Spring 2006 EE 5304/EETS 7304 Internet Protocols

TO 1-17-06 p. 1

Spring 2006

EE 5304/EETS 7304 Internet Protocols

1-24-2006

Tom OhDept of Electrical Engineering

[email protected]

TO 1-17-06 p. 2

Administrative Issues

Course Website: www.engr.smu.edu/eets/7304

Obtain lectures, video clip, syllabus, homework and homework solutions

Please fill out the Student Information Sheet and email it to me as soon as possible.

If you have any question regarding DVD delivery and other adminstrative issues, please contact Gary McClesky ([email protected], 214-768-3108).

TA: Mr. Poonawala will be my TA for this semester.

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Grade

Because of huge enrollment in this class, I will not include final report as a part of your semester grade.

So final grade will consists of three test grades only.

Grade distribution:

Test 1: 30% Test 2: 30% Test 3: 40% Homework will not be part of your grade.

All tests will be close notes and books.

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Grade (cont)

Test Dates

Test 1 : Feb. 28 (Materials from week 1, 2, 3, 4, 5) Test 2 : Apr. 4 (Materials from week 6, 7, 8, 10 and 11)

• Week 9 is Spring Break Test 3: Final week (May 5- 11) (Materials from week 12,

13, 14, 15 and 16)

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DVD and Distance Learning Students

DVD and DLS students will have additional week to finish the test. For example, DVD students must take Test 1 by Mar. 7.

For DVD and DLS students: If you don’t have proctor at your site, please ask Gary McClesky about test proctor in your area.

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Outline

Types of networks

History

Standards

Terminology

OSI protocol reference model

Text Book (Comer): Ch. 16 Protocols and Layering: pg. 251-pg. 269

Protocol layering principles

Text Book (Comer): Ch. 16 Protocols and Layering: pg. 251-pg. 269

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Outline (cont)

TCP/IP protocol architecture

Text Book (Comer): Ch. 17 Internetworking: Concepts, Architecture, and

Protocols: pg. 273-283)

B-ISDN protocol reference model

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OSI Protocol Reference Model (cont)

7 layers:

7. Application layer

User program that generates or uses data What the user sees

6. Presentation layer

Changes syntax (data format) of information, as necessary

5. Session layer

Establishes and synchronizes sessions (dialogues) between applications

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4. Transport layer

Reliable end-to-end transfer of information between users Establishes and manages connections between users Segments and reassembles information to/from subnet End-to-end error recovery and flow control

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3. Network layer

Establishes and manages connections through network Carries packets from source to destination by routing

between nodes Congestion control

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2. Data link layer

Reliable error-free transmission on link, with flow control and synchronization

Organizes bits into frames, adds acknowledgements and retransmissions

MAC sublayer: controls access to shared medium (ie, in LANs)

1. Physical layer

Point-to-point transmission of unstructured bitstream Depends on physical medium

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OSI Layering Principles

Layers are independent and strictly vertical

Each layer provides services to next higher layer and use services from next lower layer

“Protocol entities" are active in each layer

“Peer entities" in same layer communicate with each other

To send, data is passed down the layers, each layer adds protocol information (eg, header)

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OSI Layering Principles (cont)

To receive, data is passed up the layers, each layer removes protocol information

Protocol defines how the information is used between peer entities in same layer

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Physical

Sending application

Application

Presentation

Session

Transport

Network

Data link

Physical

Receiving application

Application

Presentation

Session

Transport

Network

Data link

DataAH

DataPH

DataSH

DataTH

DataNH

DataDH DT

Bits

Host A Host B

Protocol data unit exchanged

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Difficulties with OSI Model

Not best or only way to divide layers

Session and presentation layers are often minor, network layer is complex

Not followed exactly in practice, more useful as reference

Error control and flow control are repeated in different layers

Not obvious which layer is responsible for encryption, network management, internetworking

OSI model is strictly vertical: all networks conform to single standard protocol in each layer

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TCP/IP Protocol Architecture

1974 DoD standardized protocol suite for connecting other networks to ARPAnet

Internetworking is very important design consideration

TELNETApplication layer HTTP FTP SMTP SNMP

Transport layer TCP UDP

Internet layer IP ICMP

Network access layer Ethernet Radio ... etc (any network protocol)

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TCP/IP Protocol Architecture (cont)

4 layers:

