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(Paper Presentation) DSDV

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Rajesh Piryani
Rajesh Piryani

(Paper Presentation on Research Paper) Highly dynamic destination sequenced destination vector routing (DSDV) for Mobile computing

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Paper Presentation on Research Paper Charles E. Perkins Pravin Bhagwat IBM, T.J. Watson Research Center Computer Science Department Hawthorne, NY 10562 University of Maryland College Park, MD 20742 Presented By Rajesh Piryani South Asian University
 Abstract  Introduction  Overview of Routing Methods  DSDV  Example of DSDV  Properties of DSDV  Comparison with other Methods  Current Status and Future Work  Summary 8/6/2014 2
 Each node knows the distance (=cost) to its directly connected neighbors  A node sends periodically a list of routing updates to its neighbors.  If all nodes update their distances, the routing tables eventually converge  New nodes advertise themselves to their neighbors 8/6/2014 3
 A routing loop is a common problem with various types of networks, particularly computer networks.  They are formed when an error occurs in the operation of the routing algorithm.  And as a result, in a group of nodes, the path to a particular destination forms a loop. 8/6/2014 4
 node A is transmitting data to node C via node B  If the link between nodes B and C goes down and B has not yet informed node A about the breakage  node A transmits the data to node B assuming that the link A-B-C is operational and of lowest cost.  Node B knows of the broken link and tries to reach node C via node A, thus sending the original data back to node A.  node A receives the data that it originated back from node B and consults its routing table.  Node A's routing table will say that it can reach node C via node B (because it still has not been informed of the break)  thus sending its data back to node B creating an infinite loop 8/6/2014 5
 The Routing Information Protocol (RIP) is a distance-vector routing protocol, which employs the hop count as a routing metric.  RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination.  The maximum number of hops allowed for RIP is 15.  This hop limit, however, also limits the size of networks that RIP can support.  A hop count of 16 is considered an infinite distance and used to deprecate inaccessible, inoperable, or otherwise undesirable routes in the selection process.  Originally each RIP router transmitted full updates every 30 seconds. 8/6/2014 6
8/6/2014 7
 An ad-hoc network is the cooperative engagement of a collection of Mobile Hosts without the required intervention of any centralized Access Point.  This paper present an innovative design of such network.  The basic idea of the design is to operate each Mobile Host as a specialized router, which periodically advertises its view of the interconnection topology with other Mobile Hosts within the network.  This amounts to a new sort of routing protocol.  Investigated modifications to the basic Bellman-Ford routing mechanisms, as specified by RIP , to make it suitable for a dynamic and self-starting network mechanism as is required by users wishing to utilize ad-hoc networks. 8/6/2014 8
 Their modifications address some of the previous objections to the use of Bellman-Ford  Related to the poor looping properties of such algorithms in the face of broken links and the resulting time dependent nature of the interconnection topology describing the links between the Mobile Hosts. 8/6/2014 9
 As people begin to have mobile computers handy, for whatever purposes, sharing information between the computers will become a natural requirement.  Currently, such sharing is made difficult by the need for users to perform administrative tasks and set up static, bidirectional links between their computers.  if the wireless communications systems in the mobile computers support a broadcast mechanism, much more flexible and useful ways of sharing information can be imagined.  one of our primary motivations is to allow the construction of temporary networks with no wires and no administrative intervention required.  In this paper, such a interconnection between the mobile computers will be called an ad-hoc network, in conformance with current. Usage within the IEEE 802.11 subcommittee. 8/6/2014 10
 to provide service in the most general situation, we do not assume that every computer is within communication range of every other computer.  Currently, there is no method available which enables mobile computers with wireless data communications equipment to freely roam about while still maintaining connections with each other, unless special assumptions are made about the way the computers are situated with respect to each other.  Routing protocols for existing networks have not been designed specifically to provide the kind of dynamic, self-starting behavior needed for ad-hoc networks. 8/6/2014 11
 the wireless medium differs in important ways from wired media,  mobile computers may well have only a single network interface adapter,  Whereas most existing routers have network interfaces to connect two separate networks together  wireless media are of limited and variable range, in distinction to existing wired media. 8/6/2014 12
 Two primary classes 1. link-state and 2. distance-vector 8/6/2014 13
 closer to the centralized version of the shortest path computation method.  Each node maintains a view of the network topology with a cost for each link.  To keep these views consistent, each node periodically broadcasts the link costs of its outgoing links to all other nodes using a protocol such as flooding.  As a node receives this information, it updates its view of the network topology and applies a shortest-path algorithm to choose its next hop for each destination.  Some of the link costs in a node’s view can be incorrect because of long propagation delays, partitioned network, etc.  Such inconsistent views of network topologies might lead to formation of routing loops.  These loops, however, are short-lived, because they disappear in the time it takes a message to traverse the diameter of the network. 8/6/2014 14
