IETF Meeting - OSPF WG
MANET Extension of OSPF
Using CDS Flooding
draft-ogier-manet-ospf-extension-03.txt
Richard Ogier
Presented by Thomas Henderson
March 8, 2005
Ogier - 1
Basic Idea – Generalize Designated Router to
Multihop Wireless Networks
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In an OSPF broadcast network, the 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).
In a multihop wireless network, a CDS can
have multiple nodes. These are again the
interior nodes of a spanning tree.
The CDS nodes generalize the notion of a
DR, and the edges of the spanning tree
become the adjacencies.
The set of DRs can again be kept small by
selecting nodes with largest (RtrPri, RID).
For faster convergence, the tree can be
“approximate”, with more than n-1 edges,
based on 2-hop neighbor information.
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Also Generalize Backup Designated Router
for Biconnected Redundancy
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In an OSPF broadcast network, a BDR is
added for redundancy.
The adjacencies of the DR and BDR form a
biconnected subgraph.
Each non-DR/BDR node is adjacent with the
DR and the BDR.
In a multihop wireless network, BDRs can be
added so that each node is a neighbor of at
least two DR/BDR nodes.
Additional adjacencies can then be added to
form a biconnected subgraph.
Each non-DR/BDR node is adjacent with
one DR and one BDR (or a 2nd DR).
The DR/BDR selection and adjacencies
shown in the example were computed by the
proposed Essential CDS algorithm.
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Essential CDS Algorithm
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Uses 2-hop neighbor information obtained from Hellos.
Essential CDS Algorithm:
– If the router has a larger (RtrPri, RID) than all of its neighbors, it selects
itself as a DR, and selects all neighbors as Dependent Neighbors, which
become adjacent. (Same as DR selection in OSPF.)
– Else, if there does not exist a path (based on 2-hop neighbor information)
from the neighbor j with largest (RtrPri, RID) to some other neighbor, via
neighbors with larger values of (RtrPri, RID), then the router selects itself
as a DR, and selects j and each neighbor for which such a path does not
exist, as Dependent Neighbors, which become adjacent.
– BDR selection is similar, but requires 2 node-disjoint paths from neighbor j
to each other neighbor, to avoid selecting itself as BDR.
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“Persistent” version gives higher priority to existing DRs and BDRs.
Algorithm runs in O(d2) time, where d is the number of neighbors.
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Example of Essential CDS Algorithm
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Node numbers indicate RIDs.
Thin lines indicate neighbors.
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Red nodes indicate DRs.
Green nodes indicate BDRs.
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Red lines indicate adjacencies associated with
DRs, which form a tree in this case.
Green lines indicate adjacencies added to form
a biconnected subgraph.
Note that each non-DR/BDR node is adjacent to
one DR and one BDR (or a 2nd DR).
For example, node 6 does not select itself as
DR, since there is a path from node 8 to each
other neighbor via nodes with larger ID. But
node 6 selects itself as BDR, since there do not
exist two such paths from node 8 to neighbors
2, 3, 4, and 7.
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Family of Interoperable CDS Algorithms
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A larger CDS may be preferred in some cases, e.g., to provide flooding
along shorter paths (smaller stretch factor).
The following optional algorithms are interoperable because the CDS
computed by each algorithm contains the CDS computed by the
Essential CDS algorithm.
– Maximum Priority Neighbor (MPN) CDS algorithm: Requires that
the path computed from the neighbor with largest (RtrPri, RID) to
each other neighbor in the Essential algorithm, be at most k hops.
– All Neighbor Pairs (ANP) CDS algorithm: Requires that the path
computed from each neighbor to each other neighbor be at most k
hops.
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Simulation results show that the MPN and ANP algorithms (with k = 2
and 3) reduce stretch factor at the cost of a larger CDS.
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Choice of Router Priority
Each router can select its Router Priority dynamically, e.g., using the
following criteria:
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The Bandwidth capacity of node: higher priority for higher bandwidth
Neighbor stability (average lifetime of neighbors): selecting nodes with
stable neighbors will increase the lifetime of adjacencies.
Node degree: higher priority for larger degree reduces number of DRs
for small networks, but not for large networks (see simulation results).
Makes CDS less stable since node degree changes.
Willingness to be a relay (e.g., due to battery life).
A router that has been a DR or BDR for a certain amount of time can
reduce its Router Priority, so that the burden of being a DR/BDR can be
shared among all routers.
