Network Working Group R. Ogier
Internet-Draft SRI International
Intended status: Experimental P. Spagnolo
Expires: August 17, 2008 Boeing
February 17, 2008
MANET Extension of OSPF using CDS Flooding
draft-ietf-ospf-manet-mdr-00.txt
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Abstract
This document specifies an extension of OSPF for IPv6 to support
mobile ad hoc networks (MANETs). The extension, called OSPF-MDR, is
designed as a new OSPF interface type for MANETs. OSPF-MDR is based
on the selection of a subset of MANET routers, consisting of MANET
Designated Routers (MDRs) and Backup MDRs. The MDRs form a connected
dominating set (CDS), and the MDRs and Backup MDRs together form a
biconnected CDS for robustness. This CDS is exploited in two ways.
First, to reduce flooding overhead, an optimized flooding procedure
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is used in which only (Backup) MDRs flood new LSAs back out the
receiving interface; reliable flooding is ensured by retransmitting
LSAs along adjacencies. Second, adjacencies are formed only between
(Backup) MDRs and a subset of their neighbors, allowing for much
better scaling in dense networks. The CDS is constructed using 2-hop
neighbor information provided in a Hello protocol extension. The
Hello protocol is further optimized by allowing differential Hellos
that report only changes in neighbor states. Options are specified
for originating router-LSAs that provide full or partial topology
information, allowing overhead to be reduced by advertising less
topology information.
Table of Contents
1 Introduction ................................................. 4
1.1 Terminology .................................................. 5
2 Overview ..................................................... 7
2.1 Selection of MDRs, BMDRs, Parents, and Adjacencies ........... 7
2.2 Flooding Procedure ........................................... 9
2.3 Link State Acknowledgments ................................... 9
2.4 Routable Neighbors ........................................... 9
2.5 Partial and Full Topology LSAs .............................. 10
2.6 Hello Protocol .............................................. 11
3 Interface and Neighbor Data Structures ...................... 11
3.1 Changes to Interface Data Structure ......................... 11
3.2 New Configurable Interface Parameters ....................... 12
3.3 Changes to Neighbor Data Structure .......................... 14
4 Hello Protocol .............................................. 16
4.1 Sending Hello Packets ....................................... 16
4.2 Receiving Hello Packets ..................................... 19
4.3 Neighbor Acceptance Condition ............................... 22
5 MDR Selection Algorithm ..................................... 23
5.1 Phase 1: Creating the Neighbor Connectivity Matrix .......... 25
5.2 Phase 2: MDR Selection ...................................... 25
5.3 Phase 3: Backup MDR Selection ............................... 27
5.4 Phase 4: Selection of the (Backup) MDR Parent ............... 27
5.5 Phase 5: Optional Selection of Non-Flooding MDRs ............ 28
6 Interface State Machine ..................................... 28
6.1 Interface states ............................................ 28
6.2 Events that cause interface state changes ................... 29
6.3 Changes to Interface State Machine .......................... 29
7 Adjacency Maintenance ....................................... 30
7.1 Changes to Neighbor State Machine ........................... 31
7.2 Whether to Become Adjacent .................................. 32
7.3 Whether to Eliminate an Adjacency ........................... 33
7.4 Sending Database Description Packets ........................ 33
7.5 Receiving Database Description Packets ...................... 33
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8 Flooding Procedure .......................................... 34
8.1 LSA Forwarding Procedure .................................... 35
8.2 Sending Link State Acknowledgments .......................... 38
8.3 Retransmitting LSAs ......................................... 39
8.4 Receiving Link State Acknowledgments ........................ 39
9 Originating LSAs ............................................ 40
9.1 Routable Neighbors .......................................... 41
9.2 Partial and Full Topology LSAs .............................. 42
10 Calculating the Routing Table ............................... 44
11 Security Considerations ..................................... 45
12 IANA Considerations ......................................... 45
13 Acknowledgments ............................................. 45
14 Normative References ........................................ 45
15 Informative References ...................................... 46
A Packet Formats .............................................. 46
A.1 Options Field ............................................... 46
A.2 Link-Local Signaling ........................................ 46
A.3 Hello Packet DR and Backup DR Fields ........................ 51
A.4 LSA Formats and Examples .................................... 51
B Detailed Algorithms for MDR/BMDR Selection .................. 55
B.1 Detailed Algorithm for Step 2.4 (MDR Selection) ............. 55
B.2 Detailed Algorithm for Step 3.2 (BMDR Selection) ............ 56
C Min-Cost LSA Algorithm ...................................... 58
D Non-Ackable LSAs for Periodic Flooding ...................... 62
Authors Addresses ........................................... 62
Intellectual Property and Copyright Statements .............. 63
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1. Introduction
This document specifies an extension of OSPF for IPv6 [RFC2328,
RFC2740], to support a new interface type for mobile ad hoc networks
(MANETs), i.e., for broadcast-capable, multihop wireless networks in
which routers and hosts can be mobile. This extension is also
applicable to non-mobile mesh networks using layer-3 routing.
Existing OSPF interface types do not perform adequately in such
environments, due to scaling issues regarding the flooding protocol
operation, inability of the Designated Router election protocol to
converge in all scenarios, and large numbers of adjacencies when
using a Point-to-Multipoint interface type.
An OSPF implementation that is extended with this MANET interface
type does not preclude the use of any existing interface types, and
is fully compatible with a legacy OSPF implementation. MANET
networks are represented externally as Point-to-Multipoint networks,
although the design borrows concepts used by the OSPF broadcast
interface type.
The approach taken is to generalize the concept of an OSPF Designated
Router (DR) and Backup DR to multihop wireless networks, in order to
reduce overhead by reducing the number of routers that must flood new
LSAs and reducing the number of adjacencies. The generalized
(Backup) Designated Routers are called (Backup) MANET Designated
Routers (MDRs). The MDRs form a connected dominating set (CDS), and
the MDRs and Backup MDRs together form a biconnected CDS for
robustness. By definition, each router in the MANET either belongs
to the CDS or is one hop away from it. A distributed algorithm is
used to select and dynamically maintain the biconnected CDS.
Adjacencies are established only between (Backup) MDRs and a subset
of their neighbors, thus resulting in a dramatic reduction in the
number of adjacencies in dense networks, compared to the approach of
forming adjacencies between all neighbor pairs. The OSPF extension
is called OSPF-MDR.
Hello packets are modified, using OSPF link-local signaling [LLS],
for two purposes: to provide neighbors with 2-hop neighbor
information that is required by the MDR selection algorithm, and to
allow differential Hellos that report only changes in neighbor
states. Differential Hellos can be sent more frequently without a
significant increase in overhead, in order to respond more quickly to
topology changes.
Each MANET router advertises a subset of its MANET neighbors as
point-to-point links in its router-LSA. The choice of which
neighbors to advertise is flexible, allowing overhead to be reduced
by advertising less topology information. Options are specified for
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originating router-LSAs that provide full or partial topology
information.
This document is organized as follows. Section 2 presents an overview
of OSPF-MDR, Section 3 presents the new interface and neighbor data
items that are required for the extension, Section 4 describes the
Hello protocol, including procedures for maintaining the 2-hop
neighbor information, Section 5 describes the MDR selection
algorithm, Section 6 describes changes to the Interface state
machine, section 7 describes the procedures for forming adjacencies
and deciding which neighbors should become adjacent, Section 8
describes the flooding procedure, Section 9 specifies the
requirements and options for what to include in router-LSAs, and
Section 10 describes changes in the calculation of the routing table.
The appendix specifies packet formats, detailed algorithms for the
MDR selection algorithm, an algorithm for the selection of a subset
of neighbors to advertise in the router-LSA to provide shortest-path
routing, and a proposed option that uses "non-ackable" LSAs to
provide periodic flooding that reduces overhead in highly mobile
networks.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In addition, this document uses the following terms:
MANET Interface
A new OSPF interface type that supports broadcast-capable,
multihop wireless networks. Two neighboring routers on a MANET
interface may not be able to communicate directly with each other.
A neighboring router on a MANET interface is called a MANET
neighbor. MANET neighbors are discovered dynamically using a
modification of OSPF's Hello protocol, which takes advantage of
the broadcast capability.
MANET Router
An OSPF router that has at least one MANET interface.
Differential Hello
A Hello packet that reduces the overhead of sending full Hellos,
by including only the Router IDs of neighbors whose state changed
recently.
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2-Hop Neighbor Information
Information that specifies the bidirectional neighbors of each
neighbor. The modified Hello protocol provides each MANET router
with 2-hop neighbor information, which is used for selecting MDRs
and Backup MDRs.
MANET Designated Router (MDR)
One of a set of routers responsible for flooding new LSAs, and for
determining the set of adjacencies that must be formed. The set
of MDRs forms a connected dominating set and is a generalization
of the DR found in the broadcast network.
Backup MANET Designated Router (Backup MDR or BMDR)
One of a set of routers responsible for providing backup flooding
when neighboring MDRs fail, and for determining the set of
adjacencies that must be formed. The set of MDRs and Backup MDRs
forms a biconnected dominating set. The Backup MDR is a
generalization of the Backup DR found in the broadcast network.
MDR Other
A router is an MDR Other for a particular MANET interface if it is
neither an MDR nor a Backup MDR for that interface.
(Backup) MDR Parent
Each Backup MDR and MDR Other selects a Parent, which will be a
neighboring MDR if one exists. If the option of biconnected
adjacencies is chosen, then each MDR Other also selects a Backup
Parent, which will be a neighboring MDR/BMDR if one exists that is
not the Parent. Each router forms an adjacency with its Parent
and its Backup Parent (if it exists).
Bidirectional Neighbor
A neighboring router whose neighbor state is 2-Way or greater.
The set of such neighbors is called the Bidirectional Neighbor Set
(BNS).
Routable Neighbor
A bidirectional MANET neighbor becomes routable if its state is
Full, or if the SPF calculation has produced a route to the
neighbor and the neighbor satisfies a quality condition. Once a
neighbor becomes routable, it remains routable as long as it
remains bidirectional. Only routable MANET neighbors can be used
as next hops in the SPF calculation, and can be included in LSAs
originated by the router.
Non-flooding MDR
An MDR that does not immediately flood received LSAs back out the
receiving interface. Some MDRs may declare themselves non-
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flooding in order to reduce flooding overhead.
2. Overview
This section provides an overview of OSPF-MDR, including motivation
and rationale for some of the design choices.
OSPF-MDR was motivated by the desire to extend OSPF to support
MANETs, while keeping the same design philosophy as OSPF and using
techniques that are similar to those of OSPF. For example, OSPF
reduces overhead in a broadcast network by electing a Designated
Router (DR) and Backup DR, and by having two neighboring routers form
an adjacency only if one of them is the DR or Backup DR. This idea
can be generalized to a multihop wireless network by forming a
spanning tree, with the edges of the tree being the adjacencies and
the interior (non-leaf) nodes of the tree being the generalized DRs,
called MANET Designated Routers (MDRs).
To provide better robustness and fast response to topology changes,
it was decided that a router should decide whether it is an MDR based
only on local information that can be obtained from neighbor's
Hellos. The resulting set of adjacencies therefore does not always
form a tree globally, but appears to be a tree locally. Similarly,
the Backup DR can be generalized to Backup MDRs (BMDRs), to provide
robustness through biconnected redundancy. The set of MDRs forms a
connected dominating set (CDS), and the set of MDRs and BMDRs forms a
biconnected dominating set.
The following subsections provide an overview of each of the main
features of OSPF-MDR, starting with a summary of how MDRs, BMDRs, and
adjacencies are selected.
2.1. Selection of MDRs, BMDRs, Parents, and Adjacencies
The MDR selection algorithm is distributed; each router selects
itself as an MDR, BMDR, or other router (called an "MDR Other") based
on information about its one-hop neighborhood, which is obtained from
Hello packets received from neighbors. Routers are ordered
lexicographically based on the tuple (RtrPri, MDR Level, RID), where
RtrPri is the Router Priority, MDR Level represents the current state
of the router (2 for an MDR, 1 for a BMDR, and 0 for an MDR Other),
and RID is the Router ID. Routers with lexicographically larger
values of (RtrPri, MDR Level, RID) are given preference for becoming
MDRs.
The MDR selection algorithm can be summarized as follows. If the
router itself has a larger value of (RtrPri, MDR Level, RID) than all
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of its neighbors, it selects itself as an MDR. Otherwise, let Rmax
denote the neighbor with the largest value of (RtrPri, MDR Level,
RID). The router then selects itself as an MDR unless each neighbor
can be reached from Rmax in at most k hops via neighbors that have a
larger value of (RtrPri, MDR Level, RID) than the router itself,
where k is the parameter MDRConstraint, whose default value is 3.
This parameter serves to control the density of the MDR set, since
the MDR set need not be strictly minimal.
Similarly, a router that does not select itself as an MDR will select
itself as a BMDR unless each neighbor can be reached from Rmax via
two node-disjoint paths, using as intermediate hops only neighbors
that have a larger value of (RtrPri, MDR Level, RID) than the router
itself.
When a router selects itself as an MDR, it also decides which MDR
neighbors it should become adjacent with, to ensure that the set of
MDRs and the adjacencies between them form a connected backbone.
Each non-MDR router selects and becomes adjacent with an MDR neighbor
called its parent, thus ensuring that all routers are connected to
the MDR backbone.
If the option of biconnected adjacencies is chosen (AdjConnectivity =
2), then additional adjacencies are selected to ensure that the set
of MDRs and BMDRs, and the adjacencies between them, form a
biconnected backbone. In this case, each MDR Other selects and
becomes adjacent with an MDR/BMDR neighbor called its backup parent,
in addition to its MDR parent.
OSPF-MDR also provides the option of full-topology adjacencies
(AdjConnectivity = 0). If this option is selected, then each router
forms an adjacency with each bidirectional neighbor.
Prioritizing routers according to (RtrPri, MDR Level, RID) allows
neighboring routers to agree on which routers should become an MDR,
and gives higher priority to existing MDRs, which increases the
lifetime of MDRs and the adjacencies between them. In addition,
parents are selected to be existing adjacent neighbors whenever
possible, to avoid forming new adjacencies unless necessary. Once a
neighbor becomes adjacent, it remains adjacent as long as the
neighbor is bidirectional and either the neighbor or the router
itself is an MDR or BMDR (similar to OSPF). The above rules reduce
the rate at which new adjacencies are formed, which is important
since database exchange must be performed whenever a new adjacency is
formed.
