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Computer Networks Chapter 3 Data Link Layer Prof. Jerry Breecher CSCI 280 Spring 2002 The Weeks Ahead Date Lecture Lab/Project Feb 11 Exam 1 Feb 13 MAC Feb 18 The Data Link Layer Feb 20 The Data Link Layer Feb 21 Project 1 DUE Chapter Overview Data Link Layer

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computer networks

Computer Networks

Chapter 3

Data Link Layer

Prof. Jerry Breecher

CSCI 280

Spring 2002

the weeks ahead
The Weeks Ahead

Date Lecture Lab/Project

Feb 11 Exam 1

Feb 13 MAC

Feb 18 The Data Link Layer

Feb 20 The Data Link Layer

Feb 21 Project 1 DUE

Chap. 4- MAC

chapter overview
Chapter Overview

Data Link Layer

The main goal of this layer is providing reliability to the layers above it.

3.1 DLL Design Issues

What are the services provided by DLL?

3.2 Error Detection and Correction

Adding redundancy in order to find and correct errors.

3.3 DLL Protocols

xxx

3.4 Sliding Window Protocols

xxx

3.5 Protocol Specification and Verification

xxx.

3.6 Examples

Chap. 4- MAC

dll design

Overview

DLL Design
  • The concerns at the Data Link Layer include:
  • What services should be provided to upper layers?
  • Framing,
  • Error Control.
  • Flow Control.

3.1 DLL Design Issues

3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and Verification

Chap. 4- MAC

dll design5

Overview

DLL Design

The goal of the data link layer is to provide reliable, efficient communication between adjacent machines connected by a single communication channel. Specifically:

1. Group the physical layer bit stream into units called frames. Note that frames are nothing more than "packets" or "messages". By convention, we'll use the term "frames" when discussing DLL packets.

2. Sender checksums the frame and transmits checksum together with data. The checksum allows the receiver to determine when a frame has been damaged in transit.

3. Receiver re-computes the checksum and compares it with the received value. If they differ, an error has occurred and the frame is discarded.

4. Perhaps return a positive or negative acknowledgment to the sender. A positive acknowledgment indicate the frame was received without errors, while a negative acknowledgment indicates the opposite.

5. Flow control. Prevent a fast sender from overwhelming a slower receiver. For example, a supercomputer can easily generate data faster than a PC can consume it.

6. In general, provide service to the network layer. The network layer wants to be able to send packets to its neighbors without worrying about the details of getting it there in one piece.

At least, the above is what the OSI reference model suggests. As we will see later, not everyone agrees that the data link layer should perform all these tasks.

Chap. 3- DLL

dll design6

Overview

DLL Design

There are several possible kinds of services that can be provided to network layers.

The Figure is a reminder of the difference between virtual and actual communications between layers.

Chap. 3- DLL

dll design7

Acknowledged

UN-Acknowledged

Connection-Less

“Best Effort”

Better Quality

Connection

Oriented

Reliable Delivery

SERVICES PROVIDED TO THE NETWORK LAYER

DLL Design

Delivery Mechanisms:

Chap. 3- DLL

dll design8

SERVICES PROVIDED TO THE NETWORK LAYER

DLL Design
  • Unacknowledged Connection-less Service -- Best Effort:
  • The receiver does not return acknowledgments to the sender, so the sender has no way of knowing if a frame has been successfully delivered.
      • When would such a service be appropriate?
  • 1. When higher layers can recover from errors with little loss in performance. That is, when errors are so infrequent that there is little to be gained by the data link layer performing the recovery. It is just as easy to have higher layers deal with occasional lost packets.
  • 2. For real-time applications requiring "better never than late" semantics. Old data may be worse than no data. For example, should an airplane bother calculating the proper wing flap angle using old altitude and wind speed data when newer data is already available?

Chap. 3- DLL

dll design9

SERVICES PROVIDED TO THE NETWORK LAYER

DLL Design
  • Acknowledged Connection-less Service -- Acknowledged Delivery:
  • The receiver returns an acknowledgment frame to the sender indicating that a data frame was properly received. The sender keeps connection state, but may not necessarily retransmit unacknowledged frames.
  • Likewise, the receiver may hand received frames to higher layers in the order in which they arrive, regardless of the original sending order.
  • Typically, each frame is assigned a unique sequence number, which the receiver returns in an acknowledgment frame to indicate which frame the ACK refers to. The sender must retransmit unacknowledged (e.g., lost or damaged) frames.

Chap. 3- DLL

dll design10

SERVICES PROVIDED TO THE NETWORK LAYER

DLL Design
  • Acknowledged Connection-Oriented Service -- Reliable Delivery:
  • Frames are delivered to the receiver reliably and in the same order as generated by the sender.
  • Connection state keeps track of sending order and which frames require retransmission. For example, receiver state includes which frames have been received, which ones have not, etc.

