1 Department of Computer Science University of Virginia New Directions in Reliability, Security and...

1

Department of Computer Science

University of Virginia

New Directions in Reliability, Security and Privacy

in Radio Frequency Identification Systems

Leonid Bolotnyy

lbol@cs.virginia.edu www.cs.virginia.edu/~lb9xk

Gabriel Robins

robins@cs.virginia.edu www.cs.virginia.edu/robins

2

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-Tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

3

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-Tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

4

General RFID System

Tag IDTag ID

TagsReader

Local Server

5

Introduction to RFID

passive semi-passive active

• Tags types:

• Frequencies: Low (125KHz), High (13.56MHz), UHF (915MHz)

• Coupling methods:

readerantenna

signal signal

Inductive coupling Backscatter coupling

6

RFID History

1935 1973

1999

199920042006

1960

What’s next?

7

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

8

Obstacles of Reliable Identification

• Bar-codes vs. RFID– line-of-sight– scanning rate

• Object detection obstacles– radio noise is ubiquitous– liquids and metals are opaque to RF

• milk, water, juice• metal-foil wrappers

– temperature and humidity– objects/readers moving speed– object occlusion– number of objects grouped together– tag variability and receptivity– tag aging

9

Case Studies

• Defense Logistics Agency trials (2001)– 3% of moving objects did not reach destination– 20% of tags recorded at every checkpoint– 2% of a tag type detected at 1 checkpoint– some tags registered on arrival but not departure

• Wal-Mart experiments (2005)– 90% tag detection at case level– 95% detection on conveyor belts– 66% detection inside fully loaded pallets

10

Multi-Tag RFID

Use Multiple tags per object to increase reliability of object detection/identification

11

The Power of an Angle• Inductive coupling: distance ~ (power)1/6

• Far-field propagation: distance ~ (power)1/2

61.86

47.98

58.11

32.7

30

35

40

45

50

55

60

65

1 2 3 4# of Tags

Exp

ecte

d a

ng

le (

Deg

rees

)

22

1 20 0

180 1[ , ]sin( )

2

Max d d

22

0 0

180 1( )sin( )

2 2

d d

• Optimal Tag Placement:

1

4

32

B-field

β

power ~ sin2(β)

12

Equipment and Setup

• Setup– empty room– 20 solid non-metallic & 20 metallic and liquid objects– tags positioned perpendicular to each other– tags spaced apart– software drivers

• Equipment

x1

x1x8

x4

x100’s x100’s

13

Experiments• Read all tags in reader’s field• Randomly shuffle objects• Compute average detection rates

• Variables– reader type– antenna type– tag type– antenna power– object type– number of objects– number of tags per object– tags’ orientation– tags’ receptivity

14

Linear Antennas

Antenna Pair #1, Power = 31.6dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ilit

y

1Tag: 58%

2Tags: 79%

3Tags: 89%

4Tags: 93%

15

Circular Antennas

Antenna Pair #1, Power = 31.6dBm

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ilit

y

1Tag: 75%

2Tags: 94% 3Tags: 98%

4Tags: 100%

16

Linear Antennas vs. Multi-tags

Power = 31.6dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

De

tec

tio

n P

rob

ab

ilit

y

1 Reader, 1 Tag 58.0%

2 Readers, 1 Tag 64.9%

1 Reader, 2 Tags 79.3%

2 Readers, 2 Tags 84.5%

Δ=21.3%

Δ=19.8%Δ= 5.2%

Δ=14.4%

Δ= 6.9%

17

Importance of Tag Orientation

1 Tag 2 Tags 1 Tag 2 Tags180-same 0.55 0.37180-diff 0.74 0.5290-same 0.67 0.5290-diff 0.80 0.63

Circular Linear

0.47 0.3321%

-7%12%25%

18

Detection in Presence of Metals & Liquids

Power=31.6dBm, No Liquids/Metals Power=31.6dBm, With Liquids/Metals

Power=27.6dBm, No Liquids/Metals Power=27.6dBm, With Liquids/Metals

Circular Antenna

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4

Number of Tags

Det

ecti

on

Pro

bab

ilit

y

• Decrease in solid/non-liquid object detection• Significant at low power• Similar results for linear antennas