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2.2. Flooding Procedure
When an MDR receives a new LSA on a MANET interface, it immediately
floods the LSA back out the receiving interface unless it can be
determined that such flooding is unnecessary. When a Backup MDR
receives a new LSA on a MANET interface, it waits a short interval
(BackupWaitInterval), and then floods the LSA only if it has a
neighbor that did not flood or acknowledge the LSA and is not known
to be a neighbor of another neighbor (of the Backup MDR) that flooded
the LSA.
MDR Other routers never flood LSAs back out the receiving interface.
To exploit the broadcast nature of MANETs, a new LSA is processed
(and possibly forwarded) if it is received from any neighbor in state
2-Way or greater. The flooding procedure also avoids redundant
forwarding of LSAs when multiple interfaces exist.
2.3. Link State Acknowledgments
All Link State Acknowledgment packets are multicast. An LSA is
acknowledged if it is a new LSA, or if it is a duplicate LSA received
as a unicast. (A duplicate LSA received as multicast is not
acknowledged.) An LSA that is flooded back out the same interface is
treated as an implicit acknowledgment. Link state acknowledgments
may be delayed up to AckInterval seconds to allow coalescing multiple
acknowlegments in the same packet. The only exception is that
(Backup) MDRs send a multicast link state acknowledgment immediately
when a duplicate LSA is received as a unicast, in order to prevent
additional retransmissions. Only link state acknowledgments from
adjacent neighbors are processed, and retransmitted LSAs are sent
(via unicast) only to adjacent neighbors.
2.4. Routable Neighbors
In OSPF, a neighbor must typically be fully adjacent (in state Full)
for it to be used in the SPF calculation. An exception exists for an
OSPF broadcast network, to avoid requiring all pairs of routers in
such a network to form adjacencies, which would generate a large
amount of overhead. In such a network, a router can use a non-
adjacent neighbor as a next hop as long as both routers are fully
adjacent with the Designated Router. We define this neighbor
relationship as a "routable neighbor" and extend its usage to the
MANET interface type. All fully adjacent neighbors are routable, but
some neighbors for which a full adjacency does not exist may be
routable if other criteria are met.
A MANET neighbor becomes routable if its state is Full, or if it is
bidirectional and the SPF calculation has produced a route to the
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neighbor. (A flexible quality condition may also be required.) 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. The idea is that if the SPF calculation has produced a route
to the neighbor, then it makes sense to take a "shortcut" and forward
packets directly to the neighbor.
The routability condition is a generalization of the way that
neighbors on broadcast networks are treated in the SPF calculation.
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.
2.5. Partial and Full Topology LSAs
This specification allows routers to originate both full-topology
LSAs, which advertise links to all routable neighbors, and partial-
topology LSAs, which advertise only a subset of such links. In a
dense network, partial-topology LSAs are typically much smaller than
full-topology LSAs, thus achieving better scalability.
Each router advertises a subset of its routable neighbors as point-
to-point connections in its router-LSA. The choice of which
neighbors to advertise is flexible, and is determined by the
configurable parameter LSAFullness. As a minimum requirement, each
router must advertise a minimum set of "backbone" neighbors in its
router-LSA. This minimum choice corresponds to LSAFullness = 0.
This choice results in the minimum amount of LSA flooding overhead,
but does not provide routing along shortest paths. At the other
extreme, if LSAFullness = 4, then each router originates a full LSA,
which includes all routable neighbors.
Setting LSAFullness to 1 or 2 results in min-cost LSAs, which provide
routing along shortest (minimum-cost) paths. Each router decides
which neighbors to include in its router-LSA based on 2-hop neighbor
information obtained from its neighbors' Hellos. Each router
includes in its LSA the minimum set of neighbors necessary to provide
a shortest path between each pair of its neighbors. If LSAFullness =
2, then redundant paths are provided, to increase robustness and/or
allow multiple equal-cost routes to each destination.
Setting LSAFullness to 3 results in MDR full LSAs. Each (Backup) MDR
originates a full LSA that includes all routable neighbors, while
each MDR Other originates minimal LSAs. This choice does not provide
routing along shortest paths, but simulations have shown that it
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provides routing along nearly shortest paths with relatively low
overhead.
The above LSA options are interoperable with each other, because they
all require the router-LSA to include a minimum set of neighbors, and
because the construction of the router-LSA (described in Section
9.2.3) ensures that the router-LSAs originated by different routers
are consistent.
2.6. Modified Hello Protocol
OSPF-MDR uses the same Hello format as OSPFv3, but appends additional
information to Hello packets using link-local signaling (LLS), in
order to indicate the set of bidirectional neighbors and other
information that is used by the MDR selection algorithm and the min-
cost LSA algorithm. In addition to full Hellos, which include the
same set of neighbor IDs as OSPFv3 Hellos, OSPF-MDR allows the use of
differential Hellos, which include only the IDs of neighbors whose
state (or other information) has recently changed (within the last
HelloRepeatCount Hellos).
Differential Hellos are sent every HelloInterval seconds, except when
full Hellos are sent, which happens once every 2HopRefresh Hellos.
The default value of 2HopRefresh is 1, i.e., the default is to send
only full Hellos. The default value for HelloInterval is 2 seconds.
Differential Hellos are used to reduce overhead and to allow Hellos
to be sent more frequently, for faster reaction to topology changes.
3. Interface and Neighbor Data Structures
3.1. Changes to Interface Data Structure
The following modified or new data items are required for the
Interface Data Structure of a MANET interface:
Type
A router that implements this extension can have one or more
interfaces of type MANET, in addition to the OSPF interface types
defined in RFC 2328.
State
The possible states for a MANET interface are the same as for a
broadcast interface. However, the DR and Backup states now imply
that the router is an MDR or Backup MDR, respectively.
MDR Level
The MDR Level is equal to MDR (value 2) if the router is an MDR,
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Backup MDR (value 1) if the router is a Backup MDR, and MDR Other
(value 0) otherwise. The MDR Level is used by the MDR selection
algorithm.
MDR Parent
Each non-MDR router selects an MDR Parent, as described in Section
5.4. The MDR Parent will be a neighboring MDR, if one exists.
The MDR Parent is initialized to 0.0.0.0, indicating the lack of
an MDR Parent. A non-MDR router includes the Router ID of its MDR
Parent in the DR field of each Hello sent on the interface.
Backup MDR Parent
If the option of biconnected adjacencies is chosen, then each MDR
Other selects a Backup MDR Parent, as described in Section 5.4.
The Backup MDR Parent will be a neighboring MDR/BMDR, if one
exists that is not the MDR Parent. The Backup MDR Parent is
initialized to 0.0.0.0, indicating the lack of a Backup MDR
Parent. An MDR Other includes the Router ID of its Backup MDR
Parent in the Backup DR field of each Hello sent on the interface.
Router Priority
An 8-bit unsigned integer. A router with a larger Router Priority
is more likely to be selected as an MDR. The Router Priority for
a MANET interface can be changed dynamically based on any
criteria, including bandwidth capacity, willingness to be a relay
(which can depend on battery life, for example), number of
neighbors (degree), and neighbor stability. A router that has
been a (Backup) MDR for a certain amount of time can reduce its
Router Priority so that the burden of being a (Backup) MDR can be
shared among all routers. If the Router Priority for a MANET
interface is changed, then the interface variable
MDRNeighborChange must be set.
Hello Sequence Number (HSN)
The 16-bit sequence number carried by the Hello TLV. The HSN is
incremented by 1 every time a (differential or full) Hello is sent
on the interface.
MDRNeighborChange
A single-bit variable set to 1 if a neighbor change has occurred
that requires the MDR selection algorithm to be executed.
3.2. New Configurable Interface Parameters
The following new configurable interface parameters are required for
a MANET interface. The default values for HelloInterval,
RouterDeadInterval, and RxmtInterval for a MANET interface are 2, 6,
and 7 seconds, respectively.
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The default configuration for OSPF-MDR uses uniconnected adjacencies
(AdjConnectivity = 1) and partial-topology LSAs that provide
shortest-path routing (LSAFullness = 1). This is the most scalable
configuration that provides shortest-path routing. Other
configurations may be preferable in special circumstances. For
example, setting LSAFullness to 4 provides full-topology LSAs, and
setting LSAFullness to 0 provides minimal LSAs that minimize overhead
but do not ensure shortest-path routing. Setting AdjConnectivity to
2 increases robustness by providing a biconnected adjacency subgraph,
and setting AdjConnectivity to 0 results in full-topology
adjacencies.
Although all routers should preferably choose the same values for the
new configurable interface parameters, this is not required. OSPF-
MDR was carefully designed so that correct interoperation is achieved
even if each router sets these parameters independently of the other
routers.
AdjConnectivity
If equal to the default value of 1, then the set of adjacencies
forms a (uni)connected graph. If equal to the optional value of 2,
then the set of adjacencies forms a biconnected graph. If
AdjConnectivity is 0, then adjacency reduction is not used, i.e.,
the router becomes adjacent with all of its neighbors.
MDRConstraint
A parameter of the MDR selection algorithm, which affects the
number of MDRs selected. The default value of 3 results in nearly
the minimum number of MDRs. The optional value 2 results in a
larger number of MDRs.
BackupWaitInterval
The number of seconds that a Backup MDR must wait after receiving
a new LSA, before it decides whether to flood the LSA. Default
value is 0.5 second.
LSAFullness
Determines which neighbors a router should advertise in its
router-LSA. The value 0 results in minimal LSAs that include only
"backbone" neighbors. The values 1 and 2 result in partial-
topology LSAs that provide shortest-path routing, with value 2
providing redundant paths. The value 3 results in (Backup) MDRs
originating full LSAs and other routers originating minimal LSAs.
The value 4 results in all routers originating full LSAs. The
default value is 1.
AckInterval
The maximum number of seconds that an acknowledgment may be held
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before it is multicast so that acknowledgments may be coalesced.
The default value is 2 seconds.
2HopRefresh
One out of every 2HopRefresh Hellos sent on the interface must be
a full Hello. All other Hellos are differential. The default
value is 1, i.e., the default is to send only full Hellos. If
differential Hellos are used, the recommended value of 2HopRefresh
is 3.
HelloRepeatCount
The number of consecutive Hellos in which a neighbor must be
included when its state changes, if differential Hellos are used.
This parameter must be set to 3.
3.3. Changes to Neighbor Data Structure
The neighbor states are the same as for OSPF. However, the data for
a MANET neighbor that has transitioned to the Down state must be
maintained for at least HelloInterval * HelloRepeatCount seconds, to
allow the state change to be reported in differential Hellos. The
following new data items are required for the Neighbor Data Structure
of a neighbor on a MANET interface.
Neighbor Hello Sequence Number (NHSN)
The Hello sequence number contained in the last Hello received
from the neighbor.
A-bit
The A-bit copied from the Hello TLV of the last Hello received
from the neighbor. This bit is 1 if the neighbor is not using
adjacency reduction.
FullHelloRcvd
A single-bit variable equal to 1 if a full Hello has been received
from the neighbor.
Neighbor's MDR Level
The MDR Level of the neighbor, based on the DR and Backup DR
fields of the last Hello packet received from the neighbor or from
the MDR TLV in a DD packet received from the neighbor.
Neighbor's MDR Parent
The neighbor's choice for MDR Parent, obtained from the DR field
of the last Hello packet received from the neighbor or from the
MDR TLV in a DD packet received from the neighbor.
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Neighbor's Backup MDR Parent
The neighbor's choice for Backup MDR Parent, obtained from the
Backup DR field of the last Hello packet received from the
neighbor or from the MDR TLV in a DD packet received from the
neighbor.
Child
A single-bit variable equal to 1 if the neighbor is a child, i.e.,
if the neighbor has selected the router as a (Backup) MDR Parent.
Dependent Neighbor
A single-bit variable equal to 1 if the neighbor is a Dependent
Neighbor, which is decided by the MDR selection algorithm.
Dependent Neighbors become adjacent. The set of all Dependent
Neighbors is called the Dependent Neighbor Set (DNS).
Dependent Selector
A single-bit variable equal to 1 if the neighbor has selected the
router to be Dependent.
Selected Advertised Neighbor (SAN)
A single-bit variable equal to 1 if the neighbor is a selected
advertised neighbor. The set of all Selected Advertised Neighbors
is called the Selected Advertised Neighbor Set (SANS). The SANS
consists of neighbors that the router has selected to be included
in the router-LSA, along with other neighbors that are required to
be included.
Routable
A single-bit variable equal to 1 if the neighbor is routable. A
neighbor is routable if either its state is Full, or the routing
table includes a route to the neighbor. Only routable neighbors
are included in the router-LSA and are allowed as next hops in the
routing table.
Neighbor's Bidirectional Neighbor Set (BNS)
The neighbor's set of bidirectional neighbors, which is updated
when a Hello is received from the neighbor.
Neighbor's Dependent Neighbor Set (DNS)
The neighbor's set of Dependent Neighbors, which is updated when a
Hello is received from the neighbor.
Neighbor's Selected Advertised Neighbor Set (SANS)
The neighbor's set of Selected Advertised Neighbors, which is
updated when a Hello is received from the neighbor.
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Neighbor's Link Metrics
The link metric for each of the neighbor's bidirectional
neighbors, obtained from the Metric TLV appended to each Hello.
4. Hello Protocol
The MANET interface utilizes Hellos for neighbor discovery and for
enabling neighbors to learn 2-hop neighbor information. The protocol
is flexible because it allows the use of full or differential Hellos.
Full Hellos list all neighbors in state Init or above, as in OSPFv3,
whereas differential Hellos list only neighbors whose status as a
bidirectional neighbor, Dependent Neighbor, or Selected Advertised
Neighbor has recently changed. Differential Hellos are used to
reduce overhead, and they allow Hellos to be sent more frequently
(for faster reaction to topology changes). If differential Hellos
are used, full Hellos are sent less frequently to ensure that all
neighbors have current 2-hop neighbor information.
4.1. Sending Hello Packets
Hello packets are sent according to [RFC2740] Section 3.2.1.1 and
[RFC2328] Section 9.5 with the following MANET specific
specifications beginning after paragraph 3 of Section 9.5. The Hello
packet format is defined in [RFC2740] A.3.2, except for the ordering
of the Neighbor IDs and the meaning of the DR and Backup DR fields as
described below.