Chap. 3- DLL

dll design11

FRAMING

DLL Design
  • The DLL translates the physical layer's raw bit stream into discrete units (messages) called frames. How can frame be transmitted so the receiver can detect frame boundaries? That is, how can the receiver recognize the start and end of a frame? We will discuss four ways:
  • Character Count:
  • Bit Stuffing:
  • Character stuffing:
  • Encoding Violations:

Chap. 3- DLL

dll design12

FRAMING

DLL Design
  • Character Count:
  • Make the first field in the frame's header be the length of the frame. That way the receiver knows how big the current frame is and can determine where the next frame ends.
  • Disadvantage: Receiver loses synchronization when bits become garbled. If the bits in the count become corrupted during transmission, the receiver will think that the frame contains fewer (or more) bits than it actually does.
  • Although checksum will detect the frames are incorrect, the receiver will have difficulty re-synchronizing to the start of a new frame. This technique is not used anymore, since better techniques are available.

Chap. 3- DLL

dll design13

FRAMING

DLL Design

Bit Stuffing:

IDEA: Use reserved bit patterns to indicate the start and end of a frame. For instance, use the 4-bit sequence of 0111 to delimit consecutive frames. A frame consists of everything between two delimiters.

Problem: What happens if the reserved delimiter happens to appear in the frame itself? If we don't remove it from the data, the receiver will think that the incoming frame is actually two smaller frames!

Solution: Use bit stuffing. Within the frame, replace every occurrence of two consecutive 1's with 110. E.g., append a zero bit after each pair of 1's in the data. This prevents 3 consecutive 1's from ever appearing in the frame.

Chap. 3- DLL

dll design14

FRAMING

DLL Design
  • Bit Stuffing:
  • The receiver converts two consecutive 1's followed by a 0 into two 1's, but recognizes the 0111 sequence as the end of the frame.
  • Example: The frame "1 0 1 1 1 0 1" would be transmitted over the physical layer as "0 1 1 1 1 0 1 1 0 1 0 1 0 1 1 1".
  • Note: When using bit stuffing, locating the start/end of a frame is easy, even when frames are damaged. The receiver simply scans arriving data for the reserved patterns.
  • The receiver will re-synchronize quickly with the sender as to where frames begin and end, even when bits in the frame get garbled.
  • The main disadvantage with bit stuffing is the insertion of additional bits into the data stream, wasting bandwidth. How much expansion? The precise amount depends on the frequency in which the reserved patterns appear as user data.

Chap. 3- DLL

dll design15

FRAMING

DLL Design

Character stuffing:

Same idea as bit-stuffing, but operates on bytes instead of bits.

Use reserved characters to indicate the start and end of a frame. For instance, use the two-character sequence DLE STX (Data-Link Escape, Start of TeXt) to signal the beginning of a frame, and the sequence DLE ETX (End of TeXt) to flag the frame's end.

Problem: What happens if the two-character sequence DLE ETX happens to appear in the frame itself?

Solution: Use character stuffing within the frame, replace every occurrence of DLE with the two-character sequence DLE DLE. The receiver reverses the process, replacing every occurrence of DLE DLE with a single DLE.

Example: If the frame contained "A B DLE D E DLE", the characters transmitted over the channel would be "DLE STX A B DLE DLE D E DLE DLE DLE ETX".

Disadvantage: A octet is the smallest unit that can be operated on; not all architectures are 8-bit oriented.

Chap. 3- DLL

dll design16

FRAMING

DLL Design

Encoding Violations:

Send a signal that doesn't conform to any legal bit representation. In Manchester encoding, for instance, 1-bits are represented by a high-low sequence, and 0-bits by low-high sequences. The start/end of a frame could be represented by the signal low-low or high-high.

The advantage of encoding violations is that no extra bandwidth is required as in bit or character stuffing. The IEEE 802.4 standard uses this approach.

Finally, some systems use a combination of these techniques. IEEE 802.3, for instance, has both a length field and special frame start and frame end patterns.

Chap. 3- DLL

dll design17

ERROR CONTROL

DLL Design
  • Must insure that all frames are eventually delivered (possibly in order) to a destination. Three components are required to do this:
  • Acknowledgments, Timers, and Sequence Numbers
  • Acknowledgments:
  • Reliable delivery is achieved using the "acknowledgments with retransmission" paradigm.
  • The receiver returns a special acknowledgment (ACK) frame to the sender indicating the correct receipt of a frame.
  • In some systems, the receiver also returns a negative acknowledgment (NACK) for incorrectly-received frames.
  • This is only a hint to the sender so that it can retransmit a frame right away without waiting for a timer to expire.

Chap. 3- DLL

dll design18

ERROR CONTROL

DLL Design
  • Timers:
  • One problem that simple ACK/NACK schemes fail to address is recovering from a frame that is lost, and as a result, fails to solicit an ACK or NACK.
  • What happens if an ACK or NACK becomes lost?
  • Retransmission timers are used to resend frames that don't produce an ACK. When sending a frame, schedule a timer to expire at some time after the ACK should have been returned. If the timer goes off, retransmit the frame.
  • Sequence Numbers:
  • Retransmissions introduce the possibility of duplicate frames.
  • To suppress duplicates, add sequence numbers to each frame, so that a receiver can distinguish between new frames and repeats of old frames.
  • Bits used for sequence numbers depend on the number of frames that can be outstanding at any one time.