4. Application layer

Telnet, ftp, SMTP, SNMP, HTTP Corresponds to OSI layers 6-7, some 5

3. Transport (host-host) layer

Allows end-to-end communication Connection establishment, error control, flow control Transmission control protocol (TCP), user datagram

protocol (UDP) Corresponds to OSI layers 4, some 5

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2. Internet layer

Route data between different networks Includes addressing, segmentation/reassembly, error

control Implemented in gateways and routers Internet protocol (IP)

• IP packet is basic data unit in Internet Roughly corresponds to OSI layer 3

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1. Network access (host-to-network) layer

Lets host pass data to network with flow control, error control, priority, security

Corresponds to OSI layers 1-2, some 3 Can be any protocol (eg, Ethernet, ATM, X.25) to carry IP

packets

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OSI vs TCP/IP Models

OSI model can be generally applied to most protocol suites, TCP/IP model is specific to Internet

Hierarchy versus layering

OSI does not allow different protocols in same layer OSI allows interactions only between adjacent layers

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OSI vs TCP/IP Models (cont)

TCP/IP recognizes importance of internetworking different networks

Internetworking was inserted into OSI model as sublayer in layer 3

TCP/IP recognizes importance of connectionless service

OSI model recognizes only connection-oriented transport layer

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B-ISDN Protocol Reference Model

3 planes:

User plane: protocols for user data

Control plane: protocols for control (signaling) data

Management plane: protocols for management data (e.g., OAM)

Higher layers Higher layers

ATM adaptation layerCS

SAR

ATM layer

Physical layer

Management plane

Control plane User plane

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B-ISDN Protocol Reference Model (cont)

4 layers in user plane:

4. Services

Originally Class A, B, C, D (obsolete) Now CBR, real-time and nonreal-time VBR, ABR, UBR

3. ATM adaptation layer (AAL)

Originally AAL 1, 2, 3, 4 corresponding to services classes A, B, C, D

Now AAL1, AAL2, AAL3/4, AAL5 Translate user data into ATM cells

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2 sublayers:• Convergence sublayer (CS): service-specific

• Segmentation and reassembly (SAR): divides user messages into 48-byte cell payloads and recombines at receiver

2. ATM layer

Connection-oriented transfer of ATM cells (53-byte packets) across network

Establish, maintain, and terminate virtual connections Routing and congestion control

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1. physical (transmission) layer

Physical signal and framing, e.g., SONET Assumed to be digital, highly reliable, low bit errors

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Wrap Up

Protocol layering is a central design principle in all modern networks, although approaches may differ in some aspects

Examples: OSI reference model, TCP/IP protocol suite

While TCP/IP is prevalent in practice, OSI reference model provides a common conceptual understanding and terminology

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Spring 2006

EE 5304/EETS 7304 Internet Protocols

Tom OhDept of Electrical Engineering

[email protected]

Data Link Layer, ErrorDetection, ARQ

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Outline

Data link layer

Error detection

Stop-and-wait ARQ

Go-back-N ARQ

Selective repeat ARQ

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Data Link Layer (OSI Layer 2)

Physical

Data link

Network

- Unguaranteed, unstructured bitstream transmission on physical channel- Electrical/optical/radio

- Hides details of physical layer from network- Adds reliability, synchronization, flow control

- Routing, congestion control

Layer 1

Layer 2

Layer 3

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Data Link Layer (cont)

Adds synchronization (framing), reliability (error control), and flow control

Depends on quality of physical layer Eg, wireless links are unreliable needing strong error

control, optical fiber links are very reliable needing little error control

Framing

Break bitstream into identifiable frames Frame = block of data + control info. (frame header/trailer)

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Framing

Break bitstream into identifiable frames = block of data + control info. (frame header/trailer)

Commonly use special byte pattern (eg, 01111110) to mark beginning and end of frame

Same byte can mark end of one frame and start of next

Node NodeB

Frame

E B

Frame

E

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Framing (cont)

What if this byte pattern appears in data?

