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<strong>Distributed</strong> <strong>Security</strong><br />

<strong>En<strong>for</strong>cement</strong> <strong>for</strong> <strong>Trusted</strong><br />

<strong>Cluster</strong> <strong>and</strong> <strong>Grid</strong> Computing<br />

Keynote address at the IEEE<br />

International Conference on <strong>Cluster</strong> Computing<br />

(<strong>Cluster</strong> 2003), Hong Kong, Dec. 2, 2003<br />

Kai Hwang<br />

Internet <strong>and</strong> <strong>Grid</strong> Computing Lab<br />

University of Southern Cali<strong>for</strong>nia<br />

Los Angeles, CA. 90089 USA<br />

2/17/2004 1


Presentation Outline:<br />

1. <strong>Distributed</strong> <strong>Grid</strong>Sec Architecture<br />

2. Virtual Private Networks <strong>for</strong> <strong>Distributed</strong><br />

<strong>Security</strong> <strong>En<strong>for</strong>cement</strong><br />

3. Anomaly Intrusion Detection with<br />

Datamining over Network Traffic<br />

4. <strong>Security</strong>-Assured Resource Allocation<br />

5. Wireless Access Control in <strong>Grid</strong> Computing<br />

2/17/2004, Kai Hwang, USC 2


Evolutional Path of<br />

Network-based Computing<br />

Application<br />

to eliminate Resource Isl<strong>and</strong>s<br />

<strong>Grid</strong><br />

grid://…<br />

Application<br />

Web Page<br />

Computer<br />

Web<br />

http://.........<br />

Internet<br />

telnet://.... ,<br />

ftp://.....<br />

Web Page<br />

Computer<br />

Courtesy: Dr. Zhiwei Xu, Chinese Academy of Sciences<br />

2/17/2004, Kai Hwang, USC 3


Trust, <strong>Security</strong>, <strong>and</strong> Privacy<br />

• Trust vs. Risk – The foundation of security <strong>and</strong><br />

privacy in both human society <strong>and</strong> Cyberspace<br />

• <strong>Distributed</strong> Computing <strong>Security</strong> –<br />

More effective with a centralized policy en<strong>for</strong>ced<br />

with a distributed control<br />

• Internet Privacy – Must be protected with<br />

tradeoffs between security constraints <strong>and</strong><br />

privacy dem<strong>and</strong><br />

2/17/2004, Kai Hwang, USC 4


<strong>Security</strong> vs. Scalability<br />

Fully<br />

Secured<br />

Increasing<br />

<strong>Security</strong><br />

No<br />

Protection<br />

A distributed security system supporting<br />

fine-grain resource access with<br />

automatic intrusion prevention,<br />

<strong>Cluster</strong>s/Intranets<br />

detection, <strong>and</strong> responses<br />

protected by gateway based on dynamic security<br />

firewalls under a<br />

policies, adaptive<br />

static policy<br />

cryptographic engines,<br />

Web sites<br />

or LANs<br />

with limited<br />

security<br />

or privacy<br />

protection<br />

with fixed<br />

cryptography<br />

<strong>and</strong> privacy<br />

protection<br />

privacy protection,<br />

<strong>and</strong> special network<br />

security interfaces,<br />

etc.<br />

Web sites/LANs <strong>Cluster</strong>s/Intranets <strong>Grid</strong>s/Internet<br />