Chap. 3- DLL

dll design19

FLOW CONTROL

DLL Design

Flow control deals with throttling the speed of the sender to match that of the receiver. Usually, this is a dynamic process, as the receiving speed depends on such changing factors as the load, and availability of buffer space.

One solution is to have the receiver extend credits to the sender. For each credit, the sender may send one frame. Thus, the receiver controls the transmission rate by handing out credits.

LINK INITIALIZATION:

In some cases, the data link layer service must be "opened" before use:

The data link layer uses open operations for allocating buffer space, control blocks, agreeing on the maximum message size, etc.

Synchronize and initialize send and receive sequence numbers with its peer at the other end of the communications channel.

Chap. 3- DLL

error detection control

Overview

Error Detection & Control

This section is about putting in enough redundancy along with the data to be able to detect (and correct) data errors.

3.1 DLL Design Issues

3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and Verification

Chap. 4- MAC

error detection control21

ERROR CORRECTING CODES

Error Detection & Control

In data communication, line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as lightning, and the telephone worker). Moreover, noise usually occurs as bursts rather than independent, single bit errors. For example, a burst of lightning will affect a set of bits for a short time after the lightning strike.

Detecting and correcting errors requires redundancy - sending additional information along with the data.

There are two types of attacks against errors:

Error Detecting Codes: Include enough redundancy bits to detect errors and use ACKs and retransmissions to recover from the errors.

Error Correcting Codes: Include enough redundancy to detect and correct errors.

We will introduce some concepts, and then consider both detection and correction.

To understand errors, consider the following:

Messages (frames) consist of m data (message) bits and r redundancy bits, yielding an n = ( m + r ) bit codeword

Chap. 3- DLL

error detection control22

ERROR CORRECTING CODES

Error Detection & Control

Hamming Distance. Given any two codewords, we can determine how many of the bits differ. Simply exclusive or (XOR) the two words, and count the number of 1 bits in the result. This count is the Hamming Distance.

Significance? If two codewords are d bits apart, d errors are required to convert one to the other.

A code's Hamming Distance is defined as the minimum Hamming Distance between any two of its legal codewords (from all possible codewords).

In general, all 2m possible data words are legal. However, by choosing check bits carefully, the resulting codewords will have a large Hamming Distance. The larger the Hamming distance, the better the codes are able to detect errors.

To detect d 1-bit errors requires having a Hamming Distance of at least d + 1 bits. Why?

To correct d errors requires 2d + 1 bits. Intuitively, after d errors, the garbled messages is still closer to the original message than any other legal codeword.

Chap. 3- DLL

error detection control23

ERROR CORRECTING CODES

Error Detection & Control

Parity Bits

A single parity bit is appended to each data block (e.g. each character in ASCII systems) so that the number of 1 bits always adds up to an even (odd) number.

1000000(1) 1111101(0)

The Hamming Distance for parity is 2, and it cannot correct even single-bit errors (but can detect single-bit errors). As another example, consider a 10-bit code used to represent 4 possible values:

"00000 00000", "00000 11111", "11111 00000", and "11111 11111".

Its Hamming distance is 5, and we can correct 2 single-bit errors:

For instance, "10111 00010" becomes "11111 00000" by changing only two bits.

However, if the sender transmits "11111 00000" and the receiver sees "00011 00000", the receiver will not correct the error properly.

Finally, in this example we are guaranteed to catch all 2-bit errors, but we might do better: if "00111 00111" contains 4 single-bit errors, we will reconstruct the block correctly.

Chap. 3- DLL

error detection control24

ERROR CORRECTION

Error Detection & Control

What's the fewest number of bits needed to correct single bit errors? Let us design a code containing n = m + r bits that corrects all single-bit errors (remember m is the number of message (data) bits and r is number of redundant (check) bits):

There are 2m legal messages (e.g., legal bit patterns).

Each of the m messages has n illegal codewords a distance of 1 from it. That is, if we systematically invert each bit in the corresponding n-bit codeword, we get illegal codewords a distance of 1 from the original. Thus, each message requires n + 1 bits dedicated to it (n that are one bit away and 1 that is the message).

The total number of bit patterns is ( n + 1 ) * 2m< 2n. That is, all (n+1) * 2m encoded messages should be unique, and there can't be fewer messages than the 2n possible code-words.

Since n = m + r , we get:

( m + r + 1) * 2m< 2m+r or

( m + r + 1) < 2r

This formula gives the absolute lower limit on the number of bits required to detect (and correct!) 1-bit errors.

Chap. 3- DLL

error detection control25

ERROR DETECTION

Error Detection & Control

Error correction is relatively expensive (computationally and in bandwidth.)

For example, 10 redundancy bits are required to correct 1 single-bit error in a 1000-bit message. In contrast, detecting a single bit error requires only a single-bit, no matter how large the message. The most popular error detection codes are based on polynomial codes or cyclic redundancy codes(CRCs).