Similar to [RFC2328], the DR and Backup DR fields indicate whether
the router is an MDR or Backup MDR. If the router is an MDR, then
the DR field is the router's own Router ID, and if the router is a
Backup MDR, then the Backup DR field is the router's own Router ID.
These fields are also used to advertise the router's MDR Parent and
Backup MDR Parent, as specified in Section A.3 and Section 5.4.
Hellos are sent every HelloInterval seconds. Full Hellos are sent
every 2HopRefresh Hellos, and differential Hellos are sent at all
other times. For example, if 2HopRefresh is equal to 3, then every
third Hello is a full Hello. If 2HopRefresh is set to 1, then all
Hellos are full (the default).
The neighbor IDs included in the body of each Hello are divided into
the following five disjoint lists of neighbors (some of which may be
empty), and must appear in the following order:
List 1. Neighbors whose state recently changed to Down (included
only in differential Hellos).
List 2. Neighbors in state Init.
List 3. Dependent Neighbors.
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List 4. Selected Advertised Neighbors.
List 5. Unselected bidirectional neighbors, defined as bidirectional
neighbors that are neither Dependent nor Selected Advertised
Neighbors.
Note that all neighbors in Lists 3 through 5 are bidirectional
neighbors. These lists are used to update the neighbor's
Bidirectional Neighbor Set (BNS), Dependent Neighbor Set (DNS), and
Selected Advertised Neighbor Set (SANS) when a Hello is received.
Note that the above five lists are disjoint, so each neighbor can
appear in at most one list. Also note that some or all of the five
lists can be empty.
Link-local signaling (LLS) is used to append up to two TLVs to each
MANET Hello packet. The format for LLS is given in Section A.2. The
Hello TLV is appended to each (full or differential) MANET Hello
packet. It indicates whether the Hello is full or differential, and
gives the Hello Sequence Number (HSN) and the number of neighbor IDs
in each of Lists 1 through 4 defined above. The size of List 5 is
then implied by the packet length field of the Hello. The format of
the Hello TLV is given in Section A.2.3.
In both full and differential Hellos, the appended Hello TLV is built
as follows.
o The Sequence Number field is set to the current HSN for the
interface; the HSN is then incremented.
o The D-bit of the Hello TLV is set to 1 for a differential Hello
and 0 for a full Hello.
o The A-bit of the Hello TLV is set to 1 if AdjConnectivity is 0
(the router is not using adjacency reduction); otherwise it is set
to 0.
o The N1, N2, N3, and N4 fields are set to the number of neighbor
IDs in the body of the Hello that are in List 1, List 2, List 3,
and List 4, respectively. (N1 is always zero in a full Hello.)
If LSAFullness is 1 or 2, a Metric TLV is appended to each MANET
Hello packet. It advertises link costs to neighbors, to allow the
selection of neighbors to include in partial-topology LSAs. The
format of the Metric TLV is given in Section A.2.5. The I bit of the
Metric TLV can be set to 0 or 1. If the I bit is set to 0, then the
Metric TLV does not contain neighbor IDs, and contains the metric for
each bidirectional neighbor listed in the (full or differential)
Hello, in the same order. If the I bit is set to 1, then the Metric
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TLV includes the neighbor ID and metric for each bidirectional
neighbor listed in the Hello whose metric is not equal to the Default
Metric field of the TLV.
The I bit should be chosen to minimize the size of the Metric TLV.
This can be achieved by choosing the the I bit to be 1 if and only if
the number of bidirectional neighbors listed in the Hello whose
metric differs from the Default Metric field is less than 1/3 of the
total number of bidirectional neighbors listed in the Hello.
For example, if all neighbors have the same metric, then the I bit
should be set to 1, with the Default Metric equal to this metric,
avoiding the need to include neighbor IDs and corresponding metrics
in the TLV. At the other extreme, if all neighbors have different
metrics, then the I bit should be set to 0 to avoid listing the same
neighbor IDs in both the body of the Hello and the Metric TLV.
In both full and differential Hello packets, the L bit is set in the
Hello's option field to indicate LLS.
4.1.1. Full Hello Packet
In a full Hello, the neighbor ID list includes all neighbors in state
Init or higher, in the order described above. The Hello TLV is built
as described above, and if LSAFullness is 1 or 2, the Metric TLV is
built as specified in Section A.2.5.
4.1.2. Differential Hello Packet
In a differential Hello, the five neighbor ID lists defined in
Section 4.1 are populated as follows:
List 1 includes all neighbors in state Down that transitioned to this
state within the last HelloRepeatCount Hellos.
List 2 includes all neighbors in state Init that transitioned to this
state within the last HelloRepeatCount Hellos.
List 3 includes all Dependent Neighbors that became Dependent within
the last HelloRepeatCount Hellos.
List 4 includes all Selected Advertised Neighbors that became
Selected Advertised Neighbors within the last HelloRepeatCount
Hellos.
List 5 includes all unselected bidirectional neighbors (defined in
Section 4.1) that became unselected bidirectional neighbors within
the last HelloRepeatCount Hellos.
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In addition, if a Metric TLV is appended to the Hello, then each
bidirectional neighbor whose link metric changed within the last
HelloRepeatCount Hellos must also be included in the body of the
Hello (in the appropriate list).
4.2. Receiving Hello Packets
A Hello packet received on a MANET interface is processed as
described in [RFC2740] Section 3.2.2.1 and the first two paragraphs
of [RFC2328] Section 10.5, followed by the processing specified
below.
The source of a received Hello packet is identified by the Router ID
found in the Hello's OSPF packet header. If a matching neighbor
cannot be found in the interface's data structure, one is created
with the Neighbor ID set to the Router ID found in the OSPF packet
header, the state initialized to Down, all MANET-specific neighbor
variables (specified in Section 3.3) initialized to zero, and the
neighbor's DNS, SANS, and BNS initialized to empty sets.
The neighbor structure's Router Priority is set to the value of the
corresponding field in the received Hello packet. The Neighbor's MDR
Parent is set to the value of the DR field, and the Neighbor's Backup
MDR Parent is set to the value of the Backup DR field.
Now the rest of the Hello Packet is examined, generating events to be
given to the neighbor and interface state machines. These state
machines are specified either to be executed or scheduled (see
[RFC2328] Section 4.4 "Tasking support"). For example, by specifying
below that the neighbor state machine be executed in line, several
neighbor state transitions may be affected by a single received
Hello.
o If the L bit in the options field is not set, then an error has
occurred and the Hello is discarded.
o If the LLS contains a Hello TLV, the neighbor state machine is
executed with the event HelloReceived. Otherwise, an error has
occurred and the Hello is discarded.
o The Hello Sequence Number and the A-bit in the Hello TLV are
copied to the neighbor's data structure.
o The DR and Backup DR fields are processed as follows.
(1) If the DR field is equal to the neighbor's Router ID,
set the neighbor's MDR Level to MDR.
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(2) Else if the Backup DR field is equal to the neighbor's
Router ID, set the neighbor's MDR Level to Backup MDR.
(3) Else, set the neighbor's MDR Level to MDR Other.
(4) If the DR or Backup DR field is equal to the router's own
Router ID, the neighbor's Child variable is set to 1;
otherwise it is set to zero.
The neighbor ID list of the Hello is divided as follows into the five
lists defined in Section 4.1, where N1, N2, N3, and N4 are obtained
from the corresponding fields of the Hello TLV. List 1 is defined to
be the first N1 neighbor IDs, List 2 is defined to be the next N2
neighbor IDs, List 3 is defined to be the next N3 neighbor IDs, List
4 is defined to be the next N4 neighbor IDs, and List 5 is defined to
be the remaining neighbor IDs in the Hello.
Further processing of the Hello depends on whether it is full or
differential, which is indicated by the value of the D-bit of the
Hello TLV.
4.2.1. Full Hello Packet
If the received Hello is full (the D-bit of the Hello TLV is 0), the
following steps are performed:
o If the N1 field of the Hello TLV is not zero, then an error has
occurred and the Hello is discarded. Otherwise, set FullHelloRcvd
to 1.
o In the neighbor structure, modify the neighbor's DNS to equal the
set of neighbor IDs in the Hello's List 3, modify the neighbor's
SANS to equal the set of neighbor IDs in the Hello's List 4, and
modify the neighbor's BNS to equal the set of neighbor IDs in the
union of Lists 3, 4, and 5.
o If the router itself appears in the Hello's neighbor ID list, the
neighbor state machine is executed with the event 2-WayReceived
after the Hello is processed. Otherwise, the neighbor state
machine is executed with the event 1-WayReceived after the Hello
is processed.
4.2.2. Differential Hello Packet
If the received Hello is differential (the D-bit of the Hello TLV is
1), the following steps are performed:
(1) For each neighbor ID in List 1 or List 2 of the Hello:
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o Remove the neighbor ID from the neighbor's DNS, SANS,
and BNS, if it belongs to the neighbor set.
(2) For each neighbor ID in List 3 of the Hello:
o Add the neighbor ID to the neighbor's DNS and BNS, if it
does not belong to the neighbor set.
o Remove the neighbor ID from the neighbor's SANS, if it
belongs to the neighbor set.
(3) For each neighbor ID in List 4 of the Hello:
o Add the neighbor ID to the neighbor's SANS and BNS, if it
does not belong to the neighbor set.
o Remove the neighbor ID from the neighbor's DNS, if it
belongs to the neighbor set.
(4) For each neighbor ID in List 5 of the Hello:
o Add the neighbor ID to the neighbor's BNS, if it does not
belong to the neighbor set.
o Remove the neighbor ID from the neighbor's DNS and SANS, if
it belongs to the neighbor set.
(5) If the router's own RID appears in List 1, execute the neighbor
state machine with the event 1-WayReceived after the Hello is
processed.
(6) If the router's own RID appears in List 2, 3, 4, or 5, execute
the neighbor state machine with the event 2-WayReceived after
the Hello is processed.
(7) If the router's own RID does not appear in the Hello's neighbor
ID list, and the neighbor state is 2-Way or greater, and the
Hello Sequence Number is less than or equal to the previous
sequence number plus HelloRepeatCount, then the neighbor state
machine is executed with the event 2-WayReceived after the Hello
is processed (the state does not change).
(8) If 2-WayReceived is not executed, then 1-WayReceived is executed
after the Hello is processed.
4.2.3. Additional Processing for Both Hello Types
The following applies to both full and differential Hellos.
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If the router itself appears in the neighbor's DNS, the neighbor's
Dependent Selector variable is set to 1; otherwise it is set to 0.
The receiving interface's MDRNeighborChange variable is set to 1 if
any of the following changes occurred as a result of processing the
Hello:
o The neighbor's state changed from less than 2-Way to 2-Way or
greater, or vice versa.
o The neighbor is bidirectional and any of the following neighbor
variables has changed: MDR Level, Router Priority, FullHelloRcvd,
and Bidirected Neighbor Set (BNS).
The neighbor state machine is scheduled with the event AdjOK? if any
of the following changes occurred as a result of processing the
Hello:
o The neighbor's state changed from less than 2-Way to 2-Way or
greater.
o The neighbor is bidirectional and its MDR Level has changed, or
its Child variable or Dependent Selector variable has changed from
0 to 1.
If the LLS contains a Metric TLV, it is processed by updating the
neighbor's link metrics according to the format of the Metric TLV
specified in Section A.2.5.
If LSAFullness is 1 or 2 (partial-topology LSAs), then the min-cost
LSA algorithm (Appendix C) is executed and a new router-LSA is
possibly originated as specified in Section 9.2.3 if any of the
following changes occurred as a result of processing the Hello:
o The neighbor's state changed from less than 2-Way to 2-Way or
greater, or vice versa.
o The neighbor is bidirectional and any of the following neighbor
variables has changed: MDR Level, Router Priority, FullHelloRcvd,
DNS, SANS, BNS, and MDR Parent(s).
o Any of the neighbor's link metrics has changed as a result of
processing the Metric TLV.
4.3. Neighbor Acceptance Condition
In wireless networks, a single Hello can be received from a neighbor
with which a poor connection exists, e.g., because the neighbor is
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almost out of range. To avoid accepting poor quality neighbors, and
to employ hysteresis, a router may require that a stricter condition
be satisfied before changing the state of a MANET neighbor from Down
to Init or greater. This condition is called the "neighbor
acceptance condition", which by default is the reception of a single
Hello or DD packet. For example, the neighbor acceptance condition
may require that 2 consecutive Hellos be received from a neighbor
before changing the neighbor's state from Down to Init. Other
possible conditions include the reception of 3 consecutive Hellos, or
the reception of 2 of the last 3 Hellos. The neighbor acceptance
condition may also impose thresholds on other measurements such as
received signal strength.
The neighbor state transition for state Down and event HelloReceived
is thus modified (see Section 7.1) to depend on the neighbor
acceptance condition.
5. MDR Selection Algorithm
This section describes the MDR selection algorithm, which determines
determines whether the router is an MDR, Backup MDR, or MDR Other on
a given interface. The algorithm also selects the Dependent
Neighbors and the (Backup) MDR Parent, which are used to decide which
neighbors should become adjacent (see Section 7).
The MDR selection algorithm is executed just before sending a Hello
if the MDRNeighborChange bit is set for the interface; the bit is
then cleared. To simplify the implementation, the MDR selection
algorithm MAY be executed just before sending each Hello, to avoid
having to determine when the MDRNeighborChange bit should be set.
After running the MDR selection algorithm, the AdjOK? event may be
invoked for some or all neighbors as specified in Section 7.
The purpose of the MDRs is to provide a minimal set of relays for
flooding LSAs, and the purpose of the Backup MDRs is to provide
backup relays to flood LSAs when flooding by MDRs does not succeed.
The set of MDRs forms a CDS, and the set of (Backup) MDRs forms a
biconnected CDS. Note that there may be fewer Backup MDRs than MDRs,
since the MDRs themselves may already provide some redundancy.
Each MDR becomes adjacent with a subset of MDR neighbors called
Dependent Neighbors, forming a connected backbone. Each non-MDR
router connects to this backbone by selecting and becoming adjacent
with an MDR neighbor called its MDR Parent.