E B D G H F A C link costs
 In this algorithm, every node i maintains, for each destination x, a set of distances {dx i,j } where j ranges over the neighbors of i.  Node i treats neighbor k as a next-hop for a packet destined for x if {dx i,k } equals minj {dx i,j }.  The succession of next hops chosen in this manner lead to x along the shortest path.  In order to keep the distance estimates up-to-date, each node monitors the cost of its outgoing links and periodically broadcasts, to each one its neighbors, its current estimate of the shortest distance to every other node in the network 8/6/2014 16
 It is the classical Distributed Bellman-Ford (DBF) algorithm  Compared to link-state method, it is computationally more efficient, easier to implement and requires much less storage space  The primary cause for formation of routing loops  nodes choose their next-hops in a completely distributed fashion based on information which can possibly be stale and, therefore, incorrect.  Almost all proposed modifications to DBF algorithm eliminate the looping problem by forcing all nodes in the network to participate in some form of internodal coordination protocol.  within an ad-hoc mobile environment enforcing any such internodal coordination mechanism will be difficult due to the rapidly changing topology of the underlying routing network 8/6/2014 17
 The usefulness of RIP within ad-hoc environment, however, is limited as it was not designed to handle rapid topological changes.  design goal  to design a routing method for ad-hoc networks which preserves the simplicity of RIP, yet at the same time avoids the looping problem.  their approach –to tag each route table entry with a sequence number so that nodes can quickly distinguish stale routes from the new ones and thus avoid formation of routing loop 8/6/2014 18
8/6/2014 19
 Allows Collections of mobile computers –which are not close to any base stations  can exchange data along changing and arbitrary paths of interconnection  Solution must remain compatible with operation in cases where a base station is available. 8/6/2014 20
 Packets are transmitted between the stations  using routing tables -stored at each station of the network.  Each route table entry is tagged with a sequence number which is originated by the destination station.  To maintain the consistency of routing tables in a dynamically varying topology, each station periodically transmits updates, and  transmits updates immediately when significant new information is available.  not assume that the mobile hosts are maintaining any sort of time synchronization  packets indicate which stations are accessible from each station and the number of hops necessary to reach these accessible stat ions, as is often done in dist ant-vector routing algorithms 8/6/2014 21
 The packets may be transmitted containing either layer 2 (MAC) addresses or layer 3 (network) addresses  Routing information is advertised by broadcasting or multicasting the packets  Data is also kept about the length of time between arrival of the first and the arrival of the best route for each particular destination.  Based on this data, a decision may be made to delay advertising routes which are about to change soon, thus damping fluctuations of the route tables.  The advertisement of routes which may not have stabilized-delayed in order to reduce the number of rebroadcasts of possible route entries that normally arrive with the same sequence number. 8/6/2014 22
 requires each mobile station to advertise, to each of its current neighbors, its own routing table (for instance, by broadcasting its entries).  The entries in this list may change fairly dynamically over time,  so the advertisement must be made often enough to ensure that every mobile computer can almost always locate every other mobile computer of the collection  each mobile computer agrees to relay data packets to other computers upon request  agreement places a premium on the ability to determine the shortest number of hops for a route to a destination  In this way a mobile computer may exchange data with any other mobile computer in the group even if the target of the data is not within range for direct communication 8/6/2014 23
 In a wireless medium, it is important to keep in mind that broadcasts are limited in range by the physical characteristics of the medium  Broadcast address contain for each new route  The destination’s address  The number of hops required to reach the destination and  The sequence number of the information received regarding that destination, as originally stamped by the destination; 8/6/2014 24
 It include a sequence number created by the transmitter.  Routes with more recent sequence numbers  preferred as the basis for making forwarding decisions but not necessarily advertised.  Path with the same sequence number, those with the smallest metric will be used.  Routing tables are propagated, the sequence number is sent to all mobile computers  Routes received in broadcasts advertised by the receiver when it subsequently broadcasts its routing information;  the receiver adds an increment to the metric before advertising the route, since incoming packets will require one more hop to reach the destination. 8/6/2014 25
 detected by if no broadcasts have been received for a while from a former neighbor.  A broken link is described by a metric of ∞  When a link to a next hop has broken, any route through that next hop is immediately assigned an ∞ metric and assigned an updated sequence number.  such modified routes are immediately disclosed in a broadcast routing information packet.  Building information to describe broken links is the only situation when the sequence number is generated by any Mobile Host other than the destination Mobile Host. 8/6/2014 26