Allows us to acknowledge correctly received frames and to discard incorrect ones.

Tanenbaum and you have worked several examples.

Chap. 3- DLL

dll protocols

Overview

DLL PROTOCOLS

How can two DLL layers communicate in order to assure reliability?

We will look at increasingly complex protocols to see how this is done.

3.1 DLL Design Issues

3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and Verification

Chap. 4- MAC

dll protocols27

Overview

DLL Protocols

ELEMENTARY DATA LINK PROTOCOLS:

The DLL provides these services to the Network Layer above it:

Data handed to a DLL by a Network Layer on one module, are handed to the Network Layer on another module by that DLL.

The remote Network Layer peer should receive the identical message generated by the sender (e.g., if the data link layer adds control information, the header information must be removed before the message is passed to the Network Layer).

The Network Layer may want to be sure that all messages it sends, will be delivered correctly (e.g., none lost, no corruption). Note that arbitrary errors may result in the loss of both data and control frames.

The Network Layer may want messages to be delivered to the remote peer in the exact same order as they are sent.

Note: It is not always clear that we really want our data link layer protocol to provide this type of service. What if we run real-time applications across the link?

Nonetheless, the ISO reference model suggests that the data link layer provide such a service, and we now examine the protocols that do so.

Chap. 3- DLL

dll protocols28

OUR METHOD

DLL Protocols

THE METHOD WE WILL USE:

Look at successive data link protocols of increasing complexity to provide reliable, in order, message delivery to the network layer.

Environment:

Assume DLL executes as a process (scheduleable entity) with routines to communicate with the Network Layer above and the Physical Layer below.

Frames are the unit of transmission. Consists of data plus control bits (header information).

Look at data structures and prototypes on the next few pages – this is Figure 3.8.

Of special interest is typedef struct frame;

void wait_for_event( event_type *event );

wait_for_event() suspends the process until an event occurs. Possible events include requests from the network layer, the physical layer and the timer.

Chap. 3- DLL

dll protocols29

BUILDING BLOCKS

DLL Protocols

#define MAX PKT 1024 /* determines packet size in bytes */

typedef enum {false, true} boolean; /* boolean type */

typedef unsigned int seq_nr; /* sequence or ack numbers */

typedef struct {

unsigned char data[MAX PKT];

} packet; /* packet definition */

typedef enum {data, ack, nak} frame_kind; /* frame kind definition */

typedef struct { /* frames are transported in this layer */

frame_kind kind; /* what kind of a frame is it? */

seq_nr seq; /* sequence number */

seq_nr ack; /* acknowledgement number */

packet info; /* the network layer packet */

} frame;

Chap. 3- DLL

dll protocols30

BUILDING BLOCKS

DLL Protocols

/* 1. Wait for an event to happen; return its type in event. */

void wait_for_event(event_type *event );

/* 2. Fetch a packet from the network layer for transmission on the channel. */

void from_network_layer( packet *p);

/* 3. Deliver information from an inbound frame to the network layer. */

void to_network_layer( packet *p);

/* 4. Go get an inbound frame from the physical layer and copy it to r. */

void from_physical_layer( packet *p);

/* 5. Pass the frame to the physical layer for transmission. */

void to_physical_layer( packet *p);

/* 6. Start the clock running and enable the timeout event. */

void start_timer(seq_nr k);

/* 7. Stop the clock and disable the timeout event. */

void stop_timer(seq_nr k);

/* 8. Start an auxiliary timer and enable the ack_timeout event. */

void start_ack_timer(void);

/* 9. Stop the auxiliary timerand disable the ack_timeout event. */

void stop_ack_timer(void);

/* 10. Allow the network layer to cause a network_layer_event. */

void enable_network_layer( void );

/* 11. Forbid the network layer from causing a network_layer_event. */

void disable_network_layer( void );

Chap. 3- DLL

dll protocols31

AN UNRESTRICTED SIMPLEX PROTOCOL

DLL Protocols

Assumptions:

Data transmission in one direction only (simplex).

No errors take place on the physical channel.

The sender/receiver can generate/consume an infinite amount of data.

Always ready for sending/receiving.

See the code on the next page == Figure 3.9.

Chap. 3- DLL

dll protocols32

AN UNRESTRICTED SIMPLEX PROTOCOL

DLL Protocols

/* Protocol 1 (utopia) provides for data transmission in one direction only, from

sender to receiver. The communication channel is assumed to be error free,

and the receiver is assumed to be able to process all the input infinitely fast.

Consequently, the sender just sits in a loop pumping data out onto the line as

fast as it can. */

typedef enum {frame_arrival} event_type;

#include "protocol.h"

void sender1(void)

{

frame s; /* buffer for an outbound frame */

packet buffer; /* buffer for an outbound packet */

while (true) {

from_network_layer(&buffer); /* go get something to send */

s.info = buffer; /* copy it into s for transmission */

to_physical_layer(&s); /* send it on its way */

}

}

void receiver1(void)

{

frame r;

event_type event; /* filled in by wait, but not used here */

while (true) {

wait_for_event(&event); /* only possibility is frame arrival */

From_physical_layer(&r); /* go get the inbound frame */

To_network_layer(&r.info); /* pass the data to the network layer */

}

}

Chap. 3- DLL

dll protocols33

SIMPLEX STOP-AND-WAIT PROTOCOL

DLL Protocols

Assumptions:

No longer assume receiver can process incoming data infinitely fast.

