Final Report. Nuno José Pereira Farias Rodrigues. Orientadores: Prof. Manuel Ricardo Pedro Fortuna

Final Report Nuno José Pereira Farias Rodrigues Orientadores: Prof. Manuel Ricardo Pedro Fortuna Mestrado Integrado em Engenharia Electrotécnica e de Computadores Faculdade de Engenharia da Universidade do Porto 8 de Fevereiro de 2008

Index 1. Introduction...3 2. Ad-Hoc...4 3. Wireless Mesh Networks...6 4. IEEE 802.11s...8 4.1. Introduction...8 4.2. Architecture...8 4.3. Frame Formats...11 4.4. WLAN Mesh Medium Access Control...13 4.4.1. Multichannel Medium Access Control...13 4.4.2. Mesh Deterministic Access...15 4.5. Boot Sequence...16 4.5.1. Neighbor Discovery...17 4.5.2. Peer Link Setup...18 4.5.3. Link State Discovery...19 4.5.4. Path Selection...20 4.6. Hybrid Wireless Mesh Protocol (HWMP)...22 4.6.1. On-Demand Routing in HWMP...23 4.6.2. Tree based Routing in HWMP...24 4.6.3. Radio-Aware Optimized Link-State Routing Protocol...26 4.7. WLAN Mesh Security...28 4.8. Conclusion...29 5. Discrete Events Simulation...30 6. References...32 Nuno José Rodrigues Page 2

1. Introduction This document describes the work in progress for the course. The course objectives are to prepare the MSc dissertation. This includes selecting the dissertation subject, to research the state-of-the-art, and to define a work plan. The goal of this dissertation is to study the IEEE 802.11s Mesh Networks [1] in a metropolitan scenario. The main goal is to determine if IEEE 802.11s Mesh Networks scale up to a few thousands of nodes. The study will be conducted by means of simulation using the NS-3 [2] for a network size up to 5000 nodes. The scalability will be determined by simulating a metropolitan scenario various times with an increasing number of 802.11s enabled nodes, and measuring performance metrics such as the end-to-end delay, packet loss ratio, and overhead due to control plane traffic; and by studying how the value of these parameters change. To help this goal, this document exposes IEEE 802.11s and the networks it is based on. This document is organized as follows: Section 2 introduces the principles of Ad-Hoc networking. Next, in Section 3, Wireless Mesh Networks are briefly explained. Next, in Section 4 the 802.11s Wireless Mesh Extension is detailed. Finally, Section 5 provides a brief reference to Discrete Event Simulation. Nuno José Rodrigues Page 3

2. Ad-Hoc An Ad-hoc network is the cooperative engagement of a collection of mobile nodes without the required intervention of any centralized access point or existing infrastructure. Topology may change dynamically due to node mobility. A Mobile Ad-Hoc NETwork (MANET) can be defined as an autonomous collection of mobile nodes which communicate over wireless links. MANETs have a wide range of applications from military to emergency networks. Ad-Hoc networks forward packets at layer 3 using IP Ad-Hoc Routing protocols. Other Ad-Hoc networks characteristics are: automatic network configuration, it can be used in as an intranet or internet network and using routing network techniques as every node forward data. An Ad-Hoc node builds a forwarding table dynamically so it can calculate the path with lower cost between it and other nodes. Ad-Hoc networks have frequent changes in its quality of the connection and its topology might transform frequently. These changes make the nodes consume more energy because they cannot enter in a power saving state as the infrastructureless networks protocol does. To solve these problems, MANETs use 2 types of Ad- Hoc routing protocols: reactive and proactive. Reactive Ad-Hoc routing protocols calculate routes on-demand, by flooding the network with a Route Request (RREQ) packet. This introduces further delay because nodes have to calculate the route before sending packets but nodes only use network resources when it is necessary. The most common used reactive protocol in Ad-Hoc networks is the Ad-Hoc On- Demand Distance Vector (AODV) Routing [8]. Proactive Ad-Hoc routing protocols try to maintain permanent routes by using continuous traffic control to calculate optimal paths. It also detects the connection to node Nuno José Rodrigues Page 4

neighbors, using HELLO messages, and floods the network with MultiPoint Relaying which is a sort of optimized flood because it limits the number of nodes which retransmit packets. The most used proactive protocol in Ad-Hoc networks is Optimized Link-State Routing protocol (OLSR). OLSR performs a distributed election of a set of multipoint distribution relays (MPRs) that play the role of designated routers as in Ad-Hoc networks, there is no notion of a link; hence, this approach is needed in order to optimize the flooding process. The election process leads to a set of MPRs that has the property that any node is the neighbor of an MPR, and that any two MPRs are at most separated by a single node (i.e. the 2-neighbour graph of the MPRs is connected). Nuno José Rodrigues Page 5

