Network Working Group R. Ogier
Request for Comments: 5614 SRI International
Category: Experimental P. Spagnolo
Boeing
August 2009
Mobile Ad Hoc Network (MANET) Extension of OSPF
Using Connected Dominating Set (CDS) Flooding
Abstract
This document specifies an extension of OSPFv3 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
is used in which only (Backup) MDRs flood new link state
advertisements (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.
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Ogier & Spagnolo Experimental [Page 1]RFC 5614 MANET Extension of OSPF August 2009
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Table of Contents
1. Introduction ....................................................4
1.1. Terminology ................................................5
2. Overview ........................................................7
2.1. Selection of MDRs, BMDRs, Parents, and Adjacencies .........8
2.2. Flooding Procedure .........................................9
2.3. Link State Acknowledgments ................................10
2.4. Routable Neighbors ........................................10
2.5. Partial and Full Topology LSAs ............................11
2.6. Hello Protocol ............................................12
3. Interface and Neighbor Data Structures .........................12
3.1. Changes to Interface Data Structure .......................12
3.2. New Configurable Interface Parameters .....................13
3.3. Changes to Neighbor Data Structure ........................15
4. Hello Protocol .................................................17
4.1. Sending Hello Packets .....................................17
4.2. Receiving Hello Packets ...................................20
4.3. Neighbor Acceptance Condition .............................24
5. MDR Selection Algorithm ........................................25
5.1. Phase 1: Creating the Neighbor Connectivity Matrix ........27
5.2. Phase 2: MDR Selection ....................................27
5.3. Phase 3: Backup MDR Selection .............................29
5.4. Phase 4: Parent Selection .................................29
5.5. Phase 5: Optional Selection of Non-Flooding MDRs ..........30
Ogier & Spagnolo Experimental [Page 2]RFC 5614 MANET Extension of OSPF August 2009
6. Interface State Machine ........................................31
6.1. Interface States ..........................................31
6.2. Events that Cause Interface State Changes .................31
6.3. Changes to Interface State Machine ........................32
7. Adjacency Maintenance ..........................................32
7.1. Changes to Neighbor State Machine .........................33
7.2. Whether to Become Adjacent ................................34
7.3. Whether to Eliminate an Adjacency .........................35
7.4. Sending Database Description Packets ......................35
7.5. Receiving Database Description Packets ....................36
8. Flooding Procedure .............................................37
8.1. LSA Forwarding Procedure ..................................38
8.2. Sending Link State Acknowledgments ........................41
8.3. Retransmitting LSAs .......................................42
8.4. Receiving Link State Acknowledgments ......................42
9. Router-LSAs ....................................................43
9.1. Routable Neighbors ........................................44
9.2. Backbone Neighbors ........................................45
9.3. Selected Advertised Neighbors .............................45
9.4. Originating Router-LSAs ...................................46
10. Calculating the Routing Table .................................47
11. Security Considerations .......................................49
12. IANA Considerations ...........................................50
13. Acknowledgments ...............................................51
14. Normative References ..........................................51
15. Informative References ........................................51
Appendix A. Packet Formats .......................................52
A.1. Options Field ............................................52
A.2. Link-Local Signaling .....................................52
A.3. Hello Packet DR and Backup DR Fields .....................57
A.4. LSA Formats and Examples .................................57
Appendix B. Detailed Algorithms for MDR/BMDR Selection ...........62
B.1. Detailed Algorithm for Step 2.4 (MDR Selection) ..........62
B.2. Detailed Algorithm for Step 3.2 (BMDR Selection) .........63
Appendix C. Min-Cost LSA Algorithm ...............................65
Appendix D. Non-Ackable LSAs for Periodic Flooding ...............68
Appendix E. Simulation Results ...................................69
Ogier & Spagnolo Experimental [Page 3]RFC 5614 MANET Extension of OSPF August 2009
1. Introduction
This document specifies an extension of OSPFv3 [RFC5340] 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. Note that OSPFv3 is specified by describing the
modifications to OSPFv2 [RFC2328]. This MANET extension of OSPFv3 is
also applicable to non-mobile mesh networks using layer-3 routing.
This extension does not preclude the use of any existing OSPF
interface types, and is fully compatible with legacy OSPFv3
implementations.
Existing OSPF interface types do not perform adequately in MANETs,
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.
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 (if the network itself is biconnected). 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; see
[RFC5613]), 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
originating router-LSAs that provide full or partial topology
information.