Sender ships one frame and then waits for acknowledgment (stop and wait.)

The contents of the acknowledgment frame are unimportant.

Data transmission is one directional, but must have bi-directional line. Could have a half-duplex (one direction at a time) physical channel.

See the code on the next page == Figure 3.10. 

Chap. 3- DLL

dll protocols34

SIMPLEX STOP-AND-WAIT PROTOCOL

DLL Protocols

/* Protocol 2 (stop-and-wait) also provides for a one-directional flow of data from

sender to receiver. The communication channel is once again assumed to be error

free, as in protocol 1. However, this time, the receiver has only a finite buffer

capacity and a finite processing speed, so the protocol must explicitly prevent

the sender from flooding the receiver with data faster than it can be handled. */

typedef enum {frame_arrival} event_type;

#include "protocol.h"

void sender2(void)

{

frame s; /* buffer for an outbound frame */

packet buffer; /* buffer for an outbound packet */

event_type event; /* frame_arrival is the only possibility */

while (true) {

from_network_layer(&buffer); /* go get something to send */

s.info = buffer; /* copy it into s for transmission */

to_physical_layer(&s); /* send it on its way */

wait_for_event(event(&event); /* do not proceed until given the go ahead */

}

void receiver2(void)

{

frame r, s;

event_type event; /* filled in by wait, but not used here */

while (true) {

wait_for_event(&event); /* only possibility is frame arrival */

From_physical_layer(&r); /* go get the inbound frame */

To_network_layer(&r.info); /* pass the data to the network layer */

to_physical_layers); /* send a dummy frame to awaken sender */

}

}

CHECK THIS CODE!!

Chap. 3- DLL

dll protocols35

SIMPLEX PROTOCOL FOR A NOISY CHANNEL

DLL Protocols
  • SIMPLEX PROTOCOL FOR A NOISY CHANNEL:
  • Assumptions:
  • The channel is noisy and we can lose frames (they never arrive).
  • Simple approach, add a time-out to the sender so if no ACK after a certain period, it retransmits the frame.
  • Scenario of a bug that could happen if we’re not careful:
  • A transmits frame one
  • B receives A1
  • B generates ACK
  • ACK is lost
  • A times out, retransmits
  • B gets duplicate copy of A1 (and sends it on to network layer.)
  • Use a sequence number. How many bits? 1-bit is sufficient for this simple case because only concerned about two successive frames.
  • Positive Acknowledgment with Retransmission (PAR): Sender waits for positive acknowledgment before advancing to the next data item. (Numerous alternatives to this we will see later.)

Chap. 3- DLL

dll protocols36

SIMPLEX PROTOCOL FOR A NOISY CHANNEL

DLL Protocols

/* Protocol 3 (par) allows unidirectional data flow over an unreliable channel. */

#define MAX_SEQ 1 /* must be 1 for protocol 3 */

typedef enum {frame_arrival, cksum_err, timeout } event_type;

#include "protocol.h“

void sender3(void)

{

seq_nr next_frame_to_send; /* Seq number of next outgoing frame */

frame s; /* buffer for an outbound frame */

packet buffer; /* buffer for an outbound packet */

event_type event; /* frame_arrival is the only possibility */

next_frame_to_send = 0;

from_network_layer(&buffer); /* go get something to send */

while (true) {

s.info = buffer; /* copy it into s for transmission */

s.seq = next_frame_to_send; /* insert sequence number in frame */

to_physical_layer(&s); /* send it on its way */

start_timer( s.seq); /* if answer takes too long, time out */

wait_for_event(event(&event); /* frame arrival or cksum err, or timeout */

if ( event == frame_arrival) {

from_physical_layers(&s); /* Get the ACK */

if ( s.ack == next_frame_to_send ) {

from_network_layer( &buffer ); /* get the next one to send */

inc( next_frame_to_send ); /* invert next_frame_to_send */

}

}

}

}

This is Figure 3.11

Chap. 3- DLL

dll protocols37

SIMPLEX PROTOCOL FOR A NOISY CHANNEL

DLL Protocols

void receiver3(void)

{

seq_nr frame_expected;

frame r, s;

event_type event;

while (true) {

wait_for_event(&event); /* only possibility is frame arrival */

if ( frame == event_arrival ) { /* A valid frame has arrived */

from_physical_layer(&r); /* go get the inbound frame */

if ( r.seq == frame_expected ) { /* This is what we’ve been waiting for */

to_network_layer(&r.info); /* pass the data to the network layer */

inc(frame_expected); /* next time expect the other seq # */

}

s.ack = 1 – frame_expected;

to_physical_layer(&s); /* send a dummy frame to awaken sender */

}

}

}

Chap. 3- DLL

dll protocols38

SIMPLEX PROTOCOL FOR A NOISY CHANNEL

DLL Protocols

A Problem unresolved by this protocol is this:

How long should the timer be?

