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Transcript
Lecture 4 Advance Topics in Networking McGraw-Hill Technology Education Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved. Lecture Overview • • • • • • • Presenting a Research Topic Sample Thesis Topics More Thesis Topics Ad hoc Networking Reviewers Guidelines Paper Review Guidelines First Papers: For this week Presenting a Research Topic Typical steps of thesis research • accumulate background – network track courses – independent studies, research group meetings • define problem, search literature, and develop solution • implement a prototype (in JAVA or C/C++) • measure and analyze performance of prototype • summarize results in one technical paper / thesis Sample Thesis Topics • Programming of CISCO Routers – How to deploy new services without modifying IOS? • Policy-based Networking – How to efficiently detect conflicts among policy rules specified • Smart Routing and Rerouting Algorithms – How to reduce call blocking probability and data loss rate? • Study of Distributed Denial of Service attacks – How to identify sources of attacks? – How to filter out malicious traffic early? More Thesis Topics • Security Protocols for Wireless LANs – How to strengthen WEP? – How to detect intrusions? • Extreme (Ad hoc) Networking – How to mitigate effect of large propagation delays? – How to guarantee performance to selected traffic? More Thesis Topics • Mobile Agents and Survivable Networking – How to make a service ubiquitous, i.e., available while moving around the network and regenerating if necessary • Software Architecture for Dynamically Reconfigurable Systems – How to reduce programming complexity of these systems? 802.11a/b/g Networks Some slides taken from UIUC Wireless Networking Group 802.11a/b/g • Operates in 2 different modes: – Infrastructure mode • Associates with an access point • All communication goes through the access point • Used for wireless access at a company or campus – Peer-to-Peer Ad Hoc Mode • If two nodes are within range of each other they can communicate directly with no access point • A few users in a room could quickly exchange files with no access point required Infrastructure Access • Access Points: – Provide infrastructure access to mobile users – Cover a fixed area – Wired into LAN Peer to Peer Ad Hoc Mode Infrastructure Access Infrastructure Access 802.11a/b/g are multi-rate devices 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps MAC Layer Fairness Models • Per Packet Fairness: If two adjacent senders continuously are attempting to send packets, they should each send the same number of packets. • Temporal Fairness: If two adjacent senders are continuously attempting to send packets, they should each be able to send for the same amount of medium time. • In single rate networks these are the SAME! Temporal Fairness Example Per Packet Fairness 11 Mbps 802.11 Packet Fairness OAR Temporal Fairness 11 Mbps Link 0.896 3.533 1 Mbps Link 0.713 0.450 Total Throughput 1 Mbps Temporal Fairness 11 Mbps 1.609 3.983 1 Mbps 802.11b Channels • 11 available channels (in US) • Only 3 are non-overlapping! 802.11b Channel Usage Channel 1 Channel 6 Channel 11 802.11b Channel Reuse Problems • Access Point placement depends on wired network availability • Obstructions make it difficult to provide total coverage of an area • Site surveys are performed to determine coverage areas • Security Concerns: rogue access points in companies etc.. • Each Access Point has limited range Peer to Peer Ad Hoc Mode Peer to Peer Ad Hoc Mode X Problems • Communication is only possible between nodes which are directly in range of each other Problems for both Infrastructure and Ad hoc Mode • If nodes move out of range of the access point (Infrastructure Mode) OR • nodes are not in direct range of each other (Ad Hoc Mode) • Then communication is not possible!! What if ?? Multi-hop Infrastructure Access Multi-hop Ad Hoc Network OR Multi-hop Infrastructure Access • Nodes might be out of range of the access point, BUT in range of other nodes. • The nodes in range of the access point could relay packets to allow out of range nodes to communicate. • NOT part of 802.11 Multi-hop Ad Hoc Network • If communication is required between two nodes which are out of range of each other, intermediary nodes can forward the packets. • NOT part of 802.11 Source Destination How can this be done? -< ROUTING!! – Wired Networks: • Hierarchical Routing – Network is divided into subnets – Nodes look at network address and determine if the address is directly reachable. If not, just forward to the default gateway. – Different protocols for different levels of the hierarchy » RIP, OSPF, BGP Wireless Routing • Flat routing – You can’t assume that since a node is in your subnet that it is directly accessible – Node must maintain or discover routes to the destination – All nodes are routers Ad Hoc Networking 29 Initial Architectures - Low power sensors networks “surveillance” web - small, relatively static, embedded ad hoc networks `“bluetooth-type” networks - Small-to-medium sized, mobile ad hoc networks “802.11-style” Terminlology Mobile Ad Hoc Networking = = Mobile, Multi hop, Wireless Networking = Mobile Mesh Networking = Mobile Packet