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Adhoc Networks Routing Tutorial - Part II -

Adhoc Networks Routing Tutorial - Part II -. Internet Computing Laboratory @ KUT ( http://icl.kut.ac.kr ) Youn-Hee Han. It is licensed under a Creative Commons Attribution 2.5 License. Reactive Routing Protocols. Reactive Approach - Principle. On-Demand

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Adhoc Networks Routing Tutorial - Part II -

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  1. Adhoc Networks Routing Tutorial- Part II - Internet Computing Laboratory @ KUT (http://icl.kut.ac.kr) Youn-Hee Han It is licensed under a Creative Commons Attribution 2.5 License

  2. Reactive Routing Protocols Computer Network

  3. Reactive Approach - Principle • On-Demand • Re-active, become active only when needed • The routes are created when required • The source has to discover a route to the destination. • The source and intermediate nodes have to maintain a route as long as it is used. • Routes have to be repaired in case of topology changes. • Normally 3 Phases • Route Discovery • Route Maintain • Route Delete • More Scalable than Table Drive (Proactive) Protocols Computer Network

  4. Reactive Approach - Principle • Representative Protocols • DSR– Dynamic Source Routing • AODV– Ad Hoc On-demand Distance Vector Routing • TORA – Temporally Ordered Routing Algorithm Computer Network

  5. Dynamic Source Routing (DSR) • Source routing • Full Routes are kept in each data packet • Sender of a packet determines the complete sequence of nodes to forward the packet • Reaction to topology changes more rapid • node caches multiple routes to destination • Avoids need to perform Route Discovery each time a route breaks • Each host participating in the ad hoc network maintains a route cache in which it caches source routes for destination nodes Computer Network

  6. Dynamic Source Routing (DSR) • Route Discovery: • allows any host to dynamically discover a route to any other host in the ad hoc network. • a host initiating a route discovery broadcasts a route request packet, Route Request (RREQ) • each route request packet contains a route record, request ID • If a host saw the packet (with the same request ID) before, discards it. • Otherwise, the host looks up its route caches to look for a route to destination, If not find, appends its address into the packet, rebroadcast, • If finds a route in its route cache, sends a route reply packet, which is sent to the source by route cache, RREQ’s info. or the route discovery. • if successful, initiating host receives a route reply packet, Route Reply (RREP) • Any node which has a route to the destination replies with a route reply • the route is saved in the cache for future use • Any route that is seen through a node is cached Computer Network

  7. Dynamic Source Routing (DSR) • Route maintenance: • A host monitors the correct operation of routes in use • A host is able to detect that the network topology has changed such that it can no longer use its route to D because a link along the route no longer works • A Route ERROR is sent to the Source • All nodes along the path remove that route • 예를들어 S – E – F – J – D • 임의의 노드 이동 (if F) • E와 J는 Route cache에 저장된 경로중 F가 포함된 경로는 모두 삭제 • E와 J는 각각 S와 D에게 Route ERROR를 보냄 • S와 D도 Route cache에 저장된 경로중 F가 포함된 경로는 모두 삭제 • If receives a Route ERROR, the source uses a cached alternate route to destination or sends out request packets for a new route Computer Network

  8. DSR –Route Discovery • When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery • Source node S floods Route Request (RREQ) • Each node appends its own address when forwarding RREQ 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 Computer Network

  9. DSR –Route Discovery • Originating a RREQ RREQ Y Broadcast transmission (ACKed) Z [S] S E F B C M L J A G H D K I N Represents transmission of RREQ Represents a node that has received RREQ for D from S Computer Network

  10. DSR –Route Discovery • Forwarding RREQ Y RREQ Z Path Accumulation S [S,E] E F B C M L J A G [S,C] H D K I N • Node H receives packet RREQ from two neighbors B and C • Node C and E send RREQ to each other • Duplicate RREQ are discarded [X,Y] Represents list of Node ID appended to RREQ Computer Network

  11. DSR –Route Discovery • Forwarding RREQ Y Z S E F [S,E,F] B C M L J A G H D K [S,B,A] [S,C,G] I N [S,C,H] • Node C receives RREQ from G and H, but does not forward • it again, because node C has already forwarded RREQ once • Duplicate RREQ are discarded Computer Network

