Background of Ad hoc Wireless Networks Student Presentations Wireless Communication Technology and Research Ad hoc Routing and Mobile IP and Mobility Wireless Sensor and Mesh Networks Mobile and Ad hoc Networks Adhoc Network Routing

Routing Overview  Network with nodes, edges  Goal: Devise scheme for transferring message from one node to another  Which path?  Who decides – source or intermediate nodes?

Which path?  Routing generally tries to optimize something:  Shortest path (fewest hops)  Shortest time (lowest latency)  Shortest weighted path (utilize available bandwidth)  Etc…

Who determines route? Two general approaches:  Source (“path”) routing  Source specifies entire route: places complete path to destination in message header: A – D – F – G  Intermediate nodes just forward to specified next hop: D would look at path in header, forward to F  Like airline travel – get complete set of tickets to final destination before departing…

 Destination (“hop-by-hop”) routing  Source specifies only destination in message header: G  Intermediate nodes look at destination in header, consult internal tables to determine appropriate next hop  Like postal service – specify only the final destination on an envelope, and intermediate post offices select where to forward next…

Comparison  Source routing  Moderate source storage (entire route for each desired dest.)  No intermediate node storage  Higher routing overhead (entire path in message header, route discovery messages)  Destination routing  No source storage  High intermediate node storage (table with routing instructions for all possible dests.)  Lower routing overhead (just dest in header, only routers need to deal with route discovery)

Ad Hoc Routing  Every node participates in routing: no distinction between “routers” and “end nodes”  No external network setup: “self-configuring”  Especially useful when network topology is dynamic (frequent network changes – links break, nodes come and go)

Common Application  Mobile wireless hosts  Only subset within range at given time  Want to communicate with any other node

Ad Hoc Routing Protocols  Standardization effort led by IETF Mobile Ad-hoc Networks (MANET) task group  charter.html charter.html  9 routing protocols in draft stage, 4 drafts dealing with broadcast / multicast / flow issues  Other protocols being researched  utilize geographic / GPS info, Ant-based techniques, etc.

10 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

11 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

12 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

Leading MANET Contenders  DSR: Dynamic Source Routing  Source routing protocol  AODV: Ad-hoc On-demand Distance Vector Routing  “Hop-by-hop” protocol  Both are “on demand” protocols: route information discovered only as needed

Dynamic Source Routing  Draft RFC at drafts/draft-ietf-manet-dsr-07.txthttp:// drafts/draft-ietf-manet-dsr-07.txt  Source routing: entire path to destination supplied by source in packet header  Utilizes extension header following standard IP header to carry protocol information (route to destination, etc.)

DSR Protocol Activities  Route discovery  Undertaken when source needs a route to a destination  Route maintenance  Used when link breaks, rendering specified path unusable  Routing (easy!)

Route Discovery  Route Request:  Source broadcasts Route Request message for specified destination  Intermediate node:  Adds itself to path in message  Forwards (broadcasts) message toward destination  Route Reply  Destination unicasts Route Reply message to source  will contain complete path built by intermediate nodes

Details, details…  Intermediate nodes cache overheard routes  “Eavesdrop” on routes contained in headers  Reduces need for route discovery  Intermediate node may return Route Reply to source if it already has a path stored

Route Maintenance  Used when link breakage occurs  Link breakage may be detected using link-layer ACKs, DSR ACK request  Route Error message sent to source of message being forwarded when break detected  Intermediate nodes “eavesdrop”, adjust cached routes  Source deletes route; tries another if one cached, or issues new Route Request  Piggybacks Route Error on new Route Request to clear intermediate nodes’ route caches, prevent return of invalid route

19 Dynamic Source Routing (DSR) [Johnson96]  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 own identifier when forwarding RREQ

20 Route Discovery in DSR B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L

21 Route Discovery in DSR B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L [S] [X,Y] Represents list of identifiers appended to RREQ

22 Route Discovery in DSR B A S E F H J D C G I K Node H receives packet RREQ from two neighbors: potential for collision Z Y M N L [S,E] [S,C]

