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OSPF-MDR: Extension of OSPF for Mobile Ad Hoc Networks draft-ietf-ospf-manet-mdr-02.txt. Richard Ogier September 17, 2008. Review of OSPF.

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ospf mdr extension of ospf for mobile ad hoc networks draft ietf ospf manet mdr 02 txt

OSPF-MDR: Extension of OSPFfor Mobile Ad Hoc Networksdraft-ietf-ospf-manet-mdr-02.txt

Richard Ogier

September 17, 2008

review of ospf
Review of OSPF
  • Open Shortest-Path First (OSPF) is a standard IP routing protocol that performs reliable flooding of link-state advertisements (LSAs) that describe the topology of the network.
  • Each router creates a router-LSA that describes its links to neighbors, and originates/floods a new LSA (with a larger sequence number) when the contents change (subject to MinLSInterval = 5 sec).
  • OSPF allows multiple areas (clusters) for hierarchical routing. Each router in an area is provided with the full topology for the area, and Dijkstra’s algorithm is used to compute a shortest-path tree for the area.
  • OSPF supports four interface types: point-to-point, broadcast, NBMA, and point-to-multipoint. OSPF does not support MANET interfaces.
  • A MANET interface can be considered a generalization of a broadcast interface in which two neighbors need not be neighbors of each other (multi-hop network).
  • OSPF-MDR is an extension of OSPFv3 [RFC 2740], which is a modification of OSPFv2 [RFC 2328] that supports IPv6.
review of ospf cont
Review of OSPF (cont.)
  • Each router forms an adjacency with each new neighbor (except in broadcast and NBMA networks) by performing Database Exchange to synchronize databases.
  • Each router describes its database to the other router by sending Database Description (DD)packets, which list all LSA headers.
  • Link State Request packets are used to request LSAs for which the neighbor has a more recent instance.
  • When all requested LSAs are received, the neighbor becomes fully adjacent (neighbor state Full).
review of ospf cont1
Review of OSPF (cont.)
  • RFC 5243 describes a new database exchange optimization that was originally part of OSPF-MDR, but applies to OSPF in general.
    • A router does not list an LSA in DD packets sent to a neighbor, if the same or a more recent instance of the LSA was listed in a DD packet already received from the neighbor.
    • Reduces the DD overhead by about 50% in large networks.
  • OSPF flooding procedure:
    • Each router normally forwards (floods) each new LSA to each adjacent neighbor (except the one from which it was received).The case of a broadcast network is considered in the next slide.
    • An LSA is retransmitted as a unicast to an adjacent neighbor when an Ack for the LSA has not been received within RxmtInterval.
review of ospf cont2
Review of OSPF (cont.)
  • To reduce overhead in a broadcast network, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR):
    • Adjacencies are formed only between the DR/BDR and its neighbors, resulting in O(n) instead of O(n2) adjacencies.
    • The DR is responsible for flooding new LSAs to all routers on the broadcast network.
    • If a router that is not DR/BDR originates a new LSA or receives a new LSA from another interface, it forwards the LSA to the DR and BDR, then the DR broadcasts the LSA to all routers on the network. (The BDR retransmits LSAs as unicasts based on Acks.)