  12. DSR –Route Discovery • Forwarding RREQ Y Z S E F [S,E,F,J] B C M L J A G H D [S,C,H,I] K I N [S,C,G,K] Computer Network

  13. DSR –Route Reply • Destination D on receiving the first RREQ, sends a Route Reply (RREP) • RREP includes the route from S to D on which RREQ was received by node D • Route Reply is sent by reversing the route in Route Request (RREQ) • this requires bi-directional link • to ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional • If IEEE 802.11 MAC is used to send data, then links have to be bi-directional (since ACK is used) • If unidirectional (asymmetric) links are used, then RREP may need a route discovery for S from node D • Piggyback RREP on RREQ to S Computer Network

  14. DSR –Route Reply • RREP delivery Y RREP [S,E,F,J,D] Z S E F B C M L J A G H D K I N RREP control message Computer Network

  15. DSR Optimization - Route Caching • Each node caches a new route it learns • When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F • When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S • When node F forwards Route Reply RREP[S,E,F,J,D], node F learns route [F,J,D] to node D • When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D • Other optimisations: • Piggybacking on route discoveries • data on route requests • RREP on Route Discovery from D to S • RREP Hop Limit Computer Network

  16. DSR - Summary • Advantages • Reactive: routes maintained only between nodes who need to communicate • Route caching can further reduce route discovery overhead • a single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches • Disadvantages • Packet header size grows with route length due to source routing • Flood of route requests may potentially reach all nodes in the network • Care must be taken to avoid collisions between route requests and route reply propagated by neighboring nodes Computer Network

  17. DSR - Summary • Increased contention if too many route replies come back due to nodes replying using their local cache • Route Reply Storm problem • Reply storm may be eased by preventing a node from sending RREP if it hears another RREP with a shorter route • Route reply storms also prevented by randomising delay time before sending route replies Computer Network

  18. DSR–one more example source broadcasts a packet containing address of source and destination source (1,4) 1 4 The destination sends a reply packet to source. 8 (1,3) destination 3 7 (1,4,7) 2 The node discards the packets having been seen (1,2) 6 5 (1,3,5,6) (1,3,5) The route looks up its route caches to look for a route to destination If not find, appends its address into the packet Computer Network

  19. AODV • AODV: Ad hoc On-demand Distance Vector routing protocol • an IETF Experimental RFC • References • C. E. Perkins, E. M. Belding-Royer, and S. R. Das, “Ad hoc On-Demand Distance Vector (AODV) Routing,” IETF RFC 3561, July, 2003. • C. E. Perkins and E. M. Royer, “Ad hoc On-Demand Distance Vector Routing,” Proceedings 2nd IEEE Workshop on Mobile Computing Systems and Applications, February 1999, pp. 90-100. Computer Network

  20. Adhoc On-demand Distance Vector (AODV) • Pure on-demand routing protocol • A node does not perform route discovery or maintenance until it needs a route to another node or it offers its services as an intermediate node • Nodes that are not on active paths do not maintain routing information and do not participate in routing table exchanges • Uses a broadcast route discovery mechanism • Uses hop-by-hop routing • Routes are based on dynamic table entries maintained at intermediate nodes • Similar to Dynamic Source Routing (DSR), but DSR uses source routing Computer Network

  21. Adhoc On-demand Distance Vector (AODV) • Designed for MANETs with 10,000 to 100,000 nodes • Uses a broadcast route discovery mechanism as in DSR • Each node only keeps next-hop information • Improves scalability and performance • Reduces dissemination of control traffic • Eliminates overhead on data traffic • From DSR • Route discovery • Route maintenance • From DSDV • Hop-by-hop routing • Sequence numbers Computer Network

  22. AODV – overview • Route Table Entry • Destination IP Address (KEY) • Destination Sequence Number • it is used to determine the freshness of the information contained from the originating node. • crucial to avoiding routing loops • Valid Destination Sequence Number flag • Other state and routing flags • valid, invalid, repairable, being repaired • Network Interface • Hop Count • number of hops needed to reach destination • Next Hop • List of Precursors • Neighboring nodes to which a RREP was forwarded. • Lifetime (expiration or deletion time of the route) Computer Network