 Thus, there will be two routing tables kept at each Mobile Host; one for use with forwarding packets, and another to be advertised via incremental routing information packets.  To determine the probability of coming up arrival of routing information showing a better metric,  the Mobile Host has to keep a history of the weighted average time that routes to a particular destination fluctuate until the route with the best metric is received. 8/6/2014 27
 operation at Layer 3 will use network layer addresses for the next hop and destination addresses,  operation at Layer 2 will use Layer 2 Media Access Control (MAC) addresses  The difficulty is that Layer 3 network protocols provide communication  based on network addresses, and a way must be provided to resolve these Layer 3 addresses into MAC addresses.  PROPOSED SOLUTION  Each destination host would advertise which Layer 3 protocols it supports, and  Each Mobile Host advertising reachability to that destination would include along, with the advertisement, the  information about the Layer 3 protocols supported at that destination. 8/6/2014 28
 Mobile computers will frequently be used in conjunction with base stations, which allow them to exchange data with other computers connected to the wired network.  By participating in the DSDV protocol, base stations can extend their coverage beyond the range imposed by their wireless transmitters.  When a base station participates in DSDV, it is shown as a default route in the tables transmitted by a mobile station. 8/6/2014 29
 Consider Mh4. Suppose the address of each Mobile Host is represented as MHi  Suppose all sequence numbers are denoted SNNN_MHi,  where MHi specifies the computer that created the sequence number  and SNNN is a sequence number value 8/6/2014 30
 Suppose that there are entries for all other Mobile Hosts, with sequence numbers SNNN.MHi, before MH1 moves away from MH2.  The install time field helps determine when to delete stale routes.  With this protocol, the deletion of stale routes should rarely occur,  since the detection of link breakages should propagate through the ad-hoc network immediately  we expect to continue to monitor for the existence of stale routes and take appropriate action  all the computers became available to MH4 at about the same time, since its install-time for most of them is about the same.  none of the links between the computers were broken,  because all of the sequence number fields have times with even digits in the units place.  Ptrl-MHi would all be pointers to null structures,  because there are not any routes in Figure 1 which are likely to be superseded or compete with other possible routes 8/6/2014 31
8/6/2014 32
 suppose that MH1 moves into the general vicinity of MH5 and MH7, and away from the others (especially MH2).  When MH1 moves, it triggered an immediate incremental routing information update which was then broadcast to MH6.  MH6, determined that significant new routing information had been received, also triggered an immediate update which carried along the new routing information for MH1. 8/6/2014 33
 this advertisement, the information for MH4 comes first, since it is doing the advertisement.  The information for MH1 comes next, not because it has a lower address, but because MH1 is the only one which has any significant route changes affecting it.  As a general rule, routes with changed metrics are first included in each incremental packet.  The remaining space is used to include those routes whose sequence numbers have changed. 8/6/2014 34
 In this example, one node has changed its routing information, since it is in a new location.  All nodes have transmitted new sequence numbers recently.  If there were too many updated sequence numbers to fit in a single packet, only the ones which fit would be transmitted.  These would be selected with a view to fairly transmitting them in their turn over several incremental update intervals.  The frequency of transmitting full updates would be reduced if the volume of data began to consume a significant fraction of the available capacity of the medium 8/6/2014 35
 Settling Time = Time between arrival of first route and the best route with a given seq. nr.  describes how the settling time table is used to prevent fluctuations of routing table entry advertisements.  The general problem arises because route updates are selected according to the following criteria:  Routes are always preferred if the sequence numbers are newer;  Otherwise, routes are preferred if the sequence numbers are the same and yet the metric is better (lower). 8/6/2014 36
 suppose that two routes with identical sequence numbers are received by a Mobile Host, but in the wrong order.  In other words, suppose that MH4 receives the higher metric next hop first, soon after gets another next hop with a lower metric but the same sequence number.  This could happen when there are a lot of Mobile Hosts, transmitting their updates not quite regularly.  Alternatively, if the Mobile Hosts are acting independently and with markedly different transmission intervals, the situation could occur with correspondingly fewer hosts. 8/6/2014 37
 Suppose, in any event, in Figure 2 that there are enough Mobile Hosts to cause the problem, in two separate collections of Mobile Hosts both connected to a common destination MH9, but with no other Mobile Hosts in common.  Suppose further that all Mobile Hosts are transmitting updates approximately every 15 seconds, that Mobile Host MH2 has a route to MH9 with 12 hops, and Mobile Host MH6 has a route to MH9 with 11 hops,  Moreover, suppose that the routing information update from MH2 arrives at MH4 approximately 10 seconds before the routing information update from MH6. 8/6/2014 38