3. Wireless Mesh Networks Wireless mesh network (WMN) is a communications network made up of radio nodes in which there are at least two pathways of communication to each node. The coverage area of the radio nodes working as a single network becomes a mesh cloud. Access to this mesh cloud is dependent on the radio nodes working in harmony with each other to create a radio network. A mesh network is reliable and offers redundancy. The main characteristic of wireless mesh networking is the communication between nodes over multiple wireless hops on a meshed network graph similar to an Ad-Hoc wireless network. Efficient routing protocols provide paths through the wireless mesh and react to dynamic changes in the topology, so that mesh nodes can communicate with each other even if they are not in direct wireless range. Intermediate nodes on the path will forward the packets to the destination. Unlike cellular networks where the failure of a single base station (BS) may lead to the unavailability of communication services over a certain geographical area, WMNs provide fault tolerance even when a number of nodes fail. Although Ad-Hoc wireless networks are similar to WMNs, the protocols and architectures designed for the Ad-Hoc wireless networks perform poorly when applied in the WMNs. In addition, factors such as the inefficiency of protocols, interference from external sources sharing the spectrum, and the scarcity of electromagnetic spectrum further reduce the capacity of a single-radio WMN. In order to improve the capacity of WMNs and for supporting the traffic demands raised by emerging applications for WMNs, multi-radio WMNs (MR-WMNs) are under intense research. Therefore, recent advances in Nuno José Rodrigues Page 6

WMNs are mainly based on a multi-radio approach. While MR-WMNs promise higher capacity compared with single-radio WMNs, they also face several challenges. Nuno José Rodrigues Page 7

4. IEEE 802.11s 4.1. Introduction In 2003, interests in the Institute of Electronics and Electrical Engineering (IEEE) 802.11 Working Group (WG) led to the formation of Task Group (TG) S. IEEE 802.11s is being developed to improve WMNs. This group is actively working to provide wireless mesh networking extensions to their standards. The goal of the 802.11s Working Group is to amend the IEEE 802.11 MAC protocol to enable both broadcast/multicast and unicast frame delivery services at the MAC layer using radio-aware metrics. 4.2. Architecture A WLAN mesh network is a fully IEEE 802.11-based wireless network that employs multihop communications to forward traffic to and from wired Internet entry points, between nodes and between different mesh networks. A WLAN mesh network uses 802.11 based physical layer device and medium access (MAC) for providing the functionality of an Extended Service Set (ESS) mesh network. The 802.11 Access Point (AP), known as mesh point (MP) when used in WLAN mesh, establishes wireless links among each other to enable automatic topology learning and dynamic path configuration. The MP-to-MP links form a wireless backbone as mesh backhaul, which provides users with low-cost, high bandwidth and seamless multi-hop interconnection services with a limited number of Internet entry points and with other users Nuno José Rodrigues Page 8

within the network. Each MP may optionally provide services to support communication with legacy mobile stations (STAs). These devices are called mesh access points (MAPs). As shown below, the IEEE 802.11 basic service set (BSS) consists in one stationary Access Point (AP) and in the mobile stations (STAs). To cover a large area, an Extended Service Set (ESS) Figure 1 - IEEE 802.11s Architectural Model is required. An ESS is formed by several APs interconnected by the Distribution Service (DS), usually a wired network. The IEEE 802.11s introduces new elements to the ESS: Mesh Point (MP), Mesh Access Point (MAP), and Mesh Portal (MPP). MP establishes links with other MP neighbors, full participant in WLAN Mesh services. Otherwise, MAP has the same functionality of a MP and it provides BSS services to support communication with STAs. MPP is a point at which MAC Service Data Units (MSDUs) exit and enter a WLAN Mesh. There is an extra device, Nuno José Rodrigues Page 9

called Light Weight MP (LWMP) that participates in subset of the WLAN Mesh services primarily for neighbor-link communication. Figure 2 illustrates the relation between the different mesh nodes types. Figure 2 - Relation between different IEEE 802.11 mesh nodes MPs to communicate with Access Points or wired internet points are through MAP. MAPs have a fundamental role in IEEE 802.11s because they make this protocol backwards compatible. MAPs can also connect MPs from a different mesh. MPP is also very important because in it relies (on higher layer) bridging between the mesh network and the wired network. Nuno José Rodrigues Page 10