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Adjacencies
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Adjacencies are formed only between each DR/BDR router and its
(Backup) Dependent Neighbors as selected by the CDS algorithm.
The following enhancement greatly reduces the rate at which new
adjacencies are formed, when the persistent version of the Essential
CDS algorithm is used:
– Each DR/BDR router forms an adjacency with each (Backup)
Dependent Neighbor that is a DR/BDR router.
– Each non-DR/BDR router selects two DR/BDR neighbor (one of
which must be a DR), and forms adjacencies only with these two
neighbors. These adjacencies are kept as long as both are
DR/BDR neighbors and one of them is a DR.
– The two selected DR/BDR neighbors are included in the DR and
BDR fields of each Hello, as in legacy OSPF.
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Link State Advertisements
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Since a network LSA cannot describe a general topology, each router
originates a router LSA that represents neighbors as point-to-point links.
The choice of which neighbors to include in LSAs is flexible, allowing
partial or full topology information to be disseminated.
Each router MUST include all fully adjacent neighbors in its router LSA.
(Provides suboptimal paths to all nodes.)
Each router may include any “synchronized” neighbor in its router LSA,
where a neighbor j is considered synchronized if a path to node j can be
calculated from the link-state database (implying there exists a path to
node j via fully adjacent links).
To disseminate partial topology information from which min-hop paths
can be computed, each router may include all “Min-Hop Dependent
Neighbors” in its LSA (computed using the ANP algorithm as described
in the draft).
To disseminate full topology information, each router may include all
synchronized neighbors in its router LSA.
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Flooding Procedure
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Each DR forwards a new LSA the first time it is received, regardless of
which neighbor it was received from (source independent flooding).
– If the router has multiple interfaces, then the new LSA is forwarded only on
interfaces associated with Dependent Neighbors that are not covered by the
neighbor from which the LSA was received.
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Each BDR forwards a new LSA after a small delay, if the neighbors
from which the LSA has been received do not cover all Backup
Dependent Neighbors.
– If the router has multiple interfaces, then the new LSA is forwarded only on
interfaces associated with Backup Dependent Neighbors that are not
covered by neighbors from which the LSA was received.
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Source-dependent flooding is allowed as an option, to provide flooding
along min-hop paths if desired.
Note that the flooding procedure is described without mentioning
adjacencies. This allows it to be used in other MANET applications.
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Retransmitting LSAs
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LSAs are retransmitted only along adjacencies, as in OSPF.
Thus, retransmitted LSAs are sent only between DR/BDR routers and
non-DR/BDR routers.
One advantage: DR/BDR routers can group together several
retransmitted LSAs into the same LSU packet, thus reducing overhead
and avoiding a large number of retransmitted LSAs between nonDR/BDR routers.
In addition, a non-DR/BDR router never retransmits an LSA that it did
not originally transmit as a broadcast (see draft for details).
Link State Acknowledgments are always multicast. An LSA received as
a multicast is acknowledged only the first time it is received, and must
be processed even if received from a 2-way neighbor that is not
adjacent.
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Differential Hellos
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Differential Hellos report only changes in neighbor states.
Hellos also report full or partial neighbor information periodically, to
provide 2-hop neighbor information to neighbors.
Using differential Hellos allows Hellos to be sent more frequently,
allowing faster response to topology changes. Changes to 2-hop
neighbor information are reported quickly in differential Hellos.
The CDS algorithms require only partial 2-hop neighbor information,
i.e., the (Backup) Dependent Neighbors of each neighbor.
Thus, only Dependent Neighbors need to be reported periodically in
Hellos. In particular non-DR/BDR routers (which have no Dependent
Neighbors) only need to send differential Hellos, and never need to
send full Hellos or report any neighbor information periodically.
Advantage of CDS approach: A non-DR/BDR router never needs to
include all of its neighbors in either a Hello or an LSA.
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Proposed Enhancements
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Non-ackable LSAs for periodic flooding
– For LSAs that are updated frequently due to topology changes.
– Each DR periodically retransmits each non-ackable LSA as a
multicast (until a new instance is received).
– Non-ackable LSAs are never ACKed or retransmitted as a unicast.
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Multicast retransmitted LSAs
– Avoids separate unicast retransmissions to several neighbors.
– Includes a list of neighbors that must ACK the LSA.
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Optimized database exchange.
– Only the master needs to include all LSA headers in DD packets.
– After receiving DD packets from the master, the slave only sends
LSA headers that the master does not have.