What if too long? (inefficient)

What if too short? A problem because the ACK does not contain the sequence number of the frame which is being ACK'd. So, which frame is being ACK’d?

Scenario:

A sends frame A0

time out of A

resend frame A0

B receives A0, ACKS

B receives A0 again, ACKS again (does not accept)

A gets A0 ACK, sends frame A1

A1 gets lost

A gets second A0 ACK (assumes it’s ACK of A1), sends A2

B gets A2 (rejects, not correct seq. number)

Will lose two frames before getting back on track (with A3)

Chap. 3- DLL

sliding window protocols

Overview

Sliding Window Protocols

These methods provide much more realism!

General method provides buffering with ACKs.

3.1 DLL Design Issues

3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and Verification

Chap. 4- MAC

sliding window protocols40

FEATURES

Sliding Window Protocols
  • Assumptions:
  • Use more realistic Two-way communication.
  • We now have two kinds of frames (containing a "kind" field):
  • Data
  • ACK containing (sequence number of last correctly received frame).
  • Piggybacking - add acknowledgment to data frames going in reverse direction.
  • Piggybacking issue: For better use of bandwidth, how long should we wait for outgoing data frame before sending the ACK on its own.

Chap. 3- DLL

sliding window protocols41

EXAMPLE

Sliding Window Protocols

Example of a sliding window protocol. Contains a sequence number whose maximum value, MaxSeq, is 2n - 1.

For stop-and-wait sliding window protocol, n = 1.

Essentially same as Simplex Protocol, except

ACKs are numbered, which solves early time out problem.

Two-way communication.

Protocol works, all frames delivered in correct order.

Requires little buffer space.

Poor line utilization due to stop-and-wait. (To be solved in next example.)

<<< Figure 3.13 >>>

Chap. 3- DLL

sliding window protocols42

EXAMPLE

Sliding Window Protocols

/* Protocol 4 (sliding window) is bi-directional and is more robust than protocol 3 */

#define MAX-SEQ 1 /* must be 1 for protocol 4 */

typedef enum {frame-arrival, cksum-err, timeout} event-type;

#include "protocol.h"

void protocol4 (void) {

seq-nr next-frame-to-send; /* 0 or 1 only */

seq-nr frame-expected; /* 0 or 1 only */

frame r, s; /* scratch variables */

packet buffer; /* current packet being sent */

event-type event;

next-frame-to-send = 0; /* next frame on the outbound stream */

frame-expected = 0; /* number of frame arriving frame expect */

from-network-layer(&buffer); /* fetch a packet from the network layer */

s.info = buffer; /* prepare to send the initial frame */

s.seq = next-frame-to-send; /* insert sequence number into frame */

s.ack = 1 -frame-expected; /* piggybacked ack */

to-physical-layer(&s); /* transmit the frame */

start-timer(s.seq); /* start the timer running */

Chap. 3- DLL

sliding window protocols43

EXAMPLE

Sliding Window Protocols

while (true) {

wait-for-event(&event); /* frame-arrival, cksum-err, or timeout */

if (event == frame-arrival) { /* a frame has arrived undamaged. */

from-physical-layer(&r); /* go get it */

if (r.seq == frame-expected) {

* Handle inbound frame stream. */

to-network-layer(&r.info); /* pass packet to network layer */

inc(frame-expected); /* invert sequence number expected next */

}

if (r.ack == next-frame-to-send) { /* handle outbound frame stream. */

from-network-layer(&buffer); /* fetch new pkt from network layer */

inc(next-frame-to-send); /* invert sender's sequence numbe

}

}

s.info = buffer; /* construct outbound frame */

s.seq = next-frame-to-send; /* insert sequence number into it */

s.ack = 1 -frame-expected; /* seq number of last received frame */

to-physical-layer(&s); /* transmit a frame */

start-timer(s.seq); /* start the timer running */

}

}

Chap. 3- DLL

sliding window protocols44

OTHER ISSUES

Sliding Window Protocols

Problem with stop and wait protocols is that sender can only have one unACKed frame outstanding.

Example:

1000 bit frames

1 Mbs channel (satellite)

270 ms propagation delay

Frame takes 1msec ( 1000 bits/(1,000,000 bits/sec) = 0.001 sec = 1 msec ) to send. With propagation delay the ACK is not seen at the sender again until time 541msec. Very poor channel utilization. Several solutions are possible:

We can use larger frames, but the maximum size is limited by the bit error rate of the channel. The larger the frame, the higher the probability that it will become damaged during transmission.

Use pipelining: allow multiple frames to be in transmission simultaneously.

Chap. 3- DLL

sliding window protocols45

PIPELINING

Sliding Window Protocols

Sender does not wait for each frame to be ACK'ed. Rather it sends many frames with the assumption that they will arrive. Must still get back ACKs for each frame.

Provides more efficient use of transmit bandwidth, but error handling is more complex.

