CyclexAn Efficient Cheating-Resistant MAC Protocol

Jim Pugh

Mentors: Imad Aad, Mario Čagalj, Prof. Jean-Pierre Hubaux

Outline

• Problem with current protocol

• Cyclex protocol description

• How Cyclex solves current problem

• Pros and cons of Cyclex

IEEE 802.11 Cheating Problem

• Nodes can modify their protocol to allow themselves to gain throughput without incurring any penalty (cheat)

• Instead of choosing a randomized backoff time, nodes will choose some low number

• Cheating nodes send packets more often than non-cheating nodes

• Difficult to detect, due to random nature of 802.11

Our Solution: Cyclex

• Alternative to IEEE 802.11 for infrastructure mode

• Uses a turn-based approach to achieve time division multiple access (TDMA) cycle structure

• Cycle structure allows for easy cheating detection

• Operates on two different channels instead of one

• Uses Fast Collision Recovery to preserve order in cycle

System Model

• Infrastructure mode with 1 access point, many nodes

• Reliable communication channels

• Nodes may join or leave network at any time

Two Channel Approach

• Based on “A Jamming-Based MAC Protocol to Improve the Performance of Wireless Multihop Ad Hoc Networks” by Ye et al

• Upon hearing transmission from node on channel A, access point (AP) begins sending “busy tone” on channel B

• Eliminates hidden node problem

Two Channel Approach

Fast Collision Recovery

• Can detect collisions at nodes more quickly using two channel approach

• If error detected in packet header, AP will change from busy tone to “error tone” on channel B

• Transmitting nodes that hear error tone will know collision occurred immediately; can attempt to transmit again sooner than nodes in IEEE 802.11

Fast Collision Recovery

Maintaining Order with Fast Collision Recovery

• In IEEE 802.11, colliding nodes may have long delay before next transmission

• With Fast Collision Recovery, colliding nodes can transmit soon after collision; all other nodes wait longer

• If the maximum random backoff is short enough, we can guarantee that colliding nodes will transmit before any other nodes

• Preserves order, which is important for turn-based algorithm

Full Protocol Description

• Nodes take turn transmitting; specific order

• Transmissions form a “cycle”

• Acknowledgements from access point include node identity; possible for all nodes to track who transmits

Cyclex Operation

Joining the Cycle

• To join, node must either wait X consecutive slot counts of channel idle time or hear a complete “cycle”

• If no nodes present, after X slot counts, node will start a cycle

• If nodes present, after hearing cycle (same node transmit twice), node will insert itself into cycle by preempting next node transmission

Joining the Cycle

Leaving the Cycle

• If node misses transmission, may have no data or may have left network

• If node misses transmission, next node waits one slot count, then transmits; if two nodes in a row miss transmission, next node waits two slot counts; etc

• If node misses Y transmissions in a row, it is removed from cycle

Leaving the Cycle

Collisions

• Nodes in cycle don’t collide since ordered

• Joining nodes don’t collide with nodes in cycle since preempting is faster

• Only collisions occur from nodes joining the cycle simultaneously

• Worst case analysis: 0.15% loss of time

• In case of glitch, fast collision recovery allows order of network to be maintained

Cyclex Cheating Detection

• Cheating difficult to detect in IEEE 802.11 due to random nature

• Cyclex is ordered; easy cheating detection

• Active nodes listen to who transmits between their transmissions; if some node transmits twice, it is cheating

• Once cheating detected, measures can be taken to punish node

Cheating Detection

Pros of Cyclex

• Solves hidden node problem; less collisions occur, and time saved from not using RTS and CTS

• Cheating-resistant

• Very little lost transmission time in stable network operation

Cons of Cyclex

• Two channels needed instead of one

• Busy tone means more power used by access point

• Current design only works in infrastructure mode

Possible Modifications of Protocol

• Consecutive slot counts required before joining network

• Missed transmissions before node removal from cycle

• Adapt protocol for use in ad hoc networks

References• M. Raya, J. P. Hubaux, and I. Aad, “DOMINO: A System to Detect

Greedy Behavior in IEEE 802.11 Hotspots.”• M. Cagalj, S. Ganeriwal, I. Aad, and J. P. Hubaux, “On Cheating in

CSMA/CA Ad Hoc Networks.”• C. E. Koksal, H. Kassab, and H. Balakrishnan, “An Analysis of

Short-Term Fairness in Wireless Media Access Protocols,” in Proc. ACM SIGMETRICS’00, Santa Clara, CA, June 2000.

• J. L. Sobrinho and A. S. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple Access Wireless Networks,” IEEE Journal on Selected Areas in Communications, Vol. 17, No. 8, August 1999.

• S. R. Ye, Y. C. Wang, and Y. C. Tseng, “A Jamming-Based MAC Protocol to Improve the Performance of Wireless Multihop Ad Hoc Networks.”