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Simulation Results for CDS Size
Radius
0.3
0.5
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CDS Algorithm
Number of Nodes (placed randomly in unit square)
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200
300
Essential w/o degree
17.50
20.36
22.14
23.26
Essential w/ degree
13.79
18.66
27.42
33.14
Essential w/o degree
7.02
7.59
8.21
8.46
Essential w/ degree
5.14
8.03
13.47
18.54
Table shows average number of DRs computed by Essential CDS algorithm.
Better scalability to 300 nodes is achieved if node degree is not used (probably
because nodes with largest degree are concentrated near the center of the cluster).
Another advantage of not using node degree: CDS is more stable.
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Simulation in Mobile Networks
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Algorithm simulated: Essential CDS algorithm, persistent version.
Mobile network model:
– Nodes are placed randomly in unit square.
– Random direction is selected initially for each node. At each step, each
node moves 1/10 of transmission radius in its selected direction, bouncing
off sides of square as in billiards. Simulation is run for 500 steps. (Each
step can be 1 sec for very fast mobility.)
– Two values used for transmission radius: 0.3 and 0.5.
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Performance measures:
– Average number of DRs and BDRs.
– Average number of adjacencies, compared to average number of total
edges.
– Average rate of new adjacencies per step, compared to average rate of new
edges per step. (Important since database exchange must be performed
for each new adjacency.)
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Simulation in Mobile Networks (cont.)
100 nodes placed
randomly in unit
square (elongated
for easier viewing).
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Simulation in Mobile Networks (cont.)
For radius = 0.5,
there are about 2384
edges (shown).
Should all edges be
adjacencies?
Probably not.
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Simulation in Mobile Networks (cont.)
Biconnected backbone
consisting of DRs (red),
BDRs (green), and
adjacencies between
them (red lines).
Computed by Essential
CDS algorithm.
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Simulation in Mobile Networks (cont.)
Gray lines indicate
adjacencies of nonDR/BDR routers.
Note that each nonDR/BDR router is
adjacent to exactly two
DR/BDR routers.
Total number of
adjacencies for this
scenario is about 200
(twice the number of
nodes).
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Simulation in Mobile Networks (cont.)
Animation showing
movement of nodes in
simulation.
Note that most
adjacencies are
relatively stable as
nodes move.
In this scenario, the rate
of new edges is 7.19
times the rate of new
adjacencies.
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Simulation in Mobile Networks (cont.)
Results for persistent version of Essential CDS algorithm, for radius 0.5
Measure
Number of Nodes (placed randomly in unit square)
50
100
200
300
Avg # DRs
7.03
7.55
8.07
8.14
Avg # BDRs
5.19
5.33
5.57
5.65
Avg # edges
592.67
2384.30
9617.78
21646.11
Avg # adjacencies
101.37
201.72
402.46
602.41
Ratio edges/adj
5.85
11.82
23.90
35.93
New edge rate
34.17
138.25
553.70
1245.3
New adjacency rate
10.88
19.22
35.10
51.90
Ratio edge/adj rate
3.04
7.19
15.77
24.00
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Conclusions from Simulations
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While the number of edges (and rate of new edges) increases
quadratically with the number of nodes, the number of adjacencies (and
rate of new adjacencies) with the CDS algorithm increases only linearly.
Also, the number of DRs and BDRs increases only slightly as the
number of nodes increases from 100 to 300.
Therefore, the CDS approach is more scalable than the approach of
forming adjacencies with all neighbors.
Main conclusion: The DR/BDR approach of OSPF generalizes
naturally and efficiently to MANETs. Therefore, it is not necessary to
replace this approach with something completely different.
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Summary of Features
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A scalable CDS algorithm (and family of interoperable CDS algorithms) is
proposed, which generalizes the OSPF concept of a DR and BDR in a natural way.
The set of DRs forms a CDS, and the set of DRs and BDRs forms a biconnected
CDS for robustness.
The number of DRs and BDRs, the number of adjacencies, and the rate at which
new adjacencies are formed, are all found to be scalable in simulations.
The CDS algorithm uses only 2-hop neighbor information from differential Hellos,
allowing fast convergence following topology changes.
Only partial 2-hop neighbor information is required. Thus, a non-DR/BDR router
never needs to include all of its neighbors in either a Hello or an LSA.
The computed CDS provides source-independent flooding of LSAs (or other
packets). The flooding procedure also allows source-dependent flooding as an
option, if it is desired to flood packets along min-hop paths.
The choice of which neighbors to include in LSAs is flexible, allowing dissemination
of partial or full topology information.
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