23 Route Discovery in DSR B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Z Y M N L [S,C,G] [S,E,F]

24 Route Discovery in DSR B A S E F H J D C G I K Z Y M Nodes J and K both broadcast RREQ to node D Since nodes J and K are hidden from each other, their transmissions may collide N L [S,C,G,K] [S,E,F,J]

25 Route Discovery in DSR B A S E F H J D C G I K Z Y Node D does not forward RREQ, because node D is the intended target of the route discovery M N L [S,E,F,J,M]

26 Route Discovery in DSR  Destination D on receiving the first RREQ, sends a Route Reply (RREP)  RREP is sent on a route obtained by reversing the route appended to received RREQ  RREP includes the route from S to D on which RREQ was received by node D

27 Route Reply in DSR B A S E F H J D C G I K Z Y M N L RREP [S,E,F,J,D] Represents RREP control message

28 Route Reply in DSR  Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi- directional  To ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional  If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for S from node D  Unless node D already knows a route to node S  If a route discovery is initiated by D for a route to S, then the Route Reply is piggybacked on the Route Request from D.  If IEEE MAC is used to send data, then links have to be bi-directional (since Ack is used)

29 Dynamic Source Routing (DSR)  Node S on receiving RREP, caches the route included in the RREP  When node S sends a data packet to D, the entire route is included in the packet header  hence the name source routing  Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

30 Data Delivery in DSR B A S E F H J D C G I K Z Y M N L DATA [S,E,F,J,D] Packet header size grows with route length

31 When to Perform a Route Discovery  When node S wants to send data to node D, but does not know a valid route node D

32 DSR Optimization: Route Caching  Each node caches a new route it learns by any means  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  A node may also learn a route when it overhears Data packets

33 Use of Route Caching  When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request  Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D  Use of route cache  can speed up route discovery  can reduce propagation of route requests

34 Use of Route Caching B A S E F H J D C G I K [P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format) M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] Z

35 Use of Route Caching: Can Speed up Route Discovery B A S E F H J D C G I K Z M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] RREQ When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route [K,G,C,S] RREP

36 Use of Route Caching: Can Reduce Propagation of Route Requests B A S E F H J D C G I K Z Y M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] RREQ Assume that there is no link between D and Z. Route Reply (RREP) from node K limits flooding of RREQ. In general, the reduction may be less dramatic. [K,G,C,S] RREP

37 Route Error (RERR) B A S E F H J D C G I K Z Y M N L RERR [J-D] J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails Nodes hearing RERR update their route cache to remove link J-D

38 Route Caching: Beware!  Stale caches can adversely affect performance  With passage of time and host mobility, cached routes may become invalid  A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route  An illustration of the adverse impact on TCP is discussed in tutorial by [Holland99]

39 Dynamic Source Routing: Advantages  Routes maintained only between nodes who need to communicate  reduces overhead of route maintenance  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

40 Dynamic Source Routing: 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 propagated by neighboring nodes  insertion of random delays before forwarding RREQ  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

41 Dynamic Source Routing: Disadvantages  An intermediate node may send Route Reply using a stale cached route, thus polluting other caches  This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated.  For some proposals for cache invalidation, see [Hu00Mobicom]  Static timeouts  Adaptive timeouts based on link stability

Issues  Scalability  Discovery messages broadcast throughout network  Broadcast / Multicast  Use Route Request packets with data included  Duplicate rejection mechanisms prevent “storms”  Multicast treated as broadcast; no multicast-tree operation defined  Scalability issues  manet-simple-mbcast-01.txt manet-simple-mbcast-01.txt

Ad-hoc On-demand Distance Vector Routing  Draft RFC at drafts/draft-ietf-manet-aodv-10.txthttp:// drafts/draft-ietf-manet-aodv-10.txt  “Hop-by-hop” protocol: intermediate nodes use lookup table to determine next hop based on destination  Utilizes only standard IP header

AODV Protocol Activities  Route discovery  Undertaken whenever a node needs a “next hop” to forward a packet to a destination  Route maintenance  Used when link breaks, rendering next hop unusable  Routing (easy!)