Increasing Scalability<br />

2/17/2004, Kai Hwang, USC 5


<strong>Distributed</strong> <strong>Grid</strong>Sec Architecture<br />

Host<br />

Host<br />

3<br />

<strong>Grid</strong><br />

Resource<br />

Domain 1<br />

Host<br />

Host<br />

3<br />

3<br />

Host<br />

3<br />

1<br />

<strong>Grid</strong><br />

Resource<br />

Domain 2<br />

3<br />

3<br />

Host<br />

2<br />

Host<br />

Host<br />

<strong>Security</strong><br />

Manager<br />

3<br />

2<br />

<strong>Grid</strong><br />

Resource<br />

Domain 3<br />

3<br />

3<br />

Host<br />

<strong>Grid</strong><br />

Boundary<br />

Step 1:<br />

Intrusion detected<br />

by a local microfirewall<br />

Step 2:<br />

All security<br />

managers alerted<br />

with the intrusion<br />

Step 3:<br />

<strong>Security</strong> managers<br />

broadcast<br />

response<br />

comm<strong>and</strong> to all<br />

hosts under their<br />

jurisdiction.<br />

(Source: Hwang, et al [1])<br />

2/17/2004, Kai Hwang, USC 6


<strong>Grid</strong>Sec Design Objectives:<br />

• Remove the security barrier hindering distributed grid<br />

computing - Offering a new trust model<br />

• <strong>Grid</strong>Sec offers distributed intelligence in trust<br />

management on top of Globus, AppLes, NimRod etc.<br />

• Dynamic grid resource allocation optimized with respect<br />

to computing power, security dem<strong>and</strong>, <strong>and</strong> cost limit<br />

• Benefiting E-commerce, digital government, public<br />

safety, <strong>and</strong> global economy over the Internet using<br />

<strong>Grid</strong>Sec-based VPN tunneling<br />

2/17/2004, Kai Hwang, USC 7


USC NetShield Defense System<br />

Protecting <strong>Grid</strong> Computing Resources<br />

ISP<br />

The The<br />

Internet<br />

Network<br />

Router<br />

The<br />

NetShield<br />

System<br />

Firewall<br />

Datamining <strong>for</strong><br />

Anomaly Intrusion<br />

Detection (IDS)<br />

Risk<br />

Assessment<br />

System (RAS)<br />

Intrusion<br />

Response<br />

System (IRS)<br />

Victim’s<br />

Internal<br />

Network<br />

2/17/2004, Kai Hwang, USC 8


<strong>Grid</strong>Sec VPN : Combining both IPSec <strong>and</strong> MPLS<br />

Features <strong>for</strong> Federated <strong>Security</strong><br />

Internet Traffic<br />

VPN Traffic<br />

The Internet<br />

<strong>Grid</strong><br />

Resource<br />

Sites<br />

A VPN specially configured on a public Infrastructure based on tunneling at the IPSec<br />

network layer. Same policies as a private network supported by service provider <strong>and</strong><br />

using IPSec, MPLS, PKI, <strong>Grid</strong>Sec, attribute certificates, etc.<br />

2/17/2004, Kai Hwang, USC 9


A<br />

Secure <strong>Grid</strong> Resource Management<br />

supported by VPN<br />

2<br />

3<br />

B<br />

3<br />

2<br />

2<br />

C<br />

7<br />

3<br />

D<br />

2<br />

Resource<br />

Manager<br />

Trust<br />

Manager<br />

F<br />

2<br />

SARA<br />

3<br />

2<br />

3<br />

1<br />

4<br />

5<br />

E<br />

6<br />

VPN<br />

Step 1: Two-way authentication <strong>and</strong> User<br />

request submission to resource manager<br />

(RMgr) in <strong>Grid</strong> resource site F (GRS F).<br />

Step 2: RMgr in GRS F broadcast this request<br />

RMgrs in other GRSs.<br />

Step 3: RMgrs in other GRSs send reply to<br />

RMgr in GRS F with the current available<br />

resource in<strong>for</strong>mation.<br />

Step 4: RMgr in GRS F generates several<br />

possible resource allocation solutions based on<br />

the received in<strong>for</strong>mation, <strong>and</strong> sent back to user.<br />

Step 5: User selects one solution based on its<br />

computing power requirement <strong>and</strong> budget<br />

constraints, <strong>and</strong> reply to RMgr in GRS F.<br />

Step 6: Suppose user selects a resource<br />

allocation combination {A, E, F}, VPN<br />

connections are built between them, <strong>and</strong> user<br />

application is executed at these three GRSs.<br />

Step 7: RMgr in GRS F monitored the<br />

execution of user application <strong>and</strong> update the<br />

trust vector according to the execution quality.<br />

Step 8: Trust propagation: TMgr in each GRS<br />

broadcasts its trust vector periodically. TMgrs<br />

in other GRSs will update their trust vector<br />

accordingly.<br />

2/17/2004, Kai Hwang, USC 10


Developing Virtual Private Networks<br />

<strong>for</strong> Securing <strong>Grid</strong> Computing<br />

• Create encrypted tunnels between private<br />

networks used to <strong>for</strong>m the <strong>Grid</strong> computing<br />

infrastructure<br />

• The <strong>Grid</strong>Sec project chooses an approach<br />

combining the advantages of both IPsec-based<br />

<strong>and</strong> MPLS-based VPNs<br />

• Aimed to satisfy the IPv6 st<strong>and</strong>ards proposed <strong>for</strong><br />

both wired <strong>and</strong> wireless networks <strong>for</strong> the nextgeneration<br />