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RFC 5614 MANET Extension of OSPF August 2009
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 the contents of router-LSAs, and Section
10 describes changes in the calculation of the routing table.
The appendices specify 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, a proposed option that uses non-ackable LSAs to provide
periodic flooding without the overhead of Link State Acknowledgments,
and simulation results that predict the performance of OSPF-MDR in
mobile networks with up to 200 nodes. Additional information and
resources for OSPF-MDR can be found at http://www.manet-routing.org.
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 MANET Interface is 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.
MANET Router
A MANET Router is an OSPF router that has at least one MANET
interface.
Differential Hello
A Differential Hello is a Hello packet that reduces the overhead
of sending full Hellos, by including only the Router IDs of
neighbors whose state changed recently.
Ogier & Spagnolo Experimental [Page 5]RFC 5614 MANET Extension of OSPF August 2009
2-Hop Neighbor Information
This information 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)
A MANET Designated Router is 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 broadcast networks.
Each router runs the MDR selection algorithm for each MANET
interface, to decide whether the router is an MDR, Backup MDR, or
neither for that interface.
Backup MANET Designated Router (Backup MDR or BMDR)
A Backup MANET Designated Router is one of a set of routers
responsible for providing backup flooding when neighboring MDRs
fail. The set of MDRs and Backup MDRs forms a biconnected
dominating set. The Backup MDR is a generalization of the Backup
DR found in broadcast networks.
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.
Parent
Each router selects a Parent for each MANET interface. The Parent
of a non-MDR router will be a neighboring MDR if one exists. The
Parent of an MDR is always the router itself. Each non-MDR router
becomes adjacent with its Parent. The Router ID of the Parent is
advertised in the DR field of each Hello sent on the interface.
Backup Parent
If the option of biconnected adjacencies is chosen, then each MDR
Other selects a Backup Parent, which will be a neighboring MDR or
BMDR if one exists that is not the Parent. The Backup Parent of a
BMDR is always the router itself. Each MDR Other becomes adjacent
with its Backup Parent if it exists. The Router ID of the Backup
Parent is advertised in the Backup DR field of each Hello sent on
the interface.
Bidirectional Neighbor
A bidirectional neighbor is a neighboring router whose neighbor
state is 2-Way or greater.
Ogier & Spagnolo Experimental [Page 6]RFC 5614 MANET Extension of OSPF August 2009
Routable Neighbor
A bidirectional MANET neighbor becomes routable 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 and Full neighbors can be used as next hops in the SPF
calculation, and can be included in the router-LSA originated by
the router.
Non-Flooding MDR
A non-flooding MDR is an MDR that does not automatically flood
received LSAs back out the receiving interface, but performs
backup flooding like a BMDR. Some MDRs may declare themselves
non-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 neighbors'
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 (if the network itself is biconnected).
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.
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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
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 Parent.
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RFC 5614 MANET Extension of OSPF August 2009
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. Although BMDR
selection is optional if AdjConnectivity is 0 or 1, it is recommended
since BMDRs improve robustness by providing backup flooding.
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.
2.2. Flooding Procedure
When an MDR receives a new link state advertisement (LSA) on a MANET
interface, it floods the LSA back out the receiving interface unless
it can be determined that such flooding is unnecessary (as specified
in Section 8.1). The router MAY delay the flooding of the LSA by a
small random amount of time (e.g., less than 100 ms). The delayed
flooding is useful for coalescing multiple LSAs in the same Link
State Update packet, and it can reduce the possibility of a collision
in case multiple MDRs received the same LSA at the same time.
However, such collisions are usually avoided with wireless MAC
protocols.
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.
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RFC 5614 MANET Extension of OSPF August 2009
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 to allow coalescing multiple acknowledgments 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.
A MANET neighbor becomes routable if it is bidirectional and the SPF
calculation has produced a route to the neighbor. (A flexible
quality condition may also be required.) Only routable and Full
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 routes or require a large number of adjacencies.
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RFC 5614 MANET Extension of OSPF August 2009
2.5. Partial and Full Topology LSAs
OSPF-MDR allows routers to originate both full-topology LSAs, which
advertise links to all routable and Full 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 neighbors as point-to-point
links in its router-LSA. The choice of which neighbors to advertise
is flexible. As a minimum requirement, each router must advertise a
minimum set of "backbone" neighbors in its router-LSA. An LSA that
includes only this minimum set of neighbors is called a minimal LSA
and corresponds to LSAFullness = 0. This choice results in the
minimum amount of LSA flooding overhead, but does not ensure routing
along shortest paths. However, it is useful for achieving
scalability to networks with a large number of nodes.