If AdjConnectivity = 2, then each (Backup) MDR becomes adjacent with
additional (Backup) MDR neighbors to form a biconnected backbone, and
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each MDR Other selects and becomes adjacent with a second (Backup)
MDR neighbor called its Backup MDR Parent, thus becoming connected to
the backbone via two adjacencies.
The MDR selection algorithm is a distributed CDS algorithm that uses
2-hop neighbor information obtained from Hellos. More specifically,
it uses as inputs the set of bidirectional neighbors (in state 2-Way
or greater), the triplet (MDR Level, Router Priority, Router ID) for
each such neighbor and for the router itself, and the neighbor
variables Bidirectional Neighbor Set (BNS) and FullHelloRcvd for each
such neighbor. The MDR selection algorithm can be implemented in
O(d^2) time, where d is the number of neighbors.
The above triplet will be abbreviated as (RtrPri, MDR Level, RID).
The triplet (RtrPri, MDR Level, RID) is said to be larger for Router
A than for Router B if the triplet for Router A is lexicographically
greater than the triplet for Router B. Routers that have larger
values of this triplet are preferred for selection as an MDR. The
algorithm therefore prefers routers that are already MDRs, resulting
in a longer average MDR lifetime.
The MDR selection algorithm consists of five phases, the last of
which is optional. Phase 1 creates the neighbor connectivity matrix,
which determines which pairs of neighbors are neighbors of each
other. Phase 2 decides whether the calculating router is an MDR, and
which MDR neighbors are Dependent. Phase 3 decides whether the
calculating router is a Backup MDR and, if AdjConnectivity = 2, which
additional MDR/BMDR neighbors are Dependent. Phase 4 selects the MDR
Parent and Backup MDR Parent.
The algorithm simplifies considerably if AdjConnectivity is 0 (full-
topology adjacencies). In this case, Phase 4 (parent selection) is
not executed, and the set of Dependent Neighbors is empty. Also,
Phase 3 (BMDR selection) is not required if AdjConnectivity is 0 or
1. However, Phase 3 MUST be executed if AdjConnectivity is 2, and
SHOULD be executed if AdjConnectivity is 0 or 1, since BMDRs improve
robustness by providing backup flooding.
A router that has selected itself as an MDR in Phase 2 MAY execute
Phase 5 to possibly declare itself a non-flooding MDR. A non-
flooding MDR is the same as a flooding MDR except that it does not
immediately flood received LSAs back out the receiving interface,
because it has determined that neighboring MDRs exist that will
ensure all neighbors are covered. Instead, a non-flooding MDR
performs backup flooding just like a BMDR. A non-flooding MDR
maintains its MDR level (rather than being demoted to a BMDR) in
order to maximize the stability of adjacencies. (The decision to
form an adjacency does not depend on whether an MDR is non-flooding.)
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By having MDRs declare themselves to be non-flooding when possible,
flooding overhead is reduced. The resulting overhead reduction can
be dramatic for certain regular topologies, but has been found to be
about 15% for random topologies.
For convenience, in the following description, the term "bi-neighbor"
will be used as an abbreviation for "bidirectional neighbor".
5.1. Phase 1: Creating the Neighbor Connectivity Matrix
The neighbor connectivity matrix (NCM) assigns a value of 0 or 1 for
each pair of bi-neighbors, depending on the Bidirectional Neighbor
Set (BNS) and the value of FullHelloRcvd for each neighbor. NCM is a
symmetric matrix that defines a topology graph for the set of bi-
neighbors. A value of 1 for a given pair of neighbors indicates that
the neighbors are assumed to be bi-neighbors of each other in the MDR
selection algorithm. Letting i denote the router itself, NCM(i,j)
and NCM(j,i) are set to 1 for each bi-neighbor j. The value of the
matrix is set as follows for each pair of bi-neighbors j and k.
(1.1) If FullHelloRcvd is 1 for both neighbors j and k: NCM(j,k) =
NCM(k,j) is 1 only if j belongs to the BNS of neighbor k and k
belongs to the BNS of neighbor j.
(1.2) If FullHelloRcvd is 1 for neighbor j and is 0 for neighbor k:
NCM(j,k) = NCM(k,j) is 1 only if k belongs to the BNS of
neighbor j.
(1.3) If FullHelloRcvd is 0 for both neighbors j and k: NCM(j,k) =
NCM(k,j) = 0.
In step 1.1 above, two neighbors are considered to be bi-neighbors of
each other only if they both agree that the other router is a bi-
neighbor. This provides faster response to the failure of a link
between two neighbors, since it is likely that one router will detect
the failure before the other router. In step 1.2 above, only neighbor
j has reported its full BNS, so neighbor j is believed in deciding
whether j and k are bi-neighbors of each other. As Step 1.3
indicates, two neighbors are assumed not to be bi-neighbors of each
other if neither neighbor has reported its full BNS.
5.2. Phase 2: MDR Selection
Phase 2 depends on the parameter MDRConstraint, which affects the
number of MDRs selected. The default value of 3 results in nearly the
minimum number of MDRs, while the value 2 results in a larger number
of MDRs. If AdjConnectivity = 0 (full-topology adjacencies), then
the following steps are modified in that Dependent Neighbors are not
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selected.
(2.1) The set of Dependent Neighbors is initialized to be empty.
(2.2) If the router has a larger value of (RtrPri, MDR Level, RID)
than all of its bi-neighbors, the router selects itself as an
MDR, selects all of its MDR bi-neighbors as Dependent
Neighbors, and if AdjConnectivity = 2, selects all of its BMDR
bi-neighbors as Dependent Neighbors. Else, proceed to Step
2.3.
(2.3) Let Rmax be the bi-neighbor with the largest value of (RtrPri,
MDR Level, RID).
(2.4) Using NCM to determine the connectivity of bi-neighbors,
compute the minimum number of hops, denoted hops(u), from Rmax
to each other bi-neighbor u, using only intermediate nodes that
are bi-neighbors with a larger value of (RtrPri, MDR Level,
RID) than the router itself. If no such path from Rmax to u
exists, then hops(u) equals infinity. (See Appendix B for a
detailed algorithm using breadth-first search.)
(2.5) If hops(u) is at most MDRConstraint for each bi-neighbor u, the
router selects no Dependent Neighbors, and sets its MDR Level
as follows: If the MDR Level is currently MDR, then it is
changed to BMDR if Phase 3 will be executed and to MDR Other if
Phase 3 will not be executed. Otherwise, the MDR Level is not
changed.
(2.6) Else, the router sets its MDR Level to MDR and selects the
following neighbors as Dependent Neighbors: Rmax, each MDR bi-
neighbor u such that hops(u) is greater than MDRConstraint, and
if AdjConnectivity = 2, each BMDR bi-neighbor u such that
hops(u) is greater than MDRConstraint.
(2.7) If steps 2.1 through 2.6 resulted in the MDR Level changing to
BMDR, or to MDR with AdjConnectivity equal to 1 or 2, then
execute steps 2.1 through 2.6 again. (This is necessary
because the change in MDR Level can cause the set of Dependent
Neighbors and the BFS tree to change.)
Step 2.4 can be implemented using a breadth-first search (BFS)
algorithm to compute min-hop paths from node Rmax to all other bi-
neighbors, modified to allow a node as an intermediate node only if
its value of (RtrPri, MDR Level, RID) is larger than that of the
router itself. A detailed description of this algorithm, which runs
in O(d^2) time, is given in Appendix B.
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5.3. Phase 3: Backup MDR Selection
(3.1) If the MDR Level is MDR (after running Phase 2) and
AdjConnectivity is not 2, then proceed to Phase 4. (If the MDR
Level is MDR and AdjConnectivity = 2, then Phase 3 may select
additional Dependent Neighbors to create a biconnected
backbone.)
(3.2) Using NCM to determine the connectivity of bi-neighbors,
determine whether or not there exist two node-disjoint paths
from Rmax to each other bi-neighbor u, using only intermediate
nodes that are bi-neighbors with a larger value of (RtrPri, MDR
Level, RID) than the router itself. (See Appendix B for a
detailed algorithm.)
(3.3) If there exist two such node-disjoint paths from Rmax to each
other bi-neighbor u, then the router selects no additional
Dependent Neighbors and sets its MDR Level to MDR Other.
(3.4) Else, the router sets its MDR Level to Backup MDR unless it
already selected itself as an MDR in Phase 2, and if
AdjConnectivity = 2, adds each of the following neighbors to
the set of Dependent Neighbors: Rmax, and each MDR/BMDR bi-
neighbor u such that step 3.2 did not find two node-disjoint
paths from Rmax to u.
(3.5) If steps 3.1 through 3.4 resulted in the MDR Level changing
from MDR Other to BMDR, then run Phases 2 and 3 again. (This
is necessary because running Phase 2 again can cause the MDR
Level to change to MDR.)
Step 3.2 can be implemented in O(d^2) time using the algorithm given
in Appendix B. A simpler approximate algorithm is also given, which
results in a larger number of BMDRs.
5.4. Phase 4: Selection of the (Backup) MDR Parent
Each BMDR and MDR Other selects (for each MANET interface) a Parent,
which will be a neighboring MDR if one exists. If AdjConnectivity =
2, then each MDR Other also selects a Backup Parent, which will be a
neighboring MDR/BMDR if one exists that is not the Parent. Each
router forms an adjacency with its Parent and its Backup Parent (if
it exists).
For a given MANET interface, let Rmax denote the router with the
largest value of (RtrPri, MDR Level, RID) among all bidirectional
neighbors, if such a neighbor exists that has a larger value of
(RtrPri, MDR Level, RID) than the router itself. Otherwise, Rmax is
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null.
If the calculating router has selected itself as an MDR, then the
Parent is equal to the router itself, and the Backup Parent is Rmax.
If the router has selected itself as a BMDR, then the Backup Parent
is equal to the router itself.
If the router is a BMDR or MDR Other, the Parent is selected to be
any adjacent neighbor that is an MDR, if such a neighbor exists. If
no adjacent MDR neighbor exists, then the Parent is selected to be
Rmax. (By giving preference to neighbors that are already adjacent,
the formation of a new adjacency is avoided when possible.)
If AdjConnectivity = 2 and the calculating router is an MDR Other,
then the Backup Parent is selected to be any adjacent neighbor that
is an MDR or BMDR, other than the selected Parent, if such a neighbor
exists. If no such neighbor exists, then the Backup Parent is
selected to be the bidirectional neighbor, excluding the selected
Parent, with the largest value of (RtrPri, MDR Level, RID).
5.5. 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. An MDR that
does not execute the following steps is by default a flooding MDR.
(5.1) If the router has a larger value of (RtrPri, MDR Level, RID)
than all of its bi-neighbors, the router is a flooding MDR. Else,
proceed to Step 5.2.
(5.2) Let Rmax be the bi-neighbor that has the largest value of
(RtrPri, MDR Level, RID).
(5.3) Using NCM to determine the connectivity of bi-neighbors,
compute the minimum number of hops, denoted hops(u), from Rmax to
each other bi-neighbor u, using only intermediate nodes that are MDR
bi-neighbors with a smaller value of (RtrPri, RID) than the router
itself. (This can be done using BFS as in Step 2.4).
(5.4) If hops(u) is at most MDRConstraint for each bi-neighbor u,
then the router is a non-flooding MDR. Else, it is a flooding MDR.
6. Interface State Machine
6.1. Interface states
No new states are defined for a MANET interface. However, the DR and
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Backup states now imply that the router is an MDR or Backup MDR,
respectively. The following modified definitions apply to MANET
interfaces:
Waiting
In this state, the router learns neighbor information from the
Hello packets it receives, but is not allowed to run the MDR
selection algorithm until it transitions out of the Waiting state
(when the Wait Timer expires). This prevents unnecessary changes
in the MDR selection resulting from incomplete neighbor
information. The length of the Wait Timer is 2HopRefresh *
HelloInterval seconds (the interval between full Hellos).
DR Other
The router has run the MDR selection algorithm and determined that
it is not an MDR or a Backup MDR.
Backup
The router has selected itself as a Backup MDR.
DR
The router has selected itself as an MDR.
6.2. Events that cause interface state changes
All interface events defined in RFC 2328, Section 9.2 apply to MANET
interfaces, except for BackupSeen and NeighborChange. BackupSeen is
never invoked for a MANET interface (since seeing a Backup MDR does
not imply that the router itself cannot also be an MDR or Backup
MDR).
The event NeighborChange is replaced with the new interface variable
MDRNeighborChange, which indicates that the MDR selection algorithm
must be executed due to a change in neighbor information (see Section
4.2.3).
6.3. Changes to Interface State Machine
This section describes the changes to the interface state machine for
a MANET interface. The two state transitions specified below are for
state-event pairs that are described in RFC 2328, but have modified
action descriptions because MDRs are selected instead of DRs. The
state transition in RFC 2328 for the event NeighborChange is omitted;
instead the new interface variable MDRNeighborChange is used to
indicate when the MDR selection algorithm needs to be executed. The
state transition for the event BackupSeen does not apply to MANET
interfaces, since this event is never invoked for a MANET interface.
The interface state transitions for the events Loopback and UnloopInd
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are unchanged from RFC 2328.
State: Down
Event: InterfaceUp
New state: Depends on action routine.
Action: Start the interval Hello Timer, enabling the periodic
sending of Hello packets out the interface. If the router
is not eligible to become an MDR (Router Priority is 0),
the state transitions to DR Other. Otherwise, the state
transitions to Waiting and the single shot Wait Timer is
started.
State: Waiting
Event: WaitTimer
New state: Depends on action routine.
Action: Run the MDR selection algorithm, which may result in a
change to the router's MDR Level, Dependent Neighbors,
and (Backup) MDR Parent. As a result of this calculation,
the new interface state will be DR Other, Backup, or DR.
As a result of these changes, the AdjOK? neighbor event
may be invoked for some or all neighbors. (See
Section 7.)
7. Adjacency Maintenance
Adjacency forming and eliminating on non-MANET interfaces remain
unchanged. Adjacency maintenance on a MANET interface requires
changes to transitions in the neighbor state machine ([RFC2328]
Section 10.3), to deciding whether to become adjacent ([RFC2328]
Section 10.4), sending of DD packets ([RFC2328] Section 10.8), and
receiving of DD packets ([RFC2328] Section 10.6). The specification
below relates to the MANET interface only.