Internet<br />

(Reference: Hwang, et al [1])<br />

2/17/2004, Kai Hwang, USC 11


<strong>Grid</strong>Sec VPN Design: Built with Encrypted Tunnels,<br />

IPSec, <strong>and</strong> PKI over <strong>Grid</strong> or P2P Resource Sites<br />

Protocol<br />

In VPN<br />

Applications<br />

<strong>Security</strong><br />

Level<br />

<strong>Security</strong><br />

Mechanisms<br />

Where in<br />

Network<br />

IPSec<br />

VPN<br />

Site-to-site VPNs,<br />

off-net VPNs,<br />

extranets, sessions<br />

(DSL, dial-in, etc.)<br />

High<br />

Strong encryption<br />

(3DES), data<br />

authentication (HMAC<br />

<strong>and</strong> SHA-1), user<br />

authentication<br />

(RADIUS <strong>and</strong> PKI)<br />

Best at local<br />

loop <strong>and</strong> Edge,<br />

apply IPSec<br />

tunneling <strong>and</strong><br />

encryption<br />

MPLS<br />

VPN<br />

Site-to-site VPNs<br />

Ultra High<br />

“Tunnel” between<br />

end-points with<br />

same VPN ID<br />

Best within an<br />

ISP’s core<br />

network<br />

<strong>Grid</strong>Sec<br />

VPN<br />

VPNs built over<br />

distributed <strong>Grid</strong> or<br />

P2P networks with<br />

multiple resource<br />

sites<br />

Ultra High<br />

IPSec with multi-site<br />

authentication, VPN<br />

tunnels at network<br />

layer <strong>and</strong> using PKI,<br />

AC, GSI, etc.<br />

Intranet or<br />

extranet within a<br />

common virtual<br />

organization<br />

2/17/2004, Kai Hwang, USC 12


Anomaly-based IDS Architecture<br />

Audit data<br />

Data<br />

preprocessor<br />

Data mining<br />

Engine<br />

Attack-free<br />

episode rules<br />

Intrusion<br />

Detection<br />

Engine<br />

Feature<br />

extraction<br />

Signature<br />

Database<br />

Rules from realtime<br />

traffic<br />

Anomaly<br />

Detection<br />

Engine<br />

Normal<br />

profile<br />

database<br />

Alarm<br />

generator<br />

Alarm generation<br />

<strong>Security</strong> policy<br />

(Ref.: Qin <strong>and</strong> Hwang [3])<br />

2/17/2004, Kai Hwang, USC 13


Testing of the Base-Support Mining Algorithm<br />

on Normal TCP Traffic Connections<br />

from the 1999 DARPA Intrusion Detection Evaluation<br />

Data Sets collected in the first 10 Days<br />

400<br />

Number of episode rules<br />

generated<br />

350<br />

300<br />

250<br />

200<br />

150<br />

100<br />

50<br />

Base-support algorithm, minimum<br />

base-support=0.1<br />

Base-support algorithm w ith<br />

minimum base-support=0.3<br />

level-w ise algorithm, initial support<br />

value=0.1<br />

level-w ise algorithm, initial support<br />

value=0.3<br />

0<br />

1 4 7 10<br />

Training sets in days<br />

Using our base-support mining algorithm with a minimum<br />

confidence value of 0.6 <strong>and</strong> a window size of 30 sec,<br />

compared with using Lee’s Level-wise mining algorithm<br />

2/17/2004, Kai Hwang, USC 14


Pruning of Ineffective Episode Rules<br />

• Transposition Law: The rule: L 1<br />

, L 2,<br />

…, L n<br />

→ R 1<br />

,…., R m<br />

is more effective than using the rule:<br />

L 1<br />

, L 2,<br />

…, L n-1<br />

→ L n<br />

, R 1 ,…., R m<br />

• Elimination Law: The rule L 1<br />

, L 2<br />

→ R 1<br />

(c 1<br />

, s 1<br />

) is less<br />

effective than using : L 2<br />

→R 1<br />

(c 2<br />

, s 2<br />

), if c1 ≈ c2<br />

• Transitive Reconstruction Law: The rule: L 1<br />

→R 1<br />

, R 2<br />

becomes ineffective, if we have the following rules<br />

L 1<br />

→ R 1<br />

<strong>and</strong> R 1<br />

→R 2<br />

already in the rule set<br />

2/17/2004, Kai Hwang, USC 15


Effects of Pruning on the Growth of Frequent Episode<br />

Rules <strong>for</strong> Inter-LAN <strong>and</strong> Intra-LAN Traffic Events<br />