 This will occur every time that a new sequence number is issued from Mobile Host MH9.  In fact, the time differential can be drastic if any Mobile Host in collection II begins to issue its sequence number updates in multiple incremental update intervals, as would happen,  for instance, when there are too many hosts with new sequence number updates for them all to fit within a single incremental packet update.  In general, the larger the number of hops, the more drastic differentials between delivery of the updates can be expected in Figure 2 8/6/2014 39
 The settling time data is stored in a table with the following fields, keyed by the first field:  Destination address  Last settling time  Average settling time  The settling time is calculated by maintaining a running, weighted average over the most recent updates of the routes, for each destination. 8/6/2014 40
 Suppose a new routing information update arrives at MH4.  The sequence number in the new entry is the same as the sequence number in the currently used entry, and the newer entry has a worse (i.e., higher) metric.  Then MH4 must use the new entry in making subsequent forwarding decisions.  However, MH4 does not have to advertise the new route immediately  and can consult its route settling time table to decide  How long to wait before advertising it.  The average settling time is used for this determination.  For instance, MH4 may decide to delay (average.settling_time x 2) before advertising a route. 8/6/2014 41
 if a link via Mobile Host MH6 truly does break, the advertisement of a route via MH2 should proceed immediately.  To achieve this when there is a history of fluctuations at Mobile Host MH4, the link breakage should be detected fast enough so that an intermediate host in Collection II  finds out the problem and begins a triggered incremental update showing an ∞ metric for the path along the way to Mobile Host MH9.  Routes with an ∞ metric are required by this protocol to be advertised immediately, without delay. 8/6/2014 42
 Settling time of a particular route must be counted with a higher weighting factor than are less recent measurements.  When a new routing update is received from a neighbor, during the same time that the updates are applied to the table, processing also occurs to delete stale entries.  the routing algorithm could be applied in any situation where  reduced memory requirements are desired (compared to link-state routing algorithms).  The operation of an ad-hoc network could be applied to wired as well as wireless mobile computers.  In general, then, we provide a new destination-sequenced routing algorithm, and this algorithm is supplemented by a technique for damping fluctuations. 8/6/2014 43
 the DSDV protocol guarantees loop free paths to each destination.  To see why this property holds  consider a collection of N mobile hosts forming an instance of an ad-hoc style network.  assume that the system is in steady-state,  i.e. routing tables of all nodes have already converged to the actual shortest paths.  At this instant, the next node indicators to each destination induce a tree rooted at that destination.  Thus, routing tables of all nodes in the network can be collectively visualized as forming N trees, one rooted at each destination. 8/6/2014 44
 Potentially a loop may form each time node i changes its next-hop.  This can happen in two cases.  First, when node i detects that the link to its next-hop is broken,  The node resets px i to nil.Clearly, this action cannot form a loop involving 2.  The second scenario occurs when node i receives, from one of its neighbors k, a route to x, with sequence number sx k ; and metric m,  which is selected to replace the current route it has through px i 8/6/2014 45
 Let sx i denote the value of the sequence number stored at node i  and dx i denote the distance estimate from i to x just prior to receiving route from k.  i will change its next-hop from px i; to k only if either of the following two happens.  the new route contains a newer sequence number, i.e., sx k>sx I  the sequence number sx k is same as sx i, but the new route offers a shorter path to x, i.e, m < dix 8/6/2014 46
8/6/2014 47
 They have implemented a preliminary version of this protocol for use with mobile computers in our lab.  They are making necessary modifications to the MARS simulator [1] for use in creating the appropriation simulation environment for our needs.  They hope to discover good operational values via simulation for the following quantities:  Average convergence times  Incremental update period  Settling time averaging method  Full update period  Settling time applied to triggered updates  measurement of the convergence times may depend heavily on many interesting parameters, such as the average velocity of the mobile hosts, update periods, size of the mobile host population, geographical placement of mobile hosts, existence of base stations, and average processing loads at the mobile computers 8/6/2014 48
8/6/2014 49  Providing convenient connectivity for mobile compters is a challenge that is only now bieng met.  We make good use of the properties of wireless broadcast medium.  In the latter case certain additional information should be included along with the routing tables for the most convenient and efficient operation.  The information in the routing table is similar to what is found in the routing table with the todays distance vector algorithm.  All sequence numbers are generated by the destination computer in each route table entry.  The latter case describe by infinity metric and a sequence number which cannot be correctly generated any destination computer.  By the natural join operation of protocol,the sequence number chosen to represent broken links will be suspended by real routes propagated from the newlylocated destination as soon as possibal.
