Jaringan Komputer. Data Link Layer. The Data Link Layer. Study the design principles

Transcription

1 Jaringan Komputer The Data Link Layer Data Link Layer Study the design principles Algorithms for achieving reliable, efficient communication between two adjacent machines at the data link layer Adjacent = two machines are connected by a communication channel that acts conceptually like a wire (e.g., a coaxial cable, telephone line, or point-topoint wireless channel) Essential: ''wirelike' = the bits are delivered in exactly the same order in which they are sent. 2

2 Data Link Layer Machine A just puts the bits on the wire, and machine B just takes them off? Communication circuits make errors occasionally They have only a finite data rate There is a nonzero propagation delay between the time a bit is sent and the time it is received These limitations have important implications for the efficiency of the data transfer. The protocols used for communications must take all these factors into consideration 3 Data Link Layer Protocols The nature of errors, their causes, and how they can be detected and corrected Series of increasingly complex protocols, each one solving more and more of the problems present in this layer Examination of protocol modeling and correctness Some examples of data link protocols 4

3 Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control 5 Functions Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders 6

4 Packet & Frame Data link layer takes packets from the network layer and encapsulates them into frames for transmission Each frame contains a frame header, a payload field for holding the packet, and a frame trailer Frame management forms the heart of what the data link layer does 7 Packet & Frame Relationship between packets and frames 8

5 Principles Many principles, such as error control and flow control, are found in transport and other protocols as well In many networks, these functions are found only in the upper layers and not in the data link layer The principles are pretty much the same In the data link layer they often show up in their simplest and purest forms, making this a good place to examine them in detail 9 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication. 10

6 Services The data link layer can be designed to offer various services The actual services offered can vary from system to system 3 reasonable possibilities that are commonly provided are: Unacknowledged connectionless service Acknowledged connectionless service Acknowledged connection-oriented service 11 Unacknowledged connectionless service Properties Source machine send independent frames to the destination machine without having the destination machine acknowledge them No logical connection is established beforehand or released afterward If a frame is lost due to noise on the line, no attempt is made to detect the loss or recover from it in the data link layer 12

7 Use Unacknowledged connectionless service Appropriate when the error rate is very low (recovery is left to higher layers) Appropriate for real-time traffic, such as voice, in which late data are worse than bad data Most LANs use unacknowledged connectionless service in the data link layer 13 Acknowledged connectionless service Properties More reliable No logical connections Each frame sent is individually acknowledged. Use The sender knows whether a frame has arrived correctly. If it has not arrived within a specified time interval, it can be sent again. unreliable channels, such as wireless systems. 14

8 Connection-oriented service Establish a connection before data are transferred Each frame sent over the connection is numbered, and the data link layer guarantees that each frame sent is indeed received Each frame is received exactly once and that all frames are received in the right order Transfers go through three distinct phases: connection is established one or more frames are actually transmitted connection is released 15 Example Placement of the data link protocol. 16

9 Framing Data link layer use the service provided to it by the physical layer Physical layer is accept a raw bit stream and attempt to deliver it to the destination This bit stream is not guaranteed to be error free The number of bits received may be less than, equal to, or more than the number of bits transmitted, and they may have different values It is up to the data link layer to detect and, if necessary, correct errors 17 Framing Approach for error Data link layer break the bit stream up into discrete frames and compute the checksum for each frame When a frame arrives at the destination, the checksum is recomputed If the newly-computed checksum is different from the one contained in the frame, the data link layer knows that an error has occurred and takes steps to deal with discarding the bad frame sending back an error report 18

10 Framing Possible way: insert time gaps between frames, much like the spaces between words in ordinary text. Networks rarely make any guarantees about timing - too risky to count on timing to mark the start and end of each frame Other methods have been devised: Character count Flag bytes with byte stuffing Starting and ending flags, with bit stuffing Physical layer coding violations 19 Character Count Idea: specify the number of characters in the frame Problem: the count can be garbled by a transmission error A character stream. (a) Without errors. (b) With one error. 20

