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A Routing Protocol for WLAN Mesh

A Routing Protocol for WLAN Mesh. Hang Liu, Jun Li, Saurabh Mathur {hang.liu, jun.li, saurabh.mathur}@thomson.net. Abstract. This presentation will outline a hybrid mesh routing protocol Support unicast and multicast Support on-demand routing and proactive routing. Outline.

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A Routing Protocol for WLAN Mesh

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  1. A Routing Protocol for WLAN Mesh Hang Liu, Jun Li, Saurabh Mathur{hang.liu, jun.li, saurabh.mathur}@thomson.net

  2. Abstract • This presentation will outline a hybrid mesh routing protocol • Support unicast and multicast • Support on-demand routing and proactive routing

  3. Outline • Introducing the Hybrid Mesh Routing Protocol (HMRP) • Unicast Route Establishment and Maintenance • Multicast Route Establishment and Maintenance • QoS Support

  4. Introducing the Hybrid Mesh Routing Protocol (HMRP) • Based on the well-studied IP layer Ad Hoc On-Demand Distance-Vector (AODV) protocol • Modified for automatic topology learning and dynamic path selection in WLAN mesh networks based on MAC addresses • Modified to simultaneously support on-demand and proactive routing • Support unicast and multicast • Support QoS

  5. On-demand Routing vs. Proactive Routing • On-demand Routing • Pros • Reduce the effects of stale routes due to topology changes (mobility, mesh point failures) • No need to maintain unused routes • Cons • Extra route discovery delay and data buffering • Excessive control traffic if each of mesh points individually discovers the route to a special mesh point (e.g. a mesh portal) • Proactive Routing • Pros • Little delay • Cons • High routing overhead to keep the routing information current • especially when network topology changes frequently • Solution: a hybrid on-demand/proactive routing protocol

  6. On-demand Route Establishment for Unicast • On-demand routing based on Route Request/Route Reply messages, similar to AODV • When a mesh point wants to send a frame to some destination mesh point, it checks its routing table for a path. • If there is a path, it forwards the frame to the next hop. • If no path, it initiates a route discovery by broadcasting a RouteRequest (RREQ) message • Information in RREQ • Originator MAC address, current sequence number, optional layer 3 info • Destination MAC address, last known destination sequence number, optional layer 3 info • Message ID • Metric • Time-to-Live (TTL) • Route Lifetime

  7. On-demand Route Discovery • A mesh point receives a RREQ, • Update the metric field to the originator • If this is the first RREQ • Establish a reverse route to this originator in its routing table. • If the mesh point is the destination, • responds by unicasting a Route Reply (RREP) back to the originator. • If the mesh point has a valid route for the destination and the sequence number for that destination is at least as great as that indicated in the RREQ • responds by unicasting a Route Reply (RREP) back to the originator. • If the mesh point is not able to satisfy the above requirements, broadcasts the RREQ with the new metric. • If this is not the first RREQ • If the new metric is better • Updates the reverse route • Replies RREP to the originator if the above requirements for replying is satisfied, or broadcast the RREQ with the new metric. • Otherwise, discards the RREQ

  8. Forward Path Setup in On-demand Routing • If the mesh point is the destination or if the mesh point has a valid route for the destination and the sequence number for that destination is at least as great as that indicated in the RREQ, • Responds by unicasting a RouteReply (RREP) back to the originator. • RREP message contains • Originator MAC address • Destination MAC address, destination sequence number and optional layer 3 information of this destination. • Metric • Route Lifetime • Time-to-live

  9. Receiving RREP • When an intermediate mesh point receives the RREP, • Updates the metric • Sets up a forward path in its routing table. • Forwards the RREP to the originator of the RREQ • If a mesh point receives more than one RREP, • Forwards the first RREP • Updates the routing table and forwards the new RREP only if the new RREP contains a greater destination sequence number or the same destination sequence number with a better metric. Otherwise discards the new RREP. • The originator can start data transmission as soon as the first RREP is received and can later update its routing information if a better route is discovered.

  10. RREQ flooding Destination Reverse Route Source Destination RREP unicast Source Forward Route On-demand Unicast Route Establishment Flooding of the route request (RREQ) message and reverse path establishment. Unicast of the route reply (RREP) message and forward path establishment.