 This allow real route data to quickly supersede temperory link outages when a mobile computers move from one place to another.  This include movement from place to place,as well as disapperance of mobile host from the interconnect topology.  We have found that mobile computers,modelled as routers.  We hope to explore further the necessary application level support needed to automatically use of the network layer route capabilities to provide simple access to conferencing and workplace tools for collaboration and information sharing. 8/6/2014 50
8/6/2014 51

More Related Content

(Paper Presentation) DSDV

  • 1. Paper Presentation on Research Paper Charles E. Perkins Pravin Bhagwat IBM, T.J. Watson Research Center Computer Science Department Hawthorne, NY 10562 University of Maryland College Park, MD 20742 Presented By Rajesh Piryani South Asian University
  • 2.  Abstract  Introduction  Overview of Routing Methods  DSDV  Example of DSDV  Properties of DSDV  Comparison with other Methods  Current Status and Future Work  Summary 8/6/2014 2
  • 3.  Each node knows the distance (=cost) to its directly connected neighbors  A node sends periodically a list of routing updates to its neighbors.  If all nodes update their distances, the routing tables eventually converge  New nodes advertise themselves to their neighbors 8/6/2014 3
  • 4.  A routing loop is a common problem with various types of networks, particularly computer networks.  They are formed when an error occurs in the operation of the routing algorithm.  And as a result, in a group of nodes, the path to a particular destination forms a loop. 8/6/2014 4
  • 5.  node A is transmitting data to node C via node B  If the link between nodes B and C goes down and B has not yet informed node A about the breakage  node A transmits the data to node B assuming that the link A-B-C is operational and of lowest cost.  Node B knows of the broken link and tries to reach node C via node A, thus sending the original data back to node A.  node A receives the data that it originated back from node B and consults its routing table.  Node A's routing table will say that it can reach node C via node B (because it still has not been informed of the break)  thus sending its data back to node B creating an infinite loop 8/6/2014 5
  • 6.  The Routing Information Protocol (RIP) is a distance-vector routing protocol, which employs the hop count as a routing metric.  RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination.  The maximum number of hops allowed for RIP is 15.  This hop limit, however, also limits the size of networks that RIP can support.  A hop count of 16 is considered an infinite distance and used to deprecate inaccessible, inoperable, or otherwise undesirable routes in the selection process.  Originally each RIP router transmitted full updates every 30 seconds. 8/6/2014 6
  • 8.  An ad-hoc network is the cooperative engagement of a collection of Mobile Hosts without the required intervention of any centralized Access Point.  This paper present an innovative design of such network.  The basic idea of the design is to operate each Mobile Host as a specialized router, which periodically advertises its view of the interconnection topology with other Mobile Hosts within the network.  This amounts to a new sort of routing protocol.  Investigated modifications to the basic Bellman-Ford routing mechanisms, as specified by RIP , to make it suitable for a dynamic and self-starting network mechanism as is required by users wishing to utilize ad-hoc networks. 8/6/2014 8
  • 9.  Their modifications address some of the previous objections to the use of Bellman-Ford  Related to the poor looping properties of such algorithms in the face of broken links and the resulting time dependent nature of the interconnection topology describing the links between the Mobile Hosts. 8/6/2014 9
  • 10.  As people begin to have mobile computers handy, for whatever purposes, sharing information between the computers will become a natural requirement.  Currently, such sharing is made difficult by the need for users to perform administrative tasks and set up static, bidirectional links between their computers.  if the wireless communications systems in the mobile computers support a broadcast mechanism, much more flexible and useful ways of sharing information can be imagined.  one of our primary motivations is to allow the construction of temporary networks with no wires and no administrative intervention required.  In this paper, such a interconnection between the mobile computers will be called an ad-hoc network, in conformance with current. Usage within the IEEE 802.11 subcommittee. 8/6/2014 10
  • 11.  to provide service in the most general situation, we do not assume that every computer is within communication range of every other computer.  Currently, there is no method available which enables mobile computers with wireless data communications equipment to freely roam about while still maintaining connections with each other, unless special assumptions are made about the way the computers are situated with respect to each other.  Routing protocols for existing networks have not been designed specifically to provide the kind of dynamic, self-starting behavior needed for ad-hoc networks. 8/6/2014 11
  • 12.  the wireless medium differs in important ways from wired media,  mobile computers may well have only a single network interface adapter,  Whereas most existing routers have network interfaces to connect two separate networks together  wireless media are of limited and variable range, in distinction to existing wired media. 8/6/2014 12
  • 13.  Two primary classes 1. link-state and 2. distance-vector 8/6/2014 13
  • 14.  closer to the centralized version of the shortest path computation method.  Each node maintains a view of the network topology with a cost for each link.  To keep these views consistent, each node periodically broadcasts the link costs of its outgoing links to all other nodes using a protocol such as flooding.  As a node receives this information, it updates its view of the network topology and applies a shortest-path algorithm to choose its next hop for each destination.  Some of the link costs in a node’s view can be incorrect because of long propagation delays, partitioned network, etc.  Such inconsistent views of network topologies might lead to formation of routing loops.  These loops, however, are short-lived, because they disappear in the time it takes a message to traverse the diameter of the network. 8/6/2014 14