Number of Rules Generated<br />

700<br />

600<br />

500<br />

400<br />

300<br />

200<br />

100<br />

0<br />

Inter-LA N FE R s<br />

Pruned inter-LAN FERs<br />

Intra-LA N FE R s<br />

Pruned intra-LAN FERs<br />

0 20 40 60 80 100 120 140 160<br />

Window Size(Sec)<br />

The base-support = 0.1, the minimum confidence = 0.6, the reference<br />

attributes = destination, <strong>and</strong> axis attributes = service<br />

2/17/2004, Kai Hwang, USC 16


Anomaly Intrusion Detection Rate<br />

35<br />

Total number of attacks detected<br />

30<br />

25<br />

20<br />

15<br />

10<br />

5<br />

0<br />

DoS R2L Probe<br />

Attack category<br />

Intrusions detected by<br />

single packet/connection<br />

anomalies<br />

Intrusions detected by<br />

FER anomalies<br />

Intrusive attacks detected by single packet per<br />

connection versus checking the frequent episode rules<br />

2/17/2004, Kai Hwang, USC 17


Effect of Pruning on Reducing the False<br />

Alarm Rate in Anomaly Intrusion Detection<br />

number of false alarm raised<br />

16<br />

14<br />

12<br />

10<br />

8<br />

6<br />

4<br />

2<br />

0<br />

100 300 500 1000 7200<br />

Scanning period (sec)<br />

Blue bar: Detection without rule pruning<br />

Purple bar: Detection with rule purning<br />

2/17/2004, Kai Hwang, USC 18


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

p0<br />

DDoS<br />

p1<br />

p2<br />

Single-Packet Attack<br />

Multiple-Packet Attack<br />

1. Drop Pack<br />

p3<br />

p4<br />

p5<br />

2. Block Host IP<br />

New<br />

Packets<br />

Running<br />

Process<br />

Stop<br />

Attackers<br />

1. Buffer Packet,<br />

2. Limit Resource<br />

3. Lower Priority<br />

4. Drop Packet<br />

5. Denial Request<br />

from Attacker<br />

6. Block IP<br />

1. Lower Processes Priority or<br />

or Limit Resource Usage<br />

2. Kill Processes<br />

3. Clean Up Service Table<br />

or Memory Usage<br />

4. Restart Service<br />

1. Trace<br />

Back<br />

2. Push<br />

back<br />

(The<br />

Internet)<br />

2/17/2004, Kai Hwang, USC 19


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 20


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 21


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 22


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 23


Optimal or Suboptimal Resource<br />

Allocation Vectors (x1, x2) with<br />

different Per<strong>for</strong>mance/Cost Ratios<br />

.<br />

2/17/2004, Kai Hwang, USC 24


SARA: A Trust Model <strong>for</strong><br />

Securing <strong>Grid</strong> Resources Allocation<br />

GRS i<br />

Local<br />

Resource<br />

Monitor<br />

Resource<br />

Allocator<br />

SARA <strong>for</strong> GRS i<br />

User<br />

<strong>Security</strong><br />

Manager<br />

<strong>for</strong> GRS i<br />

Trust<br />

Manager<br />

Authentication<br />

Manager<br />

Interface<br />

2/17/2004, Kai Hwang, USC 25


Example: Allocating Resources from Two <strong>Grid</strong> Sites<br />

Application Dem<strong>and</strong>: (P o<br />

, T o<br />

, C o<br />

) = (4Tflops, 0.6, $2.25M)<br />

Resource Sit No. 1: R 1<br />

= (1.6Tflops, 0.8, $500K, 6 hosts)<br />

Resource Sit No. 2: R 2<br />

= (1.2Tflops, 0.7, $220K, 5 hosts)<br />