19

Varying Number of Objects

Experiment 1: 15 solid non-metallic & 15 liquids and metals

Experiment 2: 20 solid non-metallic & 20 liquids and metals

Effect of the Number of Objects on Detection Probability

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags

1 Antenna 2 Antennas 3 Antennas 4 Antennas

Det

ecti

on

Pro

bab

ilit

y

15/15 experiment

20/20 experiment

15/15 experiment

20/20 experiment

Metals & Liquids∆ : 3%-13%

20

Applications of Multi-TagsReliability Availability

Safety

Localization

21

More Applications

Tagging Bulk MaterialsPackaging

Theft PreventionSecurity

22

Economics of Multi-TagsPassive Tag Cost Trend

$0.00$0.20$0.40$0.60$0.80$1.00

2001 2002 2003 2004 2005 2006 2007 2008 2011

Year

Ta

g C

os

t

Historical Cost Prediction Cost

2001 $1.042002 $0.812003 $0.452004 $0.192005 $0.132006 $0.082007 $0.062008 $0.052011 $0.01

Year Cost

• Rapid decrease in passive tag cost• 5 cent tag expected in 2008• 1 penny tag in a few years

23

Cost Trends

Time

24

Multi-Tag Conclusion• Unreliability of object detection

– radio noise is ubiquitous– liquids and metals are opaque to RF

• milk, water, juice• metal-foil wrappers

– temperature and humidity– objects/readers moving speed– object occlusion– number of objects grouped together– tag variability and receptivity– tag aging

• Many useful applications

• Favorable economics$0.00

$0.20

$0.40

$0.60

$0.80

$1.00

2001 2002 2003 2004 2005 2006 2007 2008 2011

Historical Cost Prediction Cost

25

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

26

Motivation• Digital crypto implementations require 1000’s of gates

• Low-cost alternatives– Pseudonyms / one-time pads– Low complexity / power hash function designs– Hardware-based solutions

MD4

7350

MD5

8400

SHA-256

10868

Yuksel

1701

AES

3400

algorithm

# of gates

27

PUF-Based Security

• Physical Unclonable Function [Gassend et al 2002]• PUF security is based on

– wire delays– gate delays– quantum mechanical fluctuations

• PUF characteristics– uniqueness– reliability– unpredictability

• PUF assumptions– Infeasible to accurately model PUF– Pair-wise PUF output-collision probability is constant– Physical tampering will modify PUF

28

Individual Privacy in RFID

• Privacy

A B C

Alice was here: A, B, C

privacy

29

Hardware Tampering Privacy Models

1. Restrict memory tampering functions- allow bit flips

read-proof

tamper-proof

Allow adversary to tamper with tag’s memory

3. Detect privacy compromise - detect PUF modification

2. Purely physical privacy - no digital secrets

Cannot provide privacy without restricting adversary - simple secret overwrite allows tag tracking

30

Private Identification Algorithm

• Assumptions– no denial of service attacks (e.g., passive adversaries, DoS

detection/prevention mechanisms)– physical compromise of tags not possible

• It is important to have – a reliable PUF– no loops in PUF chains– no identical PUF outputs

ID

Requestp(ID)

ID

Database

ID1, p(ID1), p2(ID1), …, pk(ID1)

...IDn, pn(IDn), pn

2(IDn), …, pnk(IDn)

31

PUF-Based Ownership Transfer

• Ownership Transfer

• To maintain privacy we need– ownership privacy– forward privacy

• Physical security is especially important

• Solutions– public key cryptography (expensive)– knowledge of owners sequence– short period of privacy– trusted authority

32

PUF-Based MAC Algorithms

• MAC based on PUF– Motivation: “yoking-proofs”, signing sensor data– large keys (PUF is the key)– cannot support arbitrary messages

• MAC = (K, τ, υ)

K

K

• valid signature σ : υ (M, σ) = 1

• forged signature σ’ : υ (M’, σ’) = 1, M = M’

• Assumptions– adversary can adaptively learn poly-many (m, σ) pairs– signature verifiers are off-line– tag can store a counter (to timestamp signatures)

33

Large Message Space

σ (m) = c, r1, ..., rn, pc(r1, m), ..., pc(rn, m)

Assumption: tag can generate good random numbers (can be PUF-based)

Signature verification• requires tag’s presence• password-based or in radio-protected environment (Faraday Cage)• learn pc(ri, m), 1 ≤ i ≤ n• verify that the desired fraction of PUF computations is correct

To protect against hardware tampering• authenticate tag before MAC verification• store verification password underneath PUF

Key: PUF

34

Small Message SpaceAssumption: small and known a priori message space

Key[p, mi, c] = c, pc(1)(mi), ..., pc

(n) (mi)