4.3. Frame Formats WLAN mesh frame formats reuse IEEE 802.11 MAC frame formats defined [3] and extend them appropriately for supporting ESS mesh services. Unlike WMNs based on Ad-Hoc [4, 6], 802.11s transparently supports any higher layer protocols. As exposed below data frames transmitted from one MP to another use the 802.11-1999 four address format as a basis, as this is a multi-hop protocol. Octets: 2 2 6 6 6 2 6 2 3 0-2312 4 Frame Duration Address 1 Address 2 Address 3 Sequence QoS Mesh Forwarding Address 4 Body FCS Control ID Control Control Control MAC Header Figure 3 Mesh MAC frame format The four address fields contain 48-bit long MAC addresses. Address 1 is the receiver address which defines the mesh point that is going to receive the wireless transmission. Consequently, address 2 is the transmitter address which identifies the mesh point that sent this data frame. Address 3 is the destination address which identifies the final of the data frame. Finally, address 4 is the source address which identifies the source of the data frame. The MAC frame header is appended with a mesh forwarding control field, a 24-bit field that includes a time to live (TTL) field, and a mesh end-to-end sequence number. The TTL is used as mechanism to avoid loops in the mesh network, and the sequence number is used mainly to control broadcast flooding. The mesh forwarding field is present in all frames of type extended with subtype mesh data. There are two new control frames proposed by the IEEE Nuno José Rodrigues Page 11

802.11s [1]: request to switch (RTX) frame and the clear to switch (CTX) frame. These frames are used to perform backhaul channel change operations. The exchange of IEEE 802.11 management frames shall be supported between neighboring MPs. Octets: 2 2 6 6 6 2 0-2312 4 Frame Duration Sequence Frame DA SA BSSID FCS control ID Conrol Body MAC Header Figure 4 Mesh management frame format As these frames are to exchange between MPs, they only have two address fields (one-hop transmission): DA (Destination Address) and SA (Source Address). SA field is the receiving MP MAC address and SA field is the transmitting MP MAC address. BSSID field is not used between Mesh Points (should be all 0 s to backward compatibility). All 802.11 management frames are extended to include mesh-specific information elements (IEs). A non exhaustive list of these IEs consists of: Mesh ID, WLAN Mesh Capability, Neighbor List, MPP Reachability, Peer Request, Peer Response and Active Profile Announcement among others. Neighbor discovery, congestion control, HWMP routing, beaconing and synchronization, and mesh deterministic access (MDA) use IEEE 802.11 management action frames and encode IEs defined by each mechanism. For example, mesh routing-specific route request (RREQ), route response (RREP), route acknowledgment (RREP- ACK), route error (RERR) IEs are defined each and encoded into a specific action frame to be used for mesh routing and forwarding [1]. Nuno José Rodrigues Page 12

4.4. WLAN Mesh Medium Access Control MAC of the IEEE 802.11 is contention based, using distributed coordination function (DCF) mechanisms, or contention-free, using point coordination function (PCF) mechanisms. Contention-based MAC protocols are robust against environmental interference and noise, making them suitable for use in WLAN mesh networks. Two optional mechanisms for more suitably adapting 802.11 MAC services in a WLAN mesh network [1] are Multichannel MAC and MDA. 4.4.1. Multichannel Medium Access Control 802.11 MAC has been based on a single channel, i.e., all the devices on the network share the same channel. Using a multichannel MAC, where transmissions can take place simultaneously on orthogonal channels, the aggregate throughput can be increased considerably. Multichannel MAC protocols are traditionally developed for multi-radio devices. However, the common channel framework (CCF) described in [1] enables the operation of a single-radio device in a multichannel environment. Known methods for channel access, e.g., DCF or enhanced distributed coordination function (EDCF), can be used within this framework. MPs can utilize the common channel to select an available channel as shown in Figure 5. This, in essence, is a dynamic channel allocation scheme. The destination channel information (channel a) is exchanged using the RTX and CTX frames followed by data frame transmission on the destination channel a. While the data frame transmission is ongoing on channel a, another Nuno José Rodrigues Page 13