At the other extreme, if LSAFullness = 4, then the router originates
a full-topology LSA, which includes all routable and Full neighbors.
Setting LSAFullness to 1 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.
Setting LSAFullness to 2 also provides shortest-path routing, but
allows the router to advertise additional neighbors to provide
redundant routes.
Setting LSAFullness to 3 results in MDR full LSAs, causing each MDR
to originate a full-topology LSA while other routers originate
minimal LSAs. This choice does not provide routing along shortest
paths, but simulations have shown that it 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 9.4)
ensures that the router-LSAs originated by different routers are
consistent. Routing along shortest paths is provided if and only if
every router selects LSAFullness to be 1, 2, or 4.
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RFC 5614 MANET Extension of OSPF August 2009
2.6. 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).
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. 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 [RFC2328].
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,
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.
Parent
The Parent replaces the Designated Router (DR) data item of OSPF.
Each router selects a Parent as described in Section 5.4. The
Parent of an MDR is the router itself, and the Parent of a non-MDR
Ogier & Spagnolo Experimental [Page 12]RFC 5614 MANET Extension of OSPF August 2009
router will be a neighboring MDR, if one exists. The Parent is
initialized to 0.0.0.0, indicating the lack of a Parent. Each
router advertises the Router ID of its Parent in the DR field of
each Hello sent on the interface.
Backup Parent
The Backup Parent replaces the Backup Designated Router data item
of OSPF. The Backup Parent of a BMDR is the router itself. If
the option of biconnected adjacencies is chosen, then each MDR
Other selects a Backup Parent, which will be a neighboring
MDR/BMDR if one exists that is not the Parent. The Backup Parent
is initialized to 0.0.0.0, indicating the lack of a Backup Parent.
Each router advertises the Router ID of its Backup 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 MDR-Hello TLV. The HSN
is incremented by 1 (modulo 2^16) every time a Hello packet 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.
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
Ogier & Spagnolo Experimental [Page 13]RFC 5614 MANET Extension of OSPF August 2009
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 may improve robustness by providing a biconnected adjacency
subgraph, and setting AdjConnectivity to 0 results in full-topology
adjacencies.
All possible configurations of the new interface parameters are
functional, except that if AdjConnectivity is 0 (full-topology
adjacencies), then LSAFullness must be 1, 2, or 4 (see Section 9.3).
Differential Hellos should be used to reduce the size of Hello
packets when the average number of neighbors is large (e.g., greater
than 50). Differential Hellos are obtained by setting the parameter
2HopRefresh to an integer greater than 1, with the recommended value
being 3. Good performance in simulated mobile networks with up to
160 nodes has been obtained using the default configuration with
differential Hellos. Good performance in simulated mobile networks
with up to 200 nodes has been obtained using the same configuration
except with minimal LSAs (LSAFullness = 0). Simulation results are
presented in Appendix E.
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 and must be an integer greater than or
equal to 2. The default value of 3 results in nearly the minimum
number of MDRs. Values larger than 3 result in slightly fewer
MDRs, and the 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. The default
value is 0.5 second.
Ogier & Spagnolo Experimental [Page 14]RFC 5614 MANET Extension of OSPF August 2009
AckInterval
The interval between Link State Acknowledgment packets when only
delayed acknowledgments need to be sent. AckInterval MUST be less
than RxmtInterval, and SHOULD NOT be larger than 1 second. The
default value is 1 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 the value 2
providing redundant routes. The value 3 results in MDRs
originating full-topology LSAs and other routers originating
minimal LSAs. The value 4 results in all routers originating
full-topology LSAs. The default value is 1.
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 MDR-Hello TLV of the last Hello received
from the neighbor. This bit is 1 if the neighbor is using full-
topology adjacencies, i.e., is not using adjacency reduction.
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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-DD TLV in a Database Description (DD) packet received from
the neighbor.
Neighbor's Parent
The neighbor's choice for Parent, obtained from the DR field of
the last Hello packet received from the neighbor or from the MDR-
DD TLV in a DD packet received from the neighbor.