  • 16.  In this algorithm, every node i maintains, for each destination x, a set of distances {dx i,j } where j ranges over the neighbors of i.  Node i treats neighbor k as a next-hop for a packet destined for x if {dx i,k } equals minj {dx i,j }.  The succession of next hops chosen in this manner lead to x along the shortest path.  In order to keep the distance estimates up-to-date, each node monitors the cost of its outgoing links and periodically broadcasts, to each one its neighbors, its current estimate of the shortest distance to every other node in the network 8/6/2014 16
  • 17.  It is the classical Distributed Bellman-Ford (DBF) algorithm  Compared to link-state method, it is computationally more efficient, easier to implement and requires much less storage space  The primary cause for formation of routing loops  nodes choose their next-hops in a completely distributed fashion based on information which can possibly be stale and, therefore, incorrect.  Almost all proposed modifications to DBF algorithm eliminate the looping problem by forcing all nodes in the network to participate in some form of internodal coordination protocol.  within an ad-hoc mobile environment enforcing any such internodal coordination mechanism will be difficult due to the rapidly changing topology of the underlying routing network 8/6/2014 17
  • 18.  The usefulness of RIP within ad-hoc environment, however, is limited as it was not designed to handle rapid topological changes.  design goal  to design a routing method for ad-hoc networks which preserves the simplicity of RIP, yet at the same time avoids the looping problem.  their approach –to tag each route table entry with a sequence number so that nodes can quickly distinguish stale routes from the new ones and thus avoid formation of routing loop 8/6/2014 18
  • 20.  Allows Collections of mobile computers ���which are not close to any base stations  can exchange data along changing and arbitrary paths of interconnection  Solution must remain compatible with operation in cases where a base station is available. 8/6/2014 20
  • 21.  Packets are transmitted between the stations  using routing tables -stored at each station of the network.  Each route table entry is tagged with a sequence number which is originated by the destination station.  To maintain the consistency of routing tables in a dynamically varying topology, each station periodically transmits updates, and  transmits updates immediately when significant new information is available.  not assume that the mobile hosts are maintaining any sort of time synchronization  packets indicate which stations are accessible from each station and the number of hops necessary to reach these accessible stat ions, as is often done in dist ant-vector routing algorithms 8/6/2014 21
  • 22.  The packets may be transmitted containing either layer 2 (MAC) addresses or layer 3 (network) addresses  Routing information is advertised by broadcasting or multicasting the packets  Data is also kept about the length of time between arrival of the first and the arrival of the best route for each particular destination.  Based on this data, a decision may be made to delay advertising routes which are about to change soon, thus damping fluctuations of the route tables.  The advertisement of routes which may not have stabilized-delayed in order to reduce the number of rebroadcasts of possible route entries that normally arrive with the same sequence number. 8/6/2014 22
  • 23.  requires each mobile station to advertise, to each of its current neighbors, its own routing table (for instance, by broadcasting its entries).  The entries in this list may change fairly dynamically over time,  so the advertisement must be made often enough to ensure that every mobile computer can almost always locate every other mobile computer of the collection  each mobile computer agrees to relay data packets to other computers upon request  agreement places a premium on the ability to determine the shortest number of hops for a route to a destination  In this way a mobile computer may exchange data with any other mobile computer in the group even if the target of the data is not within range for direct communication 8/6/2014 23
  • 24.  In a wireless medium, it is important to keep in mind that broadcasts are limited in range by the physical characteristics of the medium  Broadcast address contain for each new route  The destination’s address  The number of hops required to reach the destination and  The sequence number of the information received regarding that destination, as originally stamped by the destination; 8/6/2014 24
  • 25.  It include a sequence number created by the transmitter.  Routes with more recent sequence numbers  preferred as the basis for making forwarding decisions but not necessarily advertised.  Path with the same sequence number, those with the smallest metric will be used.  Routing tables are propagated, the sequence number is sent to all mobile computers  Routes received in broadcasts advertised by the receiver when it subsequently broadcasts its routing information;  the receiver adds an increment to the metric before advertising the route, since incoming packets will require one more hop to reach the destination. 8/6/2014 25
  • 26.  detected by if no broadcasts have been received for a while from a former neighbor.  A broken link is described by a metric of ∞  When a link to a next hop has broken, any route through that next hop is immediately assigned an ∞ metric and assigned an updated sequence number.  such modified routes are immediately disclosed in a broadcast routing information packet.  Building information to describe broken links is the only situation when the sequence number is generated by any Mobile Host other than the destination Mobile Host. 8/6/2014 26