transmission can be initiated on another destination channel b. A single-radio MP on the SIFS DIFS SIFS Switching delay DIFS SIFS Common channel Data RTX CTX RTX CTX SIFS RTX CTX Channel a Data DATA ACK Channel b Data Switching delay DIFS DATA ACK Switching delay DIFS SIFS Figure 5 - Common channel framework common channel cannot communicate with MPs on other channels. At the same time, singleradio MPs on other channels cannot know the network status on the common channel and vice versa. A multichannel MAC protocol designed for single radio should therefore: Facilitate connectivity among arbitrary MPs that may be on different channels and facilitate protection of the ongoing transactions in order to address the foregoing issues the concept of a channel coordination window (CCW) is available in CCF. At the start of CCW, CCF-enabled MPs tune to the common channel. This enables arbitrary MPs to establish communication with each other. Secondly, at the start of CCW, the channel occupancy status is reset and MPs can renegotiate channels. CCW is repeated with a period T, and the duration of CCW is usually a fraction of T. A channel coordination mechanism is used with the help of a common control channel (CCC). Nuno José Rodrigues Page 14

4.4.2. Mesh Deterministic Access MDA is an optional access method that allows supporting MPs to access the channel with lower contention than otherwise in selected times. The method sets up time periods in mesh neighborhoods when a number of MDA supporting MPs that may potentially interfere with each others transmissions or receptions are set to not initiate any transmission sequences. For each such time period, supporting MPs that set up the state for the use of these time periods are allowed to access the channel using MDA access parameters (CW Min, CW Max, and AIFSN). In order to use the MDA method for access, an MP must be a synchronized MP. The MDA method is described in detail in [1]. Nuno José Rodrigues Page 15

4.5. Boot Sequence When powering up, MPs must perform a sequence of operations before starting to exchange information. An ingenious way of understanding IEEE 802.11s mechanism is to observe that booting sequence: Neighbor Discovery: 1. Passive or Active scanning to discover others MP(s); 2. Channel selection; 3. Mesh beaconing; Peer Link Setup: 4. Neighbor MP link establishment (for each neighbor); Link State Discovery: 5. Local link state measurement; Path Selection: Figure 6 Boot Sequence 6. Path selection initialization; Access Point Initialization: 7. Optionally AP initialization if it is a MAP. Nuno José Rodrigues Page 16

4.5.1. Neighbor Discovery The main reason of this procedure is to discover neighbor MP devices and their properties. For that purpose, MPs may use passive or active scanning, using modified beacon or probe messages, respectively, to discover a mesh network. To do that, MPs exchange beacons or probe messages, MPs may use one or more channels for communication. The specific channel selection scheme used in a WLAN mesh network may vary with different topology and application requirements. A group of MP radio interfaces that are connected to each other by a common channel are referred to as a unified channel graph (UCG). The same device may belong to different UCGs. An MP logical radio interface that is in simple unification mode selects a channel in a controlled way such that it enables the formation of a UCG that becomes merged and hence fully connected. The MP logical radio interface thus establishes links with neighbors that match the mesh ID and mesh profile and selects its channel based on the highest channel precedence value. To identify the mesh network, is used the mesh ID which is attached in the beacon or probe response frames as a new IE for passive and active scanning, respectively. The function of the mesh ID is similar to SSID (Service Set Identifier), but SSID cannot be used for identifying a mesh network. One of the reasons is that mesh ID can prevent legacy STAs from being associated with MPs without AP functionality. For the same reason, a non-ap MP beacon should not include a valid SSID but use a wildcard value for the SSID IE. Nuno José Rodrigues Page 17

