Presentation on theme: "Datornätverk A – lektion 8 Kapitel 11: Flow control and Error control. (Kapitel 12: Point-to-point access PPP. Översiktligt.)"— Presentation transcript:
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: 1.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. 2.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....
11.1 Normal operation ACK n = Acknowledgement. Expecting frame number n
Numbered acknowledgments are needed if an acknowledgment is delayed and the next frame is lost. Note:
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
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 10 8 m/sec ○Transmission time is 1000/10000000 (Takes 0.1 msec to transmit frame) ○Propagation time is 40000/ 2 x 10 8 (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 2 m and the sequence numbers range from 0 to 2 m -1. In that case the maximum window size is N = 2 m -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 Sender Receiver Frame 0 Frame 1 Frame 0 ACK 0 ACK 1 ACK 0...... Time Sender Receiver...... Time...... Sequence numbers are 1 bit long (0 or 1) Sequence numbers from 0 to 2 m - 1. m-bit field for the seq. num. Transmiss ion time for the packet Transmission + propagation time for the ACK propa- gation time Transmission + propagation time for the packet Frame 0 Frame 1 Frame 2 ACK 1 ACK 2 ACK 3 Frame consumption delay Frame consump- tion delay Window size N=3.
Sender and Receiver Prospective The window size is 7
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.
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 10 6 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 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) FlagAddressControlFlagFCSInformation
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 ○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
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 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 Flag 01111110 Address 11111111 Control 00000011 Protocol Flag 01111110 CRC Payload 1 1 11 or 2 2 or 4 1 variable Number of bytes in a field
Table 22.1 Well-known ports used by UDP PortProtocolDescription 7EchoEchoes a received datagram back to the sender 9DiscardDiscards any datagram that is received 11UsersActive users 13DaytimeReturns the date and the time 17QuoteReturns a quote of the day 19ChargenReturns a string of characters 53NameserverDomain Name Service 67BootpsServer port to download bootstrap information 68BootpcClient port to download bootstrap information 69TFTPTrivial File Transfer Protocol 111RPCRemote Procedure Call 123NTPNetwork Time Protocol 161SNMPSimple Network Management Protocol 162SNMPSimple Network Management Protocol (trap)
Table 22.2 Well-known ports used by TCP PortProtocolDescription 7EchoEchoes a received datagram back to the sender 9DiscardDiscards any datagram that is received 11UsersActive users 13DaytimeReturns the date and the time 17QuoteReturns a quote of the day 19ChargenReturns a string of characters 20FTP, DataFile Transfer Protocol (data connection) 21FTP, ControlFile Transfer Protocol (control connection) 23TELNETTerminal Network 25SMTPSimple Mail Transfer Protocol 53DNSDomain Name Server 67BOOTPBootstrap Protocol 79Finger 80HTTPHypertext Transfer Protocol 111RPCRemote Procedure Call
UDP is a connectionless, unreliable protocol that has no flow and error control. It uses port numbers to multiplex data from the application layer. Note:
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)
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. Note:
Table 22.3 Description of flags in the control field FlagDescription URGThe value of the urgent pointer field is valid. ACKThe value of the acknowledgment field is valid. PSHPush the data. RSTThe connection must be reset. SYNSynchronize sequence numbers during connection. FINTerminate the connection.
Table 22.4 States for TCP StateDescription CLOSEDThere is no connection. LISTENThe server is waiting for calls from the client. SYN-SENTA connection request is sent; waiting for acknowledgment. SYN-RCVDA connection request is received. ESTABLISHEDConnection is established. FIN-WAIT-1The application has requested the closing of the connection. FIN-WAIT-2The other side has accepted the closing of the connection. TIME-WAITWaiting for retransmitted segments to die. CLOSE-WAITThe server is waiting for the application to close. LAST-ACKThe server is waiting for the last acknowledgment.
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. 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. Note:
Some points about TCP’s sliding windows: Note: 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.