Bit stuffing: insert 0 bit after every 5 consecutive 1's Receiver recognizes this pattern and deletes the stuffed 0

bit

Pattern of 6 consecutive 1's can only mean frame beginning/end

More than 6 consecutive 1's is illegal conditionoriginal:

transmitted after stuffing :

received after destuffing :

0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0

0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 000

stuffed bits

0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0

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Error Control

To detect random bit errors and possibly correct them

Bit errors can cause lost frame (unrecognizable frame) or errored frame (certain bits are wrong)

Commonly handled by ARQ (automatic repeat request) schemes involving error detection + acknowledgements + retransmissions per frame

Stop-and-wait ARQ Go-back-N ARQ Selective-reject (selective repeat) ARQ

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Error Control (cont)

Error control = error detection or error correction

Typically only detection -- more efficient to retransmit (ARQ- automatic repeat request) than correct (FEC- forward error correction) if bit error rate is low

ARQ involves overhead cost per packet + costs for ACKs + costs for retransmissions, but retransmissions will be rare if bit error rate is low

FEC requires more code bits per frame → more overhead cost for every packet even if no errors

Error correction must be used when retransmissions cannot be requested, e.g., simplex links, deep space, audio CDs

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In ARQ schemes, acknowledgements can be positive or negative

Positive acknowledgement: ACK only frames received error-free

No ACK means need to retransmit Sender will retransmit frame after time-out

• Could be slow to respond to lost frame Lost ACKS → unnecessary retransmissions

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Negative acknowledgement (NACK): NACK only errored frames

NACK means need to retransmit Saves bandwidth if errors are rare What if NACK is lost?

• Possible complications

Or both positive + negative acks

Sender will retransmit for NACK or lost ACK/NACK

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Error Detection

For data of m bits, add an error check of r bits

Error check includes some redundancy of data enough to detect errors but not which error

n-bit codeword = m data bits

+ r error check bits

Data

Calculate checksum

Transmit data + checksum

Recalculate checksum from received data

Compare to received checksum

Error No error

=°

Sender

Receiver

m data bits r check bits

2m

“legal” codewords

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Hamming Distance

Hamming distance = minimum number of bit errors to change legal codeword into another legal codeword

To detect d errors, need distance (d+1) code

Takes d+1 or more bit errors to receive legal codeword that was another codeword originally

To correct d errors, need distance (2d+1) code

With d or fewer errors, can always look for closest legal codeword

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Hamming Distance (cont)

Eg, codewords: 0000000000, 0000011111, 1111100000, 1111111111

Distance = 5 → detect up to 4 bit errors

C1 C2

C3 C4

distance = 5

Codewords5

4

5Up to 4 bit errors are detectable as illegal

codewords

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Hamming Distance (cont)

Codewords: 0000000000, 0000011111, 1111100000, 1111111111

Distance = 5 → correct up to 2 bit errors

C1 C2

C3 C4

distance = 5

Up to 2 bit errors are correctable to nearest legal codeword

Codewords5

2

5

2

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Single (even) Parity Bit

Parity=0 if number of 1 bits in data is even, parity=1 if number is odd (ie, parity bit = binary addition of data bits modulo 2)

Valid codewords should always have even number of 1’s

Distance = 2 code → can detect single bit errors (or odd number of bit errors), no error correction

For long frames or bursty errors, even or odd number of bit errors are equally likely → about equally likely to detect errors as not

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3 bits parity bit

0000 00010011 00100101 01000110 01111001 10001010 10111100 11011111 1110

legal illegalExample:

Single bit error will change valid word into an invalid word (detectable);double bit error will change valid wordinto another valid word (undetectable)

→ 23 legal codewordsout of

24 possible codewords

transmission

Received codewords:

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Bit-Interleaved Parity (BIP-N)

1st error check bit = parity bit over 1st bits of all N-bit sequences

2nd error check bit = parity bit over 2nd bits of all N-bit sequences, etc.

Better for detecting bursts, eg, an error burst of length up to N is easily detected

1 1 1 1 1 1 0 0

: : : : : : :

BIP-8: add parity bits for each column

: : : : : : :

:

:

1 1 0 0 1 0 0 1

1 0 0 0 1 0 0 0

last data byte

first data byte

:

:

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Horizontal/Vertical Parity Checks

Arrange data in M x N array, calculate parity bits for M rows and N columns

Lower right corner can be parity check over column or row parity bits

horizontal checks

0 0 1 0 0 1 1 0

1 1 1 1 1 1 0 0

0 0 1 1 0 1 1

vertical checks

1 0 1 0 1 1 0

0

1

1 1 0 0 1 0 0 1

1 0 0 0 1 0 0 0

1

0

0

1

0

0

0 0 1 0 0 1 1 0

1 1 1 1 1 1 0 0

0 0 0 1 0 0 1

4 bit errors may not be detected

1 0 1 0 1 1 0

0

1

1 1 1 0 1 1 0 1

1 0 0 0 1 0 0 0

1

0

0

1

0

0

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Horizontal/Vertical Parity Checks (cont)