  • 27.  Thus, there will be two routing tables kept at each Mobile Host; one for use with forwarding packets, and another to be advertised via incremental routing information packets.  To determine the probability of coming up arrival of routing information showing a better metric,  the Mobile Host has to keep a history of the weighted average time that routes to a particular destination fluctuate until the route with the best metric is received. 8/6/2014 27
  • 28.  operation at Layer 3 will use network layer addresses for the next hop and destination addresses,  operation at Layer 2 will use Layer 2 Media Access Control (MAC) addresses  The difficulty is that Layer 3 network protocols provide communication  based on network addresses, and a way must be provided to resolve these Layer 3 addresses into MAC addresses.  PROPOSED SOLUTION  Each destination host would advertise which Layer 3 protocols it supports, and  Each Mobile Host advertising reachability to that destination would include along, with the advertisement, the  information about the Layer 3 protocols supported at that destination. 8/6/2014 28
  • 29.  Mobile computers will frequently be used in conjunction with base stations, which allow them to exchange data with other computers connected to the wired network.  By participating in the DSDV protocol, base stations can extend their coverage beyond the range imposed by their wireless transmitters.  When a base station participates in DSDV, it is shown as a default route in the tables transmitted by a mobile station. 8/6/2014 29
  • 30.  Consider Mh4. Suppose the address of each Mobile Host is represented as MHi  Suppose all sequence numbers are denoted SNNN_MHi,  where MHi specifies the computer that created the sequence number  and SNNN is a sequence number value 8/6/2014 30
  • 31.  Suppose that there are entries for all other Mobile Hosts, with sequence numbers SNNN.MHi, before MH1 moves away from MH2.  The install time field helps determine when to delete stale routes.  With this protocol, the deletion of stale routes should rarely occur,  since the detection of link breakages should propagate through the ad-hoc network immediately  we expect to continue to monitor for the existence of stale routes and take appropriate action  all the computers became available to MH4 at about the same time, since its install-time for most of them is about the same.  none of the links between the computers were broken,  because all of the sequence number fields have times with even digits in the units place.  Ptrl-MHi would all be pointers to null structures,  because there are not any routes in Figure 1 which are likely to be superseded or compete with other possible routes 8/6/2014 31
  • 33.  suppose that MH1 moves into the general vicinity of MH5 and MH7, and away from the others (especially MH2).  When MH1 moves, it triggered an immediate incremental routing information update which was then broadcast to MH6.  MH6, determined that significant new routing information had been received, also triggered an immediate update which carried along the new routing information for MH1. 8/6/2014 33
  • 34.  this advertisement, the information for MH4 comes first, since it is doing the advertisement.  The information for MH1 comes next, not because it has a lower address, but because MH1 is the only one which has any significant route changes affecting it.  As a general rule, routes with changed metrics are first included in each incremental packet.  The remaining space is used to include those routes whose sequence numbers have changed. 8/6/2014 34
  • 35.  In this example, one node has changed its routing information, since it is in a new location.  All nodes have transmitted new sequence numbers recently.  If there were too many updated sequence numbers to fit in a single packet, only the ones which fit would be transmitted.  These would be selected with a view to fairly transmitting them in their turn over several incremental update intervals.  The frequency of transmitting full updates would be reduced if the volume of data began to consume a significant fraction of the available capacity of the medium 8/6/2014 35
  • 36.  Settling Time = Time between arrival of first route and the best route with a given seq. nr.  describes how the settling time table is used to prevent fluctuations of routing table entry advertisements.  The general problem arises because route updates are selected according to the following criteria:  Routes are always preferred if the sequence numbers are newer;  Otherwise, routes are preferred if the sequence numbers are the same and yet the metric is better (lower). 8/6/2014 36
  • 37.  suppose that two routes with identical sequence numbers are received by a Mobile Host, but in the wrong order.  In other words, suppose that MH4 receives the higher metric next hop first, soon after gets another next hop with a lower metric but the same sequence number.  This could happen when there are a lot of Mobile Hosts, transmitting their updates not quite regularly.  Alternatively, if the Mobile Hosts are acting independently and with markedly different transmission intervals, the situation could occur with correspondingly fewer hosts. 8/6/2014 37
  • 38.  Suppose, in any event, in Figure 2 that there are enough Mobile Hosts to cause the problem, in two separate collections of Mobile Hosts both connected to a common destination MH9, but with no other Mobile Hosts in common.  Suppose further that all Mobile Hosts are transmitting updates approximately every 15 seconds, that Mobile Host MH2 has a route to MH9 with 12 hops, and Mobile Host MH6 has a route to MH9 with 11 hops,  Moreover, suppose that the routing information update from MH2 arrives at MH4 approximately 10 seconds before the routing information update from MH6. 8/6/2014 38
  • 39.  This will occur every time that a new sequence number is issued from Mobile Host MH9.  In fact, the time differential can be drastic if any Mobile Host in collection II begins to issue its sequence number updates in multiple incremental update intervals, as would happen,  for instance, when there are too many hosts with new sequence number updates for them all to fit within a single incremental packet update.  In general, the larger the number of hops, the more drastic differentials between delivery of the updates can be expected in Figure 2 8/6/2014 39