PUFmessage

counter

σ(m) = c, pc(1)(m), ..., pc

(n) (m),

..., c+q-1, pc+q-1

(1)(m), pc+q-1(n)(m)

sub-signature

Verify that the desired number of sub-signatures are valid

PUF reliability is again crucial

35

Attacks on MAC Protocolsoriginal clone

• Impersonation attacks– manufacture an identical tag– obtain (steal) existing PUFs

• Hardware-tampering attacks– physically probe wires to learn the PUF– physically read-off/alter keys/passwords

• Side-channel attacks– algorithm timing– power consumption

• Modeling attacks– build a PUF model to predict PUF’s outputs

36

Conclusions and Future Work

Hardware primitive for RFID security

Identification, MAC, Ownership Transfer, and Tag Authentication Algorithms

• Properties:– Physical keys– Protect tags from physical attacks– New attack models

• Future Work:– Design new PUF– Manufacture and test PUF– Develop PUF theory– New attack models

37

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

38

Inter-Tag Communication in RFID

• Idea: Heterogeneity in ubiquitous computing

• Applications:

39

“Yoking-Proofs”

• Applications – verify that:– medicine bottle sold together with instructions– tools sold together with safety devices– matching parts were delivered together

– several forms of ID were presented

• Problem Statement: Generate proof that a group of passive tags were identified nearly-simultaneously

• Key Observation: Passive tags can communicate with each other through reader

• Yoking: joining together / simultaneous presence of multiple tags

40

Assumptions and Goals• Assumptions

– Tags are passive– Tags have limited computational abilities– Tags can compute a keyed hash function– Tags can maintain some state– Verifier is trusted and powerful

• Solution Goals– Allow readers to be adversarial– Make valid proofs improbable to forge– Allow verifier to verify proofs off-line– Detect replays of valid proofs

• Timer on-board a tag– Capacitor discharge can implement timeout

41

Generalized “Yoking-Proof” Protocol

1

3

2

45

Anonymous Yoking: tags keep their identities private

Idea: construct a chain of mutually dependent MACs

42

Related Work on “Yoking-Proofs”

• Saito and Sakurai [2005]– solution relies on timestamps generated by trusted database– violates original problem statement– one tag is assumed to be more powerful than the others– vulnerable to “future timestamp” attack

• Piramuthu [2006]– discusses inapplicable replay-attack problem of Juels’ protocol– independently observes the problem with Saito/Sakurai protocol– proposed fix only works for a pair of tags– violates original problem statement

• Juels [2004]– protocol is limited to two tags

– no timely timer update (minor/crucial omission)

43

Talk Outline• Introduction to RFID

• Reliable Object Identification– Multi-tag RFID Systems

• Physical Security and Privacy– PUF-Based Algorithms

• Inter-Tag Communication– Generalized Yoking-Proofs

• Common Themes and Conclusion

44

Generalized “Yoking-Proofs”

Multi-Tags PUF-BasedSecurity and Privacy

RFID

Common Themes

45

Conclusion and Future Research

• Contributions

• Future Research– More multi-tag tests– Object localization using multi-tags– Split tag functionality between tags– Prevent adversarial merchandize inventorization– PUF design– More examples of inter-tag communication– Applications of RFID

46

Publications• L. Bolotnyy and G. Robins, Multi-tag Radio Frequency Identification Systems, IEEE Workshop on Automatic

Identification Advanced Technologies (Auto-ID), Oct. 2005.

• L. Bolotnyy and G. Robins, Randomized Pseudo-Random Function Tree Walking Algorithm for Secure Radio-Frequency Identification, IEEE Workshop on Automatic Identification Advanced Technologies (Auto-ID), Oct. 2005.

• L. Bolotnyy and G. Robins, Generalized “Yoking Proofs” for a Group of Radio Frequency Identification Tags , International Conference on Mobile and Ubiquitous Systems (Mobiquitous), San Jose, CA, July 2006.

• L. Bolotnyy and G. Robins, Physically Unclonable Function -Based Security and Privacy in RFID Systems , IEEE International Conference on Pervasive Computing and Communications (PerCom), New York, March 2007.

• L. Bolotnyy, S. Krize, and G. Robins, The Practicality of Multi-Tag RFID Systems, International Workshop on RFID Technology - Concepts, Applications, Challenges (IWRT), Madeira, Portugal, June 2007.

• L. Bolotnyy and G. Robins, The Case for Multi-Tag RFID Systems, International Conference on Wireless Algorithms, Systems and Applications (WASA), Chicago, Aug. 2007.

• L. Bolotnyy and G. Robins, Multi-Tag RFID Systems, International Journal of Internet and Protocol Technology, Special issue on RFID: Technologies, Applications, and Trends, 2(3/4), 2007.

