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Monica Gallardo
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Besides, the IEEE 802.16j amendments define signalling messages that are used when discovering routes with a predefined QoS level but do not specify an algorithm that manages the routes establishment and maintenance in case of a handover while providing the required QoS level. Therefore, specific call admission control (CAC) algorithms and route maintenance in case of mobility algorithms should be defined by the network operators with regard to their specific needs. Meanwhile, the IEEE 802.16j amendments do not indicate particular procedures that enable the QoS estimation over the links. For instance, it is not indicated how to evaluate the delay or the throughput value over a wireless link. Besides, it is not indicated how to verify the pretended QoS values over the wireless links.
In this article, we propose an architecture called 3TCA for MMR networks. The 3TCA architecture enables a trusted QoS estimation over wireless links while compensating for the impact of mobile RSs handoff on the affected flows. Thanks to the 3TCACs, the proposed architecture achieves also timely-based and DoS attacks detection and continues delivering the nearly same QoS level despite such attacks. A comparison of the 3TCA architecture and the existing research work is given in Table 1.
The 3TCA architecture is a novel architecture designed for MMR networks which provides QoS and intrusion-tolerance despite group mobility and probable attacks that may target the access nodes (i.e., RSs and MR-BSs). This architecture is based on trusted 3TCACs which offer time-related, security-related and intrusion-related services in order to form a secure communication environment with QoS guarantees. QoS provision is achieved first through securely estimating the QoS parameters using the trusted 3TCACs services and second through compensating the negative impact of handover and a set of attacks on the previously agreed QoS level. The TTCB component defined in the frame of the MAFTIA project lacks threshold management and compensation processing and offers useless services (i.e., such as trusted agreement) for the relay context; therefore, it needs to be revised in order to be adopted within the 3TCA architecture.
After collecting the QoS information of each link, the MR-BS deduces the global QoS that can be guaranteed over the whole path (branch) as the maximum value of delays over the links forming the path, the minimum values of rates over the links forming the path and the maximum value of jitters over the links forming the path. Besides, each MR-BS periodically estimates the QoS over the wireless links between it and its neighboring MR-BSs. The period value may be fixed by the network operator. Note that the QoS estimation procedure should be initiated from the leaf fixed RS to the root MR-BS in order to minimize the number of transmitted signalling messages (i.e., overhead).
When a mobile RS enters a new coverage area, the mobile RS needs to serve multiple service flows issued by its managed SSs. A separate admission control procedure should be executed for each service flow. We think that the available QoS between the managed SSs and the mobile RS needs to be re-estimated since the physical properties of the wireless link may change. The mobile RS needs to adapt the provided QoS in order to compensate the handover disturbance and the probable modification of the available QoS on the edge links (i.e., links between it and the managed SSs). For that reason, we propose that each mobile RS entering a new coverage area initiates an edge QoS re-estimation with each managed SS and then adapts the QoS requirements of each flow issued by the concerned managed SS. After that, the mobile RS sends a DSA-REQ on its name to its superordinate for each flow. That request encompasses the source identifier, the destination identifier and the required QoS in terms of minimum rate, maximum delay and tolerated jitter. Note that in our case, the DSA-REQ will travel from a subordinate mobile RS to its superordinate in order to minimize the overhead.
In our model, the MR-BS already has an estimate of the QoS on the sub-path from the mobile RS to it, the MR-BS can also receive QoS information of other paths on the backbone. The MR-BS may directly route flows which can not tolerate delays using the QoS information of the candidate paths. The MR-BS may also send a DSA-REQ to all the RSs on the candidate path(s) in order to request an admission control decision as specified by the IEEE 802.16j amendments because the QoS information can be updated, especially when the configured period of QoS re-estimation is relatively high.
In this section, we detail the procedures adopted in order to estimate the QoS parameters in a reliable manner using the 3TCAC modules. We also overview the mechanisms that will be used to fulfill handover disturbance compensation.
