MANET Extension of OSPF Using CDS Flooding draft-ogier-manet-ospf-extension-05.txt

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MANET Extension of OSPF Using CDS
Floodingdraft-ogier-manet-ospf-extension-05.txt
IETF Meeting - OSPF WG

• Richard Ogier, SRI International
• Phil Spagnolo, Boeing
• November 8, 2005

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Basic Idea Generalize Designated Router to
MANET Designated Routers (MDRs)

• Broadcast Network
• The DR is the only interior node of a tree and is
  a connected dominating set (CDS).
• DR and its adjacencies form a tree with n-1
  edges.
• All nodes agree on a single DR, by selecting the
  node with largest (RtrPri, RID).

• Multihop Wireless Network
• The CDS nodes generalize the notion of a DR to
  MDRs
• MDR set and its adjacencies form a tree with
  approximately n-1 edges.
• The set of MDRs are chosen by selecting nodes
  with largest (RtrPri, RID).

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Also Generalize Backup Designated Router for
Biconnected Redundancy

• Broadcast network
• 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.

• Multihop Wireless Network
• Backup MDRs are added for redundancy.
• A biconnected backbone is created consisting of
  MDRs and Backup MDRs.
• Each MDR Other selects two (Backup) MDR neighbors
  to forms adjacencies with.

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Similarities that show (Backup) MDRs are a
Natural Generalization of (Backup) DR

• Single-hop MANET The MDR-OSPF and OSPFs DR
  election algorithm select the same two routers as
  DR/MDR and Backup DR/MDR.
• Adjacencies DR/MDR Other routers form
  adjacencies with two (Backup) DR/MDR neighbors.
• Adjacency End-Points One Adjacency end-point
  must be a DR/MDR or Backup DR/MDR.
• Interface States OSPF-MDR uses the same
  interface states as broadcast OSPF DR MDR
  and Backup Backup MDR
• Next Hop Routing OSPF (in a broadcast network)
  and OSPF-MDR both allow a non-adjacent neighbor
  to be used as a next hop.

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OSPF-MDR History
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Changes from Version 03 to Version 04

• Draft Rewrite Specified complete details.
• Packet Formats Now specified
• Single MDR Selection Algorithm Specified
• runs in O(d2) time, where d is the number of
  neighbors.
• MDR and Backup MDR Parent Selection Procedure
  added as phase 4 of the MDR selection algorithm.
• Routable Neighbor Neighbors that can be
  advertised in router-LSAs and used as next hops.

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Changes from Version 04 to Version 05

• Simplified Flooding LSA Forwarding independent
  of dependent neighbors.
• Neighbor Quality Stricter quality condition
  satisfied before Down to Init transition.
• Routable Neighbor Quality Stricter quality
  condition satisfied before neighbor is declared
  routable.
• Commonly Used Terms New section.

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OSPF-MDR Details
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Hello Protocol and 2-Hop Neighbor Info

• Hellos append LLS to advertise 2-hop Neighborhood
  and enable differential hellos
• Full State Hello
• Includes all neighbors in state Init or greater.
• Sent every 2HopRefresh Hellos (on each MANET
  interface).
• Differential Hello
• Reduce overhead and allow for faster response to
  topology changes
• Includes only neighbors changed within the last
  HelloRepeatCount Hellos.
• 2-hop Neighbor Information Maintained by each
  router and used by the MDR selection algorithm.

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MDR Selection Algorithm

• Phase 1 Create the Neighbor Connectivity Matrix
• Phase 2 MDR and Dependent Neighbors Selection
• MDR Level, RtrPri, RID used to prioritize MDRs
• Dependent Neighbors chosen to ensure connected
  MDR backbone
• Phase 3 Backup MDR and Backup Dependent
  Neighbor Selection
• MDR Level, RtrPri, RID used to prioritize BMDRs
• Backup Dependent Neighbors ensures biconnected
  (B)MDR backbone
• Phase 4 MDR and Backup MDR Parent Selection
• MDRs select themselves as parent (similar for
  backup)
• MDR Others becomes adjacent with their parents
• Parents are selected persistently

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Example of MDR Selection Algorithm

• MDRS Red nodes
• Backup MDRS Green nodes
• Thin black lines bidirectional neighbors.
• Red lines Adjacencies associated with MDRs
• Green lines Adjacencies added to form a
  biconnected subgraph.
• MDR Other Becomes adjacent with one MDR and one
  Backup MDR (or a 2nd MDR).

