad or out of order
  messages, no receiver buffering is required.
• More retransmission traffic with unreliable
  network.
• Selective repeat ARQ
• More receiver buffering required.
• More complex buffer management for both sender
  and receiver.
• Saves bandwidth avoids retransmission of
  correct frames.

10
Sliding Window, Go-Back-N (1)

• Assumption Frames contain a k bit sequence
  number
• Sequence numbers cycle modulo 2k
• Next sequence number after 2k 1 is 0.
• More on this later!
• Initially, sender requests a certain number of
  frame buffers from receiver, using a special
  control message.
• Receiver replies with number of buffers granted
• This defines a window size for sender.
• Each time sender sends a frame, it increments its
  sequence number, starts a timer, and decrements
  its buffer allocation.
• If timeout occurs, frame is assumed to be lost
  and must be re-sent, followed by all subsequent
  frames.

11
Sliding Window, Go-Back-N (2)

• If senders buffer allocation reaches zero, it
  must stop sending until more buffers are granted
  by receiver.
• Receiver sends acknowledgments that include
• Highest sequence number of consecutively numbered
  frames that have been successfully received (the
  Go-Back-N rule).
• Number of buffers available.
• Based on acknowledgment number, sender knows how
  many frames can be discarded from sending buffer.

12
Go-Back-N Example (1)
Assumption sequence number is 4 bits
A Message B Comment

• ? ltack 15, win 4gt ? B grants window size 4
  A sets window to 0-3
• ? ltseq 0, data m0gt ? A adjusts window to
  1-3
• ? ltseq 1, data m1gt ? A adjusts window to
  2-3
• ? ltseq 2, data m2gt ... Message lost, but
  A thinks window is 3
• ? ltack 1, win 3gt ? B grants window size 3,
  following message 1 2-4 A adjusts
  window to 3-4
• ? ltseq 3, data m3gt ? A adjusts window to
  4

13
Go-Back-N Example (2)
A Message B Comment
.

• ? ltseq 4, data m4gt ? A has to stop sending
  new frames window
• ? ltseq 2, data m2gt ? A times out for
  acknowledging frame 2 and resends
• ? ltseq 3, data m3gt ? Resend of frame 3
• ? ltseq 4, data m4gt ? Resend of frame 4
• ? ltack 4, win 0gt ? B acknowledges up
  to frame 4, but has no buffers yet, so
  A is still blocked with window

14
Go-Back-N Example (3)
A Message B Comment
.

• ? ltack 4, win 1gt ? B grants window size
  1 after message 4 A sets window to 5
• ? ltack 4, win 2gt ? B grants window size
  2 after message 4 A sets window to
  5-6
• ? ltseq 5, data m5gt ? A adjust window to
  6
• ? ltseq 6, data m6gt ? A adjust window to
• ? ltack 6, win 0gt ? Both messages
  received by B, but A is still blocked
• ... ltack 6, win 4gt ? B grants window size
  4 7-10 but message never arrives at A

15
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 15, win 4
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
16
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
17
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
18
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
19
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
0
m0
ack 1, win 3
1
1
m1
1
1
m1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
20
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
m3
3
3
m3
3
3
m3
4
4
m4
4
4
m4
21
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
m3
2
m4
3
2
2
m2
4
3
m3
3
3
m3
4
4
4
m4
4
4
m4
22
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
2
m2
2
m2
3
3
3
m3
4
4
4
m4
23
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
3
m3
3
m3
4
4
4
m4
24
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
3
m3
3
m3
4
4
4
m4
4
m4
25
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 4, win 0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
2
m2
3
3
m3
3
3
m3
3
m3
4
4
m4
4
4
m4
4
m4
26
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 4, win 1
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5
27
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 4, win 2
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5
6
6
m6
6
6
m6
28
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5
5
m5
6
6
m6
6
6
m6
29
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5
6
6
m6
6
6
m6
6
m6
30
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5
5
m5
ack 6, win 0
6
6
m6
6
6
m6
6
m6
7
7
m7
7
7
m7
8
8
m8
8
8
m8
9
9
m9
9
9
m9
10
10
m10
10
10
m10
31
Go-Back-N Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
ack 6, win 4
5
5
m5
5
5
m5
6
6
m6
6
6
m6
7
7
m7
7
7
m7
8
8
m8
8
8
m8
9
9
m9
9
9
m9
10
10
m10
10
10
m10
32
Avoiding deadlocks

• Note that control frames (the acknowledgments /
  buffer allocations) are not themselves sequenced,
  timed out, or confirmed.
• As seen at point 19 in previous example,
  deadlocks are possible if buffer allocations may
  be lost.
• To avoid this, both hosts should periodically
  send status updates
• In previous example, B would not be receiving new
  frames.
• B would send ltstatus, ack 6, win 4gt
  periodically.
• Hopefully, one of these messages will get
  through, and A will restart.
• A may also send ltstatus, ack 6, win 0gt
  periodically while blocked.
• On receiving inconsistent status message, B would
  retransmitltack 6, win 4gt.

33
Window size versus Sequence Numbers

• A sequence number consisting of m bits allows for
  n 2m sequence numbers
• Suppose we allow a window of size n 1
• example m 2, n 4, window 5
• Ensure that the sequence number range is much
  larger than potential window sizes

1 ? ltack 3, win 5gt ? B grants 5
buffers 2 ? ltseq 0, data m0gt ? Message
received 3 ? ltseq 1, data m1gt ? Message
lost 4 ? ltseq 2, data m2gt ? Message
lost 5 ? ltseq 3, data m3gt ? Message
lost 6 ? ltseq 0, data m4gt ?? A thinks it is
out of buffers 7 ? ltack 0, win 4gt ? Was it
line 2 or line 6 that this refers to?
34
Selective Repeat

• Here is the same example, but using selective
  Repeat
• In particular
• Frame 2 will be lost on the first send attempt
• Receiver will NACK frame 2 after frame 3 arrives
• However, frame 3 is kept at receiver
• Sender will resend frame 2 after NACK arrives
• Receiver should not adjust window size with a
  NACK message

35
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 15, win 4
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
36
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
37
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
38
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
39
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
0
m0
ack 1, win 3
1
1
m1
1
1
m1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
40
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
3
m3
4
4
m4
4
4
m4
41
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
nack 2
2
2
m2
2
2
m2
3
3
m3
3
3
m3
3
m3
4
4
m4
4
4
m4
42
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
m3
2
m2
3
2
2
m2
2
3
m3
3
3
m3
4
3
m3
4
4
m4
4
4
m4
43
Selective Repeat Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
1
1
m1
1
1
m1
m3
2
m2
3
2
2
m2
2
3
m3
3
3
m3
4
3
m3
4
4
m4
4
4
m4
m4
4
44
Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 4, win 0
1
1
m1
1
1
m1
2
2
m2
2
2
m2
2
m2
3
3
m3
3
3
m3
3
m3
4
4
m4
4
4
m4
4
m4
45
Example
B
A
seq.
data
seq.
data
0
0
m0
0
0
m0
ack 4, win 1
1
1
m1
1
1
m1
2
2
m2
2
2
m2
3
3
m3
3
3
m3
4
4
m4
4
4
m4
5
5
m5
5
5
m5