DR

BDR

brute force method for supporting manets
Brute Force Method for Supporting MANETs
  • OSPF can support a MANET by treating each neighbor as a point-to-point link, and forming an adjacency with each neighbor. Every router would forward (flood) each received new LSA on the MANET interface.
  • In a network with 100 nodes with average degree 50, this would require 2500 adjacencies to be formed, as shown in the figure.
  • This method generates a huge amount of overhead, and can support only very small networks.
ospf mdr approach
OSPF-MDR Approach
  • Problem: How do we extend OSPF to mobile ad hoc networks while achieving good performance, low overhead, and maximum scalability?
  • OSPF uses the Designated Router (DR) and Backup DR to achieve this goal in broadcast networks, but can we generalize the DR to achieve this goal in multihop wireless networks?
  • OSPF-MDR generalizes the DR by selecting a small subset of routers, called MANET Designated Routers (MDRs) that form a connected dominating set (CDS).
  • MDRs achieve scalability in MANETs similar to the way DRs achieve scalability in broadcast networks:
    • MDRs have primary responsibility for flooding LSAs. Backup MDRs provide backup flooding when MDRs temporarily fail.
    • Adjacency reduction may be used, in which adjacencies are formed only between MDR/BMDR routers and their neighbors.
ospf mdr approach cont
OSPF-MDR Approach (cont.)
  • OSPF’s Hello protocol is modified to provide 2-hop neighbor information required for MDR selection, and to allow differential Hellos that list only neighbors whose state has recently changed.
  • The contents of router-LSAs is flexible:
    • Partial-topology LSAs can be used to reduce the size and origination frequency of LSAs.
    • Either partial-topology or full-topology LSAs can be used, with or without adjacency reduction.
  • Simulations show that OSPF-MDR can achieve good performance in mobile networks with up to 160 nodes with shortest-path routing, and up to 200 nodes with nearly-shortest-path routing.
  • The main website for OSPF-MDR is www.manet-routing.org, whereyou can find high quality implementation and simulation code.
basic idea generalize designated router to manet designated routers mdrs
Basic Idea – Generalize Designated Router toMANET Designated Routers (MDRs)
  • In an OSPF broadcast network, a single DR (red) is elected.
  • Each router becomes adjacent with the DR, forming a tree with n-1 edges.
  • The node having the lexicographically largest value of (DR Level, RtrPri, RID) is elected as DR, where DR Level can be DR, Backup DR, or DR Other.
  • In a multihop wireless network, the DR generalizes to multiple MDRs (red) which form a connected dominating set.
  • For fast convergence, the MDRs select themselves based on 2-hop neighbor information, giving preference to nodes with largest (RtrPri, MDR Level, RID).
  • Adjacencies are formed between MDRs to form a connected backbone.
  • Each non-MDR becomes adjacent with an MDR neighbor called its Parent.
also generalize backup designated router for biconnected redundancy
Also Generalize Backup Designated Router for Biconnected Redundancy
  • In an OSPF broadcast network, a Backup DR is added for redundancy.
  • The adjacencies of the DR and Backup DR form a biconnected subgraph.
  • Each DR Other is adjacent with the DR and the Backup DR.
  • In a multihop wireless network, Backup MDRs are added so that each node is a neighbor of at least two (Backup) MDRs.
  • Backup MDRs perform backup flooding for improved robustness.
  • Optionally, additional adjacencies may be added to form a biconnected backbone consisting of MDRs and Backup MDRs, and each MDR Other may become adjacent with a second (Backup) MDR neighbor called its Backup Parent.
animation of simulated network
Animation of Simulated Network
  • Red nodes are MDRs, lines indicate adjacencies.
  • The option of uniconnected adjacencies is used.
modified hello protocol
Modified Hello Protocol
  • A Hello packet includes up to 5 neighbor lists:
    • List 1: Down neighbors (only in differential Hellos)
    • List 2: One-way neighbors (in state Init)
    • List 3: Dependent Neighbors (which become adjacent)
    • List 4: Selected Advertised Neighbors (advertised in LSA)
    • List 5: Bidirectional neighbors not included in List 3 or 4
  • A differential Hello includes only neighbors whose list type changed within the last HelloRepeatCount (default 3) Hellos.
  • If differential Hellos are used, then a full Hello must be sent every 2HopRefresh (default 3) Hellos, to inform new neighbors of the full state.
  • Each router keeps track of the following neighbor sets for each neighbor:
    • Bidirectional Neighbor Set (BNS): used for MDR selection.
    • Dependent Neighbor Set (DNS): used for adjacency formation.
    • Selected Advertised Neighbor Set (SANS): used for constructing LSA.