  23. AODV – overview Route Table Entry Node 1 에서의 routing table 4 2 5 3 1 6 Computer Network

  24. AODV – overview Route Table Entry Node 1 에서의 routing table 4 2 5 3 1 6 Computer Network

  25. AODV – overview • Basic Message Set • RREQ – Route request • RREP – Route reply • RERR – Route error • HELLO – For link status monitoring Computer Network

  26. AODV – overview • RREQ Messages • While communication routes between nodes are valid, AODV does not play any role. • A RREQ message is broadcastedwhen a node needs to discover a route to a destination. • As a RREQ propagates through the network, intermediate nodes use it to update their routing tables (in the direction of the source node) • Setup Reverse Path to Source Node • DSN is set to the value in the originator sequence number field of the RREQ, if it is greater than the stored DSN. • The RREQ also contains the most recent sequence number for the destination (DSN). • If the source node don’t know it, DSN is zero • A valid destination route must have a sequence number at least as great as that contained in the RREQ. Computer Network

  27. AODV – overview • RREQ Message Format • J: Join Flag, R: Repair Flag • G: Gratuitous RREP flag • indicates whether a gratuitous RREP should be unicast to the node specified in the Destination IP Address field • D: Destination only flag • indicates only the destination may respond to this RREQ • U: Unknown sequence number • indicates the destination sequence number is unknown • Hop Count • The number of hops from the Originator IP Address to the node handling the request. • RREQ ID • A sequence number uniquely identifying the particular RREQ when taken in conjunction with the originating node's IP address. • Destination Sequence Number • The latest sequence number received in the past by the originator for any route towards the destination. • Originator Sequence Number • The current sequence number to be used in the route entry pointing towards the originator of the route request. 1 Computer Network

  28. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (1/7) Computer Network

  29. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (2/7) RREQ 전송 Computer Network

  30. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (3/7) Reverse Path Computer Network

  31. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (4/7) Computer Network

  32. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (5/7) Computer Network

  33. Y Z S E F B C M L J A G H D K I N AODV Route Discovery (6/7) Route Table Computer Network

  34. AODV – overview • RREP Messages • When a RREQ reaches a destination node, the destination route is made available by unicasting a RREP back to the source route. • A node generates a RREP if: • It is itself the destination. • It has an active route to the destination. • Ex: an intermediate node may also respond with an RREP if it has a “fresh enough” route to the destination. • As the RREP propagates back to the source node, intermediate nodes update their routing tables (in the direction of the destination node). • Setup Forward Path to Source Node Computer Network

  35. AODV – overview • RREP Message Format • R: Repair Flag • A: Acknowledgment required • receiver of the RREP is expected to return a RREP-ACK message • Prefix Size • If nonzero, the 5-bit Prefix Size specifies that the indicated next hop may be used for any nodes with the same routing prefix (as defined by the Prefix Size) as the requested destination • Hop Count • The number of hops from the RREP generator to the Destination IP Address. • Destination Sequence Number • The destination sequence number associated to the route. • Lifetime • The time in milliseconds for which nodes receiving the RREP consider the route to be valid. • Destination IP Address • The IP address of the destination for which a route is supplied. • Originator IP Address • The IP address of the node which originated the RREQ for which the route is supplied. 2 Computer Network

  36. RREP 전송 Forward Path AODV Route Discovery (7/7) Route Table Y Z S E F B C M L J A G H D K I N Computer Network

  37. AODV – overview • RERR Messages • This message is broadcast for broken links • Generated directly by a node or passed on when received from another node • RERR Message is broadcasted when: • A node detects that a link with adjacent neighbor is broken (destination no longer reachable). • If it gets a data packet destined to a node for which it does not have an active route and is not repairing. • If it receives a RERR from a neighbor for one or more active routes. Computer Network

  38. AODV – overview • RERR Message Format • N: No delete flag • set when a node has performed a local repair of a link, and upstream nodes should not delete the route. • DestCount • The number of unreachable destinations included in the message; MUST be at least 1. • Unreachable Destination IP Address • The IP address of the destination that has become unreachable due to a link break. • Unreachable Destination Sequence Number • The sequence number in the route table entry for the destination listed in the previous Unreachable Destination IP Address field. • RREP-ACK Message Format 3 4 Computer Network