11 Flag bytes with byte/character stuffing (esc sequence) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing (flag & esc byte's bit pattern occurs in the data 21 Starting and ending flags, with bit stuffing To allow arbitrary sized characters (not 8 bits) Each frame begins and ends with a special bit pattern, (in fact, a flag byte) Whenever sender encounters five consecutive 1s, it automatically stuffs a 0 bit into the bit stream When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it automatically destuffs (i.e., deletes) the 0 bit Boundary between two frames can be unambiguously recognized by the flag pattern 22

12 Starting and ending flags, with bit stuffing Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver s memory after destuffing. 23 Error Control How to make sure all frames are delivered to the network layer at the destination in the proper order Feedback (acknowledgement) Timers (case for lost frame) Sequence number (case for retransmission) 24

13 Flow Control What to do with a sender that wants to transmit frames faster than the receiver can accept them feedback-based flow control: the receiver sends back information to the sender giving it permission to send more data rate-based flow control: built-in mechanism that limits the rate at which senders may transmit data 25 Error Detection and Correction How to deal with transmission errors Error-Correcting Codes Error-Detecting Codes Errors on some media (e.g., radio) tend to come in bursts rather than singly Advantage side: data are always sent in blocks of bits Disadvantage: much harder to correct 26

14 Error-Correcting Codes Include enough redundant information along with each block of data sent, to enable the receiver to deduce what the transmitted data must have been Often referred to as forward error correction On channels such as wireless links that make many errors, it is better to add enough redundancy to each block for the receiver to be able to figure out what the original block was, rather than relying on a retransmission, which itself may be in error 27 Parity Bit added to guarantee there is no error on received bit Even parity Odd parity Will detect single bit error Will not detect two-bits error 28

15 Hamming code Codeword: n-bit unit m data bits and r redundant check bits (n=m+r) Hamming Distance: number of difference bit in 2 codewords (simply using XOR) Example: XOR = , HD = 3 To detect n errors, a coding schema with Hamming distance of n+1 must be used To correct n error, a coding schema with Hamming distance of 2n+1 must be used 29 Principles Hamming code Bits of codeword are numbered, bit 1 at the left end Bits that powers of 2 (1, 2, 4, 8, 16, etc.) are check bits The rest (3, 5, 6, 7, 9, etc.) filled up with m data bits Each check bit forces the parity of some collection of bits, including itself, to be even (or odd) To see which check bits the data bit in position k contributes to, rewrite k as a sum of powers of 2 example, 11 = Bit is checked by check bits occurring in its expansion bit 11 is checked by bits 1, 2, and 8 30

16 Hamming code Using 4 bit check code to code 7-bit ASCII code, known as (11,7) block code Code rate = 7/11 Redundancy = 1 7/11 Example for 7 bit value of bits are: 100x110x1xx Bits 1 are in position 11 (1011),7(0111),6(0110) and 3(0011) xxxx bits are modulo 2 sum of 11,7,6, and 3 that is 1001 The transmitted codeword: Hamming code At the receiver all bit 1 position are added, and the modulo 2 sum must be zero ( ) mod 2 = 0000 Consider bit 11 is corrupted from 1 to 0, the modulo 2 sum would be ( ) mod 2 = 1011 Not zero 1011 = 11, which means bit in position 11 is error Can correct single bit error and detect two-bits error Cannot detect other multiple-bit error 32

17 Error-Correcting Codes Use of a Hamming code to correct burst errors 33 Error Detecting Code Over copper wire or fiber, the error rate is much lower - error detection and retransmission is more efficient Parity bit Polynomial code a.k.a. CRC (Cyclic Redundancy Check 34

