Lectures 8-10 Data Link Layer

Transcription

1 Computer Communications Lectures 8-10 Data Link Layer Required Reading: Tanenbaum (chapter 3), Peterson & Davie (chapter 1) The Data Link Layer Deals with communication between two neighboring computers over a physical layer that works with raw bit streams Protocols targeted toward reliable, efficient communication Need to cope with transmission errors, limited data rate and nonzero propagation delays Functions: Provide a well-defined service interface to the network layer Deal with transmission errors Regulate the flow of data from sender to receiver 2 1

2 Placement of the Data Link Protocol 3 Packets versus Frames Packets (network layer) Frames (data link layer) Bit stream (physical layer) Relationship between packets and frames. 4 2

3 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication. Transfer data from the network layer at source to the network layer at destination Common services Unacknowledged connectionless service: suitable for low error rate channels, realtime traffic Acknowledged connectionless service: useful for wireless systems Acknowledged connection-oriented service: provides the illusion of a reliable bit stream 5 Framing Breaking the bit stream into discrete frames Methods: Insert time gaps Character count Flag bytes with byte stuffing Starting and ending flags, with bit stuffing Physical layer coding violations 6 3

4 Framing using Character Count Method A character stream. (a) Without errors. (b) With one error. 7 Framing using Flag Bytes with Byte Stuffing (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing. 8 4

5 Framing using Flag Bytes with Bit Stuffing Each frame begins and ends with a special bit pattern: (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. 9 Error and Flow Control Error Control Need to consider error characteristics: rate and burstiness Elements of error control techniques» Acknowledgements (positive or negative)» Use of timers» Sequence numbers» Use of codes for error-detection or error-correction Flow Control Feedback-based Rate-based 10 5

6 Elementary Data Link Protocols Assumptions: Model of communication: process per layer and message-passing between adjacent layers A node (computer) with an infinite supply of data uses reliable, connectionoriented service to send the data to another node over a link (communication channel) Computers do not crash Three protocols (in increasing order of complexity): An Unrestricted Simplex Protocol A Simplex Stop-and-Wait Protocol A Simplex Protocol for a Noisy Channel 11 Protocol Definitions Continued Some definitions needed in the protocols to follow. These are located in the file protocol.h. 12 6

7 Protocol Definitions (ctd.) Some definitions needed in the protocols to follow. These are located in the file protocol.h. 13 Unrestricted Simplex Protocol Unidirectional transmission Network layers at sender and receiver always ready Negligible processing time Infinite buffers Error-free channel 14 7

8 Simplex Stop-and- Wait Protocol Unidirectional transmission, but half-duplex channel Non-negligible processing time Finite buffers Error-free channel 15 A Simplex Protocol for a Noisy Channel Assume that receiver can detect a erroneous frame Protocol 2, with addition of timers and sequence numbers Size of sequence number: 1 bit (0, 1) Positive Acknowledgement with Retransmission (PAR) or Automatic Repeat request (ARQ) protocol Continued 16 8

9 A Simplex Protocol for a Noisy Channel (ctd.) Positive Acknowledgement with Retransmission (PAR) or Automatic Repeat request (ARQ) protocol 17 Handling Bidirectional Traffic Interleave data and control (e.g., ACK) frames on the same physical circuit Piggybacking 18 9

10 Sliding Window Protocols Support bidirectional data transfer Multiple frames can be in transit at any time Sender (receiver) maintains a set of sequence numbers that it is permitted to send (receive), correspondingly referred to as sending (receiving) window. Three protocols (differ in efficiency, complexity and buffer requirements): A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat 19 Sliding Window Protocols (2) Buffer requirement = maximum window size 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

11 A One-Bit Sliding Window Protocol Continued 21 A One-Bit Sliding Window Protocol (ctd.) 22 11

12 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. 23 Terminology Bandwidth (of channel/link/network/end-to-end) Network engineers often use the term bandwidth Not the same bandwidth in Hz Instead in bps, refers to capacity or maximum data rate or maximum bit-rate Differentiate with throughput effective data rate Latency or delay (of channel/link/network/end-to-end) Time to send a message from point A to point B One-way versus round-trip time (RTT) Components Latency = Propagation + Transmit + Queue Propagation = Distance / SpeedOfLight Transmit = Size / Bandwidth 24 12