Route Discovery  Route Request:  Source broadcasts Route Request (RREQ) message for specified destination  Intermediate node:  Forwards (broadcasts) message toward destination  Creates next-hop entry for reverse path to source, to use when sending reply (assumes bidirectional link…)

 Route Reply  Destination unicasts Route Reply (RREP) message to source  RREP contains sequence number, hop-count field (initialized to 0)  Will be sent along “reverse” path hops created by intermediate nodes which forwarded RREQ  Intermediate node:  Create next-hop entry for destination as RREP is received, forward along “reverse path” hop  Increment hop-count field in RREP and forward  Source:  If multiple replies, uses one with lowest hop count

Details again…  Each node maintains non-decreasing sequence number  Sent in RREQ, RREP messages; incremented with each new message  Used to “timestamp” routing table entries for “freshness” comparison  Intermediate node may return RREP if it has routing table entry for destination which is “fresher” than source’s (or equal with lower hop count)  Routing table entries assigned “lifetime”, deleted on expiration  Unique ID included in RREQ for duplicate rejection

Route Maintenance  Used when link breakage occurs  Link breakage detected by link-layer ACK, AODV “Hello” messages  Detecting node may attempt “local repair”  Send RREQ for destination from intermediate node  Route Error (RERR) message generated  Contains list of unreachable destinations  Sent to “precursors”: neighbors who recently sent packet which was forwarded over broken link  Propagated recursively

49 Ad Hoc On-Demand Distance Vector Routing (AODV) [Perkins99Wmcsa]  DSR includes source routes in packet headers  Resulting large headers can sometimes degrade performance  particularly when data contents of a packet are small  AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes  AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate

50 AODV  Route Requests (RREQ) are forwarded in a manner similar to DSR  When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source  AODV assumes symmetric (bi-directional) links  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

51 Route Requests in AODV B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L

52 Route Requests in AODV B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L

53 Route Requests in AODV B A S E F H J D C G I K Represents links on Reverse Path Z Y M N L

54 Reverse Path Setup in AODV B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Z Y M N L

55 Reverse Path Setup in AODV B A S E F H J D C G I K Z Y M N L

56 Reverse Path Setup in AODV B A S E F H J D C G I K Z Y Node D does not forward RREQ, because node D is the intended target of the RREQ M N L

57 Route Reply in AODV B A S E F H J D C G I K Z Y Represents links on path taken by RREP M N L

58 Route Reply in AODV  An intermediate node (not the destination) may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S  To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used  The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR  A new Route Request by node S for a destination is assigned a higher destination sequence number. An intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply

59 Forward Path Setup in AODV B A S E F H J D C G I K Z Y M N L Forward links are setup when RREP travels along the reverse path Represents a link on the forward path

60 Data Delivery in AODV B A S E F H J D C G I K Z Y M N L Routing table entries used to forward data packet. Route is not included in packet header. DATA

61 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)

62 Link Failure Reporting  A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry  When the next hop link in a routing table entry breaks, all active neighbors are informed  Link failures are propagated by means of Route Error messages, which also update destination sequence numbers

63 Link Failure Detection  Hello messages: Neighboring nodes periodically exchange hello message  Absence of hello message is used as an indication of link failure  Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure

Issues  Scalability  No inherent “subnetting” provision in routing tables – one entry per destination  Directionality  Assumes there is at least one bidirectional path between any two nodes

Issues (cont.)  Multicast  True multicast-tree generation and maintenance  Detailed in supplementary (expired…) draft: maodv-00.txt maodv-00.txt  Broadcast  Suggested use of IP Ident field for duplicate detection  bcast-00.txt bcast-00.txt

Protocol Performance Tests  “A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Rotuing Protocols”, D. Johnson et al., MobiCom ’98 Proceedings.  By the creators of DSR  “Performance Comparison of Two On-Demand Routing Protocols for Ad Hoc Networks”, C. Perkins et al., IEEE Personal Communications, February  By the creators of AODV  Both used ns-2 simulator, simulated link layer