What if 20 frames transmitted, and the second has an error. Frames 3-20 will be ignored at receiver side? Sender will have to retransmit. What are the possibilities?

Two strategies for receive Window size:

Chap. 3- DLL

sliding window protocols46

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

Go back n - equivalent to receiver's window size of one.

If receiver sees bad frames or missing sequence numbers, subsequent frames are discarded.

No ACKs for discarded frames.

Selective repeat - receiver's window size larger than one.

Store all received frames after the bad one.

ACK only last one received in sequence.

Chap. 3- DLL

sliding window protocols47

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

Tradeoff between bandwidth and data link layer buffer space on the receiver side.

In either case will need buffer space on the sender side. Cannot release until an ACK is received.

Use a timer for each unACK'ed frame that has been sent.

Must be able to enable/disable network layer because may not be able to handle more send data if there are many unACK’d frames

Window Size Rules

Potential problem of window sizes (receiver window size of one):

MaxSeq is 7 (0 through 7) is valid. How big can sender window be?

Send 0-7.

Receive 0-7 (one at a time) and send ACKS

All ACKS are lost

Message 0 times out and is retransmitted

Receiver accepts frame 0 (why? - because that is next frame) and passes it to Network Layer.

So – sender window size must be smaller than MaxSeq.

Look at how this is all put together in <<< Figure 3.16 >>>

Chap. 3- DLL

sliding window protocols48

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

/* Protocol5 (pipelining) allows multiple outstanding frames. The sender may

transmit up to MAX-SEQ frames without waiting for an ack. In addition, unlike

the previous protocols, the network layer is not assumed to have a new packet

all the time. Instead, the network layer causes a network-layer-ready event

when there is a packet to send. */

#define MAX-SEQ 7 /* should be 2^n -1 */

typedef enum {frame-arrival, cksum-err, timeout, network-layer-ready} event-type;

#include "protocol.h"

/* Return true if (a <=b < c circularly;

false otherwise. */

static boolean between(seq-nr a, seq-nr b, seq-nr c) {

if (((a <= b) && (b < c)) || ((c < a) && (a <= b)) || ((b < c) && (c < a)))

return(true);

else

return(false);

}

static void send-data(seq-nr frame-nr,

seq-nr frame-expected, packet buffer[]) {

/* Construct and send a data frame. */

frame s; /* scratch variable */

s.info = buffer[frame-nr]; /* insert packet into frame */

s.seq = frame-nr; /* insert sequence number into frame */

s.ack = (frame-expected + MAX-SEQ) % (MAX-SEQ + 1 ); /* piggyback ack */

to-physical-layer(&s); /* transmit the frame */

start-timer(frame-nr); /* start the timer running */

}

Chap. 3- DLL

sliding window protocols49

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

void protocol5(void) {

seq-nr next-frame-to-send; /* MAX-SEQ > 1; used for outbound stream */

seq-nr ack-expected; /* oldest frame as yet unacknowledged */

seq-nr frame-expected; /* next frame expected on inbound stream */

frame r; /* scratch variable */

packet buffer[MAX-SEQ + 1 ]; /* buffers for the outbound stream */

seq-nr nbuffered; /* # output buffers currently in use */

seq-nr i; /* used to index into the buffer array */

event-type event;

enable-network-layer(); /* allow network-layer-ready events */

ack-expected = 0; /* next ack expected inbound */

next-frame-to-send = 0; /* next frame going out */

frame-expected = 0; /* number of frame expected inbound */

nbuffered = 0; /* initially no packets are buffered */

Chap. 3- DLL

sliding window protocols50

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

while (true) {

wait-for-event(&event); /* four possibilities: see event-type */

switch(event) {

case network_layer_ready: /* the network layer has a packet to send */

/* Accept, save, and transmit a new frame. */

from-network_layer(&buffer[next-frame-to-send]); /* fetch new packet */

nbuffered = nbuffered + I; /* expand the sender's window */

send-data(next_frame-to-send, frame-expected, buffer); /* transmit the frame */

inc(next_frame-to-send); /* advance sender's upper window edge */

break;

case frame-arrival: /* a data or control frame has arrived */

from_physical_layer(&r); /* get incoming frame from physical layer */

if (r.seq == frame-expected) {

/* Frames are accepted only in order. */

to_network-layer(&r.info); /* pass packet to network layer */

inc(frame-expected); /* advance lower edge of receiver's window */

}

/* Ack n implies n- 1, n -2, etc. Check this. */

while (between(ack-expected, r.ack, next_frame_to_send)) {

/* Handle piggybacked ack. */

nbuffered = nbuffered -1; /* one frame fewer buffered */

stop-timer(ack-expected); /* frame arrived intact; stop timer */

inc(ack-expected); /* contract sender's window */

}

break;

Chap. 3- DLL

sliding window protocols51

SLIDING WINDOW MECHANISMS

Sliding Window Protocols

case cksum-err: break; /* just ignore bad frames */

case timeout: /* trouble; retransmit all outstanding frames*/

next-frame-to-send = ack-expected; /* start retransmitting here */

for (i = I; i <= nbuffered; i++){

send-data(next-frame-to-send, frame-expected, buffer); /* resend 1 frame */

inc(next-frame-to-send); /* prepare to send the next one */

}

break;

}

if (nbuffered < MAX-SEQ)

enable_network_layer();

else

disable_network_layer();

}

}

Chap. 3- DLL

sliding window protocols52

PERFORMANCE

Sliding Window Protocols

A few pages back, we defined channel utilization as simply bits-transmitted/capacity. Now we’ll do it again with a bit more precision.

