Countering Selfish Misbehavior in Multi-channel MAC Protocols

al. proposed a system called DOMINO that employs a seriesof statistical detection mechanisms at a trusted access point[16]. A common implicit assumption for [4], [10], [16] is thatnearby nodes can infer the backoff values followed by theirneighbors via overhearing. This is not easily achieved whentransmissions are distributed over multiple channels. Gametheoreticformulations of the impact of MAC layer misbehaviorwere presented in [3], [9].Multi-channel MAC Protocols: Multi-channel MAC protocolscan be classified into three categories: (a) split-phase [5],[18], [23], (b) dedicated control channel [8], [12], [17], [21],and (c) frequency hopping [1], [20]. We limit our related workdescription to the fist category, as our misbehavior strategiesare primarily applicable to split-phase MMACs.So et al. proposed an MMAC protocol that addresses themulti-channel hidden terminal problem [18]. In MMAC, timeis divided to alternating control and data phases. During thecontrol phase, all nodes converge to a default channel tonegotiate the channel assignment for the upcoming data phaseusing a variant of the Distributed Coordinated Function (DCF)of IEEE 802.11. In the data phase, nodes switch to the negotiatedchannels to perform data transmissions. When a nodehas a packet for transmission, it initializes a backoff counterto a random value within [0, cw 0 ], where cw 0 denotes theminimum contention window (CW) in slots. For every elapsedidle slot, the sender decrements its backoff counter by oneunit, while the counter remains frozen when slots are sensed tobe busy. When the backoff counter becomes zero, the sendertransmits an Ad hoc Traffic Indication Message (ATIM), usedas a communication request for the desired destination node.If a collision is detected (based on the timeout of an ATIM-ACK), the sender chooses a new backoff value from [0, cw 1 ],where cw 1 = 2cw 0 . The CW is doubled with every consecutivecollision up to cw max , and is reset to cw 0 after a success.If the ATIM transmission is successful, the destination selectsa channel for the upcoming data phase and replies with anATIM-ACK. The sender confirms the reservation by broadcastingan ATIM-RES packet that echoes the destination’schannel selection. This channel selection is made accordingto a Preferable Channel List (PCL) individually maintained byeach node. At the beginning of each control phase, the priorityof every channel is set to medium (MID). A node i promotesthe priority of channel f j to HIGH if it reserves f j for thefollowing data phase, and demotes the priority of f j to LOWif f j is reserved by any other node. The priority of a channel canbe demoted multiple times (indicated by an associated counter)if multiple reservations are placed on the same channel. Thechannel with the highest priority according to the sender’s andreceiver’s PCL lists is selected, with the receiver’s PCL havinga higher priority than the sender’s (ties are resolved arbitrarily).Channel access during the data phase is contention-based usingthe DCF function, as it is possible that the same channel isselected by multiple communicating pairs.The stages of MMAC are shown in Fig. 1. A set of six nodeslocated within the same collision domain share three channels.f 3ACK RES ATIM ACK RES ATIM ACK RESf 2f 1ATIMC F(f1) C(f1) A D(f2) A(f2) B E(f3) B(f3)Control phasedataB-EdataA-DdataC-FData phaseFig. 1. Channel negotiation process in MMAC [18]. Within parenthesis, weindicate the channel selection included with each packet.Nodes A, B, and C have data packets for nodes D, E, andF , respectively. During the control phase, node C completesa negotiation with F by reserving f 1 . Nodes A, B, D, and Elower the priority of f 1 to LOW, while nodes C and F promotethe priority of f 1 to HIGH. At subsequent negotiations, pairsA-D and B-E choose channels f 2 and f 3 , respectively. Duringthe data phase, all pairs engage in parallel transmissions.Chen et al. proposed MAP [5] which extends MMAC to anadjustable data phase according to the number of successfulnegotiations during the control phase. Zhang et al. proposedTMMAC [23], a TDMA based multi-channel MAC protocolwith a split-phase design. Unlike MMAC and MAP, in TMMACthe control phase is also dynamically adjusted to accommodatevarying traffic loads.III. MODEL ASSUMPTIONSSystem model: We consider a wireless network that operatesover a set of orthogonal frequency bands (channels), denotedby F = {f 1 , f 2 , ..., f n }. All channels are assumed to havethe same bandwidth and propagation characteristics. Nodes areequipped with a single half-duplex radio transceiver with afixed communication range r, independent of the operatingfrequency. Nodes are assumed to be time-synchronized to acommon slotted system. This is a common requirement for bothsplit-phase and channel-hopping multi-channel MAC designs[18], [20]. We consider that nodes coordinate access to the setof channels using a split-phase MMAC (e.g., [5], [18], [23]).Upon detection of misbehavior, we assume that monitoringnodes can make recommendations to a reputation system [2].Nodes with low reputation can eventually be removed fromthe network via a credential revocation process. Finally, communicationsamong nodes can be authenticated using standardcryptographic techniques such as public key cryptography orsymmetric key methods [19].Adversary model: We assume an adversary aiming at gainingan unfair share of the available bandwidth by violating theMMAC specifications. Unfairness is measured in terms of thethroughput achieved by the misbehaving node compared withthe throughput of protocol-compliant nodes. The adversary isassumed to be independently acting (no collusion). Moreover,he only has access to his own cryptographic credentials andcannot compromise the credentials of other nodes. Therefore,he cannot launch impersonation attacks such as Sybil attacks,where he assumes identities of other nodes [11]. Physicallayer attacks such as selective jamming of MMAC packets arebeyond the scope of the present work.