Adjacencies are established with some subset of the router's
neighbors. Each (Backup) MDR forms adjacencies with a subset of its
(Backup) MDR neighbors to form a biconnected backbone, and each MDR
Other forms adjacencies with two selected (Backup) MDR neighbors
called "parents", thus providing a biconnected subgraph of
adjacencies.
An adjacency maintenance decision is made when any of the following
four events occur between a router and its neighbor. The decision is
made by executing the neighbor event AdjOK?.
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(1) The neighbor state changes from Init to 2-Way.
(2) The MDR Level changes for the neighbor or for the router itself.
(3) The neighbor is selected to be the (Backup) MDR Parent.
(4) The neighbor selects the router to be its (Backup) MDR Parent.
7.1. Changes to Neighbor State Machine
The following specifies new transitions in the neighbor state
machine.
State(s): Down
Event: HelloReceived
New state: Depends on action routine.
Action: If the neighbor acceptance condition is satisfied (see
Section 4.3), the neighbor state transitions to Init and
the Inactivity Timer is started. Otherwise, the neighbor
remains in the Down state.
State(s): Init
Event: 2-WayReceived
New state: 2-Way
Action: Transition to neighbor state 2-Way.
State(s): 2-Way
Event: AdjOK?
New state: Depends on action routine.
Action: Determine whether an adjacency should be formed with the
neighboring router (see Section 7.2). If not, the
neighbor state remains at 2-Way and no further action is
taken.
Otherwise, the neighbor state changes to ExStart, and the
following actions are performed. If the neighbor has a
larger Router ID than the router's own ID, and the
received packet is a DD packet with the initialize (I),
more (M), and master (MS) bits set, then execute the
event NegotiationDone, which causes the state to
transition to Exchange.
Otherwise (negotiation is not complete), the router
increments the DD sequence number in the neighbor data
structure. If this is the first time that an adjacency
has been attempted, the DD sequence number should be
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assigned a unique value (like the time of day clock). It
then declares itself master (sets the master/slave bit to
master), and starts sending Database Description Packets,
with the initialize (I), more (M) and master (MS) bits
set, the MDR TLV included in an LLS, and the L bit set.
This Database Description Packet should be otherwise
empty. This Database Description Packet should be
retransmitted at intervals of RxmtInterval until the next
state is entered (see [RFC2328] Section 10.8).
State(s): ExStart or greater
Event: AdjOK?
New state: Depends on action routine.
Action: Determine whether the neighboring router should still be
adjacent (see Section 7.3). If yes, there is no state
change and no further action is necessary. Otherwise,
the (possibly partially formed) adjacency must be
destroyed. The neighbor state transitions to 2-Way. The
Link state retransmission list, Database summary list,
and Link state request list are cleared of LSAs.
7.2. Whether to Become Adjacent
The following defines the method to determine if an adjacency should
be formed between neighbors in state 2-Way. The following procedure
does not depend on whether AdjConnectivity is 1 or 2, but the
selection of Dependent Neighbors (by the MDR selection algorithm)
depends on AdjConnectivity.
If adjacency reduction is not used (AdjConnectivity is 0), then an
adjacency is formed with each neighbor in state 2-Way. Otherwise an
adjacency is formed with a neighbor in state 2-Way if any of the
following conditions is true:
(1) The router is a (Backup) MDR and the neighbor is a (Backup)
MDR and is either a Dependent Neighbor or a Dependent Selector.
(2) The router is a (Backup) MDR and the neighbor is a child.
(3) The neighbor is a (Backup) MDR and is the router's (Backup)
Parent.
(4) The neighbor is not using adjacency reduction, as indicated
by the A-bit of the Hello TLV appended to the last Hello
received from the neighbor.
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Otherwise, an adjacency is not established and the neighbor remains
in state 2-Way.
7.3. Whether to Eliminate an Adjacency
The following defines the method to determine if an adjacency should
be eliminated between neighbors in a state greater than 2-way. An
adjacency is maintained if one of the following is true.
(1) The router is an MDR.
(2) The router is a Backup MDR.
(3) The neighbor is an MDR.
(4) The neighbor is a Backup MDR.
Otherwise, the adjacency MAY be eliminated.
7.4 Sending Database Description Packets
Sending a DD packet on a MANET interface is the same as [RFC2740]
Section 3.2.1.2 and [RFC2328] Section 10.8 with the following
additions to paragraph 3 of Section 10.8.
If the neighbor state is ExStart, the standard initialization packet
is sent with an MDR TLV appended using LLS, and the L bit is set in
the DD packet's option field. The DR and Backup DR fields of the MDR
TLV are set exactly the same as the DR and Backup DR fields of a
Hello sent on the same interface, as specified in Section A.3.
7.5. Receiving Database Description Packets
Processing a DD packet received on a MANET interface is the same as
[RFC2328] Section 10.6, except for the changes described in this
section. The following additional steps are performed before
processing the packet based on neighbor state in paragraph 3 of
Section 10.6.
o If the DD packet's L bit is set in the options field and an MDR
TLV is appended, then the MDR TLV is processed as follows.
(1) If the DR field is equal to the neighhor's Router ID,
(a) Set the MDR Level of the neighbor to MDR.
(b) Set the neighbor's Dependent Selector variable to 1.
(2) Else if the Backup DR field is equal to the neighbor's
Router ID,
(a) Set the MDR Level of the neighbor to Backup MDR.
(b) Set the neighbor's Dependent Selector variable to 1.
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(3) Else,
(a) Set the MDR Level of the neighbor to MDR Other.
(b) Set the neighbor's Dependent Selector variable to 0.
(4) If the DR or Backup DR field is equal to the router's own
Router ID, the neighbor's Child variable is set to 1,
otherwise it is zero.
o If the neighbor state is Init, the neighbor event 2-WayReceived is
executed.
o If the MDR Level of the neighbor changed, the neighbor state
machine is scheduled with the event AdjOK?.
o If the neighbor's Child status has changed from 0 to 1, the
neighbor state machine is scheduled with the event AdjOK?.
o If the neighbor's neighbor state changed from less than 2-Way to
2-Way or greater, the neighbor state machine is scheduled with the
event AdjOK?.
In addition, if the router accepts a received DD packet and processes
its contents, then the following action SHOULD be performed for each
LSA listed in the DD packet (whether the router is master or slave).
If the router has an instance of the LSA in the Database summary list
for the neighbor, which is the same or less recent than the LSA
listed in the packet, then the LSA is removed from the Database
summary list. This avoids including the LSA in a DD packet sent to
the neighbor, when the neighbor already has an instance of the LSA
that is the same or more recent. This optimization reduces overhead
due to DD packets by approximately 50% in large networks.
8. Flooding Procedure
This section specifies the changes to RFC 2328, Section 13 for
routers that support OSPF-MDR. The first part of Section 13 (before
Section 13.1) is the same except for the following three changes.
o To exploit the broadcast nature of MANETs, if the Link State
Update (LSU) packet was received on a MANET interface, then the
packet is dropped without further processing only if the sending
neighbor is in a lesser state than 2-Way. Otherwise, the LSU
packet is processed as described in this section.
o If the received LSA is the same instance as the database copy, the
following actions are performed in addition to step 7. For each
MANET interface for which a BackupWait Neighbor Set exists for the
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LSA (see Section 8.1):
(a) Remove the sending neighbor from the BackupWait Neighbor list
if it belongs to the list.
(b) For each neighbor on the receiving interface that belongs
to the BNS for the sending neighbor, remove the neighbor
from the BackupWait Neighbor list if it belongs to the list.
o Step 8, which handles the case in which the database copy of the
LSA is more recent than the received LSA, is modified as follows.
If the sending neighbor is in a lesser state than Exchange, then
the router does not send the LSA back to the sending neighbor.
There are no changes to Sections 13.1, 13.2, or 13.4. The following
subsections describe the changes to Sections 13.3 (Next step in the
flooding procedure), 13.5 (Sending Link State Acknowledgments), 13.6
(Retransmitting LSAs), and 13.7 (Receiving Link State
Acknowledgments) of RFC 2328.
8.1. LSA Forwarding Procedure
Step 1 of [RFC2328], Section 13.3 should be performed, with the
following change, so that the new LSA is placed on the Link State
retransmission list for each appropriate adjacent neighbor. Step
1(c) is replaced with the following action, so that the LSA is not
placed on the retransmission list for a neighbor that has already
acknowledged the LSA.
o If the new LSA was received from this neighbor, or a link state
acknowledgment (LS Ack) for the new LSA has already been received
from this neighbor, examine the next neighbor.
To determine whether an Ack for the new LSA has been received from
the neighbor, the router maintains an Acked LSA List for each
adjacent neighbor, as described in Section 8.4. When a new LSA is
received, the Acked LSA List for each neighbor, on each MANET
interface, should be updated by removing any LS Ack that is for an
older instance of the LSA than the one received.
The following description will use the notion of a "covered"
neighbor. A neighbor k is defined to be covered if the LSA was sent
as a multicast by a MANET neighbor j, and neighbor k belongs to the
Bidirectional Neighbor Set (BNS) for neighbor j. A neighbor k is
also defined to be covered if the LSA was sent to the multicast
address AllSPFRouters by a neighbor j on a broadcast interface on
which both j and k are neighbors. (Note that j must be the DR or
Backup DR for the broadcast network, since only these routers may
send LSAs to AllSPFRouters on a broadcast network.)
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Steps 2 through 5 of [RFC2328], Section 13.3 are unchanged if the
outgoing interface (on which the LSA may be forwarded) is not of type
MANET. If the outgoing interface is of type MANET, then steps 2
through 5 are replaced with the following steps, to determine whether
the LSA should be forwarded on each eligible MANET interface.
(2) If either of the following two conditions is satisfied for every
bidirectional neighbor on the interface, examine the next
interface (the LSA is not flooded out this 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 or broadcast interface, and
the neighbor is covered (defined above).
Note that the above two conditions do not assume the outgoing
interface is the same as the receiving interface.
(3) If the LSA was received on this interface, and the router is an
MDR Other for this interface, examine the next interface (the LSA
is not flooded out this interface).
(4) If the LSA was received on this interface, and the router is a
Backup MDR or a non-flooding MDR for this interface, then the
router waits BackupWaitInterval before deciding whether to flood
the LSA. To accomplish this, the router creates a BackupWait
Neighbor List for the LSA, which initially includes every
bidirectional neighbor on this interface that fails to satisfy
both conditions (a) and (b) in step 2. A single shot BackupWait
Timer associated with the LSA is started, which is set to expire
after BackupWaitInterval seconds plus a small amount of random
jitter. (The actions performed when the BackupWait Timer expires
are described below in Section 8.1.2.) Examine the next
interface (the LSA is not immediately flooded out this
interface).
(5) If the router is a flooding MDR for this interface, or if the LSA
was originated by the router itself, then the LSA is flooded out
the interface (whether or not the LSA was received on this
interface). The LSA is included in an LSU packet which is
multicast out the interface using the destination IP address
AllSPFRouters.
(6) If the LSA was received on a MANET or broadcast interface that is
different from this (outgoing) interface, then the following two
steps SHOULD be performed to avoid redundant flooding.
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(a) If the router has a larger value of (RtrPri, MDR Level, RID)
on the outgoing interface than every covered neighbor
(defined above) that is a neighbor on BOTH the receiving and
outgoing interfaces (or if no such neighbor exists), then the
LSA is flooded out the interface.
(b) Else, the router waits BackupWaitInterval before deciding
whether to flood the LSA on the interface, by performing the
actions in step 4 for a Backup MDR (whether or not the router
is a Backup MDR on this interface). A separate BackupWait
Neighbor List is created for each interface, but only one
BackupWait Timer is associated with the LSA. Examine the
next interface (the LSA is not immediately flooded out this
interface).
(7) If the optional step 6 is not performed, then the LSA is flooded
out the interface. The LSA is included in an LSU packet which is
multicast out the interface using the destination IP address
AllSPFRouters.
8.1.1. Note on Step 6 of LSA Forwarding Procedure
Performing the optional Step 6 can greatly reduce flooding overhead
if the LSA was received on a MANET or broadcast interface. For
example, assume the LSA was received from the DR of a broadcast
network that includes 100 routers, and 50 of the routers (not
including the DR) are also attached to a MANET. Assume that these 50
routers are neighbors of each other in the MANET, and that each has a
neighbor in the MANET that is not attached to the broadcast network
(and is therefore not covered). Then by performing Step 6 of the LSA
forwarding procedure, the number of routers that forward the LSA from
the broadcast network to the MANET is reduced from 50 to just 1
(assuming that at most one of the 50 routers is an MDR).
8.1.2. BackupWait Timer Expiration
If the BackupWait Timer for an LSA expires, then the following steps
are performed for each (MANET) interface for which a BackupWait
Neighbor List exists for the LSA.
(1) If the BackupWait Neighbor List for the interface contains at
least one router that is currently a bidirectional neighbor, the
following actions are performed.
(a) The LSA is flooded out the interface.
(b) If the LSA is on the Ack List for the interface (i.e., is
scheduled to be included in a delayed Link State
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Acknowledgment packet), then the router SHOULD remove the LSA
from the Ack List, since the flooded LSA will be treated as
an implicit Ack.
(c) If the LSA is on the Link State retransmission list for any
neighbor, the retransmission SHOULD be rescheduled (if
necessary) so that it does not occur within AckInterval plus
propagation delays.
(2) The BackupWait Neighbor list is then deleted (whether or not the
LSA is flooded).
8.2. Sending Link State Acknowledgments
This section describes the procedure for sending Link State
Acknowledgments (LS Acks) on MANET interfaces. Section 13.5 of RFC
2328 remains unchanged for non-MANET interfaces, but does not apply
to MANET interfaces. To minimize overhead due to LS Acks, and to
take advantage of the broadcast nature of MANETs, all LS Ack packets
sent on a MANET interface are multicast using the IP address
AllSPFRouters. In addition, duplicate LSAs received as a multicast
are not acknowledged.
When a router receives an LSA, it must decide whether to send a
delayed Ack, an immediate Ack, or no Ack. (However, a non-ackable
LSA is never acknowledged, as described in Appendix D.) A delayed
Ack may be delayed for up to AckInterval seconds, and allows several
LS Acks to be grouped into a single multicast LS Ack packet. An
immediate Ack is also sent in a multicast LS Ack packet, and may
include other LS Acks that were scheduled to be sent as delayed Acks.