What is the channel efficiency of a stop-and-wait protocol?

F = frame size = D + H = data bits + header bits

C = channel capacity (bps)

I = propagation delay plus processor service time (seconds)

A = ack size (bits)

Draw picture

Time between frames: Time to get frame on wire + Propagation time for frame + Time to get ACK on wire + Propagation time for ACK = F/C + I + A/C + I

Time spent sending data (doing useful stuff): D/C

Efficiency: D/C D D

--------------------- = ------------------------ = ---------------------------

F/C + 2I + A/C F + 2IC + A D + H + 2IC + A

What here helps or hinders efficiency?

Chap. 3- DLL

protocol specification verification

Overview

Protocol Specification & Verification

The issue is an age-old one. How do you specify the operation of a protocol and then assure that it is working correctly.

3.1 DLL Design Issues

3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and Verification

Chap. 4- MAC

protocol specification verification54

How Do You Represent the State You Are In?

Protocol Specification & Verification

PROTOCOL SPECIFICATION AND VERIFICATION:

The goal of this section is to learn methods of representing specs.

State Diagrams are a useful way of verifying that a design is correct and complete.

Look again at <<< Figure 3.11 >>>.

The possible states for this configuration are represented by (XYZ) where

X = <0|1>: Sequence number of frame being sent

Y = <0|1>: Sequence number of frame receiver expects

Z = <0|1|A|->: State of the channel; <Seq. 0|Seq. 1|ACK|empty>

(0,0,0) = sender has sent frame 0, the receiver expects 0, and frame 0 is on the channel.

See how this is represented in the Figure - state diagram.

Useful for determining:

Guarantee that some transitions are NOT possible.

Guarantee that no deadlock possible

(every state has a transition out of it.)

Chap. 3- DLL

examples

HDLC

Examples

HDLC - HIGH LEVEL DATA LINK CONTROL:

Adopted as part of X.25.

A connection oriented 64Kbps network using either virtual or permanent circuits.

Bit oriented (uses bit stuffing and bit delimiters)

3-bit sequence numbers

Up to 7 unACK'ed frames can be outstanding at any time (how big is the receiver's window?)

ACK's the "frame expected" rather than last frame received (any behavior difference between the two? No, as long as the sender and receiver agree on the same convention).

Look at control information in the two Figures.

Chap. 3- DLL

examples56

DLL In The Internet

Examples
  • Point-to-point lines:
  • Between routers over leased lines
  • Dial-up to a host via a modem
  • PPP - Point-to-Point Protocol
  • a Standard (RFCs 1661-1663)
  • Can be used for dial-up and leased router-router lines.
  • Provides:
  • Framing method to delineate frames. Also handles error detection.
  • Link Control Protocol (LCP) for bringing lines up, negotiation of options, bringing them down. These are distinct PPP packets.
  • Network Control Protocol (NCP) for negotiating network layer options.
  • Similar to HDLC, but is character-oriented.
  • PPP doesn’t provide reliable data transfer using sequence numbers and acknowledgments as the default. Reliable data transfer can be requested as an option (as part of LCP).
  • Allows an internet provider to reuse IP addresses. You get to use an address only for the duration of your login.

Chap. 3- DLL

examples57

DLL In ATM

Examples
  • Transmission Convergence (TC) sublayer (refer back to ATM reference model).
  • Physical layer is T1, T3, SONET, FDDI.
  • This sublayer does header check-summing and cell reception.
  • Header Checksum
  • 5-byte header consists of 4 bytes of virtual circuit and control + 1 byte of checksum.
  • Checksum 4 bytes of header information and store in 5th byte.
  • Use CRC checksum x8 + x2 + x + 1 and add a constant 01010101 bit string.
  • Low probability of error (likelihood of fiber) so keep it cheap to checksum. Upper layers can checksum payload if they like.
  • 8-bit checksum field is called Header Error Control (HEC).
  • Idle Cells:
  • May have to output dummy cells in a synchronous medium (must send cells at periodic times). Use idle cells. Also have operation and maintenance (OAM) cells. Exchange control and other information.

Chap. 3- DLL

examples58

DLL In ATM

Examples

Cell Reception:

Drop idle cells , pass along OAM cells.

Need to generate framing information for underlying technology, but no framing bits! Use a probabilistic approach of matching up valid headers and checksums in a 40-bit window.

See the Figure which describes how to get in synch. Have a state-transition diagram where we are looking for d consecutive valid headers.

If a bad cell received (flipped bit) do not immediately give up on synchronization.

Chap. 3- DLL