Objective function (Integer Programming):<br />

Subjective to the following constraints:<br />

2/17/2004, Kai Hwang, USC 26


Wireless Access Control of <strong>Grid</strong> Resources<br />

Wireless Ad Hoc Network/<br />

Wireless <strong>Grid</strong><br />

Sensor<br />

Sensor<br />

Sensor Network<br />

Sensor<br />

Internet / Wired <strong>Grid</strong><br />

Digital Camera<br />

Cell phone<br />

PDA<br />

Laptop<br />

Laptop<br />

Wireless Ad Hoc Network/<br />

Wireless <strong>Grid</strong><br />

• Air interfaces, admission control, disconnection h<strong>and</strong>ling, wireless<br />

PKI, security binding, <strong>and</strong> QoS all dem<strong>and</strong> extensive R/D<br />

• The <strong>Grid</strong>Sec VPN supports both wired <strong>and</strong> wireless communications<br />

in distributed cluster, grid , <strong>and</strong> pervasive applications<br />

2/17/2004, Kai Hwang, USC 27


The Architecture <strong>for</strong> Wireless<br />

Connection Admission Control<br />

Wireless<br />

connection<br />

requests from<br />

clients with<br />

traffic profile<br />

<strong>and</strong> the QoS<br />

requirements<br />

Effective<br />

b<strong>and</strong>width<br />

calculation<br />

Trust<br />

evaluation<br />

of client<br />

sources<br />

e ij<br />

t ij<br />

Connection<br />

Admission<br />

Control<br />

(CAC)<br />

Requested<br />

wireless<br />

connection<br />

granted or<br />

denied<br />

Trust in<strong>for</strong>mation on clients<br />

or from traffic monitor<br />

B<strong>and</strong>width constraints (C,<br />

C new <strong>and</strong> C h<strong>and</strong>off in Eqs. 1-3)<br />

Allocate the b<strong>and</strong>width to satisfy the<br />

given QoS <strong>and</strong> security requirements<br />

2/17/2004, Kai Hwang, USC 28


Secure Connection Admission<br />

based on Effective B<strong>and</strong>width Allocation<br />

25<br />

Admissible Connections<br />

20<br />

15<br />

10<br />

5<br />

PB<br />

EB1<br />

EB2<br />

0<br />

0.13M 0.19M 0.26M 0.32M<br />

Allocated B<strong>and</strong>width<br />

Maximum number of admissible connections under different traffic<br />

condition. PB: Peak b<strong>and</strong>width method with zero drop rate,<br />

EB1: Effective b<strong>and</strong>width method 1 with 0.1% loss probability,<br />

EB2: Effective b<strong>and</strong>width method-2 with 1% loss probability)<br />

2/17/2004 29


Maximum Number of Admissible Connections<br />

70<br />

Number of Admissible Connections<br />

60<br />

50<br />

40<br />

30<br />

20<br />

10<br />

100% T rust<br />

90% T rust<br />

70% T rust<br />

0<br />

PB EB1 EB2<br />

EB1: Effective b<strong>and</strong>width method with 0.1% loss probability <strong>and</strong><br />

EB2: Effective b<strong>and</strong>width method with 1% loss probability),<br />

PB: Peak b<strong>and</strong>width allocation method<br />

2/17/2004, Kai Hwang, USC 30


<strong>Grid</strong>Sec Research Team at USC<br />

<strong>and</strong> our International Collaborators:<br />

• Sponsored by a NSF/ITR Research Grant in the USA<br />

• Principal Investigator: Kai Hwang at USC<br />

Co-PI: Clif<strong>for</strong>d Neuman at In<strong>for</strong>mation Science Institute, USC<br />