  • 40.  The settling time data is stored in a table with the following fields, keyed by the first field:  Destination address  Last settling time  Average settling time  The settling time is calculated by maintaining a running, weighted average over the most recent updates of the routes, for each destination. 8/6/2014 40
  • 41.  Suppose a new routing information update arrives at MH4.  The sequence number in the new entry is the same as the sequence number in the currently used entry, and the newer entry has a worse (i.e., higher) metric.  Then MH4 must use the new entry in making subsequent forwarding decisions.  However, MH4 does not have to advertise the new route immediately  and can consult its route settling time table to decide  How long to wait before advertising it.  The average settling time is used for this determination.  For instance, MH4 may decide to delay (average.settling_time x 2) before advertising a route. 8/6/2014 41
  • 42.  if a link via Mobile Host MH6 truly does break, the advertisement of a route via MH2 should proceed immediately.  To achieve this when there is a history of fluctuations at Mobile Host MH4, the link breakage should be detected fast enough so that an intermediate host in Collection II  finds out the problem and begins a triggered incremental update showing an ∞ metric for the path along the way to Mobile Host MH9.  Routes with an ∞ metric are required by this protocol to be advertised immediately, without delay. 8/6/2014 42
  • 43.  Settling time of a particular route must be counted with a higher weighting factor than are less recent measurements.  When a new routing update is received from a neighbor, during the same time that the updates are applied to the table, processing also occurs to delete stale entries.  the routing algorithm could be applied in any situation where  reduced memory requirements are desired (compared to link-state routing algorithms).  The operation of an ad-hoc network could be applied to wired as well as wireless mobile computers.  In general, then, we provide a new destination-sequenced routing algorithm, and this algorithm is supplemented by a technique for damping fluctuations. 8/6/2014 43
  • 44.  the DSDV protocol guarantees loop free paths to each destination.  To see why this property holds  consider a collection of N mobile hosts forming an instance of an ad-hoc style network.  assume that the system is in steady-state,  i.e. routing tables of all nodes have already converged to the actual shortest paths.  At this instant, the next node indicators to each destination induce a tree rooted at that destination.  Thus, routing tables of all nodes in the network can be collectively visualized as forming N trees, one rooted at each destination. 8/6/2014 44
  • 45.  Potentially a loop may form each time node i changes its next-hop.  This can happen in two cases.  First, when node i detects that the link to its next-hop is broken,  The node resets px i to nil.Clearly, this action cannot form a loop involving 2.  The second scenario occurs when node i receives, from one of its neighbors k, a route to x, with sequence number sx k ; and metric m,  which is selected to replace the current route it has through px i 8/6/2014 45
  • 46.  Let sx i denote the value of the sequence number stored at node i  and dx i denote the distance estimate from i to x just prior to receiving route from k.  i will change its next-hop from px i; to k only if either of the following two happens.  the new route contains a newer sequence number, i.e., sx k>sx I  the sequence number sx k is same as sx i, but the new route offers a shorter path to x, i.e, m < dix 8/6/2014 46
  • 48.  They have implemented a preliminary version of this protocol for use with mobile computers in our lab.  They are making necessary modifications to the MARS simulator [1] for use in creating the appropriation simulation environment for our needs.  They hope to discover good operational values via simulation for the following quantities:  Average convergence times  Incremental update period  Settling time averaging method  Full update period  Settling time applied to triggered updates  measurement of the convergence times may depend heavily on many interesting parameters, such as the average velocity of the mobile hosts, update periods, size of the mobile host population, geographical placement of mobile hosts, existence of base stations, and average processing loads at the mobile computers 8/6/2014 48
  • 49. 8/6/2014 49  Providing convenient connectivity for mobile compters is a challenge that is only now bieng met.  We make good use of the properties of wireless broadcast medium.  In the latter case certain additional information should be included along with the routing tables for the most convenient and efficient operation.  The information in the routing table is similar to what is found in the routing table with the todays distance vector algorithm.  All sequence numbers are generated by the destination computer in each route table entry.  The latter case describe by infinity metric and a sequence number which cannot be correctly generated any destination computer.  By the natural join operation of protocol,the sequence number chosen to represent broken links will be suspended by real routes propagated from the newlylocated destination as soon as possibal.
  • 50.  This allow real route data to quickly supersede temperory link outages when a mobile computers move from one place to another.  This include movement from place to place,as well as disapperance of mobile host from the interconnect topology.  We have found that mobile computers,modelled as routers.  We hope to explore further the necessary application level support needed to automatically use of the network layer route capabilities to provide simple access to conferencing and workplace tools for collaboration and information sharing. 8/6/2014 50