When MP devices are discovered, the path selection protocol and metric should be checked for a match with the profile. If there is no match, the newly discovered device should be ignored. If an MP is unable to detect any neighbor MPs, it adopts a mesh ID from one of its profiles, and proceeds to the active state, which, in the case of an MAP is the AP initialization state. This will occur when the MP is the first device to power on (or multiple MPs power on simultaneously). Any peer MP links will be established later as part of the continuous mesh formation procedures. 4.5.2. Peer Link Setup An MP must be able to establish at least one mesh link with a peer MP, and may be able to establish many such links simultaneously. It is possible that there are more candidate peer MPs than the device is capable of being associated with simultaneously. In this case, MP must select MPs to establish peer links based on some measure of signal quality, such as gathered during the discovery phase, or other statistics received from candidate neighbor MPs. An MP will continue to look for received beacons on any of the UCGs it is operating on. On receipt of a beacon from an unknown neighboring MP, but containing a matching mesh ID, an MP will attempt to create a peer link to the new neighbor. Once a mesh node has joined a mesh network, it needs to establish peer links with its neighbors before it can start to send packets. The mesh node that has recently joined the mesh network sends an Association Request frame which has 4 fields: Mesh ID, WLAN Mesh Nuno José Rodrigues Page 18

Capability, Active Profile Announcement and MP Peer Request. The other MPs should reply with an Association Response Frame which has the same field except the last one that is MP Peer Response. Note that all active MPs shall include a WLAN Mesh Capability element in all transmitted beacon and probe response frame. This element indicates active MP status, Active Protocol ID, Active Metric ID, peer capacity field (indicates the number of additional peer associations that can be supported). Note that the MP should be able to establish many links simultaneously. Before accepting the association request, a MP shall check the state of the requesting MP in its own neighbor table. If the state is set to association pending, it shall compare the directionality value contained in the MP peer associate request element with that contained in its own table entry. If the received value is less than or equal to the transmitted value stored in the table, the MP shall reject the association request by transmitting an association response containing an MP peer response element with status set to deny. Otherwise, it may accept or reject the association request at its option. 4.5.3. Link State Discovery The purpose of the local link state discovery procedure is to populate the r (current bit rate in user, that is, the modulation mode) and e pt (packet error rate at the current bit rate for a data frame with a 1000 byte payload) fields for each peer MP in the neighbor table. There values are used by the route establishment algorithm to determine the most efficient available routes. One of the two MP is responsible for determining the link quality (calculating the previous two parameters). Nuno José Rodrigues Page 19

To announce those values, is used a Local Link state announcement which has 6 octets 4 of them to r and e pt attached to a mesh management frame. 4.5.4. Path Selection Mesh path selection describes a single-hop or multi-hop path between mesh points at the link layer using 802.11 management frames. Simple client STA nodes associate with one of the Mesh AP devices as normal. Mesh path selection services consist of baseline management messages for neighbor discovery, local link state measurement and maintenance, and identification of an active path selection protocol. Each WLAN Mesh uses a single method to determine paths through the Mesh, although a single device may be capable of supporting several. MPs use the WLAN Mesh Capability Information Element to discover which protocol and metric an established WLAN Mesh is using, allowing the MP to identify if and how it should participate in the mesh. The default protocol to path selection is Hybrid Wireless Mesh Protocol (HWMP) which is going to be described in the next section. Simplifying HWMP protocol, path selection procedure, is taken care when a S MP wants to find the route to a destination MP D, is broadcast a RREQ with the destination node D specified in the destination list and the metric field initialized to 0. When a node receives a RREQ it creates a route to S or updates its current route if the RREQ is fresh enough and is traversed through a route that offered a better metric than the current one. If a route is created or modified the RREQ is also forwarded (rebroadcast) updating the metric field. After creating or updating a route to S, the destination Nuno José Rodrigues Page 20

node D sends a unicast RREP back to S. Connectivity information is provided and maintained by periodically broadcasting routing protocol messages. If a node has not sent a broadcast message, a RREQ message, within the last hello interval, the node may broadcast a hello message. Nuno José Rodrigues Page 21