18 CRC Treating bit strings as representations of polynomials with coefficients of 0 and 1 only has 6 bits and thus represents a six-term polynomial with coefficients 1, 1, 0, 0, 0, and 1: x 5 + x 4 + x 0 Sender and receiver must agree upon a generator polynomial, G(x), in advance The idea is to append a checksum to the end of the frame that the polynomial represented by the checksummed frame is divisible by G(x) When the receiver gets the checksummed frame, it tries dividing it by G(x). If there is a remainder, there has been a transmission error 35 CRC The algorithm for computing the checksum: Let r be the degree of G(x). Append r zero bits to the low-order end of the frame so it now contains m + r bits and corresponds to the polynomial x r M(x). Divide the bit string corresponding to G(x) into the bit string corresponding to x r M(x), using modulo 2 division. Subtract the remainder (which is always r or fewer bits) from the bit string corresponding to x r M(x) using modulo 2 subtraction. The result is the checksummed frame to be transmitted. Call its polynomial T(x). 36

19 CRC Calculation of the polynomial code checksum 37 Elementary Data Link Protocols An Unrestricted Simplex Protocol A Simplex Stop-and-Wait Protocol A Simplex Protocol for a Noisy Channel 38

20 An Unrestricted Simplex Protocol A protocol that is as simple as it can be Data are transmitted in one direction only Both the transmitting and receiving network layers are always ready Processing time can be ignored Infinite buffer space is available The communication channel between the data link layers never damages or loses frames 39 A Simplex Stop-and-Wait Protocol The receiving network layer unable to process incoming data infinitely quickly No infinite amount of buffer space to store all incoming frames How to prevent the sender from flooding the receiver with data faster than the latter is able to process them sender simply insert a delay to slow it down sufficiently to keep from swamping the receiver receiver provide feedback to the sender 40

21 A Simplex Protocol for a Noisy Channel Frames may be either damaged or lost completely If frame is damaged in transit, the receiver hardware will detect this when it computes the checksum Sender put a sequence number in the header of each frame it sends, then the receiver can check the sequence number of each arriving frame to see if it is a new frame or a duplicate to be discarded Sender waits for a positive acknowledgement before advancing to the next data item Called PAR (Positive Acknowledgement with Retransmission) or ARQ (Automatic Repeat request). 41 Sliding Window Protocols A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat 42

22 Sliding Window Protocols At any instant of time: the sender maintains a set of sequence numbers corresponding to frames it is permitted to send. the receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept Deliver all frames in the order sent 43 Sliding Window Protocols Frames may be lost or damaged in transit Sender must keep all these frames in its memory for possible retransmission If the maximum window size is n, the sender needs n buffers to hold the unacknowledged frames If the window ever grows to its maximum size, the sending data link layer must forcibly shut off the network layer until another buffer becomes free 44

23 Sliding Window Protocols At receiver window, any frame falling outside the window is discarded without comment When a frame whose sequence number is equal to the lower edge of the window is received, it is passed to the network layer, an acknowledgement is generated, and the window is rotated by one Receiver's window always remains at its initial size Note that a window size of 1 means that the data link layer only accepts frames in order, but for larger windows this is not so The network layer, in contrast, is always fed data in the proper order, regardless of the data link layer's window size 45 Sliding Window Protocols 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. 46

24 One-Bit Sliding Window Protocol Uses stop-and-wait since the sender transmits a frame and waits for its acknowledgement before sending the next one For sequence number, 0 and 1 are the only possibilities 47 One-Bit Sliding Window Protocol 48

25 One-Bit Sliding Window Protocol 3/14/ One-Bit Sliding Window Protocol - example Computer A is trying to send its frame 0 to computer B and B is trying to send its frame 0 to A Suppose that A's timeout interval is a little too short Consequently, A may time out repeatedly, sending a series of identical frames, all with seq = 0 and ack = 1 When first valid frame arrives at computer B, it will be accepted and frame_expected will be set to 1 All the subsequent frames will be rejected because B is now expecting frames with sequence number 1, not 0 Since all the duplicates have ack = 1 and B is still waiting for an acknowledgement of 0, B will not fetch a new packet from its network layer. 50

