Computer Networks - -DLL

8/8/2019 Computer Networks - -DLL

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Computer NetworksChapter 3

Data Link Layer

Prof. M. Sreenivasa Rao


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Chap. 4- MAC 2

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 CorrectionAdding redundancy in order to find and correct errors.

3.3 DLL Protocols

xxx

3.4 Sliding Window Protocolsxxx

3.5 Protocol Specification and Verification

xxx.

3.6 Examples


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Chap. 4- MAC 3

DLL DesignOverview

The concerns at the Data Link Layer include:

1. What services should be provided to

upper layers?

2. Framing,

3. Error Control.

4. Flow Control.


3.2 Error Detection and Correction

3.3 DLL Protocols

3.4 Sliding Window Protocols

3.5 Protocol Specification and

Verification


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Chap. 3- DLL 4

DLL DesignOverview

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. Senderchecksums 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 errorhas 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.


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Chap. 3- DLL 5

DLL DesignOverview

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.


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Chap. 3- DLL 6

DLL DesignSERVICES PROVIDED TO THE

NETWORK LAYER

Delivery Mechanisms:

Connection-Less

Connection

Oriented

AcknowledgedUN-Acknowledged

Best Effort Better Quality

Reliable Delivery


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Chap. 3- DLL 7


NETWORK LAYER

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 datamay 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?


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Chap. 3- DLL 8


NETWORK LAYER

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.


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Chap. 3- DLL 9


NETWORK LAYER

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.


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Chap. 3- DLL 10

DLL DesignFRAMING

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:


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Chap. 3- DLL 11

DLL DesignFRAMING

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. Ifthe 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.


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Chap. 3- DLL 12

DLL DesignFRAMING

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 theincoming 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.


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Chap. 3- DLL 13

DLL DesignFRAMING

Bit Stuffing:

The receiver converts two consecutive 1's followed by a 0 into two 1's, butrecognizes 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 whenframes are damaged. The receiver simply scans arriving data for the reservedpatterns.

The receiver will re-synchronize quickly with the sender as to where frames beginand end, even when bits in the frame get garbled.

The main disadvantage with bit stuffing is the insertion of additional bits into thedata stream, wasting bandwidth. How much expansion? The precise amountdepends on the frequency in which the reserved patterns appear as user data.


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Chap. 3- DLL 14

DLL DesignFRAMING

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 theframe 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 thechannel 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.


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Chap. 3- DLL 15

DLL DesignFRAMING

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 inbit 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.


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Chap. 3- DLL 16

DLL DesignERROR CONTROL

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.


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Chap. 3- DLL 17

DLL DesignERROR CONTROL

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 beoutstanding at any one time.


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Chap. 3- DLL 18

DLL DesignFLOW CONTROL

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.


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Chap. 4- MAC 19

Error Detection &

Control

Overview

This section is about putting in enough

redundancy along with the data to be

able to detect (and correct) data

errors.



3.3 DLL Protocols



Verification


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Chap. 3- DLL 20

Error Detection &

Control

ERROR CORRECTING CODES

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 ofm data (message) bits and rredundancy bits, yielding an n = ( m + r ) bit

codeword


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Chap. 3- DLL 21

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, all2m

possible data words are legal. However, by choosing check bits carefully, the resultingcodewords 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, afterd errors, the garbled messages is still closer to the

original message than any other legal codeword.


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Chap. 3- DLL 22

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.


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Chap. 3- DLL 23

Error Detection &

Control

ERROR CORRECTION

What's the fewest number of bits needed to correct single bit errors? Let us design a code containing n =

m + rbits that corrects all single-bit errors (rememberm is the number of message (data) bits and ris

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 theoriginal. 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.


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Chap. 3- DLL 24

Error Detection &

Control

ERROR DETECTION

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.


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Chap. 4- MAC 25

DLL PROTOCOLSOverview

How can two DLL layers communicate in

order to assure reliability?

We will look at increasingly complex

protocols to see how this is done.