4.6. Hybrid Wireless Mesh Protocol (HWMP) The Hybrid Wireless Mesh Protocol (HWMP) is the default routing protocol for IEEE 802.11s. As a hybrid routing protocol, HWMP contains both reactive and proactive routing components. HWMP is an adaptation of the reactive routing AODV [4, 6] called Radio-Metric AODV (RM-AODV) and spanning tree based routing protocol. While AODV works on layer 3 with IP address and uses the hop count as routing metric, RM-AODV works on layer 2 with MAC address and uses a radio-aware routing metric for the path selection. HWMP allows MPs to perform the discovery and maintenance of optimal routes themselves or to rely on the formation of a tree structure based on a root node (logically placed in an MPP). If a mesh network has no root node configured (for example an Ad-Hoc network), on-demand rote discovery is used for all routing in the mesh network. A tree structured network is enabled by configuring an MP (typically a MPP) as a root node. In that case, other MPs proactively maintain routes to the root node and a proactive distance vector routing tree is created and maintained. On-demand routing and tree based routing run simultaneously. The main benefits of the HWMP are the flexibility to adapt to the requirements of a wide range of scenarios, ranging from fixed to mobile mesh networks and MPs discover and user the best metric path to any destination in the mesh with low complexity. In addition, when a root node is configured in the mesh, flooding of route discovery packets in the mesh is reduced if the destination is outside the mesh, the need to buffer messages at the source while on-demand route discovery is in progress is reduced, non-discovery broadcast and multicast traffic can be Nuno José Rodrigues Page 22

delivered along the tree topology and on-demand routes have the topology tree to choose if an on-demand route become unavailable or during route discovery. HWMP has a single hybrid routing feature. If a proactive tree exists, it may be used by default while on-demand route discovery is in progress for intra-mesh destinations. An MP may choose to rely exclusively on the routing tree for all intra-mesh as well as gateway traffic routing. 4.6.1. On-Demand Routing in HWMP The main characteristic of the reactive routing path is computed only if one is necessary for sending data between two mesh points. HWMP has 4 information elements: root announcement (RANN), path request (PREQ), path reply (PREP), and path error (PERR). Except for PERR, all other information elements of HWMP contain 3 important fields: destination sequence number (DSN), time-to-live (TTL), and metric. DSN and TTL can prevent the counting to infinity problem, and the metric field helps to find a better routing path than just using hop count. On-demand routing in HWMP uses a route request (RREQ) and route reply (RREP) mechanisms to establish routes between two MPs. A source mesh point that needs a path to a destination mesh point broadcasts a route request message requesting a route to the destination. The route request message is processed and forwarded by all mesh points and sets up reverse paths to the originator of the route discovery. The destination mesh point or Nuno José Rodrigues Page 23

intermediate mesh points with a path to the destination will answer with a unicast route reply message. This sets up the forward path to the destination. HWMP adapts original AODV to work at layer 2 and changes all IP and IP addressing references to MAC and MAC addresses. Apart from this adaptation, it still uses some original mechanisms of the original AODV, such as: route discovery, destination only and reply and forwarding, route maintenance, best candidate route caching, sequencing, route acknowledgment and route errors. In order to be compatible with legacy STA devices, MAPs create and administer messages on behalf of the legacy STAs that are associated with them. The functionality is similar to the situation when an MAP has multiple addresses. The associated STA addresses may be thought of as alias address for the MAP. However, STA handoffs due to roaming may cause a route to become stale. RERR mechanism needs to be modified in order to make sure that backhaul routing continues to function when there are many legacy STAs in a network and/or there are frequent and rapid STA handoffs. 4.6.2. Tree based Routing in HWMP In the proactive tree-bases routing mode, there are two mechanisms: one is based on proactive PREQ and the other is based on proactive RANN. In the proactive PREQ mechanism, if a MP (typically a MPP) in a WLAN mesh is optionally configured as a root node, other MPs proactively maintain routes to the root node using topology discovery primitives. HWMP topology formation begins when the root portal Nuno José Rodrigues Page 24

announces itself with the root announcement message, which contains the distance metric and a sequence number. The value of the metric is zero. Any MP hearing the announcement directly updates its route table as directly connected child of the root and the metric associated with the link. It then rebroadcasts the root announcement with an updated distance vector metric. Thus, the topology builds away from the root as each MP updates the distance vector to root and re-advertises to its neighbors the cumulative cost to the root portal. When a node wants to send a frame to another node, and if it has no route to that node (mapping its address to a given MP) it may send the frame to the root. The root looks up the routing and bridging tables to see if the packet is intended for a node within the mesh or outside. It forwards the message appropriately back to the mesh or its uplink. If it finds the entry inside the mesh, it sends the frame to the destination parent MP using an additional tunnel encapsulation. When the packet reaches the destination parent MP and checks the tunnel encapsulation, it knows that the address is within the mesh and may initiate an RREQ back to the source. This hybrid routing mechanism allows the initial frame to be forwarded on the tree topology path followed by establishing an optimal, on-demand route between the source destination pair for all subsequent frames among them. Any frame sent from the root follows the optimal path to any other MP in the network by the spanning tree properties. When there are multiple portals in a mesh network, a single portal takes the root role either by provisioning or by a dynamic procedure. All other portals assume non-root roles. In presence of multiple portals, the root forwards all frames with unknown addresses outside the mesh to its own uplink as well as other portals for forwarding on their uplinks. All non-root portals forward frames from outside the mesh to the root portal for further forwarding within the mesh network. Nuno José Rodrigues Page 25