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Adjacency Maintenance

• Forming biconnected adjacencies
• (Backup) MDR forms an adjacency with each
  neighboring (Backup) MDR that is (Backup)
  Dependent
• MDR Other forms an adjacency with its (Backup)
  MDR Parent
• Forming uniconnected adjacencies omit Backup
  in the above
• Existing adjacency Maintain as long as either
  the router itself or the neighbor is a (Backup)
  MDR. Otherwise it is torn down.

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Flooding Procedure

• LSA Processed By Any Bidirectional Neighbor
  Exploits the broadcast nature of MANETs.
• LSA Forwarding Exclusion
• The LSA or an ACK for the LSA has been received
  from each neighbor
• The LSA was received on a MANET interface, and
  all neighbors are covered by the neighbor from
  which the LSA was received.
• MDR Forwarding Floods the LSA back out the
  receiving interface if not excluded.
• Backup MDR Forwarding Waits BackupWaitInterval
  seconds, and then floods the LSA back out the
  receiving interface if not excluded.
• MDR Other Forwarding LSAs are never forwarded
  back out the same interface.

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Example of MDR Flooding
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Example of MDR Flooding
Delay BackupWait Interval before flooding
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Example of MDR Flooding
Delay BackupWait Interval before flooding
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Example of MDR Flooding
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Example of MDR Flooding
Cancel Backup
Flood Backup
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Sending Link State Acknowledgments

• All LSAcks sent multicast
• LSA Acknowledgement rules
• New LSA Send a delayed ACK on each MANET
  interface, unless the LSA is flooded out the
  interface.
• Duplicate multicast LSA do not send an LSAck.
• Duplicate unicast LSA
• If the router is a (Backup) MDR, send an
  immediate ACK out the receiving interface.
• If the router is an MDR Other, send a delayed ACK
  out the receiving interface.

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Receiving Link State Acknowledgments

• Acked LSA List Keeps track of any LSAs the
  adjacent neighbor has acknowledged, but which the
  router itself has not yet received.
• Necessary because each router acknowledges an LSA
  only the first time it is received as a
  multicast.
• Additional Processing Steps to RFC 2328
• Reception of an LS ACK newer than the database
  copy The LS ACK is added to the Acked LSA List
  for the sending neighbor.
• BackupWait Neighbor List Backup MDR receives an
  LSAck for an LSA on the BackupWait List, the
  sending neighbor is removed from the list of
  neighbors that have not yet been covered.

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Link State Advertisements

• Router-LSA Links The choice of which neighbors
  to include in the router-LSA is flexible, subject
  to only two requirements
• A router MUST include all Full neighbors in its
  router-LSA.
• A router MUST NOT include any non-routable
  neighbors in its router-LSA.
• Four options for the router-LSA, depending on
  LSAFullness
• Minimum LSAs
• Full LSAs
• MDR Full LSAs
• Min-Cost LSAs
• The four LSA options are interoperable with each
  other, since they all satisfy the above two
  requirements.

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Backup Charts

• Proceed to see more detail

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Changes from Version 04 to Version 05

• Simplified Flooding Forwarding a new LSA does
  not depend on which neighbors are dependent.
• Neighbor Quality A router may require that a
  stricter quality condition be satisfied before
  changing the state of a MANET neighbor from Down
  to Init or greater.
• Routable Neighbor Quality A router may require
  that a stricter quality condition be satisfied
  before declaring a neighbor to be routable.
• Commonly Used Terms Subsection 1.1 has been
  added.