• Post-doctorial Researchers at ISI/USC<br />

Dr. Tatyana Ryutov <strong>and</strong> Dr. Dongho Kim<br />

• Research Assistants at USC EE <strong>and</strong> CS Departments:<br />

Min Qin, Shanshan Song, Yongjin Kim, Rakesh Rajbanshi,<br />

Ching-Hua Chuan, Gurpreet Grewal, Mikin Macwan,<br />

Narayana Jayaram, Yushun Zhang, Rohil Tripathi, .....<br />

• International Collaborators:<br />

Prof. Michel Cosnard of INRIA, France<br />

Dr. Zhiwei Xu of Chinese Academy of Sciences<br />

Dr. Rajkumar Buyya of Melbourne Univ., Australia<br />

2/17/2004, Kai Hwang, USC 31


Global <strong>Grid</strong>Sec Testing Environment<br />

International Collaborators in USA,<br />

France, China, <strong>and</strong> Australia<br />

INRIA,<br />

Nice,<br />

France<br />

CAS/Vega<br />

Beijing,<br />

China<br />

The <strong>Grid</strong>Sec<br />

over Internet<br />

USC<br />

Gateway,<br />

Los Angeles<br />

Trojan<br />

<strong>Cluster</strong> in<br />

IGC Lab.<br />

USC/ISD<br />

Supercluster<br />

<strong>Security</strong><br />

Policy<br />

Manager<br />

<strong>Security</strong><br />

Database<br />

Melbourne<br />

University,<br />

Australia<br />

USC NetShield Defense<br />

System <strong>and</strong> Testing<br />

Facilities<br />

2/17/2004, Kai Hwang, USC 32


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 33


Intrusion Response Strategies<br />

<strong>for</strong> Defending against DDoS Attacks<br />

2/17/2004, Kai Hwang, USC 34


Conclusions:<br />

• <strong>Grid</strong>Sec <strong>for</strong> protecting distributed resources<br />

• <strong>Security</strong>-assured resource allocation (SARA)<br />

• Local resources <strong>for</strong>tified with NetShield library<br />

• Remote processing through <strong>Grid</strong>Sec VPN tunneling<br />

• Automated intrusion detection <strong>and</strong> response<br />

• Generating anomaly detection rules to build IDS<br />

• Adaptive intrusion response through risk assessment<br />

• Priority defense against DDoS <strong>and</strong> flood attacks<br />

• Continued research tasks <strong>and</strong> future directions:<br />

• Testing SARA <strong>and</strong> NetShield on <strong>Grid</strong>Sec testbed<br />

• Optimize the <strong>Grid</strong>Sec VPN architecture<br />

• Explore wireless <strong>Grid</strong> computing technology<br />

• Integrating pervasive, cluster, <strong>and</strong> <strong>Grid</strong> computing<br />

2/17/2004, Kai Hwang, USC 35


Recent Reports <strong>and</strong> Submitted Papers:<br />

1. K. Hwang, et al, “ <strong>Grid</strong>Sec: A <strong>Distributed</strong> VPN/IDS Architecture <strong>for</strong><br />

Securing <strong>Grid</strong> Computing ”, Technical Report, Internet <strong>and</strong> <strong>Grid</strong><br />

Computing Lab., Univ. of Southern Calif., Dec. 2003 (in preparation)<br />

2. S. Song, K. Hwang, <strong>and</strong> R. Rajbanshi, “<strong>Security</strong>-Assured Resource<br />

Allocation <strong>for</strong> <strong>Trusted</strong> <strong>Grid</strong> Computing”, submitted to IPDPS- 2004,<br />

October 16, 2003<br />

3. M. Qin <strong>and</strong> K. Hwang, “Effectively Generating Frequent Episode<br />

Rules <strong>for</strong> Anomaly-based Intrusion Detection”, submitted to IEEE<br />

Symposium on <strong>Security</strong> <strong>and</strong> Privacy, Nov.3, 2003<br />

4. Y. Kim <strong>and</strong> K. Hwang, “Secure Admission Control <strong>for</strong> Resolving<br />

Wireless Congestion in <strong>Grid</strong> Computing “, submitted to IEEE<br />

Internet Computing Magazine, Nov.27, 2003<br />

2/17/2004, Kai Hwang, USC 36

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