Johnson et al  Compared DSR, AODV, DSDV, TORA  Varied number of sources, node mobility, traffic load  50 nodes total, 64-byte data packets  Looked at packet delivery ratio, routing overhead  Conclusions:  DSR, AODV similar on packet delivery ratio  DSR much lower routing traffic overhead (excluding DSR’s routing header extension in each data packet)  TORA, DSDV performed very poorly in certain situations (low packet delivery ratio)

Perkins et al  Compared DSR and AODV  Varied number of sources, node mobility, traffic load  50 and 100 nodes, 512-byte data packets  Looked at packet delivery ratio, packet delay, routing overhead, total network throughput  Conclusions:  DSR outperforms with fewer nodes, lower traffic load, less node mobility  AODV outperforms when have more nodes, higher traffic load, greater node mobility  DSR always lower routing overhead (excluding routing header)  DSR poor delivery ratio when many nodes, many sources, high mobility

Linux Implementations  DSR  Sourceforge “PicoNet” project, Alex Song:  AODV  NIST “Kernel AODV” implementation, Luke Klein-Berndt:

Optimized Link State Routing (OLSR) Overview  An optimization of the link-state protocol  Introduces serial numbers to handle stale routes  The Goal is to  Reduce the flooding of topology control messages  Reduce size of topology control messages

OLSR Overview (2)  Uses MultiPoint Relays (MPRs), that relays all traffic from a node  Each node advertises the nodes that have selected it as an MPR  From this information other nodes can build a "last hop" table and calculate the routing table from this

OLSR Overview (3) A subset of 1-hop neighbors are selected as MPRs The set of MPRs must cover all 2-hop neighbors

Topology Broadcast based on Reverse Path Forwarding (TBRPF) Overview  Two modes of operation  Full Topology, (FT)  Suited for small networks with few nodes  Partial Topology, (PT)  Suited for dense networks  Node only reports changes to its source tree  Each node can compute min-hop distance to every other node  Neighbor discovery  By sending out differentiated HELLO’s  New and recently lost neighbors

TBRPF Overview (2)  Routing function  By means of a reportable sub-tree  Links to all neighbors  Branch of the source tree rooted at node j if node i determines that i is the next hop of some neighbor k to reach j ’s reportable subtree 6’s reportable subtree 10’ reportable subtree j i k

75 TBRPF Overview (3)  Link failure and rerouting  Reorganisation of 2’s and 6’s reportable subtree ’s reportable subtree 6’s reportable subtree 10’ reportable subtree

Comparison  Reactive  Suitable for networks with small subset of nodes communicating  High latency when establishing new routes  Low storage and message overhead  Difficult to implement QoS  Pro-active  Suitable for networks with large subset of nodes communicating  Low latency when establishing connections  Overhead to maintain routes to all destinations  Relatively easy to implement QoS

Reactive Protocols  DSR  Hop-by-hop route to destination stored in cache  Multiple routes possible  Route cache is only updated in terms of RREPs and RERRs.  Generally low routing load  Better performance in small netoworks  AODV  Next-hop routing towards destination  One route to each destination  Timers and sequence numbers to avoid stale routes  High routing load for small numbers of nodes  Better performance in larger network

Pro-Active Routing  OLSR  Suitable for dense networks  Use of MPRs reduce the set of active nodes  Small packet size  Dependent on special HELLO messages  TBRPF  More topology information is available to nodes  No need for special HELLO messages if network supports link sensing  Very good results in simulations

Conclusion  As more and more devices becomes network aware, good ad-hoc routing algorithms will become increasingly important  The choice of what ad-hoc routing algorithm to use is highly dependent on network layout and traffic characteristics  Ad-hoc routing is still a hot research topic

Conclusion (2)  The standardization process  AODV is generating the most interest in the research community and is furthest along in the standardization process  Not much current research on DSR  TBRPF seems to be a relatively closed project, very few independent articles on TBRPF are published  OLSR is not generating many articles either  More independent performance comparisons are needed, especially for TBRPF and OLSR

Assignment #6  Find and Compare Papers related to TBRPF and OLSR.

Q&A ??