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Changes from Version 03 to Version 04

• Draft Rewrite Specified complete details.
• Packet Formats Now specified
• MANET Designated Router (MDR) Terminology for a
  Designated Router (DR) on a MANET interface.
• Single MDR Selection Algorithm Specified
  previously called the MPN CDS algorithm, which
  includes the Essential CDS algorithm as a special
  case.
• runs in O(d2) time, where d is the number of
  neighbors.
• ANP CDS algorithm removed
• MDR and Backup MDR Parent Selection Procedure
  added as phase 4 of the MDR selection algorithm.
• Routable Neighbor "synchronized neighbors" now
  known as "routable neighbors", to reflect that
  such a neighbor is not perfectly synchronized,
  but is sufficiently synchronized to be advertised
  in router-LSAs and used as a next hop.
• Min-cost LSAs Partial-topology LSAs which
  provide minimum cost routes under certain
  assumptions are added.

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Basic Idea Generalize Designated Router to
MANET Designated Routers (MDRs)

• Broadcast Network
• DR and its adjacencies form a tree with n-1
  edges.
• The DR is the only interior node of the tree, and
  is a connected dominating set (CDS).
• All nodes agree on a single DR, by selecting the
  node with largest (RtrPri, RID).

• Multihop Wireless Network
• CDS can have multiple nodes. These are again the
  interior nodes of a spanning tree.
• The CDS nodes generalize the notion of a DR to
  MDRs, and the edges of the spanning tree become
  the adjacencies.
• The set of MDRs can again be kept small by
  selecting nodes with largest (RtrPri, RID).
• For faster convergence, the MDRs select
  themselves based on 2-hop neighbor information.
  As a result, the resulting set of adjacencies is
  not always a tree.

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Also Generalize Backup Designated Router for
Biconnected Redundancy

• Broadcast network
• 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.

• Multihop Wireless Network
• Backup MDRs are added so that each node is a
  neighbor of at least two (Backup) MDRs.
• Additional adjacencies can then be added to form
  a biconnected backbone consisting of MDRs and
  Backup MDRs.
• Each MDR Other selects two (Backup) MDR neighbors
  called parents, and forms adjacencies with its
  parents.

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Similarities that show (Backup) MDRs are a
Natural Generalization of (Backup) DR

• Single-hop MANET The MDR selection algorithm
  and OSPFs DR election algorithm both select the
  same two routers as DR/MDR and Backup DR/MDR.
  (The MDR selection algorithm also selects a
  second Backup MDR to make the backbone
  biconnected.)
• Adjacencies Each DR/MDR Other forms adjacencies
  with two (Backup) DR/MDR neighbors, and
  advertises these two neighbors in Hellos.
• Adjacency End-Points If an adjacency exists
  between two routers, then one of them must be a
  DR/MDR or Backup DR/MDR.
• Interface States OSPF-MDR uses the same
  interface states as OSPF, with the DR and
  Backup states implying that the router is an
  MDR or Backup MDR.
• Next Hop Routing OSPF (in a broadcast network)
  and OSPF-MDR both allow a non-adjacent neighbor
  to be used as a next hop, and both originate LSAs
  (network-LSA for OSPF) that imply that a router
  can use a non-adjacent neighbor as a next hop.
• Due to the general topology of MANETs, a MANET
  router must explicitly include non-adjacent
  neighbors in its router-LSA.

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Hello Protocol and 2-Hop Neighbor Info

• Hellos include
• Sequence Number TLV
• Hellos potentially include
• Heard Neighbor List TLV (neighbors in Init
  state)
• Reported Neighbor List TLV (bidirectional
  neighbors)
• Lost Neighbor List TLV (recently lost neighbors)
• Full State Hello
• Includes all neighbors in state Init or greater.
• Sent every 2HopRefresh Hellos (on each MANET
  interface).
• Differential Hello
• Includes only neighbors changed within the last
  HelloRepeatCount (default 3) Hellos.
• Reduce overhead and allow Hellos to be sent more
  frequently for faster response to topology
  changes.
• 2-hop Neighbor Information Reported Neighbor
  Lists (RNL) are maintained by each router, which
  define the 2-hop neighbor information used by the
  MDR selection algorithm.
• Neighbor Quality A router may optionally employ
  hysteresis by requiring a stricter quality
  condition (e.g., receiving two consecutive
  Hellos) before changing the state of a new
  neighbor from Down to Init.