Distance = 4 code

Can detect odd number of bit errors in any row or column Can detect up to 3 bit errors and correct any single bit

error But any rectangular pattern of 4 bit errors is undetectable

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IP Example

Internet protocol (IP) checksum

Checksum field is included in IP packet header for error detection

View packet as sequence of 16-bit words, add up all words using ones complement arithmetic

Errors are detected if checksum in header does not match with calculated checksum

• Relatively weak error detection but simple implementation• eg, misordering of 16-bit words is not detectable

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k-bit string is viewed as coefficients of (k-1)-degree polynomial, eg, 1011 represents 3rd-degree 1x3 + 0x2 + 1x + 1

All arithmetic is modulo 2, ie, XOR (binary addition/subtraction without carries/borrows)

Modulo 2 long division is like binary except subtraction is modulo 2

Sender and receiver agree on r-degree generator polynomial G(x)

Both highest order bit and last bit are 1

Cyclic Redundancy Check (CRC) or Polynomial Codes

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CRC (cont)

Compute r-bit checksum and append to m-bit data string such that resulting n-bit codeword is exactly divisible by G(x)

(1) append r zeros to end of m-bit data string M(x) to get (m+r) bits representing xrM(x)

(2) divide (modulo 2) xrM(x) by G(x) (3) r-bit remainder is checksum

Resulting n-bit codeword C(x) is exactly divisible by G(x) with no remainder

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Example: M(x)=10011010, G(x)=1101

1. Append temp. 000 to M(x) x3M(x) = 10011010000

100110100001101110110011101

10001101

10111101

11001101

10001101

101

3. Append remainder to M(x) C(x) = 10011010101

111110012. Divide by G(x)

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CRC (cont)

Receiver divides codeword by G(x), assumes no error only if remainder is 0

Consider error pattern as polynomial E(x)

Received frame = transmitted codeword C(x) + E(x) C(x)/G(x) has no remainder -> any remainder is due to

E(x)/G(x) Errors undetectable only if E(x) is exactly divisible by G(x)

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CRC (cont)

Best choices of G(x) are not evenly divisible by common error polynomials E(x), and

if G(x) has 2 or more terms, then all single-bit errors can be detected

if G(x) has a factor with at least 3 terms, then all double-bit errors can be detected

if x+1 is a factor of G(x), then any odd number of errors can be detected

r-bit checksum will detect all burst errors of length up to r

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CRC (cont)

Commonly used G(x):

CRC-12: x12 + x11 + x3 + x2 + x + 1 CRC-16: x16 + x 15 + x2 + 1 CRC-CCITT: x16 + x 12 + x5 + 1 CRC-32: x32 + x 26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7

+ x5 + x4 + x2 + x + 1

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ARQ Schemes

Automatic repeat request (ARQ) is possible when a feedback channel can be used for acknowledgements

Receiver detects errors but does not attempt to correct

Overhead (retransmissions) is incurred only when necessary, unlike constant overhead in FEC (redundant bits)

More efficient than FEC for low BER

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Stop-and-Wait ARQ

Source sends a frame and waits for ACK or NACK

Receiver returns ACK for correct frame, NACK for errored frame, nothing for destroyed frame

Source retransmits after NACK or time-out

Sends next frame only after valid ACK

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Stop-and-Wait ARQ (cont)

Frame 1 lost

Frame 0 successful

frame 0

Timeout

Timeout

ACK 1

frame 1

frame 1

ACK 0

frame 0

ACK 1

frame 0

ACK 1

Retransmit

Frame 1 successful

ACK 1 lost

Retransmit

Frame 0 successful

Sender Receiver

time

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If ACK is lost, source will retransmit

Receiver will get copy of previous frame Frames must be numbered to avoid ambiguities

Frame X receivedTimeout

frame X

ACK X

frame X

ACK X

ACK X lost

Retransmit

Frame X successful

Sender Receiver

Duplicate received

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If ACK is late, sender will retransmit

Sender may get duplicate ACKs after retransmission Duplicate ACKs must not cause confusion ACKs must be numbered (convention is to give number of

next expected frame)

