Data Link Layer Protocols

1
Data Link Layer Protocols

• Flow Control in Data Link Layer

2
Flow Control Initial Assumptions

• Simplex Channel
• Infinite buffer capacity with the receiver
• Error free transmission
• Network layer at the senders end is always ready
  with data
• No need for flow control

3
Protocol Definitions
Continued ?
Some definitions needed in the protocols to
follow. These are located in the file protocol.h.
4
Protocol Definitions(ctd.)
Some definitions needed in the protocols to
follow. These are located in the file
protocol.h.
5
Unrestricted Simplex Protocol
6
Some assumptions dropped

• Infinite capacity in the buffer of the receiver.
• Need for flow control
• Stop-n-Wait protocol
• Sender sends a frame
• And waits for a signal in the form of a dummy
  frame
• No seq no. is required since the line is still
  error free

7
Simplex Stop-and-Wait Protocol
8
Further assumptions dropped

• The channel is noisy, frames may be damaged or
  lost
• Good scene data frame reaches intact, ack sent
  back and received, next frame sent
• Bad scene
• Data frame damaged or lost ..hence no ack
  sender times out and resends .. No problems
• Data frame reaches intact but Ack lost .. Times
  out ..resends.. Receiver receives duplicate
  frames..Problem

9
Problem of Duplicate frames

• Solution
• Keep a sequence number for each frame to
  distinguish between the new frame and a duplicate
  frame.
• How large should be the sequence number? Or
• What should be minimum number of bits required
  for the sequence number?
• The only ambiguity at the receiver is between two
  successive frames.. Say m and m1 and never
  between m-1 and m 1 .. Its only after the ack
  for m-1 reaches back intact mth frame is sent..
  And once mth frame reaches intact at the
  receivers end .. Story of m-1th frame is over ..
  However depending upon whether ack of mth frame
  reaches back intact or not either mth or m1 th
  frame is sent .. hence only 1 bit is sufficient

10
A Simplex Protocol for a Noisy Channel
A positive acknowledgement with retransmission
protocol.
Continued ?
11
A Simplex Protocol for a Noisy Channel (ctd.)
A positive acknowledgement with retransmission
protocol.
12
Further assumption dropped

• Channel is simplex ..
• So, Now communication is two way .. I.e. Duplex
  channel
• Interleave data and ack .. So ..a kind field is
  required to specify whether the frame contains
  data or ack.
• Piggyback ack on data frame

13
Sliding Window Protocol(Duplex)

• At any instant of time, the sender maintains a
  set of sequence numbers corresponding to frames
  it is permitted to send this includes
• frames not yet sent
• Frames sent but not yet acknowledged .. May have
  to be sent again
• This corresponds to the senders window. When a
  packet comes from the NL ..it is appended in the
  window and when an ack arrives for a frame..it is
  deleted from the buffer(and from the window)

14
Sliding Window Protocol(Duplex)

• Similarly, the receiver also maintains a set of
  sequence numbers corresponding to frames it is
  permitted to accept.
• When a frame arrives, its seq number is
  checked..if it falls in the window it is accepted
  else discarded .. Once an ack is sent for a frame
  it is deleted from the window so that duplicate
  frames are not accepted.
• Sliding window gives the DLL the flexibility to
  accept the frames in any order but they must be
  handed over to the NL in order.

15
Sliding Window Protocols (2)

• A sliding window of size 1, with a 3-bit sequence
  number.
• (a) Initially.
• (b) After the first frame has been sent.
• (c) After the first frame has been received.
• (d) After the first acknowledgement has been
  received.

16
Sliding Window Protocols

• A One-Bit Sliding Window Protocol
• A Protocol Using Go Back N
• A Protocol Using Selective Repeat

17
A One-Bit Sliding Window Protocol
Continued ?
18
A One-Bit Sliding Window Protocol (ctd.)
19
How does it handle duplicate frames

• Say when an Ack is lost A times out and
  resends..
• Both A and B are wanting to send a packet
  numbered 0, or one can imagine they have both
  sent out packets with seq no. 1 and are waiting
  for its ack.

20

• A sends A0, seq 0, ack 1, B sends B0, seq0,
  ack 1, but B0 is lost.
• B gets the packet A0 but ack is not the one
  required .. It resends B0 with the ack of A0
  I.e. A0, seq 0 and ack 0, (This is a duplicate
  of the ack that was lost) also it increments the
  FE .I.e FE becomes 1.
• Duplicates of A0,0,1 are rejected since their seq
  number (0) dont match the FE(1). However B sends
  the duplicate of the above again.
• If this is also lost.. A duplicates and then B
  duplicates and so on.

21

• Eventually one of the duplicates of B reaches A
  ..I.e. the ack required by A (remember both are
  waiting for ack of packet 0 in the beginning) and
  A sends another packet .. A1 with seq 1 and ack
  0 that for B0 it just received.
• This was a good scene problem was only due to
  noisy channel .. Ack were being lost

22
Bad scene duplicates without transmission error

• When B sends before receiving a frame from A.

23
A One-Bit Sliding Window Protocol (2)

• Two scenarios for protocol 4. (a) Normal
  case. (b) Abnormal case. The notation is (seq,
  ack, packet number). An asterisk indicates where
  a network layer accepts a packet.

24
Further assumption dropped

• So far, implicitly, we have assumed that the time
  of transmission is negligible. I.e. the time
  frame takes to reach the receiver and the ack to
  travel back is negligible.

