View
216
Download
0
Tags:
Embed Size (px)
Citation preview
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.”