In the proactive RANN mechanism, the root node periodically floods an RANN element into the network. When a MP receives the RANN and also needs to create/refresh a route to the root, it sends a unicast PREQ to the root. When the root receives this unicast PREQ, it replies with PREP to the MP. Thus, the unicast PREQ forms the reverse route from the root to the originating MP, while the unicast PREP creates the forward route from the originating MP to the root. 4.6.3. Radio-Aware Optimized Link-State Routing Protocol This is an optional protocol in IEEE 802.11s. Radio-aware optimized link-state routing (RA-OLSR) is a unified and extensible proactive, link-state routing framework for WMNs based on the original OLSR protocol with extensions from fisheye state routing (FSR). RA-OLSR enables the discovery and maintenance of optimal routes based on a predefined metric, given that each MP has a mechanism to determine the metric cost of a link to each of its neighbors. In order to propagate the metric information between MPs, a metric field is used in RA-OLSR control messages. In disseminating topology information over the network, RA-OLSR adopts the following approaches in order to reduce the related control overhead: It uses only a subset of MPs in the network, called multipoint relays (MPRs), in flooding process and it can control (and thereby reduce) the message exchange frequencies based on the fisheye scopes. The current RA-OLSR protocol specifications also include association discovery protocol to support legacy 802.11 stations. The MAPs select paths among MAPs and MPs by running RA- OLSR protocol and complement routing information among MAPs and MPs with the Nuno José Rodrigues Page 26

information of legacy 802.11 stations associated with them. OLSR is an optimization over the classical link-state routing protocol, tailored for MANETs. It inherits the stability of a link-state routing protocol and has the advantage of having routes immediately available when needed due to its proactive nature. Nuno José Rodrigues Page 27

4.7. WLAN Mesh Security The link access protocol is based on 802.11i [9] Robust Security Network Association (RSNA) security and supports both centralized and distributed IEEE 802.1x-based authentication and key management. In a WLAN mesh, an MP performs the roles of both the supplicant and the authenticator, and may optionally perform the roles of an authentication server (AS). The AS may be collocated with an MP or be located in a remote entity with which the MP has a secure connection (this is assumed and specified by the 802.11s proposal). A node establishes RSNA in one of the three ways: 1. Centralized 802.1x authentication model 2. Distributed 802.1x authentication model 3. Pre-shared key authentication model The first two use 802.1x EAP-based authentication followed by 802.11i 4-way handshake. An authenticator is used in the first model, whereas MP MP perform mutual authentication in the second model. The pre-shared model does not quite scale to meshes where multi-hop routing is required. In particular, it is infeasible to secure routing functionality when a pre-shared key is used in a mesh with more than two nodes, because it is no longer possible to reliably determine the source of any message. IEEE 802.11s Task Group is discussing more robust security for WLAN mesh and is expected to make some substantial changes to the draft security specification. Nuno José Rodrigues Page 28

4.8. Conclusion The 802.11s [1] enables basic mesh services in a WLAN network by implementing the amendments to IEEE 802.11 protocol and processing rules without changing the PHY layer. Most of these amendments are simple extensions of existing 802.11 MAC such as power saving and beaconing. Routing, forwarding, interworking, security, and QoS are the five areas where the draft standard introduces changes to the 802.11 standard. Routing and forwarding changes enable multi-hop forwarding in the mesh network. 802.11s specifies a protocol based on earlier MANET work. 802.11i standard is [9] a single-hop security solution, and if a network only deploys 802.11i, traffic will be encrypted between STA and MAP, but not between MPs. Some of the known limitations of 802.11s are: 1. Mesh routing is layer-2 based, which may not scale well. 2. Hybrid mesh routing has not been studied yet. 3. Mesh forwarding does not have multicast optimizations. 4. Mesh networks may not work efficiently when multiple portals are connected to a larger 802 LAN. 5. Mesh interworking with other 802 technologies is not supported. 6. Mesh security mechanisms not support fast roaming, handoffs, or route convergence as the backhaul links change frequently and/or rapidly. This section presented an overview of the 802.11 standard, including basic WLAN mesh networking architecture, and routing mechanisms. Nuno José Rodrigues Page 29