• 1 conference and 1 journal paper in submission

• 2 invited book chapters in preparationSecurity in RFID and Sensor Networks, to be published by Auerbach Publications, CRC Press, Taylor&Francis Group

47

More Successes

• Deutsche Telekom (largest in EU) offered to patent our multi-tags idea.

• Received $450,000 NSF Cyber Trust grant, 2007 (PI: Gabriel Robins).

• Technical Program Committee member:International Workshop on RFID Technology - Concepts, Applications, Challenges (IWRT), Barcelona, Spain, June 2008.

• Our papers and presentation slides used in lecture-based undergraduate/graduate courses (e.g., Rice University,

George Washington University).

48

49

Thank You!

Questions?

Dissertation Committee: Gabriel Robins (advisor), Dave Evans, Paul Reynolds, Nina Mishra, and Ben Calhoun

Stephen Wilson, Blaise Gassend, Daihyun Lim,Karsten Nohl, Patrick Graydon, and Scott Krize

lbol@cs.virginia.edu www.cs.virginia.edu/~lb9xk

50

BACK UP SLIDESNOT USED DURING

PRESENTATION

51

Types of Multi-Tags

• Triple-Tags

• n-Tags

• Dual-Tags– Own Memory Only– Shared Memory Only– Own and Shared Memory

• Redundant Tags

• Complimentary Tags

52

Controlling Variables1. Radio noise

2. Tag variability

3. Reader variability

4. Reader power level

5. Distance to objects &type, # of antennas

53

Circular Antennas vs. Multi-Tags

Power = 31.6dBm

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ility

1 Reader, 1 Tag 75.9%

2 Readers, 1 Tag 91.0%

1 Reader, 2 Tags 94.2%

2 Readers, 2 Tags 99.4%

Δ=18.3%

Δ=8.4%Δ= 5.2%

Δ=3.2%

Δ= 15.1%

54

Linear Antennas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

31.6 30.6 29.6 28.6 27.6 26.6 25.6

Power (dBm)

Det

ectio

n P

rob

abili

ty

Circular Antennas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

31.6 30.6 29.6 28.6 27.6 26.6 25.6

Power (dBm)D

etec

tion

Pro

bab

ility

1 Tag 2 Tags 3 Tags 4 Tags

Power

• Decrease in detection with decrease in power• More rapid decrease in detection for circular antennas

55• Low detection probabilities• Drop in detection at low power

• Linear antennas outperform circular• Multi-tags better than multiple readers

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags

Antenna #1 Antenna #2 Antenna #1 and #2

Number of Tags

De

tec

tio

n P

rob

ab

ilit

y

Power=31.6dBm, Circular AntennasPower=31.6dBm, Linear AntennasPower=27.6dBm, Circular AntennasPower=27.6dBm, Linear Antennas

Multi-Tags on Metals and Liquids

56

Detection Delta

Change in Detection Based on # of Antennas and Tags

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1 Antenna 2 Antennas 3 Antennas 4 Antennas

Ch

ang

e in

Det

ecti

on

Pro

bab

ilit

y

1 tag

2 tags

3tags

1 tag

2 tags

3tags

1 tag

2 tags

3tags

1 tag

2 tags

3tags

0.036

0.030

0.029

0.014

57

Anti-Collision Algorithms

Binary No Effect No Effect

Binary Variant No Effect No Effect

Randomized Linear Increase** No Effect*

STAC Causes DoS No Effect*

Slotted Aloha Linear Increase** No Effect*

Algorithm Redundant Tags Connected-Tags

* Assuming tags communicate to form a single response** If all tags are detected

58

Business Case for RFID

• Costs & benefits (business case)– Moore’s law– higher employee productivity– automated business processes– workforce reduction

• Tag manufacturing yield and testing– 30% of chips damaged during manufacturing– 15% damaged during printing [U.S. GAO]– 20% tag failure rate in field [RFID Journal]– 5% of tags purchased marked defective

59

RFID Tag Demand

• Demand drivers– tag cost– desire to stay competitive

• Cost effective tag design techniques– memory design (self-adaptive silicon)– assembly technology (fluidic self assembly)– antenna design (antenna material)

Increase in RFID tag demand

Decrease in RFID tag cost

60

Thesis

Multi-tags can considerably improve reliability in RFID systems at a reasonable cost;

effective PUF implementations can enable hardware-tampering resistant algorithms for RFID security and privacy;

generalized yoking-proofs can provide auditing mechanisms for the near-simultaneous reading of multiple RFID tags.