Bidirectionalneighbors

mdr selection algorithm
MDR Selection Algorithm
  • Phase 1:Create theneighbor connectivity matrix based on information obtained from Hellos: NCM(j,k) = 1 if neighbors j and k are neighbors of each other.
  • Phase 2: Decide whether the router is an MDR, select Dependent Neighbors
    • If the router has a lexicographically larger value of (RtrPri, MDR Level, RID) than all of its neighbors, it selects itself as an MDRand selects all MDR neighbors as Dependent Neighbors, and proceeds to Phase 4.
    • Let Rmax be the neighbor with the lexicographically largest value of(RtrPri, MDR Level, RID).
    • Using BFS, compute min-hop paths from Rmax to each other neighbor, using only intermediate nodes that are neighbors with a larger value of(RtrPri, MDR Level, RID) than the router itself.
    • If all neighbors can be reached in at most MDRConstraint hops (default 3), then the router is not an MDR.
    • Otherwise, the router selects itself as an MDR and selects the following neighbors as Dependent Neighbors: Rmax and each MDR neighbor that could not be reached in at most k hops.
    • Each MDR router becomes adjacent with its Dependent Neighbors, ensuring that the backbone of MDRs is connected via adjacencies.
mdr selection algorithm cont
MDR Selection Algorithm (cont.)
  • Phase 3: Decide whether the router is a Backup MDR
    • If the router did not select itself as an MDR, it selects itself as a Backup MDR if there exists a neighbor that cannot be reached from Rmax via two node-disjoint paths, using the same graph used in Phase 2.
    • If biconnected adjacencies are used (AdjConnectivity= 2), the router selects the following neighbors as Dependent Neighbors: Rmax and each MDR/BMDR neighbor that could not be reached via two node-disjoint paths.This ensures that the backbone of MDRs and BMDRs is biconnected.
    • Disjoint paths from Rmax to all other neighbors are computed in O(n2) time, using a simplification of the algorithm of Suurballe and Tarjan [Networks, 1984].
  • Benefits of prioritizing routers according to(RtrPri, MDR Level, RID):
    • Allows neighbors to agree on which routers should become MDR.
    • Using MDR Level increases stability and lifetime of MDRs and adjacencies.
    • Using Router Priority allows certain routers to be more likely to be MDRs.
    • Router Priority can be changed dynamically so that the burden of being an MDR can be shared among all routers.
mdr selection algorithm example
MDR Selection Algorithm - Example

1

  • Node numbers indicate RIDs.
  • Thin lines indicate bidirectional neighbors.
  • Red nodes indicate MDRs.
  • Green nodes indicate Backup MDRs (BMDRs).
  • Red lines indicate adjacencies associated with MDRs, which form a tree in this case.
  • Green lines indicate adjacencies added to form a biconnected subgraph.
  • Each MDR Other becomes adjacent with one MDR (Parent) and optionally with another MDR or BMDR (Backup Parent).
  • Node 7 selects itself as MDR, since there does not exist a path from node 8 to node 4 via neighbors with larger RID.
  • Node 6 selects itself as Backup MDR, since there do not exist two disjoint paths from node 8 to neighbors 2, 3, 4, and 7 via neighbors with larger RID.

2

3

4

5

6

7

8

mdr selection algorithm cont1
MDR Selection Algorithm (cont.)

