1. Datornätverk A – lektion 8
Kapitel 11: Flow control and Error control. (Kapitel 12: Point-to-point access PPP. Översiktligt.)

2. 11.1 Flow and Error Control Flow Control (Flödesstyrning)
Error Control (Felhantering)
Båda dessa funktioner hanteras av vissa datalänkprotokoll (lager 2), i LLC-sublagret, t.ex. vid trådlös kommunikation eller vid modem.
End-to-end flödesstyrning och felkontroll hanteras av transportprotokollet TCP (lager 4).

3. Flow control
Necessary when data is being sent faster than it can be processed by receiver to avoid that the receiver’s buffer is overwhelmed.

4. Felhantering med hjälp av felrättande koder
FEC = Forward Error Correction.
Baseras på felrättande istället för felupptäckande koder.
Kräver ingen backkanal.
Två typer:
Faltningskoder (convolutional codes).
Ex:Vid Faltningskod med kodtakt (code rate) 1/3 infogas två redundanta bitar mellan varje bit i nyttomeddelandet. Dessa felrättande bitar beräknas kontinuerligt för varje inkommande bit i nyttomeddelandet.
Blockkoder (block codes)
Ex: I digital-TV-systemet används en s.k. Read Salomon-kod med beteckningen RS(204, 188, 8). Det innebär att nyttoinformationen delas upp i 188 byte stora block. För varje block beräknas en felrättande kod, som läggs till blocket så att blocket blir 204 byte. Redundanden är alltså 204 – 188 = 16 byte. Koden klarar 8 felaktiga byte.

5. Felhantering med hjälp av felupptäckande koder
Alternativ 1: Bortkastning av felaktiga paket.
Alternativ 2: ARQ = Automatic Repeat reQuest
= automatisk omsändning av paket vid bitfel, eller om paketet inte når fram.
I fortsättning kommer vi med begreppet ”error control”
eller ”felkontroll” att avse ARQ.

6. Protocols to be presented
Stop-and-wait ARQ
Sliding Window Flow Control
Go-back-N ARQ
Selective Repeat ARQ
Sliding Window Protocols

7. The Stop-and-Wait Protocol
The simplest protocol for error and flow control
How the protocol operates:
Source may not send a new frame until the receiver acknowledges previous one.
The receiver sends only positive acknowledgements (ACK) to notify the sender that the frame was received.
If the frame 0 was received, the ACK 1 is sent. In that way the sender is notified that the receiver is expecting frame 1.
The ID of the frame is called a sequence number.
1 bit sequence numbers is sufficient. Sequence:

8. ACK n = Acknowledgement. Expecting frame number n
Normal operation
ACK n = Acknowledgement. Expecting frame number n

9. 11.2 Stop-and-Wait ARQ, lost frame

10. Lost or Damaged Frame
The sender starts a timer when it sends each frame
If the ACK is not received before the timer expires, the sender resends the same frame again

11. 11.3 Stop-and-Wait ARQ, lost ACK frame

12. Lost or damaged ACK Lost ACK causes duplicate frames
A duplicate frame is recognized by the sequence number and is discarded
The receiver sends the same ACK again

13. 11.4 Stop-and-Wait ARQ, delayed ACK

14. Note:
Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost.

15. Piggybacking
Usually the communication is in both ways – this means that the sender is a receiver and the receiver is the sender, too. (both send and receive data)
To save on the processing and bandwidth the short ACKs messages are not sent as separate frames. Instead, they may be included in the frames with data.
This technique is called piggybacking

16. Piggybacking

17. Efficiency of Stop-and-Wait
Very inefficient, having in mind that most of the time the sender is idle
Example: 40 km copper cable, 10 Mbps rate, 1000 bit frame,
Signal in copper propagates at 2 x 108 m/sec
Transmission time is 1000/ (Takes 0.1 msec to transmit frame)
Propagation time is 40000/ 2 x 108 (0.2 msec delay to begin arriving at the receiver)
Total time is 0.3 msec. to get to the receiver
ACK transmission time is approximately 0 (assuming the ACK is very short (length  0)
0.2 msec is the time for the ACK to arrive at the sender
Total time is 0.5 msec before the sender can transmit again
0.5 ms for 0.1 msec frame or efficiency is 20%