25
Long propagation delays

• Consider a 50 kbps satellite channel with a round
  trip delay of 500 msec (10-3 sec).
• To send a frame of size 1000 bits ..it takes
  1000/50kbps 20msec.
• At t0, the sender sends the first bit and at
• t 20 msec (1000/50,000 sec 1/50 sec
  1000/50 msec 20msec), it sends the last bit.
• At t 270 msec, the frame completely reaches the
  receiver.
• Not before t 520 msec , the ack arrives back at
  the receiver ( it will be later than this). This
  means that the sender was blocked for 500 msec
  out of 520 msec.

26
Contd..

• That means
• Long propagation delays
• High bandwidth
• And, short frame length
• Is bad in terms of channel efficiency.
• A large window size is needed at the senders end
  whenever the bandwidth round trip delay is
  large.

27
Pipelining

• Imagine in the above example, if the sender could
  send more frames without waiting for the ack for
  the first one, the channel could have been
  utilized more efficiently. This technique is
  called pipelining.

28
Pipelining

• Imagine the scenario, when the sender always has
  something to send until the ack for the first
  frame arrives .. Best scenario, right?
• Clearly this is not Stop-n Wait.

29
Pipelining contd

• Let
• Channel capacity is b bits/sec
• Frame size l bits,
• Round trip propagation time is R
• The time required to transmit a single frame is
  l/b.
• So, in stop n wait, the line is busy for l/b
  time and idle for R time, thus the line
  utilization is
• l/b divided by l/b R l/(l bR).
• If l lt bR, the efficiency will be less than 50.

30
Pipelining over noisy channels

• What if one or more frames is errored.
• Two options
• Throw away all the subsequent frames and go back
  and start resending from the damaged frame. This
  corresponds to size 1 window in the receiver. -
  Go back N Protocol
• Buffer the subsequent frames and wait for the
  damaged frame to be resent, once that comes
  handover the frames in proper order to the
  NL-Selective Repeat Protocol

31
Comparison

• Trade off between the bandwidth utilization and
  the buffer space in the receiver.

32
A Protocol Using Go Back N

• Pipelining and error recovery. Effect on an
  error when
• (a) Receivers window size is 1.
• (b) Receivers window size is large.

33
Issues

• For a maxseq 7, Number of outstanding frames
  can only be 7 and not 8. To see why, let us
  consider what happens when 8 frames are allowed
  to be outstanding .. That is the scene is
• Sender has sent out 8 frames
• The piggybacked ack for frame 7 finally comes to
  the sender.
• Now suppose the sender has 8 outstanding frames,
  07
• Now sender sends next frame 0, if it is lost, if
  B has a frame to send, B will send the
  piggybacked ack for 7.

34

• Next, the sender sends the next frame 1 without
  waiting for the ack for 0, now suppose this is
  also lost, again B has a frame to send,then B
  will again send ack for 7.
• And so on .. Sender sends all the outstanding
  frames, 07
• Suppose all of them are lost, the receiver will
  still send the ack of the last frame received
  I.e. 7.
• Now sender has no way of knowing, whether all the
  frames were lost or since it has received ack for
  7..all the frames reached intact.

35
Solution

• The problem is solved by keeping at most 7
  instead of 8 outstanding frames, say
• 0-6 in the first batch, and
• 7,0-5 in the second
• If all the frames in the second batch are lost,
  sender may keep on receiving ack for frame 6
  which is not in senders window and hence it will
  know that frames were lost.

36
Sliding Window Protocol Using Go Back N
Continued ?
37
Sliding Window Protocol Using Go Back N
Continued ?
38
Sliding Window Protocol Using Go Back N
Continued ?
39
Sliding Window Protocol Using Go Back N
40
Sliding Window Protocol Using Go Back N (2)

• Simulation of multiple timers in software.

41
A Sliding Window Protocol Using Selective Repeat
Continued ?
42
A Sliding Window Protocol Using Selective Repeat
(2)
Continued ?
43
A Sliding Window Protocol Using Selective Repeat
(3)
Continued ?
44
A Sliding Window Protocol Using Selective Repeat
(4)
45
Issues in Selective Repeat

• 7 frames 0-6 are sent,
• Received, ack-ed, and window of the receiver
  advanced to 7,0-5
• All ack lost
• Sender times out and resends frame 0-6
• At receiver, 0-5 accepted, buffered(since next
  frame it is expecting is 7) as new frames and
  ack-ed (Ack 6) and 6 discarded and acked as a
  duplicate
• Sender advances its window to 7,0-5
• Sends packets 7,0-5

46

• 7 is accepted and passed onto the NL
• 0 and others are rejected as duplicate and the
  previous buffered frames are passed on to the NL
  - wrong packets!!!!
• Order in which packets are sent
• P0p6, p0p6, p7, p0(new)p5(new)
• Order in which packets are handed over to NL of
  the receiver
• P0p6,p7,p0p5(duplicate) and rest are discarded
  including the new frames

47
Solution

• Keep the window size 4
• That is half the range of permitted sequence
  numbers.

48
A Sliding Window Protocol Using Selective Repeat
(5)

• (a) Initial situation with a window size seven.
• (b) After seven frames sent and received, but not
  acknowledged.
• (c) Initial situation with a window size of four.
• (d) After four frames sent and received, but not
  acknowledged.

49
Programming Assignments

• Make Groups of two and,
• Implement
• Sliding window protocol with CRC on a duplex
  channel
• Go Back N protocol with Hamming Code on a duplex
  channel
• Implementation of the channel must be clearly
  defined.
• There must be two copies of the same program
  running simultaneously, ( For example, on time
  sharing basis on the same machine, say in two
  different windows.) if required some
  synchronization may be added between the two
  processes. Alternatively, threads could be used.
• Will tell you more about the points on which your
  programs will be evaluated. Few are
• Acks incorporated or not, piggybacked or not,
  channel is truly duplex or not etc

50
I Acknowledge

• Help from the following site
• http//www.cs.vu.nl/ast/
• In preparing this lecture.