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MDR Selection Algorithm (slide 1 of 2)

• Phase 1 Create the Neighbor Connectivity Matrix
• Matrix used to identify bidirectional neighbors
• Phase 2 MDR and Dependent Neighbors Selection
• Runs in O(d2) time using BFS to compute paths
  from the neighbor with largest value of (MDR
  Level, RtrPri, RID) to the other neighbors, using
  only neighbors with a larger value of (MDR Level,
  RtrPri, RID) as intermediate nodes.
• MDRConstraint (default 3) constrains the number
  of hops allowed in the computed paths. A smaller
  value (2) results in a larger CDS with a smaller
  stretch factor.
• MDR Level Gives priority to existing MDRs for
  increased stability (similar to OSPFs DR
  election algorithm).
• RtrPri Can depend on bandwidth capacity,
  battery life, node degree, neighbor stability,
  etc. RtrPri can also be changed dynamically to
  share the burden of being an MDR among all
  routers.
• Dependent Neighbors Used to determine which MDR
  neighbors to become adjacent with, to ensure the
  backbone of MDRs is connected.

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MDR Selection Algorithm (slide 2 of 2)

• Phase 3 Backup MDR and Backup Dependent
  Neighbor Selection
• Runs in O(d2) time using an algorithm that
  computes two node-disjoint paths from the
  neighbor with largest value of (MDR Level,
  RtrPri, RID) to the other neighbors.
• Backup Dependent Neighbors Used to determine
  which (Backup) MDR neighbors to become adjacent
  with, to ensure the backbone of MDRs and Backup
  MDRs is biconnected.
• Phase 4 MDR and Backup MDR Parent Selection
• (Backup) MDR Parent replaces the (Backup) DR
  interface variable of OSPF.
• If the router itself is an MDR, then the MDR
  Parent is the router itself, otherwise it is a
  neighboring MDR. (Similar for Backup MDR.)
• The (Backup) MDR Parent is advertised in the
  (Backup) DR field of each Hello.
• If the parameter AdjConnectivity 2
  (biconnected), each MDR Other becomes adjacent
  with both of its parents, ensuring that the
  adjacency graph is biconnected.
• To maximize stability of adjacencies, the parents
  are selected persistently, i.e., the existing
  (Backup) MDR Parent is kept whenever possible.

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Example of MDR Selection Algorithm

• Node numbers RIDs.
• Thin lines bidirectional neighbors.
• Red nodes MDRs.
• Green nodes Backup MDRs.
• Red lines Adjacencies associated with MDRs,
  which form a tree in this case.
• Green lines Adjacencies added to form a
  biconnected subgraph.
• MDR Other Becomes adjacent with one MDR and one
  Backup MDR (or a 2nd MDR).
• Example Node 6 does not select itself as MDR,
  since there is a path from node 8 to each other
  neighbor via nodes with larger RID. But node 6
  selects itself as Backup MDR, since there do not
  exist two such paths from node 8 to neighbors 2,
  3, 4, and 7.

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Adjacency Maintenance

• Forming biconnected adjacencies
• Each (Backup) MDR forms an adjacency with each
  neighboring (Backup) MDR that is (Backup)
  Dependent, providing a biconnected backbone.
• Each MDR Other forms an adjacency with its
  (Backup) MDR Parent, creating a biconnected
  adjacency graph.
• Forming uniconnected adjacencies omit Backup
  in the above
• Existing adjacency Maintain as long as either
  the router itself or the neighbor is a (Backup)
  MDR. Otherwise it is torn down.
• MDR TLV To form new adjacencies more quickly in
  mobile networks, initialization DD packets
  include the MDR Parent and Backup MDR Parent of
  the sending router.
• Database Exchange Optimization A router (master
  or slave) performing database exchange does not
  include an LSA header in its DD packets if it
  knows the neighbor has the same or newer instance
  of the LSA. Reduces DD packet overhead by about
  50.