18. Sliding-Window flow control
Several frames can be sent without acknowledgement being received
N is the window size – the maximum number of frames that can be sent and not being acknowledged.
The receiver must be able to buffer N frames.
Sequence numbers are used to identify each frame. They are carried in the header.
The number of different sequence numbers must be at least N+1.
If the field for sequence numbers allows m bits, the number of different sequence number is 2m and the sequence numbers range from 0 to 2m-1. In that case the maximum window size is N = 2m-1.

19. Sender sliding window
The sender window is the the set of frames that may be transmitted before an ACK. It slides when the sender has received an ACK and sent next frame.

20. 11.7 Receiver sliding window
The receiver window is the the set of frames that may be accepted before the buffer is full. While the buffer is full, the receiver sends no ACK. The window of a stuffed receiver slides when the receiver has ”consumed” a frame and thus sent an ACK.

21. Stop-and-Wait vs. Sliding Window
Window size N=3.
Sender
Receiver
Sender
Receiver
Frame 0
Transmission + propagation time for the packet
Frame 0
propa-gation time
Transmission time for the packet
Frame 1
ACK 0
Frame 2
ACK 1 .
Time
Frame 1
Frame consump-tion delay
ACK 2
Transmission + propagation time for the ACK
ACK 3
ACK 1
Time
Frame 0
ACK 0
Frame consumption delay . .
Sequence numbers are 1 bit long (0 or 1)
Sequence numbers from 0 to 2m-1. m-bit field for the seq. num.

22. Sender and Receiver Prospective
The window size is 7

23. Sliding Window Flow Control5 6 7 1 2 3 4 F0 5 6 7 1 2 3 4 F1 5 6 7 1 2 3 4 5 6 7 1 2 3 4
ACK1 F2 5 6 7 1 2 3 4 5 6 7 1 2 3 4
ACK2 F3 5 6 7 1 2 3 4 5 6 7 1 2 3 4
ACK3 5 6 7 1 2 3 4 F4 5 6 7 1 2 3 4
ACK4 5 6 7 1 2 3 4 F5 5 6 7 1 2 3 4
ACK5 5 6 7 1 2 3 4 5 6 7 1 2 3 4
ACK6 5 6 7 1 2 3 4 5 6 7 1 2 3 4
N=6

24. ARQ with Sliding Window
Problems arise when some of the frames are discarded (errors or lost frames). Two strategies are developed to deal with this problem:
Go-back-N strategy
The reciever simply discards all frames after the damaged frame without sending acknowledgement.
Selective repeat strategy
The receiver keeps all the frames after the damaged frame. It sends negative acknowledgement (NACK) for the damaged frame. When the sender finaly notice that something is wrong it retransmits the bad frame.
The two strategies are trade-offs between bandwidth and data-link buffer space.

25. Go-Back-N Strategy
If a frame is lost, the lost frame and all the frames sent after it are sent again.
Sending window of size N, receiving window of size 1.
The sender has to buffer N frames
Bandwidth is wasted.

26. 11.9 Go-Back-N ARQ, normal operation

27. 11.10 Go-Back-N ARQ, lost frame

28. 11.11 Go-Back-N ARQ: sender window size

29. Selective Repeat Strategy
Only retransmit the frames that are in error
Both sending and receiving window are of size N

30. 11.13 Selective Repeat ARQ, lost frame

31. 11.14 Selective Repeat ARQ, sender window size

32. Bandwidth – Delay Product
The product of the bit rate (bandwidth expressed as bits per seconds) and the propagation time gives the number of bits that can be on the channel and thus can give orientation about the window size
When propagation time is high (for example in satellite channels), the window size need to be larger

33. Example 1
In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link?
Solution
The bandwidth-delay product is
1  106  20  10-3 = 20,000 bits
The system can send 20,000 bits during the time it takes for the data to go from the sender to the receiver and then back again. However, the system sends only 1000 bits. We can say that the link utilization is only 1000/20,000, or 5%. For this reason, for a link with high bandwidth or long delay, use of Stop-and-Wait ARQ wastes the capacity of the link.