Neighbor's Backup Parent
The neighbor's choice for Backup Parent, obtained from the Backup
DR field of the last Hello packet received from the neighbor or
from the MDR-DD 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) 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. Each
MDR/BMDR router becomes adjacent with its Dependent Neighbors
(which are also MDR/BMDR routers) to form a connected backbone.
The set of all Dependent Neighbors on a MANET interface is called
the Dependent Neighbor Set (DNS) for the interface.
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. Selected Advertised Neighbors are neighbors
that the router has selected to be included in the router-LSA,
along with other neighbors that are required to be included. The
set of all Selected Advertised Neighbors on a MANET interface is
called the Selected Advertised Neighbor Set (SANS) for the
interface.
Routable
A single-bit variable equal to 1 if the neighbor is routable.
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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.
Neighbor's Link Metrics
The link metric for each of the neighbor's bidirectional
neighbors, obtained from the Metric TLV appended to Hello packets.
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 on the interface that are in state
Init or greater, 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 [RFC5340], Section 4.2.1.1, 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 [RFC5340], Section 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 Parent and
Backup Parent, as specified in Section A.3 and Section 5.4.
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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.
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
MDR-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 MDR-Hello TLV is given in Section A.2.3.
In both full and differential Hellos, the appended MDR-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 (modulo 2^16).
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o The D-bit of the MDR-Hello TLV is set to 1 for a differential
Hello and 0 for a full Hello.
o The A-bit of the MDR-Hello TLV is set to 1 if AdjConnectivity is 0
(the router is using full-topology adjacencies); 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.)
The MDR-Metric TLV (or Metric TLV) advertises the link cost to each
bidirectional neighbor on the interface, to allow the selection of
neighbors to include in partial-topology LSAs. If LSAFullness is 1
or 2, a Metric TLV must be appended to each MANET Hello packet unless
all link costs are 1. 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 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 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 on the
interface that are in state Init or greater, in the order described
above. The MDR-Hello TLV is built as described above. If a Metric
TLV is appended, it is built as specified in Section A.2.5.
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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 each neighbor in state Down that has not yet been
included in HelloRepeatCount Hellos since transitioning to this
state.
List 2 includes each neighbor in state Init that has not yet been
included in HelloRepeatCount Hellos since transitioning to this
state.
List 3 includes each Dependent Neighbor that has not yet been
included in HelloRepeatCount Hellos since becoming a Dependent
Neighbor.
List 4 includes each Selected Advertised Neighbor that has not yet
been included in HelloRepeatCount Hellos since becoming a Selected
Advertised Neighbor.
List 5 includes each unselected bidirectional neighbor (defined in
Section 4.1) that has not yet been included in HelloRepeatCount
Hellos since becoming an unselected bidirectional neighbor.
In addition, a bidirectional neighbor must be included (in the
appropriate list) if the neighbor's BNS does not include the router
(indicating that the neighbor does not consider the router to be
bidirectional).
If a Metric TLV is appended to the Hello, then a bidirectional
neighbor must be included (in the appropriate list) if it has not yet
been included in HelloRepeatCount Hellos since its metric last
changed.
4.2. Receiving Hello Packets
A Hello packet received on a MANET interface is processed as
described in [RFC5340], Section 4.2.2.1, 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
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RFC 5614 MANET Extension of OSPF August 2009
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
Parent is set to the value of the DR field, and the Neighbor's Backup
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 to be either 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 an MDR-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 MDR-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.
(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 and set the
neighbor's Dependent Neighbor variable to 0. (Only MDR/BMDR
neighbors can be Dependent.)
(4) If the DR or Backup DR field is equal to the router's own
Router ID, set the neighbor's Child variable to 1; otherwise,
set it to 0.
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 MDR-Hello TLV. List 1 is
defined to be the first N1 neighbor IDs, List 2 is defined to be the
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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
MDR-Hello TLV.
4.2.1. Full Hello Packet
If the received Hello is full (the D-bit of the MDR-Hello TLV is 0),
the following steps are performed:
o If the N1 field of the MDR-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 MDR-Hello TLV
is 1), the following steps are performed:
(1) For each neighbor ID in List 1 or List 2 of the Hello:
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:
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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.
If the router itself belongs to 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.
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o The neighbor is bidirectional and any of the following neighbor
variables has changed: MDR Level, Router Priority, FullHelloRcvd,
and Bidirectional 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 the LLS does not contain a Metric TLV
and LSAFullness is 1 or 2, the metric for each of the neighbor's
links is set to 1.