26 One-Bit Sliding Window Protocol - example After every rejected duplicate comes in, B sends A a frame containing seq = 0 and ack = 0 Eventually, one of these arrives correctly at A, causing A to begin sending the next packet No combination of lost frames or premature timeouts can cause the protocol to deliver duplicate packets to either network layer, to skip a packet, or to deadlock. 51 One-Bit Sliding Window Protocol - abnormal 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 52

27 Pipelining Transmission time is not negligible Long round-trip time can have important implications for the efficiency of the bandwidth utilization By allowing the sender to transmit up to w frames before blocking, instead of just 1 (pipelining) If the bandwidth is high, even for a moderate delay, sender will exhaust its window quickly unless it has a large window If the delay is high the sender will exhaust its window even for a moderate bandwidth These two factors tells what the capacity of the pipe is 53 Pipelining Pipelining frames over an unreliable communication channel raises some serious issues What happens if a frame in the middle of a long stream is damaged or lost? Large numbers of succeeding frames will arrive at the receiver before the sender even finds out that anything is wrong. When a damaged frame arrives at the receiver, it obviously should be discarded, but what should the receiver do with all the correct frames following it? Remember that the receiving data link layer is obligated to hand packets to the network layer in sequence 54

28 Pipelining Pipelining and error recovery. Effect on an error when (a) Receiver s window size is 1. (b) Receiver s window size is large. 55 Protocol Using Go Back N Dealing with errors in the presence of pipelining The receiver discard all subsequent frames, sending no acknowledgements for the discarded frames Corresponds to a receive window of size 1 If the sender's window fills up before the timer runs out, the pipeline will begin to empty Eventually, the sender will time out and retransmit all unacknowledged frames in order, starting with the damaged or lost one This approach can waste a lot of bandwidth if the error rate is high 56

29 Selective Repeat Bad frame that is received is discarded, but good frames received after it are buffered When the sender times out, only the oldest unacknowledged frame is retransmitted If that frame arrives correctly, the receiver can deliver to the network layer, in sequence, all the frames it has buffered Often combined with having the receiver send NAK when it detects an error NAKs stimulate retransmission before the corresponding timer expires and thus improve performance 57 Protocol Verification Realistic protocols and the programs that implement them are often quite complicated Need to find formal, mathematical techniques for specifying and verifying protocols using some models and techniques Models: Finite State Machined Models Petri Net Models 58

30 Finite State Machine Models Each protocol machine (i.e., sender or receiver) is always in a specific state at every instant of time Its state consists of all the values of its variables, including the program counter The state of the complete system is the combination of all the states of the two protocol machines and the channel The state of the channel is determined by its contents From each state, there are zero or more possible transitions to other states Initial state corresponds to the description of the system when it starts running 59 Finite State Machine Models Formally, a finite state machine model of a protocol can be regarded as a quadruple (S, M, I, T): S: set of states the processes and channel can be in M: set of frames that can be exchanged over channel I: set of initial states of the processes. T: set of transitions between states Reachability analysis can be used to detect a variety of errors: Incompleteness (in the protocol specification) Deadlock (a situation in which the protocol can make no more forward progress) Extraneous transition (event cannot occur) 60

31 Finite State Machine Models Simplified protocol 3. unreachable state and checksum errors are ignored here for simplicity (a) State diagram (b) Transmissions. 61 Finite State Machine Models Each state is labeled by SRC S is 0 or 1, correspond to frame sender try to send R is 0 or 1, correspond to frame the receiver expects C is 0, 1, A, or empty ( ), correspond to state of channel Initial state is (000) sender has just sent frame 0 receiver expects frame 0 frame 0 is currently on the channel 62

32 Finite State Machine Models Example of total 9 transitions: Transition 0: channel losing its contents Transition 1: channel correctly delivering packet 0 to the receiver, receiver then expect frame 1 and emitting an acknowledgement During normal operation, transitions 1, 2, 3, and 4 are repeated in order over and over Transition 7: eventually, the sender times out and the system moves back to (000) The loss of an acknowledgement is more complicated, requiring two transitions, 7 and 5, or 8 and 6, to repair the damage 63 Petri Net Models Has four basic elements: Places: represents a state which (part of) the system may be in Transitions: indicated by a horizontal or vertical bar Arcs: each transition has zero or more input arcs coming from its input places, and zero or more output arcs, going to its output places Tokens: a transition is enabled if there is at least one input token in each of its input places. Enabled transition may fire at will, removing one token from each input place and depositing a token in each output place 64