  11. Mesh Point with Multiple 802.11 Interfaces • A mesh point may have multiple IEEE 802.11 wireless interfaces. • A unique mesh point ID • Can be one of its interface’s MAC addresses • Each interface also has its own MAC address. • The mesh point ID is used in the RREQ and RREP messages and other routing control messages. • When a multi-interface mesh point broadcasts the RREQ message, it may broadcast the RREQ message on all its interfaces. • When a multi-interface mesh point responds a RREQ message by unicasting a RREP message, it sends the RREP message on the interface on which it received the corresponding RREQ message.

  12. Routing table • Routing table contains an entry for a destination. • Each entry includes • Destination MAC address, destination sequence number and optional layer 3 information (supported layer 3 protocol and address, e.g. the IP address of destination mesh point)) • Next hop MAC address • Interface to the next hop • List of upstream mesh points and interfaces using this route • State and routing flags (e.g. valid, invalid) • Metric to the destination • Route Lifetime: updated each time when the route is used. • The route becomes invalid if it is not used within the specified route lifetime.

  13. Proactive Routing • Reduce the route discovery delay. • Reduce the routing control traffic for many to one communications • If a mesh point wants to advertise the route to it • Broadcasts unsolicited Route Announcement (RANN) messages • Originator MAC address, destination sequence number, optional layer 3 info • Route Lifetime • Metric • Time-to-Live (TTL) • A mesh point receives a RANN, • Updates the metric field to the originator • If no route to the originator of the RANN • Creates the route in its routing table and broadcasts the RANN with the new metric. • If already has a valid route to the originator of the RANN • Updates its routing table and broadcasts the RANN with the new metric only if the RANN contains a greater destination sequence number or the same destination sequence number with a better metric.

  14. Proactive Routing (Cont.) • When a source wants to transmit frames to a destination, it already may have a forward path to this destination obtained from the route announcement. • It can transmit the frame immediately. • However it is possible that there is no reverse path from the destination to the source. • If bi-directional communications are needed, the source can send a gratuitous RREP in unicast to this destination. • The gratuitous RREP is routed through the forward path. • Establishes a reverse path to the source.

  15. Proactive Routing (Cont.) RANN flooding Route to the RANN Originator Originator of the RANN A mesh point initiates flooding of the route announcement (RANN) messages to proactively establish the routes to it in the mesh network. Gratuitous RREP unicast Reverse Route to the Source Destination (Originator of RANN) Source A source mesh point unicasts a gratuitous route reply (RREP) to the originator of the RANN (the destination) for establishing a reverse route to the source.

  16. Route Maintenance • If a link breaks, the Route Error (RERR) messages is sent to the affected originator of the active paths. • Initiated by the upstream mesh point of the broken link. • List all the unreachable destinations due to this link failure and their destination sequence number. • The destination sequence number is incremented. • Transmit the RERR to its one or more upstream neighbors • When a neighbor receives a RERR from its downstream mesh point, • Marks the route to the destination as invalid • Propagates the RERR to its upstream mesh points • When a originator receives the RERR • Re-initiates the route discovery. • If a mesh point receives a data frame with a destination address for which it does not have an active route • Send a RERR message

  17. Local Connectivity Management • Send beacons (Hello message) periodically • Update the route lifetime associated with the neighbor in the route table after receiving a beacon from it. • If a mesh point fails to receive any beacon from a neighbor for a given hello_life. • Link to this neighbor broken. • Updates the route information for this neighbor.

  18. Non-forwarding mesh points • Non-forwarding mesh points • Some mesh points may joint the mesh only as the source or destination, i.e. not forwarding data originated from other mesh points due to process and battery limitation or other reasons . • A non-forwarding mesh point sends the RREQ when it wants to transmit frames. • A non-forwarding mesh point replies the RREQ if it is the destination. • A non-forwarding mesh point does not reply the RREQ if it is not the destination. • A non-forwarding mesh point does not forward any routing control message (RREQ, RREP, RERR, RANN).

  19. Mesh AP with Multiple Associated Stations • Multiple stations may associate a mesh AP • Mesh AP acts as their proxy. • Routing is transparent to the non-mesh stations. • The mesh AP discovers the route to the destination when it forwards frames originated by an associated station. • It also responds a RREQ for its associated stations by unicasting a RREP to the RREQ originator. • The mesh AP may advertise the route for its associated stations by broadcasting the RANN in the mesh network. • A RANN may contain routing information for multiple destinations • Destination’s address, metric, sequence number, etc. • Each corresponds to a station.