3.3 DLL Protocols



Verification


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Chap. 3- DLL 26

DLL ProtocolsOverview

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 thedata 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.


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Chap. 3- DLL 27

DLL ProtocolsOUR METHOD

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 thenetwork layer, the physical layer and the timer.


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Chap. 3- DLL 28

DLL ProtocolsBUILDING BLOCKS

#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;


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Chap. 3- DLL 29

DLL ProtocolsBUILDING BLOCKS

/* 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 );


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Chap. 3- DLL 30

DLL ProtocolsAN UNRESTRICTED SIMPLEX

PROTOCOL

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.


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Chap. 3- DLL 31

DLL ProtocolsAN UNRESTRICTED SIMPLEX

PROTOCOL

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

}


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Chap. 3- DLL 32

DLL ProtocolsSIMPLEX STOP-AND-WAIT

PROTOCOL

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.


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Chap. 3- DLL 33

DLL ProtocolsSIMPLEX STOP-AND-WAIT

PROTOCOL

/* Protocol 2 (stop-and-wait) also provides for a one-directional flow of data fromsender 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 buffercapacity and a finite processing speed, so the protocol must explicitly preventthe 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!!


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Chap. 3- DLL 34

DLL ProtocolsSIMPLEX PROTOCOL FOR A NOISY

CHANNEL

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 were not careful:

1. A transmits frame one

2. B receives A1

3. B generates ACK

4. ACK is lost

5. A times out, retransmits

6. 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 concernedabout 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.)


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Chap. 3- DLL 35


CHANNEL

/* 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


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Chap. 3- DLL 36


CHANNEL

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 weve 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 */

}}

}


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Chap. 3- DLL 37


CHANNEL

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 ACKd?

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 A1A1 gets lost

A gets second A0 ACK (assumes its ACK of A1), sends A2

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

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


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Chap. 4- MAC 38

Sliding Window

Protocols

Overview

These methods provide much more realism!

General method provides buffering with

ACKs.



3.3 DLL Protocols



Verification


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Chap. 3- DLL 39

Sliding Window

Protocols

FEATURES

Assumptions:

Use more realistic Two-way communication.

We now have two kinds offrames (containing a "kind" field):

1. Data

2. 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.


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Chap. 3- DLL 40

Sliding Window

Protocols

EXAMPLE

Example of a sliding window protocol. Contains a sequence number whose maximum value, MaxSeq, is2n - 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.)

>


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Chap. 3- DLL 41

Sliding Window

Protocols

EXAMPLE

/* 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 */


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Chap. 3- DLL 42

Sliding Window

Protocols

EXAMPLE

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 */

}

}


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Chap. 3- DLL 43

Sliding Window

Protocols

OTHER ISSUES

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.


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PIPELINING

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:


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SLIDING WINDOW MECHANISMS

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.


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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 unACKd 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 >


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

transmit up to MAX-SEQ frames without waiting for an ack. In addition, unlikethe previous protocols, the network layer is not assumed to have a new packetall the time. Instead, the network layer causes a network-layer-ready eventwhen 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


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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 */


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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;


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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


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PERFORMANCE

A few pages back, we defined channel utilization as simply bits-transmitted/capacity. Now well do it againwith 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 onwire + 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?


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Chap. 4- MAC 52

Protocol

Specification &

Verification

Overview

The issue is an age-old one. How do you

specify the operation of a protocol and

then assure that it is working correctly.



3.3 DLL Protocols



Verification

Protocol


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Protocol

Specification &

Verification

How Do You Represent the State You Are In?

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 >.

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

X = : Sequence number of frame being sentY = : Sequence number of frame receiver expects

Z = : State of the channel;

(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.)


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ExamplesHDLC

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.


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Chap. 3- DLL 55

ExamplesDLL In The Internet

Point-to-point lines:

Between routers over leased linesDial-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 doesnt 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.


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Chap. 3- DLL 56

ExamplesDLL In ATM

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.


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ExamplesDLL In ATM

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

ford consecutive valid headers.

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

Computer Networks - -DLL

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