Hidden Problems with the Hidden Node Problem

Hidden Node Problem A B C Hidden node Outside the TX range of sender but within the range of receiver. C is hidden node! Throughput is decreased. Larger data packets are more vulnerable.

Solution in IEEE 802.11 For unicast packets it s a solved problem Four-way handshake RTS/CTS/DATA/ACK A B

Formally Duration field RTS DATA Sender CTS ACK Receiver Blue/red Node BSY Virtual CS Yellow Node BSY

Problem I: Prohibiting Parallel TX [1] Q A B P

Problem I: Prohibiting Parallel TX [1] Blue nodes can transmit in Parallel Yellow nodes can Receive in Parallel A B Red nodes are helpless!

Detecting Opportunities For Parallel TX If a node receives only RTS Can transmit in parallel If a node receives only CTS Can Receive in parallel If a node receives both RTS + CTS Can do nothing

Detecting Opportunities Does not help! Q A B P Duration field RTS DATA Sender CTS ACK Receiver Blue/red Node BSY Virtual CS Yellow Node BSY

Solution MAC-P [1] What is missing? 1. A time gap between RTS/CTS and Data so that potential nodes for parallel transmission can also exchange RTS-CTS 2. Consequently, an indication in RTS- CTS packets about the start time of Data and ACK packets

Solution MAC-P [1] Transmitting in Parallel RTS/CTS Source Dest T DATA T ACK Q R A B Master Transmission RTS T DATA T ACK DATA A CTS T DATA T ACK ACK B RTS T DATA DATA T ACK Q CTS T DATA T ACK ACK R

Solution MAC-P [1] Transmitting in Parallel RTS/CTS Source Dest T DATA T ACK Q A B P Z Master Transmission RTS T DATA T ACK DATA A CTS T DATA T ACK ACK B RTS DATA Z CTS ACK P

Principle for Parallel transmission A B Q P A B Q P There can be at most one transmitter within the reception range of a receiver A B Q P A B Q P A B Q P

MAC-P Performance Comparison

MAC-P limitations: Packet size for non-master transmission should be less than that of master transmission. Designed for static network. For Mobile Network enabling parallel transmission is still an open problem!

Problem II: RTS/CTS-induced congestion [2]

Problem II: RTS/CTS-induced congestion [2] -MAC layer takes certain number of attempts and then drops. - It looks like congestion although buffer may still have space.

Blocked Node A C B Blocked D RTS

False blocking A C B Blocked RTS D E Falsely blocked

False blocking may propagate! A E C B Blocked RTS D G F

Pseudo deadlock

Solution RTS Validation

Problem III: Virtual Jamming [3] REAL JAMMING (Continuously generate interfering signals) DoS Attack VIRTUAL JAMMING (Pretend that you are transmitting)

Way of V. Jamming Duration field Format of RTS/CTS packet Large value on Duration field. Solution! Duration field is a 16 bit integer number. Deny access -Put for as a high large cap. as 32,767 micro second. -Ignore anything Roughly 30 RTS (30 X 32,767 = 983,010 microsec) packets per second above a is threshold. enough for full Denial of service!

Second kind of attack Attacker Receiver Blue/Red Node BSY BSY Yellow Node BSY BSY

Special scenario

Simulation Model Modified existing implementation of IEEE 802.11 in NS2. TR range equals CS range. Packet size 1024 bytes. Destination is within one-hop i.e. no routing protocol. No error model.

Effect on throughput Without Attack With Attack

Solution: RTS Validation Validate RTS by sensing the channel (Backward compatible!) When to validate?? Attacker Carrier Sense Receiver Blue/Red Node X X BSY Yellow Node BSY

Partial Virtual Jamming Validate RTS by sensing the channel (Backward compatible!) When to validate?? Attacker Carrier Sense Receiver Blue/Red Node BSY Yellow Node BSY

Solution: Random RTS Validation Validate RTS by sensing the channel (Backward compatible!) When to validate?? Attacker Carrier Sense Receiver Node 1 BSY Node 2 BSY

Throughput after Random RTS validation With Attack With Random RTS Validation

More Experimental result Throughput

More Experimental result Delay

Future Direction Experiment with random scenarios. Lone CTS is a great problem! How about in mobile environment? Can we tell with reasonable confidence that the hidden node problem is a solved problem??

Refernces [1] MACA-P: A MAC for Concurrent transmissions in Multi-hop Wireless Networks, Arup Acharya, archan Misra and Sorav Bansal. [2] RTS/CTS-Induced Congestion in Ad Hoc Wireless LANS, Saikat Ray, Jeffrey B. Carruthers and David Starobinski. [3] Protecting Wireless Networks against a Denial of Service Attack Based on Virtual Jamming, Dazhi Chen, Jing Deng, and Pramod K. Varshney. [4] 802.11 Denial-of-Service Attacks: Real Vulnerabilities and Practical Solutions, John Bellardo and Stefen Savage.

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