  20. Mesh AP with Multiple Associated Stations (Cont.) Mesh AP with Proxy Infrastructure Based WLAN Station WLAN Mesh Network

  21. Multicast Routing • The same protocol supports unicast and multicast • A separate multicast routing table is maintained in a mesh point. • Multicast on-demand route discovery is based on route request (RREQ) and route reply (RREP), similar to unicast on-demand route discovery. • A mesh point can dynamically join or leave a group at any time. • Each multicast group has a group leader. • The group leader maintains the multicast group sequence number. • New group leader is created once the current group leader fails.

  22. Multicast Route Discovery • When a mesh point wants to join a multicast group, it broadcasts a RREQ • When a mesh point receives a join RREQ for a multicast group, • Updates the metric field and sets up an inactivated route for the originator of the RREQ in the multicast routing table. • No data frames will be forwarded along the inactive link for the multicast group. • Establish a unicast reverse route to the originator • A member of the multicast tree may reply to a join RREQ, if its recorded group sequence number is at least as great as that carried in the RREQ. • The group leader shall always reply to a join RREQ. • Non-multicast-tree member does not reply, but propagates the RREQ with new metric

  23. Multicast Forward Path Establish • The responding mesh point unicasts the RREP back to the originator of the RREQ along the reverse route. • Intermediate mesh point receives the RREP, • Update the metric, • Set up an inactivated forward path • Forward the RREP to the next hop.

  24. Multicast Route Activation • The originator may receive multiple RREPs from different members on the multicast group tree, each sets up a potential branch. • The originator waits for the length of a route discovery interval and tracks the received RREP messages. • It selects a path with the greatest multicast group sequence number and the best metric to the multicast tree. • It activates the path to the tree at the end of the route discovery interval • Unicast a Multicast Activation (MACT) message with the join flag set along the selected path.

  25. Group Leader Group Leader F F F N N Group Leader F F The new branch is added Join the Multicast Group New mesh point N Multicast group member Forwarding mesh point for the multicast tree Non-tree member RREPs sent back to the originator of RREQ Flooding of RREQ message Multicast tree link Group Leader F N Unicast the MACT message with join flag set to activate the path to the multicast tree

  26. Group Leader Group Leader F F F B C A Leave a Multicast Group • A leaf mesh point revokes its member status • Unicast a MACT with the prune flag set to its next hop • Once the next mesh point receives the prune message, it deletes the routing information for the sender. • If the mesh point is not a group member and becomes a leaf • prunes itself and unicasts a prune message to its next hop. • If the mesh point is a group member or it is not a leaf mesh point • Does not prune itself Unicast the MACT with prune flag set to leave the group Multicast tree after pruning

  27. Broken Tree Branch Repair • When a link break occurs, the mesh point downstream of the break (i.e. the mesh point farther from the multicast group leader) repairs it. • Send a join RREQ for the multicast group with an extension field indicating the sending mesh point’s metric to the group leader. • A mesh point can respond to the RREQ by sending a RREP if it satisfies the conditions • Part of the multicast tree • With a group sequence number at least as great as the one in the RREQ • The metric to the group leader is better than the one indicated in the RREQ • At the end of the discovery period, the repair mesh point selects its path • Unicast a MACT message to activate the path.

  28. Group Leader Group Leader Group Leader B B B A A A Group Leader Group Leader B B F F F F F F F F F F F A A Repair of Broken Tree Link Reply RREP by the qualified tree members A link is broken Initiate repair with RREQ Activate the new links with MACT Repaired multicast tree

  29. Quality of Service Support • It is necessary to support QoS in WLAN mesh networks, e.g. for multimedia and video applications. • QoS requirements, for example, maximum delay and minimum bandwidth requirements for a data flow can be carried in the optional fields of an extended RREQ message. • To respond to a RREQ with QoS extensions, a mesh point must be able to satisfy the QoS constraints. Otherwise, it discards this QoS RREQ message. • After the route is established, if any mesh point along the path detects that it no longer satisfies the requested QoS parameters • Sends a route error message to the originator. • The RERR message may also carry the currently measured bandwidth and delay parameters.

  30. Summary • Hybrid mesh routing protocol • Based on the well-studied AODV • Support unicast and multicast • Support on-demand routing and proactive routing • Support QoS

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