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

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).

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.

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.

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.

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

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: 0 1 0 1 0 ... .

ACK n = Acknowledgement. Expecting frame number n 11.1 Normal operation ACK n = Acknowledgement. Expecting frame number n

11.2 Stop-and-Wait ARQ, lost frame

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

11.3 Stop-and-Wait ARQ, lost ACK frame

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

11.4 Stop-and-Wait ARQ, delayed ACK

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

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

11.5 Piggybacking

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 1000/10000000 (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%

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.

11.6 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.

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.

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.

Sender and Receiver Prospective The window size is 7

Sliding Window Flow Control 5 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

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.

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.

11.9 Go-Back-N ARQ, normal operation

11.10 Go-Back-N ARQ, lost frame

11.11 Go-Back-N ARQ: sender window size

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

11.13 Selective Repeat ARQ, lost frame

11.14 Selective Repeat ARQ, sender window size

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

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.

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.

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

HDLC Frame Structure Flag: 01111110, at start and end Address Control FCS Information Flag: 01111110, 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)

11.18 HDLC frame types

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

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: 0001111111110111100011111011 000111110111101111000111110011 Redundant 0s

11.24 Bit stuffing and removal

11.25 Bit stuffing in HDLC

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

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 01111110 Address 11111111 Control 00000011 Flag 01111110 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

PART V Transport Layer

The transport layer is responsible for process-to-process delivery. Figure 22.1 Types of data deliveries The transport layer is responsible for process-to-process delivery.

Figure 22.9 Error control

Figure 22.2 Port numbers

Figure 22.3 IP addresses versus port numbers

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)

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

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.

Figure 22.10 User datagram format

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

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

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

Figure 22.11 Stream delivery

Figure 22.12 Sending and receiving buffers

Figure 22.13 TCP segments

Example 1 Imagine a TCP connection is transferring a file of 6000 bytes. The first byte is numbered 10010. 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)

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

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

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.

Figure 22.14 TCP segment format

Figure 22.15 Control field

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.

Figure 22.16 Three-step connection establishment

Figure 22.17 Four-step connection termination

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.

Figure 22.18 State transition diagram

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.

Figure 22.19 Sender buffer

Figure 22.20 Receiver window

Figure 22.21 Sender buffer and sender window

Figure 22.22 Sliding the sender window

Figure 22.23 Expanding the sender window

Figure 22.24 Shrinking the sender window

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.

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.

Figure 22.25 Lost segment

Figure 22.26 Lost acknowledgment

Figure 22.7 Connection establishment

Figure 22.8 Connection termination

Figure 22.27 TCP timers