Frame X receivedTimeout

frame X

ACK X

frame X

ACK X

ACK X is slow

Retransmit

Duplicate ACKs

Sender Receiver

Duplicate received

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One bit sequence number is sufficient

Only ambiguity is between consecutive frames

Simple but inefficient

tframe = time to transmit frame tprop = propagation time If no errors, it takes tframe + 2 tprop time per frame

(ignoring time for ACKs)

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Utilization U = 1/(1 + 2a) where a = tprop/tframe (normalized propagation time)

tframe = 1

ACK

Sender Receiver

Frame

tprop = a

tprop = a

time

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Go-Back-N ARQ

Efficiency can be improved by allowing multiple frames in transit before ACK

Sliding window = up to N unacknowledged frames allowed to be in transit at any one time

Sender must wait for ACK before advancing window to next frame

Example: if last ACKed frame is j, sender can send up to frame j+N before stopping and waiting for ACK of frame j+1

TO 1-17-06 p. 61

Go-Back-N ARQ (cont)

ACKs may be accumulative, ie, ACK(j) will acknowledge all frames up to j

ACKs may be “piggybacked” - carried in the header of a data frame in reverse direction

If frame j is NACKed, sender will go back and resend frame j

For simplicity, all subsequent frames are repeated Destination can discard errored frame j and all subsequent

frames until frame j is received correctly

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timeout

error

0 1 2 3 4 5 6 7 3 4 5 6 7 8 9 10

0 1 2 E D D D D 3 4 5 6 7 8 9 10

frames discardedby receiver

A

ackfram

e

B

time

TO 1-17-06 p. 63


n-bit sequence numbers can support window sizes up to 2n-1

Why not window sizes up to 2n?

Example: assume 3-bit sequence numbers and window size of 8

TO 1-17-06 p. 64

Go-Back-N (cont)

timeout

0 1 2 3 4 5 6 7 0

0 1 2 3 4 5 6 7 0

Window size = 8:

Ack(0) refers to entire window of frames or is it duplicate of previous Ack(0)?

No problem if window size is limited to 7 instead

A

ack(

0)frame

B

ack(

0)time

TO 1-17-06 p. 65

Go-Back-N (cont)

timeout

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Window size = 7:

No ambiguity because Ack(0) cannot re-occur

A

ack(

0)frame

B

ack(

7)time

TO 1-17-06 p. 66

Go-Back-N (cont)

Assume no errors and normalize:

Frame transmission time = 1 Propagation time = a

Case 1: N > 2a + 1

1 2a

N

First Ack returns before the end of window → sender can keep transmitting→ utilization U = 1

TO 1-17-06 p. 67

Go-Back-N (cont)

Assume no errors and normalize:

Frame transmission time = 1 Propagation time = a

Case 2: N < 2a + 1

1 2a

N

First Ack returns after the end of window → sender must stop and wait→ utilization U = N/(1+2a)

TO 1-17-06 p. 68

Selective Repeat ARQ

Go-back-N may be inefficient because several frames are retransmitted for every errored frame, even if some were received properly

Efficiency may be improved if sender retransmits only specific errored frames

If frame j is lost, receiver will continue to receive and buffer frames j+1, j+2,...

When frame j is received without errors, frames must be reordered in proper sequence

TO 1-17-06 p. 69

Selective Repeat (cont)

timeout

error

0 1 2 3 4 5 6 7 3 8 9 10 11 12 13 14

0 1 2 E 4 5 6 7 3 8 9 10 11 12 13 14

frames must be buffered and

reordered after frame 3 received

correctly

A

ackfram

e

B

time

TO 1-17-06 p. 70

Selective Repeat (cont)

Capable of higher efficiency than go-back-N

If no errors, efficiency is same as go-back-N If errors, each error causes one frame retransmission (go-

back-N would retransmit multiple frames)

Not as widely adopted as go-back-N because more difficult implementation

Receiver must buffer frames received after an errored frame and re-order them after that frame is retransmitted

TO 1-17-06 p. 71

Wrap Up

Data link layer adds synchronization, reliability, and flow control above physical layer if needed

Error control for reliability

Usually error detection and retransmissions Stop-and-wait ARQ, go-back-N ARQ, selective repeat

ARQ

Documents

Spring 2006 EE 5304/EETS 7304 Internet Protocols