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Flooding Procedure

• LSA Processed By Any Bidirectional Neighbor
  Exploits the broadcast nature of MANETs.
• LSA Forwarding Exclusion No forwarding is
  performed if either of the following two
  conditions are satisfied for all bidirectional
  neighbors on the interface
• The LSA or an ACK for the LSA has been received
  from each neighbor.
• The LSA was received on a MANET interface, and
  all neighbors are covered by another neighbor
  from which the LSA was received.
• MDR Forwarding Floods the LSA back out the
  receiving interface if there exists a
  bidirectional neighbor that does not satisfy
  condition (a) or (b).
• Backup MDR Forwarding Waits BackupWaitInterval
  seconds, and then floods the LSA back out the
  receiving interface if there exists a
  bidirectional neighbor that does not satisfy
  condition (a) or (b).
• MDR Other Forwarding LSAs are never forwarded
  back out the same interface.
• Multiple MANET Interfaces Optional step which
  avoids redundant forwarding when flooding occurs
  over multiple MANET interfaces.

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Example of MDR Flooding

• Node 8 originates and floods a new router-LSA.
• If node 7 (MDR) receives the new LSA, it floods
  the LSA back out the same interface.
• If node 6 (BMDR) receives the new LSA, it waits
  BackupWaitInterval seconds. 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 unless it has received an ACK for the LSA
  from nodes 2, 3, and 4. (Nodes 5 and 7 were
  covered when node 8 flooded the LSA.)
• If node 4 (MDR) receives the new LSA from any
  neighbor, it floods the LSA.
• If node 3 (BMDR) receives the new LSA from any
  neighbor, it waits BackupWaitInterval seconds,
  and then floods the LSA unless all neighbors are
  covered.

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Sending Link State Acknowledgments

• All LSAcks sent multicast LSAcks on MANET
  interfaces are sent to AllSPFRouters.
• LSA Acknowledgement rules
• New LSA Send a delayed ACK on each MANET
  interface, unless the LSA is flooded out the
  interface.
• Duplicate multicast LSA do not send an LSAck.
• Duplicate unicast LSA
• If the router is a (Backup) MDR, send an
  immediate ACK out the receiving interface.
• If the router is an MDR Other, send a delayed ACK
  out the receiving interface.
• Immediate LSAcks are sent in case (a) is to
  prevent other adjacent neighbors from
  retransmitting the LSA.

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Receiving Link State Acknowledgments

• Acked LSA List Keeps track of any LSAs the
  adjacent neighbor has acknowledged, but which the
  router itself has not yet received.
• Necessary because, unlike RFC 2328, each router
  acknowledges an LSA only the first time it is
  received as a multicast.
• Additional Processing Steps to RFC 2328
• Reception of an LS ACK newer than the database
  copy The LS ACK is added to the Acked LSA List
  for the sending neighbor.
• BackupWait Neighbor List Backup MDR receives an
  LSAck for an LSA on the BackupWait List, the
  sending neighbor is removed from the list of
  neighbors that have not yet been covered.

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Routable Neighbors

• Routable Neighbor Definition
• A neighbor in state Full, or
• The SPF calculation has produced a route to the
  neighbor and the neighbor satisfies a flexible
  quality condition.
• Routable MANET neighbors can
• be used as next hops in the SPF calculation
• be included in the router-LSA originated by the
  router
• Generalization of the Broadcast Network
• Non-adjacent neighbors are used as a next hop, if
  both routers are fully adjacent to the DR.
• 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.

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Link State Advertisements

• Router-LSA Links The choice of which neighbors
  to include in the router-LSA is flexible, subject
  to only two requirements
• A router MUST include all Full neighbors in its
  router-LSA.
• A router MUST NOT include any non-routable
  neighbors in its router-LSA.
• Four options for the router-LSA, depending on
  LSAFullness
• Minimum LSAs Include only fully adjacent
  neighbors.
• Full LSAs Include all routable neighbors.
• MDR Full LSAs Only (Backup) MDRs originate Full
  LSAs, other routers originate Minimum LSAs.
• Min-Cost LSAs Include the minimum set of
  neighbors to provide a 2-hop path between each
  pair of neighbors, based on the neighbors LSAs.
  Provides min-hop routing if all link costs are
  equal. Provides min-cost routing under certain
  assumptions.
• The four LSA options are interoperable with each
  other, since they all satisfy the above two
  requirements.