  39. AODV – overview • Hello Messages • HELLO messages are used to determine local connectivity • HELLO Message = RREP with TTL = 1 • If a node does not receive any packets (HELLO messages or otherwise) from a neighbor for more than ALLOWED_HELLO_LOSS * HELLO_INTERVAL, the node will assume that the link to this neighbor is currently lost. • A node should use HELLO messages only if it is part of an active route. Computer Network

  40. AODV – overview Basic Messaging Routing Source G A RREQ RREQ RREQ RREP RREQ B D RREQ RREP RREQ RREQ RREP F Destination C RREQ RREQ E Computer Network

  41. AODV – overview • Why Sequence Numbers in AODV • To avoid using old/broken routes • To determine which route is newer • To prevent formation of loops • Assume that A does not know about failure of link C-D because RERR sent by C is lost • Now C performs a route discovery for D. Node A receives the RREQ (say, via path C-E-A) • Node A will reply since A knows a route to D via node B • Results in a loop (for instance, C-E-A-B-C ) A B C D E A B C D E Computer Network

  42. AODV – overview • destination sequence number(DSN) used as timestamps. • DSN is updated whenever a node receives new (i.e., not stale) information about the sequence number from RREQ, RREP, or RERR messages • Source sequence number in RREQ indicates “freshness” of reverse route to the source • Destination sequence number in RREQ indicates the latest sequence number received in the past by the originator towards the destination. • Destination sequence number in RREP or RERR indicates “freshness” of route to the destination Computer Network

  43. AODV – overview • Sequence Numbers Rule • A sender node MUST increment it before it sends a RREQ • An intermediate node in RREQ path determines whether Received DSN < Stored DSN. If it is, the RREQ message MUST be discarded and the sender is notified of the new information. • Ensures RREQs not forwarded unnecessarily • Before a node sends a RREP in response to a RREQ, it MUST update its own sequence number to MAX {its current stored DSN : DSN in the RREQ packet}. Computer Network

  44. AODV – overview • When does an intermediate node sends RREP to the RREQ originator? • “an intermediate node has a more fresh information” • It means • Seq(A) < Seq(B) • Intermediate node (B) either has a newer route to endpoint than the start node (A) • If a node receives a RREQ, when does a node updates its routing table entry? • 1. Received Number > Stored Number • 2. Received Number == Stored Number, and (The number in hop count field of the message + 1) < Stored hop count number Computer Network

  45. Start to transmit data packet Broadcast RREQ packet NET_TRAVERSAL_TIME Timer set Retry ++ Generate RREQ packet(Hop Count = 0) NET_TRAVERSAL_TIME=2 * NET_TRAVERSAL_TIME AODV – Flowchart Generate RREQ packet Control Messages sent of Port 654 UDP When need to send data. Have a route to destination? No Yes Yes RREP received withinNET_TRAVERSAL_TIME? Retry < RREQ_RETRIES? No Yes No Packet Sending is not allowed End

  46. AODV – Flowchart • Receive RREQ packet Receive RREQ packet Already Process RREQ ID? Yes Discard it No Set the Reverse path Destination ? No Fresh Route Information to destination ? No Yes Yes Rebroadcast RREQ packet Reply RREP packet Computer Network

  47. AODV – Flowchart • Receive RREP packet Receive RREP packet Set Forward path Source node? No Update next hop information Yes Start to transmit data packet Unicast RREP via Reverse path Computer Network

  48. AODV – Flowchart • Detecting broken link Detecting Broken Link Generate RERR packet Precursor count = 1? No Yes Multiple unicast RERR Unicast RERR via Reverse path Computer Network

  49. Timeouts • A routing table entry maintaining a reverse path is purged after a timeout interval • timeout should be long enough to allow RREP to come back • A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval • if no is data being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid) Computer Network

  50. Link Failure Detection • Hellomessages • Absence of hello message is used as an indication of link failure • When it is sent? • Every HELLO_INTERVAL, a node checks whether it has sent a broadcast (a RREQ or an layer 2 message) within the HELLO_INTERVAL • If it has not, it broadcasts HELLO • Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure • A neighbor of node X is still considered active for if the neighbor sent a packet within active_route_timeout interval which was forwarded using an entry in the routing table Computer Network

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