Networking Ad hoc network applicability Scale Network type Commercial Small scale (few nodes) home/office personal industrial local networks Government specific Public Safety Community/urban communications networks “covert” networks Large scale (many nodes) mobile cellular like Large-scale military network local Hybrid Communication Networks Satellite overlay MANET No fixed infrastructure High speed backbone network Fixed/static infrastructure IETF MANET standardization MANET - established in 1997 chartered working group within Internet Engineering Task Force (IETF) Focussed on studying routing specification with the goal of supporting network scaling up to hundreds of routers Unicast routing protocol Multicast routing protocol Work on routing for large and small scale networks Work relies on the existing IETF standards such as mobileIP and IP addressing For large-scale MANET the lack of interest have put this work in question Flooding: work on requirements had started Mobile Ad Hoc Networking (MANET) Dynamic topologies Bandwidth-constrained Asymmetric links with variable capacity Energy constrained Multiple technologies can be used simultaneously Open issues A optimisation network layer and radio layers for different systems (incl. 802.11, HiperLAN) B QoS support C secuirity D mobility • B, C, D issues could be orthogonal, joint optimization is very difficult (system design choice) • tradeoff between centralized and distributed algorithms for B,C,D Relevant ETSI activities MESA Project - ad hoc network on future Public Safety communications BRAN - HiperLAN-2, other Standardization challenges => There is need for standard-based approach at the network layer. Mobile Ad Hoc Networks • Formed by wireless hosts which may be mobile • Without (necessarily) using a preexisting infrastructure • Routes between nodes may potentially contain multiple hops Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination Mobile Ad Hoc Networks (MANET) • Mobility causes route changes Why Ad Hoc Networks ? • Ease of deployment • Speed of deployment • Decreased dependence on infrastructure Many Applications • Personal area networking – cell phone, laptop, ear phone, wrist watch • Military environments – soldiers, tanks, planes • Civilian environments – taxi cab network – meeting rooms – sports stadiums – boats, small aircraft • Emergency operations – search-and-rescue – policing and fire fighting Challenges • Limited wireless transmission range • Broadcast nature of the wireless medium • Packet losses due to transmission errors • Mobility-induced route changes • Mobility-induced packet losses • Battery constraints • Potentially frequent network partitions • Ease of snooping on wireless transmissions (security hazard) Unicast Routing in Mobile Ad Hoc Networks Why is Routing in MANET different ? • Host mobility – link failure/repair due to mobility may have different characteristics than those due to other causes • Rate of link failure/repair may be high when nodes move fast • New performance criteria may be used – route stability despite mobility – energy consumption Unicast Routing Protocols • Many protocols have been proposed • Some have been invented specifically for MANET • Others are adapted from previously proposed protocols for wired networks • No single protocol works well in all environments – some attempts made to develop adaptive protocols Routing Protocols • Proactive protocols – Determine routes independent of traffic pattern – Traditional link-state and distance-vector routing protocols are proactive • Reactive protocols – Maintain routes only if needed • Hybrid protocols Trade-Offs • Latency of route discovery – Proactive protocols may have lower latency since routes are maintained at all times – Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y • Overhead of route discovery/maintenance – Reactive protocols may have lower overhead since routes are determined only if needed – Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating • Which approach achieves a better trade-off depends on the traffic and mobility patterns Overview of Unicast Routing Protocols Flooding for Data Delivery • Sender S broadcasts data packet P to all its neighbors • Each node receiving P forwards P to its neighbors • Sequence numbers used to avoid the possibility of forwarding the same packet more than once • Packet P reaches destination D provided that D is reachable from sender S • Node D does not forward the packet Flooding for Data Delivery Y Z S E F B C M J A L G H K D I N Represents a node that has received packet P Represents that connected nodes are within each other’s transmission range Flooding for Data Delivery Y Broadcast transmission Z S E F B C M J A L G H K D I Represents a node that receives packet P for the first time Represents transmission of packet P N Flooding for Data Delivery Y Z S E F B C M J A L G H K I • Node H receives packet P from two neighbors: potential for collision D N Flooding for Data Delivery Y Z S E F B C M J A L G H K I D N • Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P once Flooding for Data Delivery Y Z S E F B C M J A L G H K I D N • Nodes J and K both broadcast packet P to node D • Since nodes J and K are hidden from each other, their transmissions