13 Perceived Latency versus RTT 10, MB object, 1.5-Mbps link 1-MB object, 10-Mbps link 2-KB object, 1.5-Mbps link 2-KB object, 10-Mbps link 1-byte object, 1.5-Mbps link 1-byte object, 10-Mbps link RTT (ms) Impact of Round Trip Time and Bandwidth In stop-and-wait protocols, sender waits for an acknowledgement before sending another frame Implicit assumption: RTT negligible Inefficient when RTT is longer Example: 50Kbps satellite channel with 500ms round-trip propagation delay 1000-bit frames 20ms transmit time Minimum time required for sender to receive an ACK: 520ms 500ms idle time (could have send 25 more frames) Similar inefficiency even with high bandwidth channels Consider transferring a 1-MB file on a 1Gbps cross-country link with 100ms round-trip propagation delay More generally, when bandwidth-delay product (BDP) is larger 26 13

14 Delay x Bandwidth Product Amount of data in flight or in the pipe Usually relative to RTT Example: 100ms x 45Mbps = 560KB Delay Bandwidth Network as a pipe 27 Pipelining To achieve higher efficiency when BDP is large Send w frames before blocking, i.e., increase sender window size to w, instead of just 1 Match w (window size) to bandwidth-delay product (pipe volume) 28 14

15 Error Recovery with Pipelining Pipelining and error recovery. Effect on an error when (a) Receiver s window size is 1 (Go Back N). (b) Receiver s window size is large (Selective repeat w/ negative acks). 29 Sliding Window Protocol Using Go Back N Drop the assumption that network layer always has an infinite supply of data to send. Maximum number of outstanding frames is MAX_SEQ, even though (MAX_SEQ + 1) distinct sequence numbers Cumulative acknowledgements All sent, but unacknowledged frames need to be buffered at sender for possible retransmission Assume always reverse traffic to piggyback acknowledgements Timer management in software 30 15

16 Sliding Window Protocol Using Go Back N Continued 31 Sliding Window Protocol Using Go Back N Continued 32 16

17 Sliding Window Protocol Using Go Back N Continued 33 Sliding Window Protocol Using Go Back N 34 17

18 Sliding Window Protocol Using Go Back N (2) Simulation of multiple timers in software. 35 A Sliding Window Protocol Using Selective Repeat Continued 36 18

19 A Sliding Window Protocol Using Selective Repeat (2) Continued 37 A Sliding Window Protocol Using Selective Repeat (3) Continued 38 19

20 A Sliding Window Protocol Using Selective Repeat (4) 39 A Sliding Window Protocol Using Selective Repeat (5) Problem: overlap between new and old receive windows Solution: ensure no overlap by limiting the maximum window size to at most half the range of sequence numbers (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

21 A Sliding Window Protocol Using Selective Repeat (6) Additional issues: #Buffers at receiver = window size #Timers = #Buffers Use of auxiliary timer at receiver for sending separate ACKs when no reverse traffic to piggyback them» Length of the timeout? Use of negative acknowledgements (NACKs) for fast recovery 41 Error Detection and Correction Error-Detecting Codes with redundancy sufficient to detect up to a certain number of errors When error rates are low, error detection and retransmission is efficient Error-Correcting Codes with redundancy needed to also correct a certain number of errors Useful when error rate is high Lower limit on number of check bits (r) needed for correcting single errors in a message with m bits using the relation (m+r+1) <= 2 r Achieved using Hamming method Codewords, Codes and Hamming Distance (d) Need a distance d+1 code to detect d errors E.g., single parity bit can detect single errors Need a distance 2d+1 code to correct d errors E.g., code ( , , , ) can correct double errors 42 21

22 Hamming Codes Can correct single errors Can also be used to correct a single burst errors via interleaved transmission Similar approach also works with parity bits to detect a burst errors Use of a Hamming code to correct burst errors. 43 Cyclic Redundancy Check (CRC) or Polynomial Code Depending on the generator polynomial, polynomial codes can: Detect single bit errors Detect isolated double errors Detect odd number of errors Detect all bursts <= r with r check bits CRC-32 international standard for IEEE 802 Detect all bursts <= 32 Detect all bursts affecting an odd number of bits Checksum computation and verification can be done in hardware Calculation of the polynomial code checksum

23 The Data Link Layer in the Internet A home personal computer acting as an internet host. 45 PPP Point to Point Protocol The PPP full frame format for unnumbered mode operation

24 PPP Point to Point Protocol (2) A simplified phase diagram for bring a line up and down. 47 PPP Point to Point Protocol (3) The LCP frame types