61

Related Work on PUF

• Optical PUF [Ravikanth 2001]

• Silicon PUF [Gassend et al 2002]– Design, implementation, simulation, manufacturing– Authentication algorithm– Controlled PUF

• PUF in RFID– Identification/authentication [Ranasinghe et al 2004]– Off-line reader authentication using public key cryptography

[Tuyls et al 2006]

62

Privacy Model

1. A passive adversary observes polynomially-many rounds of reader-tag communications with multiple tags

2. An adversary selects 2 tags

3. The reader randomly and privately selects one of the 2 tags and runs one identification round with the selected tag

4. An adversary determines the tag that the reader selected

Experiment:

Definition: The algorithm is privacy-preserving if an adversary can notdetermine reader selected tag with probability substantially greater than ½

Theorem: Given random oracle assumption for PUFs,an adversary has no advantage in the above experiment.

63

Improving Reliability of Responses• Run PUF multiple times for same ID & pick majority

μm(1-μ)N-m )kR(μ, N, k) ≥ (1 - ∑

N Nm

N+12

m=

number of runs

chain lengthunreliabilityprobability

overallreliability

R(0.02, 5, 100) ≥ 0.992

• Create tuples of multi-PUF computed IDs &identify a tag based on at least one valid position value

∞expected numberof identifications

S(μ, q) = ∑ i [(1 – (1-μ)i+1)q - (1 – (1-μ)i)q]i=1

tuple size

S(0.02, 1) = 49, S(0.02, 2) = 73, S(0.02, 3) = 90

(ID1, ID2, ID3)

64

Choosing # of PUF Computations

α < probv ≤ 1 and probf ≤ β ≤ 1

0 ≤ t ≤ n-1

i=t+1

μi(1-μ)n-iprobv(n, t, μ) = 1 - ∑

nni

j=t+1

τj(1-τ)n-jprobf(n, t, τ) = 1 - ∑

nnj

probv(n, 0.1n, 0.02)

probf(n, 0.1n, 0.4)

65

MAC Large Message Space Theorem

Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded from above by the tag impersonation probability.

66

MAC Small Message Space Theorem

Given random oracle assumption for a PUF, the probability that an adversary could forge a signature for a message is bounded by the tag impersonation probability times the number of sub-signatures.

67

Purely Physical Ownership Transfer

oid = h(counter)

r1, a = hs(r0, r1)

r0, c1, ..., cn

(r1, a)

Challenges sent to tag in increasing order

counter = counter - 1hs(r1, new)

• Properties:– All PUF computations must be correct– PUF-based random number generator– Physical write-once counter– oid is calculated for each identification– Inherently limited # of owners

s = poid(v1) ... poid(vn)

v1 = h(c1), ..., vn = h(cn)

++

68

s2,4

s1,2

s3,9

s2,5

s3,10s3,8

Using PUF to Detect and Restore Privacy of Compromised System

1. Detect potential tag compromise2. Update secrets of affected tags

s1,0

s2,0

s1,1

s2,1

s3,1

s2,2 s2,3

s3,0 s3,4 s3,5s3,2 s3,3 s3,7s3,6

69

PUF vs. Digital Hash Function

• Reference PUF: 545 gates for 64-bit input– 6 to 8 gates for each input bit– 33 gates to measure the delay

• Low gate count of PUF has a cost– probabilistic outputs– difficult to characterize analytically– non-unique computation– extra back-end storage

• Different attack target for adversaries– model building rather than key discovery

• Physical security– hard to break tag and remain undetected

MD4

7350

MD5

8400

SHA-256

10868

Yuksel

1701

PUF

545

AES

3400

algorithm

# of gates

70

PUF Design• Attacks on PUF

– impersonation– modeling– hardware tampering– side-channel

• Weaknesses of existing PUF

• New PUF design– no oscillating circuit– sub-threshold voltage

• Compare different non-linear delay approaches

reliability

71

PUF Contribution and Motivation

Contribution• Physical privacy models• Privacy-preserving tag identification algorithm• Ownership transfer algorithm• Secure MAC algorithms• Comparison of PUF with digital hash functions

Motivation• Digital crypto implementations require 1000’s of gates• Low-cost alternatives

– Pseudonyms / one-time pads– Low complexity / power hash function designs– Hardware-based solutions

72

Speeding Up The Yoking Protocol

starting / closing tags

Idea: split cycle into several sequences of dependent MACs

Requires– multiple readers or multiple antennas

– anti-collision protocol