may collide => Packet P may not be delivered to node D at all, despite the use of flooding Flooding for Data Delivery Y Z S E F B C M J A L G H K D I • Node D does not forward packet P, because node D is the intended destination of packet P N Flooding for Data Delivery Y Z S E F B C M J A L G H • Flooding completed K I D N • Nodes unreachable from S do not receive packet P (e.g., node Z) • Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N) Flooding for Data Delivery Y Z S E F B C M J A L G H K D I • Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet) N Flooding for Data Delivery: Advantages • Simplicity • May be more efficient than other protocols when rate of information transmission is low enough so that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher – this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions • Potentially higher reliability of data delivery – Because packets may be delivered to the destination on multiple paths Flooding for Data Delivery: Disadvantages • Potentially, very high overhead – Data packets may be delivered to too many nodes who do not need to receive them • Potentially lower reliability of data delivery – Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead – Broadcasting in IEEE 802.11 MAC is unreliable – In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet – in this case, destination would not receive the packet at all Flooding of Control Packets • Many protocols perform (potentially limited) flooding of control packets, instead of data packets • The control packets are used to discover routes • Discovered routes are subsequently used to send data packet(s) • Overhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods Broadcast Storm Problem [Ni99Mobicom] • When node A broadcasts a route query, nodes B and C both receive it • B and C both forward to their neighbors • B and C transmit at about the same time since they are reacting to receipt of the same message from A • This results in a high probability of collisions D B C A 62 Broadcast Storm Problem • Redundancy: A given node may receive the same route request from too many nodes, when one copy would have sufficed • Node D may receive from nodes B and C both D B C A Solutions for Broadcast Storm • Probabilistic scheme: On receiving a route request for the first time, a node will rebroadcast (forward) the request with probability p • Also, re-broadcasts by different nodes should be staggered by using a collision avoidance technique (wait a random delay when channel is idle) – this would reduce the probability that nodes B and C would forward a packet simultaneously in the previous example 64 Solutions for Broadcast Storms • Counter-Based Scheme: If node E hears more than k neighbors broadcasting a given route request, before it can itself forward it, then node E will not forward the request • Intuition: k neighbors together have probably already forwarded the request to all of E’s neighbors D E B C F A Solutions for Broadcast Storms • Distance-Based Scheme: If node E hears RREQ broadcasted by some node Z within physical distance d, then E will not re-broadcast the request • Intuition: Z and E are too close, so transmission areas covered by Z and E are not very different – if E re-broadcasts the request, not many nodes who have not already heard the request from Z will hear the request E <d Z Summary: Broadcast Storm Problem • Flooding is used in many protocols, such as Dynamic Source Routing (DSR) • Problems associated with flooding – collisions – redundancy • Collisions may be reduced by “jittering” (waiting for a random interval before propagating the flood) • Redundancy may be reduced by selectively rebroadcasting packets from only a subset of the nodes 67 Generic On-demand Routing Protocol • Routes are maintained only between nodes which need to communicate • Route Requests (RREQ) are flooded through the network • When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source • When the intended destination receives a Route Request, it replies by sending a Route Reply • Route Reply travels along the reverse path set-up when Route Request is forwarded 68 Route Requests Phase Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S Route Requests Phase Y Broadcast transmission Z S E F B C M L J A G H D K I Represents transmission of RREQ N Route Requests Phase Y Z S E F B C M L J A G H D K I Represents links on Reverse Path N Route Requests Phase Y Z S E F B C M L J A G H D K I N • Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Route Requests Phase Y Z S E F B C M L J A G H D K I N Route Requests Phase Y Z S E F B C M L J A G H D K I • Node D does not forward RREQ, because node D is the intended target of the RREQ N Route Reply Phase Y Z S E F B C M L J A G H D K I Represents links on path taken by RREP N Data Delivery Y DATA Z S E F B C M L J A G H D K I N Routing table entries are used to forward data packet. Summary: Generic On-demand Routing Protocols • Nodes maintain routing tables containing entries only for routes that are in active use • Next-hop per destination maintained at each node • Unused routes expire even if topology does not change The End Questions? McGraw-Hill Technology Education Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.