33 Petri Net Models A Petri net with two places and two transitions. 65 Petri Net Models A Petri net model for protocol 3 66

34 Petri Net Models Transitions 1 and 2 correspond to transmission of frame 0 by the sender, normally, and on a timeout respectively Transitions 3 and 4 are analogous for frame 1 Transitions 5, 6, and 7 correspond to the loss of frame 0, an acknowledgement, and frame 1, respectively Transitions 8 and 9 occur when a data frame with the wrong sequence number arrives at the receiver Transitions 10 and 11 represent the arrival at the receiver of the next frame in sequence and its delivery to the network layer 67 Petri Net Models Can be represented in convenient algebraic form resembling a grammar Each transition contributes one rule to the grammar Each rule specifies the input and output places of the transition 1: BD AC 2: A A 3: AD BE 4: B B 5: C 6: D 7: E 8: CF DF 9: EG DG 10: CG DF 11: EF DG 68

35 Example Data Link Protocols HDLC High-Level Data Link Control: classical bit-oriented protocol Its variants have been in use for decades in many applications The Data Link Layer in the Internet PPP The Point-to-Point Protocol 69 High-Level Data Link Control Bit oriented use bit stuffing for data transparency History: Derived from the data link protocol first used in the IBM mainframe world: SDLC (Synchronous Data Link Control) protocol ANSI modified it to become ADCCP (Advanced Data Communication Control Procedure ISO modified it to become HDLC CCITT then adopted and modified HDLC for its LAP (Link Access Procedure) as part of the X.25 70

36 High-Level Data Link Control Frame format for bit-oriented protocols. 71 High-Level Data Link Control Control field is used for sequence numbers, acknowledgements, and other purposes Data field may contain any information. It may be arbitrarily long. Checksum field is a cyclic redundancy code Frame is delimited with another flag sequence ( ). On idle point-to-point lines, flag sequences are transmitted continuously Minimum frame contains three fields and totals 32 bits, excluding the flags on either end 72

37 High-Level Data Link Control Control field of (a) An information frame. (b) A supervisory frame. (c) An unnumbered frame. 73 High-Level Data Link Control Uses a sliding window, with a 3-bit sequence number Seq field is the frame sequence number Next field is a piggybacked acknowledgement instead of piggybacking the number of the last frame received correctly, they use the number of the first frame not yet received (i.e., the next frame expected) P/F: Poll/Final. Used when a computer is polling a group of terminals Computer is inviting the terminal to send data. All frames sent by the terminal, except the final one, have the P/F bit set to P The final one is set to F 74

38 The Data Link Layer in the Internet A home personal computer acting as an internet host 75 PPP PPP Point to Point Protocol Defined in RFC 1661, Further elaborated on in several other RFCs (e.g., RFCs 1662 and 1663). Handles error detection, supports multiple protocols, allows IP addresses to be negotiated at connection time, permits authentication 3 features: Framing method (+ error detection) Link Control Protocol (LCP): bring up, test, negotite options, and bring down Have different NCP (Network Control Protocol) for each network layer supported 76

39 PPP Point to Point Protocol The PPP full frame format for unnumbered mode operation PPP frame is based on HDLC, but character oriented 77 PPP Point to Point Protocol A simplified phase diagram for bring a line up and down 78

40 PPP Point to Point Protocol The LCP frame types 79 Summary Data link layers Task: convert the raw bit stream offered by the physical layer into a stream of frames for use by the network layer Framing method: character count, bit/byte stuffing Error Control: detect & retransmit, correct Flow Control: sliding window (6 protocol variants) 80