5. Discrete Events Simulation Discrete-event simulation is a computing technique for studying the behavior of eventdriven systems. A system is a collection of entities (e.g., people and machines) that interact over time. These entities and the interactions between them must be modeled. In discrete-event simulation, a set of system states is specified for the system, and the evolution of the system is viewed as a sequence of the form: < s 0, (e 0, t 0 ), s 1, (e 1, t 1 ), s 2,... > where the s i s are system states, the e i s are system events, and the t i s are non-negative numbers representing event occurrence times. The above sequence means that the system started at time 0 in state s 0 ; then event e 0 occurred at time t 0 taking the system to state s 1 ; then event e 1 occurred at time t 1 taking the system to state s 2 ; and so on. Something can be modeled with two events: one at the beginning and another at the end leading to the existence of more than one state. Each event occurrence is assumed to take zero time. The t i s are required to be non-decreasing, i.e., t i t i+1 for every i. Actually t i < t i+1 does not occur every time because it is the case in discrete-event models that two unrelated events can occur at the same time. Given the evolution of a system, it is possible to determine its properties and evaluate appropriate performance measures. In general, there is a set of system parameters, referred to as input parameters, that determines the evolution of the system, and hence the properties and performance measures. For example, the input parameters to the queuing system are the customer service requirements and arrival times. Typically, the input parameters of a system should be defined stochastically (or probabilistically), instead of deterministically. That is, random variables are chosen, taking values from some domain with some probability Nuno José Rodrigues Page 30

distribution. Each set of input parameter values originate a unique evolution. The objective is to obtain performance measures averaged over all such evolutions. There are two reasons for introducing random variables. First, for most real-life system, there aren t characterizations of the input parameters. Hence, using probabilistic inputs, the real-life system is approximated. Secondly, even if there has an exact characterization of the input parameters, it is often computationally too expensively or analytically intractable to take them into account. Discrete Events Simulation will be used to simulate WMN using NS-3 [2] environment. NS-3 is a discrete event network simulator for Internet systems written in C++. It is used to protocol design, large scale systems studies and prototyping. NS is a research community resource and over 50% of AMC and IEEE network simulation papers from 2000-2004 cite the use of ns-2. This tool already includes some of the simulation models needed to develop the simulation study proposed by this dissertation. Nuno José Rodrigues Page 31

6. References 1. IEEE P802.11-06/0328r0, Joint SEE-Mesh/Wi-Mesh Proposal to 802.11 TGs, IEEE, Draft Standard, February 2006, Work in Progress. 2. NS-3. ns-3 project. [Online] Available: http://www.nsnam.org/ 3. ANSI/IEEE Std 802.11, 1999 Edition (R2003), 802.11: Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE, LAN/MAN Standard, 1999 4. Mobile Ad-hoc Networks (MANET) Working Group. The Internet Engineering Task Force (IETF). [Online] Available: http://www.ietf.org/html.charters/manet-charter.html 5. IEEE 802.11s ESS Mesh Networking. Ko, Prof. Young-Bae, 2006. 6. Redes Ad-Hoc. Ricardo, Prof. Manuel Alberto Pereira. Porto : s.n., 2007. 7. Wireless mesh networks: a survey. Ian F. Akyildiz, Xudong Wang, Weilin Wang. 2005, ScienceDirect. 8. IETF RFC 3561, Ad hoc On-Demand Distance Vector (AODV) Routing. Perkins, C.; Belding- Royer, E.; DAS, S., 2003. 9. IEEE 802.11i-2004: Amendment 6: Medium Access Control (MAC) Security Enhancements. IEEE Standards (2004-07-23). Nuno José Rodrigues Page 32