34. Example 2
What is the utilization percentage of the link in Example 1 if the link uses Go-Back-N ARQ with a 15-frame sequence?
Solution
The bandwidth-delay product is still 20,000. The system can send up to 15 frames or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged frames, the utilization percentage is much less because frames have to be resent.

35. High-level Data Link Control Protocol
HDLC is one of the first protocols that implements mechanisms of ARQ
Supports half-duplex and full-duplex mode on point-to-point links
Uses three types of frames: information (I-frames), supervisory (S-frames) and unnumbered (U-frames)
Only I frames carry information, S frames carry transport control information and U frames are used for managing the link

36. HDLC Frame Structure Flag: 01111110, at start and end
Address
Control
FCS
Information
Flag: , at start and end
Physical Address: secondary station (for multidrop configurations)
Information: the data to be transmitted
Frame check sequence (FCS): 16- or 32-bit CRC
Control: purpose or function of frame
Information frames: contain user data
Supervisory frames: flow/error control (ACK/ARQ)
Unnumbered frames: variety of control functions (see p.220)

37. HDLC frame types

38. The Need for Bit Stuffing
The flags show the receiver the start and the end of frame
There is a problem if the flag appears in the middle of the frame as a part of data
The receiver will ”think” it is the end of frame
A technique called “bit stuffing” is used to resolve this problem

39. Bit Stuffing The sender stuffs redundant 0s Redundant 0s
Every time it encounters five 1s in a row, it inserts a redundant 0
The redundant 0 tells the receiver that the sequence is not a flag
The receiver removes all redundant 0s to restore the original frame
Example: Bit stuff the following data:
Redundant 0s

40. 11.24 Bit stuffing and removal

41. Bit stuffing in HDLC

42. PPP (Point-to-Point Protocol)
Based upon HDLC
Used for point-to-point access
Common protocol used for connecting home users to the Internet (via dial-up, DSL or cable modem or leased line)
Defines the negotiation for establishment of the link
Defines the protocol carried on the network layer
Includes authentication and a field about the type of network protocol carried within the frame

43. PPP Frame Format 1 1 1 1 or 2 variable 2 or 4 1
Number of bytes in a field 1 1 1
1 or 2
variable
2 or 4 1
Flag
Address
Control
Flag
Protocol
Payload
CRC
Physical Address field with all 1s indicate broadcasting, i.e. that all stations accept the frame
Since the Address and Control fields are constant, the two parties can negotiate to omit them, thus saving 2 bytes
Protocol field defines what is carried in the payload field (user data or other information)
CRC bits are error control bits

44. PART V
Transport Layer

45. The transport layer is responsible for process-to-process delivery.
Figure Types of data deliveries
The transport layer is responsible for process-to-process delivery.

46. Figure Error control

47. Figure Port numbers

48. Figure 22.3 IP addresses versus port numbers

49. Table 22.1 Well-known ports used by UDP
Protocol
Description
    7
Echo
Echoes a received datagram back to the sender
    9
Discard
Discards any datagram that is received
  11
Users
Active users
  13
Daytime
Returns the date and the time
  17
Quote
Returns a quote of the day
  19
Chargen
Returns a string of characters
  53
Nameserver
Domain Name Service
  67
Bootps
Server port to download bootstrap information
  68
Bootpc
Client port to download bootstrap information
  69
TFTP
Trivial File Transfer Protocol
111
RPC
Remote Procedure Call
123
NTP
Network Time Protocol
161
SNMP
Simple Network Management Protocol
162
Simple Network Management Protocol (trap)