Phase 4: Parent Selection

  • An MDR selects itself as Parent, to indicate to neighbors that it is an MDR.
  • A non-MDR router selects an adjacent MDR neighbor as Parent, if one exists (to avoid forming a new adjacency when possible); otherwise it selects Rmax as Parent.
  • A Backup MDR selects itself as Backup Parent, to indicate to neighbors that it is a BMDR.
  • If AdjConnectivity = 2, an MDR Other selects an adjacent MDR/BMDR parent as Backup Parent, if one exists.
  • The (Backup) Parent is advertised in the (Backup) DR field of each Hello sent on the interface.
  • A router becomes adjacent with its Parent and Backup Parent (if it exists and is an MDR or BMDR), thus ensuring that each router is connected (or biconnected) to the backbone.
mdr selection algorithm concluded
MDR Selection Algorithm (concluded)
  • Phase 5: Optional Selection of Non-Flooding MDRs
    • A router that has selected itself as an MDR MAY execute the following steps to possibly declare itself a non-flooding MDR.
    • Compute min-hop paths from Rmax to each other neighbor, using only intermediate nodes that are neighbors with a smaller value of(RtrPri, RID) than the router itself.
    • If the resulting hop distance to each neighbor is at most MDRConstraint (default 3), the router is a non-flooding MDR.
    • A non-flooding MDR does not immediately forward LSAs, but behaves like a Backup MDR.
    • Phase 5 can reduce flooding overhead dramatically for certain regular topologies, but the reduction has been found to be less than 15% for random topologies.
lsa flooding procedure
LSA Flooding Procedure
  • To exploit the broadcast nature of MANETs, an LSA is processed (and possibly forwarded) if it is received from any bidirectional neighbor (not just adjacent neighbors).
  • A router never forwards an LSA on a MANET interface if either of the following two conditions is satisfied for all bidirectional neighbors on the interface:
    • The LSA or an Ack for the LSA has been received from the neighbor (over any interface).
    • The LSA was received on a MANET interface, and the neighbor is “covered” by another neighbor from which the LSA was received (using 2-hop neighbor info).
  • If an MDR receives a new LSA, it floods the LSA back out the receiving interface if there exists a bidirectional neighbor that does not satisfy condition (a) or (b).
  • If a Backup MDR or non-flooding MDR receives a new LSA, it waits ½ sec plus jitter, then floods the LSA back out the receiving interface if there exists a bidirectional neighbor that does not satisfy condition (a) or (b). (½ sec = BackupWaitInterval)
  • An MDR Other never floods a received LSA back out the same interface.
  • An optional step (6) is specified, which avoids redundant forwarding when flooding occurs over multiple MANET interfaces.
example of mdr flooding
Example of MDR Flooding

1

  • Node 8 originates and floods a new router-LSA.
  • If node 7 (MDR) receives the new LSA from 8, it floods the LSA back out the same interface.
  • If node 6 (BMDR) receives the new LSA from 8, it waits ½ sec plus jitter. If during this interval it hears node 7 or 4 forward the LSA, then it does not flood the LSA since all neighbors are covered. Otherwise node 6 floods the LSA.
  • If node 4 (MDR) receives the new LSA from node 7 or node 6, it floods the LSA.
  • If node 3 (BMDR) receives the new LSA from node 7 or node 6, it waits ½ sec plus jitter, then floods the LSA unless all neighbors are covered.

2

3

4

5

6

7

8

sending link state acknowledgments
Sending Link State Acknowledgments
  • All LS Ack packets sent on a MANET interface are multicast using the IP address AllSPFRouters.
  • The following rules are used for acknowledging a received LSA:
    • If the LSA is new, send a delayed Ack on each MANET interface, unless the LSA is flooded out the interface.
    • If the LSA is a duplicate and was received as a multicast, do not send an Ack. (Each LSA is Acked only once, unless received as a unicast  retransmission.)
    • If the LSA is a duplicate and was received as a unicast:
      • If the router is an MDR (or a BMDR if AdjConnectivity = 2), send an immediate Ack out the receiving interface.
      • Otherwise, send a delayed Ack out the receiving interface.
  • Reason for sending an immediate Ack in case (a) is to prevent other adjacent neighbors from retransmitting the LSA.
  • Optimization: To reduce the number of Acks sent, each delayed Ack is delayed by at least MinLSInterval (5 sec) and at most RxmtInterval (7 sec), so that the Ack need not be sent if a new instance of the LSA is received before the scheduled time for the Ack.
receiving link state acknowledgments
Receiving Link State Acknowledgments
  • Each router maintains an Acked LSA List for each adjacent neighbor, to keep track of any LSA instances the neighbor has acknowledged, but which the router itself has not yet received. This is necessary because, unlike RFC 2328, each router acknowledges an LSA only the first time it is received as a multicast.
  • Thus, if a router receives an Ack for a given LSA before it receives the LSA itself, the router need not forward the LSA to the neighbor that already acked the LSA.
  • An LS Ack packet that is received from an adjacent neighbor is processed as in RFC 2328, with the following additional steps:
    • If the router receives an LS Ack for an LSA that is newer than the database copy, the LS Ack is added to the Acked LSA List for the sending neighbor.
    • If a Backup MDR receives an LSA Ack for an LSA for which the BackupWait Timer is set, the sending neighbor is removed from the list of neighbors that have not yet been covered.
routable neighbors
Routable Neighbors