The decision depends on whether the received LSA is new (i.e., is
more recent than the database copy) or a duplicate (the same instance
as the database copy), and on whether the LSA was received as a
multicast or a unicast (which indicates a retransmitted LSA). The
following rules are used to make this decision.
(1) If the received LSA is new, a delayed Ack is sent on each
MANET interface associated with the area, unless the LSA is
flooded out the interface.
(2) If the LSA is a duplicate and was received as a multicast,
the LSA is not acknowledged.
(3) If the LSA is a duplicate and was received as a unicast:
(a) If the router is an MDR, or AdjConnectivity = 2 and the
router is a Backup MDR, or AdjConnectivity = 0, then an
immediate Ack is sent out the receiving interface.
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(b) Otherwise, a delayed Ack is sent out the receiving
interface.
The reason that (Backup) MDRs send an immediate Ack when a
retransmitted LSA is received, is to try to prevent other adjacent
neighbors from retransmitting the LSA, since (Backup) MDRs usually
have a large number of adjacent neighbors. MDR Other routers do not
send an immediate Ack (unless AdjConnectivity = 0) because they have
fewer adjacent neighbors, and so the potential benefit does not
justify the additional overhead resulting from sending immediate
Acks.
8.3. Retransmitting LSAs
LSAs are retransmitted according to Section 13.6 of RFC 2328. Thus,
LSAs are retransmitted only to adjacent routers. Therefore, since
OSPF-MDR does not allow an adjacency to be formed between two MDR
Other routers, an MDR Other never retransmits an LSA to another MDR
Other, only to its parents, which are (Backup) MDRs.
Retransmitted LSAs are included in LSU packets that are sent directly
to an adjacent neighbor that did not acknowledge the LSA (explicitly
or implicitly). The length of time between retransmissions is given
by the configurable interface parameter RxmtInterval, whose default
is 7 seconds for a MANET interface. To reduce overhead, several
retransmitted LSAs should be included in a single LSU packet whenever
possible.
8.4. Receiving Link State Acknowledgments
A Link State Acknowledgment (LS Ack) packet that is received from an
adjacent neighbor (in state Exchange or greater) is processed as
described in Section 13.7 of RFC 2328, with the additional steps
described in this section. An LS Ack packet that is received from a
neighbor in a lesser state than Exchange is discarded.
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.
If the neighbor from which the LS Ack packet was received is in state
Exchange or greater, then the following steps are performed for each
LS Ack in the received LS Ack packet:
(1) If the router does not have a database copy of the LSA being
acknowledged, or has a database copy which is less recent than
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the one being acknowledged, the LS Ack is added to the Acked LSA
List for the sending neighbor.
(2) If the router has a database copy of the LSA being acknowledged,
which is the same as the instance being acknowledged, then the
following action is performed. For each MANET interface for which
a BackupWait Neighbor List exists for the LSA (see Section 8.1),
remove the sending neighbor from the BackupWait Neighbor list if
it belongs to the list.
9. Originating LSAs
Unlike the DR of an OSPF broadcast network, an MDR does not originate
a network-LSA, since a network-LSA cannot be used to describe the
general topology of a MANET. Instead, each router advertises a
subset of its MANET neighbors as point-to-point links in its router-
LSA. The choice of which neighbors to advertise is flexible, and is
determined by the configurable parameter LSAFullness.
If adjacency reduction is used (AdjConnectivity is 1 or 2), then as a
minimum requirement each router must advertise a minimum set of
"backbone" neighbors in its router-LSA. This minimum choice
corresponds to LSAFullness = 0, and results in the minimum amount of
LSA flooding overhead, but does not provide routing along shortest
paths.
Therefore, to allow routers to calculate shortest paths, without
requiring every pair of neighboring routers along the shortest paths
to be adjacent (which would be inefficient due to requiring a large
number of adjacencies), a router-LSA may also advertise non-adjacent
neighbors that satisfy a synchronization condition described below.
To motivate this, we note that OSPF already allows a non-adjacent
neighbor to be a next hop, if both the router and the neighbor belong
to the same broadcast network (and are both adjacent to the DR). A
network-LSA for a broadcast network (which includes all routers
attached to the network) implies that any router attached to the
network can forward packets directly to any other router attached to
the network (which is why the distance from the network to all
attached routers is zero in the graph representing the link-state
database).
Since a network-LSA cannot be used to describe the general topology
of a MANET, the only way to advertise non-adjacent neighbors that can
be used as next hops, is to include them in the router-LSA. However,
to ensure that such neighbors are sufficiently synchronized, only
"routable" neighbors are allowed to be included in LSAs, and to be
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used as next hops in the SPF calculation.
9.1. Routable Neighbors
A bidirectional MANET neighbor becomes routable if its state is Full,
or if the SPF calculation has produced a route to the neighbor and
the neighbor satisfies the routable neighbor quality condition
(defined below). Since only routable neighbors are advertised in
router-LSAs, and since adjacencies are selected to form a connected
spanning subgraph, this definition implies that there exists, or
recently existed, a path of full adjacencies from the router to the
routable neighbor. 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.
This requirement does not guarantee perfect synchronization, but
simulations have shown that it performs well in mobile networks.
This requirement avoids, for example, forwarding packets to a new
neighbor that is poorly synchronized because it was not reachable
before it became a neighbor.
To avoid selecting poor quality neighbors as routable neighbors, a
neighbor that is selected as a routable neighbor must satisfy the
routable neighbor quality condition. By default, this condition is
that the neighbor's BNS must include the router itself (indicating
that the neighbor agrees the connection is bidirectional).
Optionally, a router may impose a stricter condition. For example, a
router may require that two Hellos have been received from the
neighbor that (explicitly or implicitly) indicate that the neighbor's
BNS includes the router itself.
The single-bit neighbor variable Routable indicates whether the
neighbor is routable, and is initially set to 0. If adjacency
reduction is used, Routable is updated as follows when the state of
the neighbor changes, or the SPF calculation finds a route to the
neighbor, or a Hello is received that affects the routable neighbor
quality condition.
(1) If Routable is 0 for the neighbor and the state of the neighbor
changes to Full, Routable is set to 1 for the neighbor.
(2) If Routable is 0 for the neighbor, the state of the neighbor is
2-Way or greater, there exists a route to the neighbor, and the
routable neighbor quality condition (defined above) is satisfied,
then Routable is set to 1 for the neighbor.
(3) If Routable is 1 for the neighbor and the state of the neighbor
is less than 2-Way, Routable is set to 0 for the neighbor.
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If adjacency reduction is not used (AdjConnectivity = 0), then a
neighbor is defined to be routable if and only if its state is
Full.
9.2. Partial and Full-Topology LSAs
The choice of which MANET neighbors to include in the router-LSA is
flexible. Whether or not adjacency reduction is used, the router can
originate either partial-topology or full-topology LSAs. This
flexibility is made possible by defining two types of neighbors that
are included in the router-LSA: backbone neighbors and selected
advertised neighbors.
9.2.1. Backbone Neighbors and Selected Advertised Neighbors
A backbone neighbor is defined to be a bidirectional neighbor that is
a Dependent Neighbor, Dependent Selector, (Backup) Parent, or child.
If adjacency reduction is not used (AdjConnectivity = 0), then the
set of backbone neighbors is empty (since there are no dependent
neighbors or parents).
If adjacency reduction is used, then a router MUST include in its
router-LSA all backbone neighbors that are routable. A minimum LSA,
corresponding to LSAFullness = 0, includes only these neighbors.
This choice guarantees connectivity, but does not provide shortest
paths. However, it may be useful in large networks to achieve
maximum scalability. If adjacency reduction is not used, then
LSAFullness MUST NOT be 0, since in this case the set of backbone
neighbors is empty.
To allow flexibility while ensuring that LSAs are symmetric (i.e.,
router i advertises a link to router j if and only if router j
advertises a link to router i), each router maintains a selected
advertised neighbor list (SANS), which consists of MANET neighbors
that the router has selected to advertise in its router-LSA, not
including backbone neighbors. Since the SANS does not include
Dependent Neighbors, the lists SANS and DNS are disjoint. (Note that
both lists are advertised in Hellos.)
If LSAFullness = 0, then the SANS is empty, since only backbone
neighbors are included in the router-LSA. At the other extreme, a
full-topology LSA, corresponding to LSAFullness = 4, includes all
routable neighbors. In this case, the SANS includes all
bidirectional MANET neighbors except backbone neighbors. Note that
backbone neighbors and neighbors in the SANS need not be routable,
but only routable neighbors may be included in the router-LSA. (This
is done so that the SANS, which is advertised in Hellos, does not
depend on routability.)
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9.2.2. Choice of LSAFullness
LSAFullness affects the contents of the router-LSA by determining the
neighbors to include in the SANS. The choices of SANS corresponding
to the extreme cases of LSAFullness equal to 0 and 4 were described
above.
If LSAFullness is 1 or 2, the router originates min-cost LSAs, which
are partial-topology LSAs that (when flooded) provide each router
with sufficient information to calculate at least one shortest
(minimum-cost) path to each destination. If LSAFullness is 2, then
additional MANET neighbors are also included in the router-LSA to
provide redundant routes.
Appendix C describes the algorithm for selecting the neighbors to
include in the SANS that results in min-cost LSAs. The input to this
algorithm includes information obtained from Hellos received from
each MANET neighbor, including the Bidirectional Neighbor List (BNS),
Dependent Neighbor Set (DNS), Selected Advertised Neighbor List
(SANS), and the Metric TLV. The Metric TLV, specified in Section
A.2.2.8, is included in each Hello and advertises the link cost to
each bidirectional neighbor. To minimize overhead, it allows the
option of advertising only a single metric for the interface (equal
to the link cost to each neighbor).
If LSAFullness is 3, each (Backup) MDR originates a full LSA (which
includes all routable neighbors), while each MDR Other originates a
minimum LSA (which includes only routable backbone neighbors). If a
router has multiple MANET interfaces, its LSA includes all routable
neighbors on the interfaces for which it is a (Backup) MDR, and
includes only routable backbone neighbors on its other interfaces.
When a router changes its MDR Level from MDR Other to (Backup) MDR on
a given interface, it must originate a new LSA. This choice provides
routing along nearly shortest paths with relatively low overhead.
It is not necessary for different routers to choose the same value of
LSAFullness; the different choices are interoperable because they all
require the router-LSA to include a minimum set of neighbors, and
because the construction of the router-LSA (described below) ensures
that the router-LSAs originated by different routers are consistent.
9.2.3. Construction of the Router-LSA
When a new router-LSA is originated, it includes a point-to-point
(type 1) link for each MANET neighbor j that is routable and
satisfies at least one of the following three conditions:
(1) The router's SANS (for any interface) includes j.
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(2) Neighbor j's SANS includes the router (to ensure symmetry).
(3) Neighbor j is a backbone neighbor.
Note that if adjacency reduction is not used (AdjConnectivity = 0),
then the set of backbone neighbors is empty, and a MANET neighbor is
routable if and only if it is in the Full state. Also note that
neighbors in the SANS need not be routable, but only routable
neighbors are included in the router-LSA.
A new router-LSA is originated if any of the events specified in
Section 12.4 of [RFC2328] occurs, except that event (4), i.e., one of
the neighboring routers changes to/from the FULL state, does not
apply to MANET neighbors. For MANET neighbors, event (4) is replaced
with the following two events:
o There exists a routable MANET neighbor j that satisfies one of the
above three conditions, but is not included in the current router-
LSA.
o The current router-LSA includes a MANET neighbor that is no longer
routable.
If AdjConnectivity = 0 and LSAFullness = 4 (full LSAs), then since
only Full neighbors are routable and the SANS includes all
bidirectional neighbors in this case, the above two events are
equivalent to one of the neighboring routers changing to/from the
Full state.
10. Calculating the Routing Table
The routing table calculation is the same as specified in RFC 2328,
except for the following change to Section 16.1 (Calculating the
shortest-path tree for an area).
Recall from Section 9 that a router can use any routable neighbor as
a next hop to a destination. However, unless LSAFullness = 4 (full-
topology LSAs), the router-LSA originated by the router usually does
not include all routable neighbors. Therefore, the shortest-path
tree calculation described in Section 16.1 of RFC 2328 must be
modified to allow any routable neighbor on a MANET interface to be
used as a next hop. This is accomplished by modifying step 2 so that
the router-LSA associated with the root vertex (i.e., the router
doing the calculation) is augmented to include all routable neighbors
on each MANET interface. In addition, step 2b (checking for a link
from W back to V) must be skipped when V is the root vertex and W is
a routable MANET neighbor whose state is less than Full. However,
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step 2b MUST be executed when W is in state Full and when V is not
the root vertex, to ensure that Full neighbors are synchronized in
both directions, and to ensure compatibility with OSPFv3.
Note that, if LSAFullness is less than 4, then the set of routable
neighbors can change without causing the contents of the router-LSA
to change. This could happen, for example, if a routable neighbor
that was not included in the router-LSA transitions to the Down or
Init state. Therefore, if the set of routable neighbors changes, the
shortest-path tree must be recalculated even if the router-LSA does
not change.
After the shortest-path tree and routable table are calculated, the
set of routable neighbors must be updated, since a route to a non-
routable neighbor may have been discovered. If the set of routable
neighbors changes, then the shortest-path tree and routing table must
be calculated a second time. The second calculation will not change
the set of routable neighbors again, so two calculations are
sufficient.
11. Security Considerations
This document proposes an extension of OSPFv3 and can use the same
IPv6 security mechanisms as OSPFv3. Hence, this document does not
raise any new security concerns.
12. IANA Considerations
This document defines three new LLS TLV types (see Section A.2) to be
allocated by IANA.
13. Acknowledgments
Thanks to Aniket Desai for helpful discussions and comments,
including the suggestion that Router Priority should come before MDR
Level in the lexicographical comparison of (RtrPri, MDR Level, RID)
when selecting MDRs and BMDRs, and that the MDR calculation should be
repeated if it causes the MDR Level to change. Thanks also to Tom
Henderson for helpful discussions and comments.
14. Normative References
[RFC2328] J. Moy. "OSPF Version 2", RFC 2328, April 1998.
[RFC2740] R. Coltun, D. Ferguson, and J. Moy. "OSPF for IPv6", RFC
2740, December 1999.
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[LLS] A. Zinin, A. Roy, L. Nguyen, B. Friedman, and D. Young, "OSPF
Link-local Signaling", draft-ietf-ospf-lls-03.txt (work in
progress), August 2007.