In order to estimate the jitter on the wireless edge links between the RS and the managed SSs, the RS-3TCAC sends a QoS-REQ signalling message to its payload entity and then triggers the trusted absolute timestamping service. The 3TCAE entity forwards the received packet to the SS which should answer with a first QoS-RSP message. 3TCAE receives the QoS-RSP and then forwards it to the RS-3TCAC which computes the edge delay between the RS and the SS. As the jitter computation requires a synchronization between the entity which will send the equivalent of the RTP packets each 20 ms and the entity which will receive them, the RS-3TCAC and the SS need to have the same clock values.
The delay estimation on the wireless edge links and on the backbone is very similar to the jitter estimation one. More precisely, the delay will be estimated using the same control packets required to compute the jitter. Note that the mobile RS should re-estimate the delay on the edge links in case of handover. Besides, the requested delays for the served flows will be updated in order to compensate the handover delays.
The handover process induces delays mainly caused by the signalling exchange and the switching of the connection to a new access station. Therefore, the additional delays should be compensated in order to provide the level of QoS that was agreed. Besides, we assume that the handing over RS does not start the compensation procedure before performing handover and allowing each managed SS to perform the QoS re-estimation. We propose that when the handing over mobile RS begins the handover process, it triggers the trusted duration measurement service. When the mobile RS attaches to a new access node and the managed SSs terminate the QoS re-estimation procedure, the trusted duration measurement should be stopped in order to compute a trusted value of the duration of the handover process. The obtained delay is then subtracted from the delay values of the current flows if its value is smaller than the previously agreed value. However, if this is not the case or if we may not find an available route that offers the updated value, we may try to subtract a portion of the handover delay using either a linear approach or an exponential approach. Note that regarding the first attempt of QoS compensation, the MR-BS managing the handing over RS may select a route based on the stored values of QoS offered by the routes to the destination. Nevertheless, that first attempt of QoS compensation may fail because the MR-BS may take its decision before the periodic update of the offered QoS.
SSs are mobile entities which can either directly attach to the MR-BSs or access the MR network via RSs. Colluding SSs acting within the same coverage area may try to cause a DoS to their access point by sending multiple requests at the same time in order to saturate the processing capabilities of the managing RS or MR-BS. In order to tolerate such attack, the 3TCAC kernels will be configured to not process more than a threshold value of requests sent by the same SS within a period of time and to not process more than n requests or QoS estimations overall.
The proposed 3TCA architecture achieves a good level of intrusion tolerance since it rapidly detects and tolerates a large range of DoS and timely-based attacks. In particular, 3TCA may detect and tolerate Distributed DoS attacks (i.e., DDoS) that are mainly caused by colluding malicious SSs or colluding malicious RSs. Thanks to the configured threshold values and the synchronous behavior, a 3TCAC component is able to detect any DoS attack bombarding it or any DoS issued by malicious RSs refusing to correctly participate in the QoS estimation or in the data transfer. Moreover, the synchronous behavior of a 3TCA component enables it to easily detect replay attacks and time-based attacks aiming at tampering with the offered QoS. Attacks relative to the integrity of the transmitted messages and the authenticity of the participating entities in the route establishment and data transfer are easily detected and countered thanks to the authentication service guaranteed by the 3TCACs.
Besides, the 3TCA architecture induces a reduced number of false positives since the 3TCACs do not immediately declare an RS as malicious. They rather use two stages of variables and thresholds. If the first threshold is reached, the variable describing the malicious behavior of the RS is incremented until a second threshold is reached. More precisely, an RS is not declared malicious before the second threshold is reached. Therefore, we indirectly take into consideration the non-malicious causes of disturbance such as the saturation of a processor or the lateness or loss of data and signalling packets due to unfavorable transmission conditions. Meanwhile, the 3TCA architecture induces a reduced number of false negatives since it relies on the trusted 3TCACs. As described earlier, 3TCACs provide trusted security and time-related services which enable a rigorous control of the behavior of the malicious entities that are participating in the QoS estimation, the routes establishment and the data transfer.
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