1

  • A bidirectional MANET neighbor is defined to be routable if the SPF calculation has produced a route to the neighbor. (A quality condition can also be required.) A routable neighbor need not be adjacent.
  • Only Full and routable neighbors can be used as next hops in the SPF calculation, and can be included in the router-LSA originated by the router.
  • The idea is that, since a routable neighbor can be reached through an acceptable path, it makes sense to take a "shortcut" and forward packets directly to the routable neighbor.
  • Note that OSPF already allows a non-adjacent neighbor to be used as a next hop, if both routers are fully adjacent to the DR of a broadcast network. The routability condition is a generalization of this condition to MANETs.
  • The network-LSA of an OSPF broadcast network implies that a router can use a non-adjacent neighbor as a next hop. But a network-LSA cannot describe the general topology of a MANET, making it necessary to explicitly include non-adjacent neighbors in the router-LSA.
  • Allowing only adjacent neighbors in LSAs would either result in suboptimal paths or would require a large number of adjacencies to provide optimal paths.

2

3

routable

4

5

DR

1

2

link state advertisements
Link State Advertisements
  • The router-LSA contains a point-to-point link for each MANET neighbor that is advertised.
  • The choice of which neighbors to advertise in the router-LSA is flexible, subject to the following two rules:
    • The router-LSA can advertise any neighbor that is Fullorroutable.
    • If adjacency reduction is used, the router-LSA must advertise all Full neighbors and all routable neighbors that satisfy the condition for becoming adjacent (and will therefore become Full).
  • Main options for router-LSAs (other options are allowed):
    • Minimum LSAs: Advertise only the required neighbors.
    • Full-topology LSAs: Advertise all Full and routable neighbors.
    • Min-cost LSAs: Advertise a subset of neighbors that is sufficient to provide routing along shortest (minimum-cost) paths. Advertised neighbors are selected using 2-hop neighbor information obtained from Hellos.
  • The options are interoperable: Different routers need not select the same option for the router-LSA or for adjacency reduction.
link state advertisements cont
Link State Advertisements (cont.)
  • Min-Cost LSA Algorithm
    • Each router selects the Selected Advertised Neighbor Set (SANS), using the Bidirectional Neighbor Set (BNS), Dependent Neighbor Set (DNS), and SANS for each neighbor (obtained from Hellos).
    • A neighbor j is included in the SANS if necessary to provide a min-cost path from some other neighbor to j, using (RtrPri, RID) to break ties.
    • Adjacent neighbors are always advertised (if routable), so are not included in the SANS.

1

  • Example
    • Red nodes are MDRs, red lines are adjacencies
    • Assume all metrics are 1.
    • Adjacencies do not provide a min-cost path from node 6 to node 1.
    • Node 4 includes node 1 in its SANS to provide such a path. Node 3 does not, since it has a smaller RID.