[RFC2119] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
15. Informative References
[Lawler] E. Lawler. "Combinatorial Optimization: Networks and
Matroids", Holt, Rinehart, and Winston, New York, 1976.
[Suurballe] J.W. Suurballe and R.E. Tarjan. "A Quick Method for
Finding Shortest Pairs of Disjoint Paths", Networks, Vol. 14,
pp. 325-336, 1984.
A. Packet Formats
A.1. Options Field
A new bit, called L (for LLS) is introduced to OSPFv3 Options field
(see Figure A.1). The mask for the bit is 0x200. Routers set the L
bit in Hello and DD packets to indicate that the packet contains LLS
data block. Routers set the L bit in a self-originated router-LSA to
indicate that the LSA is non-ackable.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
| | | | | | | | | | | | | | |L|AF|*|*|DC| R| N|MC| E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
Figure A.1: The Options field
A.2. Link-Local Signaling
Link-local signaling (LLS) [LLS] describes an extension to OSPFv2 and
OSPFv3 which allows the exchange of arbitrary data using existing,
standard OSPF packet types. Here we use LLS for OSPFv3, which is
accomplished by adding an LLS data block at the end of the OSPFv3
packet.
The IPv6 header length includes the total length of the OSPFv3
header, OSPFv3 data, and LLS data, but the OSPFv3 header does not
contain the LLS data length in its length field. The IPv6 packet
format is depicted in Figure A.2 below.
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+---------------------+ --
| IPv6 Header | ^
| Length = HL+X+Y | | Header Length = HL
| | v
+---------------------+ --
| OSPFv3 Header | ^
| Length = X | |
|.....................| | X
| | |
| OSPFv3 Data | |
| | v
+---------------------+ --
| | ^
| LLS Data | | Y
| | v
+---------------------+ --
Figure A.2: Attaching LLS Data Block
The LLS data block may be attached to OSPFv3 Hello and Database
Description (DD) packets. The data included in the LLS block
attached to a Hello packet may be used for dynamic signaling, since
Hello packets may be sent at any moment in time. However, delivery of
LLS data in Hello packets is not guaranteed. The data sent with DD
packets is guaranteed to be delivered as part of the adjacency
forming process.
A.2.1 LLS Data Block
The data block used for link-local signaling is formatted as
described below (see Figure A.3).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | LLS Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| LLS TLVs |
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure A.3: Format of LLS Data Block
The Checksum field contains the standard IP checksum of the entire
contents of the LLS block.
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The 16-bit LLS Data Length field contains the length (in 32-bit
words) of the LLS block including the header and payload.
Implementations should not use the Length field in the IPv6 packet
header to determine the length of the LLS data block.
The rest of the block contains a set of Type/Length/Value (TLV)
triplets as described in the following section. All TLVs must be
32-bit aligned (with padding if necessary).
A.2.2 LLS TLV Format
The contents of LLS data block is constructed using TLVs. See Figure
A.4 for the TLV format.
The type field contains the TLV ID which is unique for each type of
TLVs. The Length field contains the length of the Value field (in
bytes) that is variable and contains arbitrary data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Value .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure A.4: Format of LLS TLVs
Note that TLVs are always padded to 32-bit boundary, but padding
bytes are not included in TLV Length field (though it is included in
the LLS Data Length field of the LLS block header). All unknown TLVs
MUST be silently ignored.
A.2.3 Hello TLV
The Hello TLV is appended to each MANET Hello. It includes the
current Hello sequence number (HSN) for the transmitting interface
and the number of neighbors of each type that are listed in the body
of the Hello (see Section 4.1). It also indicates whether the Hello
is full or differential (via the D-bit), and whether AdjConnectivity
is 0 (via the A-bit).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hello Sequence Number | Reserved |A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| N1 | N2 | N3 | N4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Set to 14.
o Length: Set to 8.
o Hello Sequence Number: A circular two octet unsigned integer
indicating the current HSN for the transmitting interface. The
HSN for the interface MUST be incremented by 1 every time a
(differential or full) Hello is sent on the interface.
o Reserved: Set to 0. Reserved for future use.
o A (1 bit): Set to 1 if AdjConnectivity is 0, otherwise set to 0.
o D (1 bit): Set to 1 for a differential Hello and 0 for a full
Hello.
o N1 (8 bits): The number of neighbors listed in the Hello that
are in state Down. N1 is zero if the the Hello is not
differential.
o N2 (8 bits): The number of neighbors listed in the Hello that
are in state Init.
o N3 (8 bits): The number of neighbors listed in the Hello that
are Dependent.
o N4 (8 bits): The number of neighbors listed in the Hello that
are Selected Advertised Neighbors.
A.2.4 MDR TLV
A new TLV is defined which includes the same two Router IDs that are
included in the DR and Backup DR fields of a Hello sent by the
router. This TLV is used in conjunction with a Database Description
packet, and is used to indicate the router's MDR Level and selected
parent(s).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| DR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup DR |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
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o Type: Set to 15.
o Length: Set to 8.
o DR: The same Router ID that is included in the DR field of a
Hello sent by the router (see Section A.3).
o Backup DR: The same Router ID that is included in the Backup DR
field of a Hello sent by the router (see Section A.3).
A.2.5 Metric TLV
If LSAFullness is 1 or 2, the Metric TLV is appended to each MANET
Hello packet. It provides the link metric for each bidirectional
neighbor listed in the body of the Hello. At a minimum, this TLV
advertises a single default metric. If the I bit is set, the Router
ID and link metric are included for each bidirectional neighbor
listed in the body of the Hello whose link metric is not equal to the
default metric. This option reduces overhead when all neighbors have
the same link metric, or only a few neighbors have a link metric that
differs from the default metric. If the I bit is zero, the link
metric is included for each bidirectional neighbor that is listed in
the body of the Hello and the neighbor RIDs are omitted from the TLV.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Default Metric | Reserved |I|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor ID (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric (1) | Metric (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Set to 16.
o Length: Set to 4 + 6*N if the I bit is 1, and to 4 + 2*N if the I
bit is 0, where N is the number of neighbors included in the TLV.
o Default Metric: If the I bit is 1, this is the link metric that
applies to every bidirectional neighbor listed in the body of
the Hello whose RID is not listed in the Metric TLV.
o Neighbor ID: If the I bit is 1, the RID is listed for each
bidirectional neighbor (Lists 3 through 5 as defined in
Section 4.1) in the body of the Hello whose link metric is not
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equal to the default metric. Omitted if the I bit is 0.
o Metric: Link metric for each bidirectional neighbor, listed in
the same order as the Neighbor IDs in the TLV if the I bit is 1,
and in the same order as the Neighbor IDs of bidirectional
neighbors (Lists 3 through 5 as defined in Section 4.1)
in the body of the Hello if the I bit is 0.
A.3. Hello Packet DR and Backup DR Fields
The Designated Router (DR) and Backup DR fields of a Hello packet are
set as follows:
o DR: This field is the router's MDR Parent, or is 0.0.0.0 if the
MDR Parent is null. The MDR Parent of an MDR is always the
router's own RID.
o Backup DR: This field is the router's Backup MDR Parent, or is
0.0.0.0 if the Backup MDR Parent is null. The Backup MDR Parent
of a BMDR is always the router's own RID.
A.4. LSA Formats and Examples
LSA formats are specified in [RFC2740] Section 3.4.3. Figure A.5
below gives an example network map for a MANET in a single area.
o Four MANET nodes RT1, RT2, RT3, and RT4 are in area 1.
o RT1's MANET interface has links to RT2 and RT3's MANET interfaces.
o RT2's MANET interface has links to RT1 and RT3's MANET interfaces.
o RT3's MANET interface has links to RT1, RT2, and RT3's MANET
interfaces.
o RT4's MANET interface has a link to RT3's MANET interface.
o RT1 and RT2 have stub networks attached on broadcast interfaces.
o RT3 has a transit network attached on a broadcast interface.
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..........................................
. Area 1.
. + .
. | .
. | 2+---+1 1+---+
. N1 |--|RT1|-----+ +---|RT4|----
. | +---+ | / +---+
. | | / .
. + | N3 / .
. | / .
. + | / .
. | | / .
. | 2+---+1 | / .
. N2 |--|RT2|-----+-------+ .
. | +---+ |1 .
. | +---+ .
. | |RT3|----------------
. + +---+ .
. |2 .
. +------------+ .
. |1 N4 .
. +---+ .
. |RT5| .
. +---+ .
..........................................
Figure A.5: Area 1 with IP addresses shown
Network IPv6 prefix
-----------------------------------
N1 5f00:0000:c001:0200::/56
N2 5f00:0000:c001:0300::/56
N4 5f00:0000:c001:0400::/56
Table 1: IPv6 link prefixes for sample network
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Router interface Interface ID IPv6 global unicast prefix
-----------------------------------------------------------
RT1 LOOPBACK 0 5f00:0001::/64
to N3 1 n/a
to N1 2 5f00:0000:c001:0200::RT1/56
RT2 LOOPBACK 0 5f00:0002::/64
to N3 1 n/a
to N2 2 5f00:0000:c001:0300::RT2/56
RT3 LOOPBACK 0 5f00:0003::/64
to N3 1 n/a
to N4 2 5f00:0000:c001:0400::RT3/56
RT4 LOOPBACK 0 5f00:0004::/64
to N3 1 n/a
RT5 to N4 1 5f00:0000:c001:0400::RT5/56
Table 2: IPv6 link prefixes for sample network
Router interface Interface ID link-local address
-------------------------------------------------------
RT1 LOOPBACK 0 n/a
to N1 1 fe80:0001::RT1
to N3 2 fe80:0002::RT1
RT2 LOOPBACK 0 n/a
to N2 1 fe80:0001::RT2
to N3 2 fe80:0002::RT2
RT3 LOOPBACK 0 n/a
to N3 1 fe80:0001::RT3
to N4 2 fe80:0002::RT3
RT4 LOOPBACK 0 n/a
to N3 1 fe80:0001::RT4
RT5 to N4 1 fe80:0002::RT5
Table 3: OSPF Interface IDs and link-local addresses
A.4.1 Router-LSAs
As an example, consider the router-LSA that node RT3 would originate.
The node consists of one MANET, one broadcast, and one loopback
interface.
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RT3's router-LSA
LS age = DoNotAge+0 ;newly originated
LS type = 0x2001 ;router-LSA
Link State ID = 0 ;first fragment
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
Options = (V6-bit|E-bit|R-bit)
Type = 1 ;p2p link to RT1
Metric = 11 ;cost to RT1
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.1 ;RT1's Router ID
Type = 1 ;p2p link to RT2
Metric = 12 ;cost to RT2
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.2 ;RT2's Router ID
Type = 1 ;p2p link to RT4
Metric = 13 ;cost to RT4
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.4 ;RT4's Router ID
Type = 2 ;connects to N4
Metric = 1 ;cost to N4
Interface ID = 2 ;RT3's Interface ID
Neighbor Interface ID = 1 ;RT5's Interface ID (elected DR)
Neighbor Router ID = 192.1.1.5 ;RT5's Router ID (elected DR)
A.4.2 Link-LSAs
Consider the link-LSA that RT3 would originate for its MANET
interface.
RT3's Link-LSA for its MANET interface
LS age = DoNotAge+0 ;newly originated
LS type = 0x0008 ;Link-LSA
Link State ID = 1 ;Interface ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
RtrPri = 1 ;default priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0001::RT3
# prefixes = 0 ;no global unicast address
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A.4.3 Intra-Area-Prefix-LSAs
A MANET node originates an intra-area-prefix-LSA to advertise its own
prefixes, and those of its attached networks or stub links. As an
example, consider the intra-area-prefix-LSA that RT3 will build.
RT2's intra-area-prefix-LSA for its own prefixes
LS age = DoNotAge+0 ;newly originated
LS type = 0x2009 ;intra-area-prefix-LSA
Link State ID = 177 ;or something
Advertising Router = 192.1.1.3 ;RT3's Router ID
# prefixes = 2
Referenced LS type = 0x2001 ;router-LSA reference
Referenced Link State ID = 0 ;always 0 for router-LSA reference
Referenced Advertising Router = 192.1.1.3 ;RT2's Router ID
PrefixLength = 64 ;prefix on RT3's LOOPBACK
PrefixOptions = 0
Metric = 0 ;cost of RT3's LOOPBACK
Address Prefix = 5f00:0003::/64
PrefixLength = 56 ;prefix on RT3's interface 2
PrefixOptions = 0
Metric = 1 ;cost of RT3's interface 2
Address Prefix = 5f00:0000:c001:0400::RT3/56 ;pad
B. Detailed Algorithms for MDR/BMDR Selection
This section provides detailed algorithms for Step 2.4 of Phase 2
(MDR Selection) and Step 3.2 of Phase 3 (BMDR Selection) of the MDR
selection algorithm described in Section 5. Step 2.4 uses a breadth-
first search (BFS) algorithm, and Step 3.2 uses an efficient
algorithm for finding pairs of node-disjoint paths from Rmax to all
other neighbors. Both algorithms run in O(d^2) time, where d is the
number of neighbors.
For convenience, in the following description, the term "bi-neighbor"
will be used as an abbreviation for "bidirectional neighbor". Also,
node i denotes the router performing the calculation.
B.1. Detailed Algorithm for Step 2.4 (MDR Selection)
The following algorithm performs Step 2.4 of the MDR selection
algorithm, and assumes that Phase 1 and Steps 2.1 through 2.3 have
been performed, so that the neighbor connectivity matrix NCM has been
computed, and Rmax is the bi-neighbor with the (lexicographically)
largest value of (RtrPri, MDR Level, RID). The BFS algorithm uses a
FIFO queue so that all nodes 1 hop from node Rmax are processed
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first, then 2 hops, etc. When the BFS algorithm terminates, hops(u),
for each bi-neighbor node u of node i, will be equal to the minimum
number of hops from node Rmax to node u, using only intermediate
nodes that are bi-neighbors of node i and that have a larger value of
(RtrPri, MDR Level, RID) than node i. The algorithm also computes,
for each node u, the tree parent p(u) and the second node r(u) on the
tree path from Rmax to u, which will be used in Step 3.2
(a) Compute a matrix of link costs c(u,v) for each pair of
bi-neighbors u and v as follows: If node u has a larger value
of (RtrPri, MDR Level, RID) than node i, and NCM(u,v) = 1,
then set c(u,v) to 1. Otherwise, set c(u,v) to infinity.
