lect8+9+10_notes

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Computer Communications

Lectures 8-10

Data Link LayerRequired Reading: Tanenbaum (chapter 3), Peterson &

Davie (chapter 1)

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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

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Placement of the Data Link Protocol

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Packets versus Frames

• Packets (network layer) ��Frames (data link layer) �� Bit stream (physical layer)

Relationship between packets and frames.

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Services Provided to Network Layer

• 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, real-time traffic– Acknowledged connectionless service: useful for wireless systems– Acknowledged connection-oriented service: provides the illusion of a reliable bit stream

(a) Virtual communication. (b) Actual communication.

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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

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Framing using Character Count Method

A character stream. (a) Without errors. (b) With one error.

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Framing using Flag Bytes with Byte Stuffing

(a) A frame delimited by flag bytes.

(b) Four examples of byte sequences before and after stuffing.

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Framing using Flag Bytes with Bit Stuffing

• Each frame begins and ends with a special bit pattern: 01111110

(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.

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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

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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, connection-oriented 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

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Protocol Definitions

Continued �

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

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Protocol Definitions(ctd.)

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

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Unrestricted Simplex Protocol

• Unidirectional transmission

• Network layers at sender and receiver always ready

• Negligible processing time

• Infinite buffers

• Error-free channel

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Simplex Stop-and-Wait Protocol

• Unidirectional transmission, but half-duplex channel

• Non-negligible processing time

• Finite buffers

• Error-free channel

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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 �

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A Simplex Protocol for a Noisy Channel (ctd.)

Positive Acknowledgement with Retransmission (PAR) or Automatic Repeat reQuest (ARQ) protocol

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Handling Bidirectional Traffic

• Interleave data and control (e.g., ACK) frames on the same physical circuit

• Piggybacking

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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

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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.

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A One-Bit Sliding Window Protocol

Continued �

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A One-Bit Sliding Window Protocol (ctd.)

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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.

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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

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Perceived Latency versus RTT

10,000

5000

2000

1000

500

200

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10010RTT (ms)

1-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

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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

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Delay x Bandwidth Product

• Amount of data “in flight” or “in the pipe”

• Usually relative to RTT

• Example: 100ms x 45Mbps = 560KB

Bandwidth

Delay

Network as a pipe

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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)

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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).

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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

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Continued �

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Continued �

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Continued �

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Sliding Window Protocol Using Go Back N (2)

Simulation of multiple timers in software.

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A Sliding Window Protocol Using Selective Repeat

Continued �

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A Sliding Window Protocol Using Selective Repeat (2 )

Continued �

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Continued �

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• 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.

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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

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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) <= 2r

– 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 (0000000000, 0000011111, 1111100000, 1111111111) can correct double errors

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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.

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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.

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The Data Link Layer in the Internet

A home personal computer acting as an internet host.

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PPP – Point to Point Protocol

The PPP full frame format for unnumbered mode operation.

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PPP – Point to Point Protocol (2)

A simplified phase diagram for bring a line up and down.

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PPP – Point to Point Protocol (3)

The LCP frame types.

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