2

4

3

5

6

link state advertisements cont1
Link State Advertisements (cont.)
  • Each router advertises the following neighbors in its router-LSA:all Full neighbors, and each routable neighbor j that satisfies any of the following three conditions:
    • The router’s SANS includes neighbor j.
    • Neighbor j's SANS includes the router (to ensure symmetry).
    • Neighbor j is satisfies the condition for becoming adjacent.
simulation results for ospf mdr
Simulation Results for OSPF-MDR
  • Results were obtained using GTNetS simulator with OSPF-MDR version 1.01, available at www.manet-routing.org
  • Scenario parameters:
    • Grid length 500 m (square), radio range 150m, 200 m, 250 m.
    • Random waypoint mobility with maximum speed 10 m/s, pause time 0.
    • UDP packet size 40 bytes, packet rate 10 packets/s. Source and destination are selected randomly for each packet.
    • Simulated MAC protocol is 802.11b.
  • The following configurations of OSPF-MDR were simulated:
    • Default configuration with uniconnected adjacencies and min-cost LSAs,except using differential Hellos.
    • Same except using minimal LSAs.
    • Same except using full-topology LSAs.
    • Same except using full-topology adjacencies and full-topology LSAs.
benefits of adjacency reduction
Benefits of Adjacency Reduction
  • The following statistics do not increase as the number of nodes increases, contributing to the low overhead achieved by OSPF-MDR:
    • The number of adjacencies per node is dramatically smaller than the number of bidirectional neighbors per node.
    • The number of adjacency changes per node per second is dramatically smaller than the number of neighbor changes per node per second.
  • The following results are for radio range 250 m and maximum speed 10 m/s, and are independent of the LSA choice.
  • The results imply that with 200 nodes, there are 2.14/.039 = 55 times as many adjacency formations with full-topology adjacencies as with uniconnected adjacencies!

Number of nodes

20 40 60 80 120 160 200

Avg no. nbrs/node 11.38 25.82 36.30 50.13 75.87 98.65 125.59

Avg no. adjs/node 2.60 2.31 2.38 2.26 2.25 2.32 2.13

Nbr changes/node/s .17 .37 .57 .75 1.22 1.65 2.14

Adj changes/node/s .035 .036 .046 .040 .032 .035 .039

summary of results
Summary of Results
  • OSPF-MDR achieves good performance for up to at least 160 nodes with min-cost LSAs and up to at least 200 nodes with minimum LSAs.
  • The results for the number of hops traveled by UDP packets show that routes obtained with minimal LSAs are only 2.3% to 4.5% longer than with min-cost LSAs when the range is 250 m, 3.5% to 7.4% longer when the range is 200 m, and 0.1% to 11.2% longer when the range is 150 m.
  • Min-cost LSAs achieve dramatically lower overhead than full-topology LSAs without significantly affecting delivery ratio or path length.
  • Both adjacency reduction and LSA reduction reduce overhead greatly. For range 250 m:
    • With no adjacency or LSA reduction, overhead is 929 kbps with only 60 nodes.
    • With uniconnected adjacencies and full-topology LSAs, overhead is 890 kbps with 80 nodes.
    • With uniconnected adjacencies and min-cost LSAs, overhead is less than 500 kbps with 120 nodes, and less than 800 kbps with 160 nodes.
    • With uniconnected adjacencies and minimal LSAs, overhead is only637 kbps with 200 nodes.
commands for running simulations
Commands for running simulations
  • ./random_waypoint_manet-opt seed=8 num_nodes=20 pktrate=10 velocity=10 radio_range=250 alpha=0.5 pause_time=0 HelloInterval=2 RxmtInterval=7 DeadInterval=6 AckInterval=1000 BackupWaitInterval=500 TwoHopRefresh=3 AdjConnectivity=1 LSAFullness=1 diff_hellos start_time=1800 stop_time=3600
  • num_nodes is 20, 40, 60, 80, 100, 120, 160, 200
  • radio_range is 150, 200, 250
  • LSAFullness = 0 for minimal LSAs, 1 for min-cost LSAs, 4 for full LSAs.
  • AdjConnectivity = 1 for uniconnected adjacencies, 0 for full-topology adjacencies.
  • stop_time changed to 2700 when num_nodes is 100 or larger.