50. Table 22.2 Well-known ports used by TCP
Protocol
Description
   7
Echo
Echoes a received datagram back to the sender
    9
Discard
Discards any datagram that is received
  11
Users
Active users
  13
Daytime
Returns the date and the time
  17
Quote
Returns a quote of the day
  19
Chargen
Returns a string of characters
  20
FTP, Data
File Transfer Protocol (data connection)
  21
FTP, Control
File Transfer Protocol (control connection)
  23
TELNET
Terminal Network
  25
SMTP
Simple Mail Transfer Protocol
  53
DNS
Domain Name Server
  67
BOOTP
Bootstrap Protocol
  79
Finger
  80
HTTP
Hypertext Transfer Protocol
111
RPC
Remote Procedure Call

51. Note:
UDP is a connectionless, unreliable protocol that has no flow and error control. It uses port numbers to multiplex data from the application layer.

52. Figure 22.10 User datagram format

53. Note:
The calculation of checksum and its inclusion in the user datagram are optional.

54. Figure 22.11 Stream delivery
TCP offers stream delivery
– virtual circuit connection
over a packet oriented network

55. Note:
UDP is a convenient transport-layer protocol for applications that provide flow and error control. It is also used by multimedia applications.

56. Figure 22.11 Stream delivery

57. Figure 22.12 Sending and receiving buffers

58. Figure TCP segments

59. Example 1
Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered What are the sequence numbers for each segment if data are sent in five segments with the first four segments carrying 1000 bytes and the last segment carrying 2000 bytes?
Solution
The following shows the sequence number for each segment:
Segment 1 ==> sequence number: 10,010 (range: 10,010 to 11,009)
Segment 2 ==> sequence number: 11,010 (range: 11,010 to 12,009)
Segment 3 ==> sequence number: 12,010 (range: 12,010 to 13,009)
Segment 4 ==> sequence number: 13,010 (range: 13,010 to 14,009)
Segment 5 ==> sequence number: 14,010 (range: 14,010 to 16,009)

60. Note:
The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.

61. Note:
The value of the sequence number field in a segment defines the number of the first data byte contained in that segment.

62. Note:
The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receive. The acknowledgment number is cumulative.

63. Figure 22.14 TCP segment format

64. Figure Control field

65. Table 22.3 Description of flags in the control field
URG
The value of the urgent pointer field is valid.
ACK
The value of the acknowledgment field is valid.
PSH
Push the data.
RST
The connection must be reset.
SYN
Synchronize sequence numbers during connection.
FIN
Terminate the connection.

66. Figure 22.16 Three-step connection establishment

67. Figure 22.17 Four-step connection termination

68. Table 22.4 States for TCP State Description CLOSED
There is no connection.
LISTEN
The server is waiting for calls from the client.
SYN-SENT
A connection request is sent; waiting for acknowledgment.
SYN-RCVD
A connection request is received.
ESTABLISHED
Connection is established.
FIN-WAIT-1
The application has requested the closing of the connection.
FIN-WAIT-2
The other side has accepted the closing of the connection.
TIME-WAIT
Waiting for retransmitted segments to die.
CLOSE-WAIT
The server is waiting for the application to close.
LAST-ACK
The server is waiting for the last acknowledgment.

69. Figure 22.18 State transition diagram

70. Note:
A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP’s sliding windows are byte-oriented.

71. Figure Sender buffer

72. Figure 22.20 Receiver window

73. Figure 22.21 Sender buffer and sender window

74. Figure 22.22 Sliding the sender window

75. Figure 22.23 Expanding the sender window

76. Figure 22.24 Shrinking the sender window

77. Note:
In TCP, the sender window size is totally controlled by the receiver window value (the number of empty locations in the receiver buffer). However, the actual window size can be smaller if there is congestion in the network.

78. Some points about TCP’s sliding windows:
Note:
Some points about TCP’s sliding windows:
The source does not have to send a full window’s worth of data.
The size of the window can be increased or decreased by the destination.
The destination can send an acknowledgment at any time.

79. Figure Lost segment

80. Figure 22.26 Lost acknowledgment

81. Figure 22.7 Connection establishment

82. Figure 22.8 Connection termination

83. Figure TCP timers