(Note that the matrix NCM(u,v) is symmetric, but the matrix
c(u,v) is not.)
(b) Set hops(u) = infinity for all bi-neighbors u other than Rmax,
and set hops(Rmax) = 0. Initially, p(u) is undefined for each
neighbor u. For each bi-neighbor u such that c(Rmax,u) = 1,
set r(u) = u; for all other u, r(u) is initially undefined.
Add node Rmax to the FIFO queue.
(c) While the FIFO queue is nonempty:
Remove the node at the head of the queue; call it node u.
For each bi-neighbor v of node i such that c(u,v) = 1:
If hops(v) > hops(u) + 1, then set hops(v) = hops(u) + 1,
set p(v) = u, set r(v) = r(u) if hops(v) > 1, and add
node v to the tail of the queue.
B.2. Detailed Algorithm for Step 3.2 (BMDR Selection)
Step 3.2 of the MDR selection algorithm requires the router to
determine whether there exist two node-disjoint paths from Rmax to
each other bi-neighbor u, via bi-neighbors that have a larger value
of (RtrPri, MDR Level, RID) than the router itself. This information
is needed to determine whether the router should select itself as a
BMDR.
It is possible to determine separately for each bi-neighbor u whether
there exist two node-disjoint paths from Rmax to u, using the well-
known augmenting path algorithm [Lawler] which runs in O(n^2) time,
but this must be done for all bi-neighbors u, thus requiring a total
run time of O(n^3). The algorithm described below makes the same
determination simultaneously for all bi-neighbors u, achieving a much
faster total run time of O(n^2). The algorithm is a simplified
variation of the Suurballe-Tarjan algorithm [Suurballe] for finding
pairs of disjoint paths.
The algorithm described below uses the following output of Phase 2:
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the tree parent p(u) of each node (which defines the BFS tree
computed in Phase 2), and the second node r(u) on the tree path from
Rmax to u.
The algorithm uses the following concepts. For any node u on the BFS
tree other than Rmax, we define g(u) to be the first labeled node on
the reverse tree path from u to Rmax, if such a labeled node exists
other than Rmax. (The reverse tree path consists of u, p(u),
p(p(u)), ..., Rmax.) If no such labeled node exists, then g(u) is
defined to be r(u). In particular, if u is labeled then g(u) = u.
Note that g(u) either must be labeled or must be a neighbor of Rmax.
For any node k that either is labeled or is a neighbor of Rmax, we
define the unlabeled subtree rooted at k, denoted S(k), to be the set
of nodes u such that g(u) = k. Thus, S(k) includes node k itself and
the set of unlabeled nodes downstream of k on the BFS tree that can
be reached without going through any labeled nodes. This set can be
obtained in linear time using a depth-first search starting at node
k, and using labeled nodes to indicate the boundaries of the search.
Note that g(u) and S(k) are not maintained as variables in the
algorithm given below, but simply refer to the definitions given
above.
The BMDR algorithm maintains a set B, which is initially empty. A
node u is added to B when it is known that two node-disjoint paths
exist from Rmax to u via nodes that have a larger value of (RtrPri,
MDR Level, RID) than the router itself. When the algorithm
terminates, B consists of all nodes that have this property.
The algorithm consists of the following two steps.
(a) Mark Rmax as labeled. For each pair of nodes u, v on the BFS
tree other than Rmax such that r(u) is not equal to r(v) (i.e.,
u and v have different second nodes), NCM(u,v) = 1, and node u
has a greater value of (RtrPri, MDR level, RID) than the router
itself, add v to B. (Clearly there are two disjoint paths from
Rmax to v.)
(b) While there exists a node in B that is not labeled, do the
following. Choose any node k in B that is not labeled, and let
j = g(k). Now mark k as labeled. (This creates a new unlabeled
subtree S(k), and makes S(j) smaller by removing S(k) from it.)
For each pair of nodes u, v such that u is in S(k), v is in
S(j), and NCM(u,v) = 1:
o If u has a larger value of (RtrPri, MDR level, RID) than the
router itself, and v is not in B, then add v to B.
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o If v has a larger value of (RtrPri, MDR level, RID) than the
router itself, and u is not in B, then add u to B.
A simplified version of the algorithm MAY be performed by omitting
step (b). However, the simplified algorithm will result in more
BMDRs, and is not recommended if AdjConnectivity = 2 since it will
result in more adjacencies.
The above algorithm can be executed in O(n^2) time, where n is the
number of neighbors. Step (a) clearly requires O(n^2) time since it
considers all pairs of nodes u and v. Step (b) also requires O(n^2)
time because each pair of nodes is considered at most once. This is
because labeling nodes divides unlabeled subtrees into smaller
unlabeled subtrees, and a given pair u, v is considered only the
first time u and v belong to different unlabeled subtrees.
C. Min-Cost LSA Algorithm
This section describes the algorithm for determining which MANET
neighbors to include in the router-LSA when LSAFullness is 1 or 2.
The min-cost LSA algorithm ensures that the link-state database
provides sufficient information to calculate at least one shortest
(minimum-cost) path to each destination. If LSAFullness is 2, then
additional MANET neighbors are also included in the router-LSA to
provide redundant routes. The algorithm assumes that a router may
have multiple interfaces, at least one of which is a MANET interface.
The algorithm becomes significantly simpler if the router has only a
single (MANET) interface.
The input to this algorithm includes information obtained from Hellos
received from each neighbor on each MANET interface, including the
neighbor's Bidirectional Neighbor Set (BNS), Dependent Neighbor Set
(DNS), Selected Advertised Neighbor Set (SANS), and link metrics.
The input also includes the link-state database if the router has a
non-MANET interface.
The output of the algorithm is the router's SANS for each MANET
interface. The SANS is used to determine the contents of the router-
LSA as described in Section 9.2.3. The min-cost LSA algorithm must
be run to update the SANS (and possibly originate a new router-LSA)
whenever any of the following events occurs:
o The state or routability of a neighbor changes.
o A Hello received from a neighbor indicates a change in its
MDR Level, Router Priority, FullHelloRcvd, BNS, DNS, SANS,
MDR Parent(s), or link metrics.
o An LSA originated by a non-MANET neighbor is received.
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Although the algorithm described below runs in O(d^3) time, where d
is the number of neighbors, an incremental version for a single
topology change runs in O(d^2) time, as discussed following the
algorithm description.
For convenience, in the following description, the term "bi-neighbor"
will be used as an abbreviation for "bidirectional neighbor". Also,
router i will denote the router doing the calculation. To perform
the min-cost LSA algorithm, the following steps are performed.
(1) Create the neighbor connectivity matrix (NCM) for each MANET
interface, as described in Section 5.1. Create the multiple-
interface neighbor connectivity matrix MNCM as follows. For each
bi-neighbor j, set MNCM(i,j) = MNCM(j,i) = 1. For each pair j, k
of MANET bi-neighbors, set MNCM(j,k) = 1 if NCM(j,k) equals 1 for
any MANET interface. For each pair j, k of non-MANET bi-
neighbors, set MNCM(j,k) = 1 if the link-state database indicates
that a direct link exists between j and k. Otherwise, set
MNCM(j,k) = 0. (Note that a given router can be a neighbor on
both a MANET interface and a non-MANET interface.)
(2) Create the inter-neighbor cost matrix (COST) as follows. For
each pair j, k of routers such that each of j and k is a bi-
neighbor or router i itself:
(a) If MNCM(j,k) = 1, set COST(j,k) to the metric of the link
from j to k obtained from j's Hellos (for a MANET interface),
or from the link-state database (for a non-MANET interface).
If there are multiple links from j to k (via multiple
interfaces), COST(j,k) is set to the minimum cost of these
links.
(b) Otherwise, set COST(j,k) to LSInfinity.
(3) Create the backbone neighbor matrix (BNM) as follows. BNM
indicates which pairs of MANET bi-neighbors are backbone
neighbors of each other, as defined in Section 9.2.1. If
adjacency reduction is not used (AdjConnectivity = 0), set all
entries of BNM to zero and proceed to step 4.
In the following, if a link exists from router j to router k on
more than one interface, we consider only interfaces for which
the cost from j to k equals COST(j,k); such interfaces will be
called "candidate" interfaces.
For each pair j, k of MANET bi-neighbors, BNM(j,k) is set to 1 if
j and k are backbone neighbors of each other on a candidate MANET
interface. That is, BNM(j,k) is set to 1 if, for any candidate
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MANET interface, NCM(j,k) = 1 and either of the following
conditions is satisfied:
(a) Router k is included in j's DNS or router j is included in
k's DNS.
(b) Router j is the (Backup) Parent of router k or router k is
the (Backup) Parent of router j.
Otherwise, BNM(j,k) is set to 0.
(4) Create the selected advertised neighbor matrix (SANM) as follows.
For each pair j, k of routers such that each of j and k is a bi-
neighbor or router i itself, SANM(j,k) is set to 1 if, for any
candidate MANET interface, NCM(j,k) = 1 and k is included in j's
SANS. Otherwise, SANM(j,k) is set to 0. Note that SANM(i,k) is
set to 1 if k is currently a selected advertised neighbor.
(5) Compute the new set of selected advertised neighbors as follows.
For each MANET bi-neighbor j, initialize the bit variable
new_sel_adv(j) to 0. (This bit will be set to 1 if j is
selected.) For each MANET bi-neighbor j:
(a) If j is a bi-neighbor on more than one interface, consider
only candidate interfaces (for which the cost to j is
minimum). If one of the candidate interfaces is a non-MANET
interface, examine the next neighbor (j is not selected since
it will be advertised anyway).
(b) If adjacency reduction is used, and one of the candidate
interfaces is a MANET interface on which j is a backbone
neighbor (see Section 9.2), examine the next neighbor (j is
not selected since it will be advertised anyway).
(c) Otherwise, if there is more than one candidate MANET
interface, select the "preferred" interface by using the
following preference rules in the given order: an interface
is preferred if (1) router i's SANS for that interface
already includes j, (2) router i's Router Priority is larger
on that interface, and (3) router i's MDR level is larger on
that interface.
(d) This step is optional, but SHOULD be performed in order to
remove redundant advertised neighbors. If SANM(i,j) = 0,
i.e., j is not currently a selected advertised neighbor, then
proceed to step (e).
Otherwise, for each bi-neighbor k (on any interface) such
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that COST(k,j) > COST(k,i) + COST(i,j), determine whether
there exists another bi-neighbor u such that either COST(k,u)
+ COST(u,j) < COST(k,i) + COST(i,j), or COST(k,u) + COST(u,j)
= COST(k,i) + COST(i,j) and any of the following three
conditions is true:
o BNM(u,j) = 1,
o SANM(j,u) > SANM(j,i), or
o SANM(j,u) = SANM(j,i), SANM(u,j) = 1, and
(RtrPri(u), MDR_Level(u), RID(u)) is lexicographically
less than (RtrPri(i), MDR_Level(i), RID(i)).
If for each such bi-neighbor k, there exists such a bi-
neighbor u, then do not select j, skip step (e), and continue
to the next neighbor j.
(e) For each bi-neighbor k (on any interface) such that COST(k,j)
> COST(k,i) + COST(i,j), determine whether there exists
another bi-neighbor u such that either COST(k,u) + COST(u,j)
< COST(k,i) + COST(i,j), or COST(k,u) + COST(u,j) = COST(k,i)
+ COST(i,j) and either of the following conditions is true:
o BNM(u,j) = 1, or
o (SANM(j,u), SANM(u,j), RtrPri(u), MDR_Level(u), RID(u))
is lexicographically greater than
(SANM(j,i), SANM(i,j), RtrPri(i), MDR_Level(i), RID(i)).
If for some such bi-neighbor k, there does not exist such a
bi-neighbor u, then set new_sel_adv(j) = 1.
(6) For each MANET interface I, update the SANS to equal the set of
all bi-neighbors j such that new_sel_adv(j) = 1 and I is the
preferred interface for j. If LSAFullness = 2, the SANS SHOULD
also include other bidirectional neighbors to provide redundant
routes.
(7) With the SANS updated, a new router-LSA may need to be
originated as described in Section 9.2.3.
The lexicographical comparison of Step 5e gives preference to links
that are already advertised, in order to improve LSA stability.
The above algorithm can be run in O(d^2) time if a single link change
occurs. For example, if link (x,y) fails where x and y are neighbors
of router i, and either SANS(x,y) = 1 or BNM(x,y) = 1, then Step 5
need only be performed for pairs j, k such that either j or k is
equal to x or y.
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D. Non-Ackable LSAs for Periodic Flooding
In a highly mobile network, it is possible that a router almost
always originates a new router-LSA every MinLSInterval seconds. In
this case, it should not be necessary to send Acks for such an LSA,
or to retransmit such an LSA as a unicast, or to describe such an LSA
in a DD packet. In this case, the originator of an LSA MAY indicate
that the router-LSA is "non-ackable" by setting the L bit in the
options field of the LSA. For example, a router can originate non-
ackable LSAs if it determines (e.g., based on an exponential moving
average) that a new LSA is originated every MinLSInterval seconds at
least 90 percent of the time. (Simulations are needed to determine
the best threshold.)
A non-ackable LSA is never acknowledged, nor is it ever retransmitted
as a unicast or described in a DD packet, thus saving substantial
overhead. However, the originating router must periodically
retransmit the current instance of its router-LSA as a multicast
(until it originates a new LSA, which will usually happen before the
previous instance is retransmitted), and each MDR must periodically
retransmit each non-ackable LSA as a multicast (until it receives a
new instance of the LSA, which will usually happen before the
previous instance is retransmitted). The retransmission interval
should be slightly larger than MinLSInterval (e.g., MinLSInterval +
1) so that a new instance of the LSA is usually received before the
previous one is retransmitted. Note that the reception of a
retransmitted (duplicate) LSA does not result in immediate forwarding
of the LSA; only a new LSA (with a larger sequence number) may be
forwarded immediately, according to the flooding procedure of Section
8.
Authors' Addresses
Richard G. Ogier
SRI International
Email: rich.ogier@earthlink.net, richard.ogier@sri.com
Phil Spagnolo
Boeing Phantom Works
Email: phillip.a.spagnolo@boeing.com
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