Transcript (Backup) MDR

IETF Meeting - OSPF WG
MANET Extension of OSPF
Using CDS Flooding
draft-ogier-manet-ospf-extension-05.txt
Richard Ogier, SRI International
Phil Spagnolo, Boeing
November 8, 2005
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Changes from Version 04 to Version 05
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The flooding procedure has been simplified so that the decision to
forward a new LSA does not depend on which neighbors are
dependent.
To avoid accepting poor quality neighbors, and to employ hysteresis, 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.
To avoid selecting poor quality neighbors as routable neighbors, a
router may require that a stricter quality condition be satisfied before
declaring a neighbor to be routable.
Subsection 1.1 has been added, which defines commonly used terms.
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Changes from Version 03 to Version 04
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The draft was rewritten to specify complete details.
Packet formats are now specified.
The term MANET Designated Router (MDR) is now used instead of Designated
Router (DR) for MANET interfaces.
Only a single parameterized MDR selection algorithm is now specified
(previously called the MPN CDS algorithm), which includes the Essential CDS
algorithm as a special case. This algorithm runs in O(d^2) time, where d is the
number of neighbors.
The optional ANP CDS algorithm has been omitted from the draft.
A procedure for selecting the MDR Parent and Backup MDR Parent has been
added as Phase 4 of the MDR selection algorithm.
The term "synchronized neighbor" has been changed to "routable neighbor", 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.
A new option for partial-topology LSAs, called min-cost LSAs, has been added,
which provides minimum cost routes under certain assumptions.
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Basic Idea – Generalize Designated Router to
MANET Designated Routers (MDRs)
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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 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
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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.
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
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In a single-hop MANET, the MDR selection algorithm and OSPF’s 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.)
Each DR/MDR Other forms adjacencies with two (Backup) DR/MDR
neighbors, and advertises these two neighbors in Hellos.
In both OSPF and OSPF-MDR, if an adjacency exists between two routers,
then one of them must be a DR/MDR or Backup DR/MDR.
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.
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
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Each Hello includes a sequence number TLV and potentially includes
three node-list TLVs:
- Heard Neighbor List (neighbors in Init state)
- Reported Neighbor List (bidirectional neighbors)
- Lost Neighbor List (recently lost neighbors)
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A full state Hello includes all neighbors in state Init or greater. (Each
neighbor is in the Heard or Reported Neighbor List.) A full state Hello is
sent every 2HopRefresh Hellos (on each MANET interface).
A differential Hello includes only neighbors whose list has changed
within the last HelloRepeatCount (default 3) Hellos. Differential Hellos
reduce overhead and allow Hellos to be sent more frequently for faster
response to topology changes.
Each router maintains each neighbor’s Reported Neighbor List (RNL)
obtained from Hellos, which defines the 2-hop neighbor information
used by the MDR selection algorithm.
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)
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Phase 1 – Create the Neighbor Connectivity Matrix. The 2-hop neighbor
information is used to determine which pairs of neighbors have a
bidirectional link between them.
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Phase 2 – Determine whether the router is an MDR, and select
Dependent Neighbors.
– 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.
– Parameter 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.
– Using MDR Level gives priority to existing MDRs for increased stability (similar to
OSPF’s DR election algorithm).
– Dependent Neighbors are used to determine which MDR neighbors to become
adjacent with, to ensure the backbone of MDRs is connected.
– 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.
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MDR Selection Algorithm
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(slide 2 of 2)
Phase 3 – Determine whether the router is a Backup MDR, and select
Backup Dependent Neighbors.
– 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 are used to determine which (Backup) MDR
neighbors to become adjacent with, to ensure the backbone of MDRs and
Backup MDRs is biconnected.
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Phase 4 – Select the MDR Parent and Backup MDR Parent.
– (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
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Node numbers indicate RIDs.
Thin lines indicate bidirectional neighbors.
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Red nodes indicate MDRs.
Green nodes indicate Backup MDRs.
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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 and one Backup MDR (or a 2nd MDR).
For 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
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Adjacencies are formed as follows when biconnected adjacencies are
used:
– 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.
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For uniconnected adjacencies, omit “Backup” in the above.
An existing adjacency is maintained as long as either the router itself or
the neighbor is a (Backup) MDR. Otherwise it is torn down.
To form new adjacencies more quickly in mobile networks, each DD
packet includes an MDR TLV, which identifies 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
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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:
(a) The LSA or an ACK for the LSA has been received from the neighbor (over
any interface).
(b) The LSA was received on a MANET interface, and the neighbor is “covered”
by another neighbor from which the LSA was received.
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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 receives a new LSA, it 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).
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.
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Example of MDR Flooding
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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.)
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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
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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.
– If the LSA is a duplicate and was received as a unicast:
(a) If the router is a (Backup) MDR, send an immediate ACK out
the receiving interface.
(b) If the router is an MDR Other, send a delayed ACK out the
receiving interface.
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Reason for sending an immediate ACK in case (a) is to prevent other
adjacent neighbors from retransmitting the LSA.
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Receiving Link State Acknowledgments
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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. (Necessary because,
unlike RFC 2328, each router acknowledges an LSA only the first time
it is received as a multicast.)
An LS ACK packet that is received from an adjacent neighbor is
processed as in RFC 2328, with the following additional steps:
(1) 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.
(2) 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.
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Routable Neighbors
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A MANET neighbor is defined to be routable if its state is Full, or if the
SPF calculation has produced a route to the neighbor and the neighbor
satisfies a flexible quality condition.
Only routable MANET neighbors can be used as next hops in the SPF
calculation, and can be included in the router-LSA originated by the
router.
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.
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Link State Advertisements
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The choice of which neighbors to include in the router-LSA is flexible,
subject to only two requirements:
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A router MUST include all Full neighbors in its router-LSA.
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A router MUST NOT include any non-routable neighbors in its router-LSA.
Four options for the router-LSA, depending on LSAFullness:
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Minimum LSAs: Include only fully adjacent neighbors.
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Full LSAs: Include all routable neighbors.
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MDR Full LSAs: Only (Backup) MDRs originate Full LSAs, other routers
originate Minimum LSAs.
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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.
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