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
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.
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5. MDR Selection Algorithm
This section describes the MDR selection algorithm, which is run for
each MANET interface to determine whether the router is an MDR,
Backup MDR, or MDR Other for that interface. The algorithm also
selects the Dependent Neighbors and the (Backup) Parent, which are
used to decide which neighbors should become adjacent (see Section
7.2).
The MDR selection algorithm must be executed just before sending a
Hello if the MDRNeighborChange bit is set for the interface. The
algorithm SHOULD also be executed whenever a bidirectional neighbor
transitions to less than 2-Way, and MAY be executed at other times
when the MDRNeighborChange bit is set. The bit is cleared after the
algorithm is executed.
To simplify the implementation, the MDR selection algorithm MAY be
executed periodically 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 MDRs and Backup MDRs
forms a biconnected CDS (if the network itself is biconnected).
Each MDR selects and becomes adjacent with a subset of its 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 Parent. Each MDR
selects itself as Parent, to inform neighbors that it is an MDR.
If AdjConnectivity = 2, then each (Backup) MDR selects and becomes
adjacent with additional (Backup) MDR neighbors to form a biconnected
backbone, and each MDR Other selects and becomes adjacent with a
second (Backup) MDR neighbor called its Backup Parent, thus becoming
connected to the backbone via two adjacencies. Each BMDR selects
itself as Backup Parent, to inform neighbors that it is a BMDR.
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 (Router Priority, MDR Level, Router ID) for
each such neighbor and for the router itself, and the neighbor
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RFC 5614 MANET Extension of OSPF August 2009
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
for the interface, 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 Parent and Backup Parent.
The algorithm simplifies considerably if AdjConnectivity is 0 (full-
topology adjacencies). In this case, the set of Dependent Neighbors
is empty and MDR Other routers need not select Parents. 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
automatically flood received LSAs back out the receiving interface,
because it has determined that neighboring MDRs are sufficient to
flood the LSA to all neighbors. 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.) By having MDRs
declare themselves to be non-flooding when possible, flooding
overhead is reduced. The resulting reduction in flooding overhead
can be dramatic for certain regular topologies, but has been found to
be less than 15% for random topologies.
The following subsections describe the MDR selection algorithm, which
is applied independently to each MANET interface. For convenience,
the term "bi-neighbor" will be used as an abbreviation for
"bidirectional neighbor".
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RFC 5614 MANET Extension of OSPF August 2009
5.1. Phase 1: Creating the Neighbor Connectivity Matrix
Phase 1 creates the neighbor connectivity matrix (NCM) for the
interface. The NCM is a symmetric matrix that defines a topology
graph for the set of bi-neighbors on the interface. The NCM assigns
a value of 0 or 1 for each pair of bi-neighbors; a value of 1
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 on the interface.
(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 selected.
(2.1) The set of Dependent Neighbors is initialized to be empty.
Ogier & Spagnolo Experimental [Page 27]RFC 5614 MANET Extension of OSPF August 2009
(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; if AdjConnectivity = 2, selects all of its BMDR bi-
neighbors as Dependent Neighbors; then proceeds to Phase 4.
(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 if it is an
MDR or BMDR; 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.) This step is not
required if the MDR selection algorithm is executed
periodically.
Step 2.4 can be implemented using a breadth-first search (BFS)
algorithm to compute min-hop paths from Rmax to all other bi-
neighbors, modified to allow a bi-neighbor to be 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.
Ogier & Spagnolo Experimental [Page 28]RFC 5614 MANET Extension of OSPF August 2009
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 if it is an MDR or BMDR,
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.) This step is not required if the MDR
selection algorithm is executed periodically.
Step 3.2 can be implemented in O(d^2) time using the algorithm given
in Appendix B. A simplified version of the algorithm is also
specified, which results in a larger number of BMDRs.
5.4. Phase 4: Parent Selection
Each router selects a Parent for each MANET interface. The Parent of
a non-MDR router will be a neighboring MDR if one exists. If the
option of biconnected adjacencies is chosen, then each MDR Other
selects a Backup Parent, which will be a neighboring MDR/BMDR if one
exists that is not the Parent. The Parent of an MDR is always the
router itself, and the Backup Parent of a BMDR is always the router
itself.
Ogier & Spagnolo Experimental [Page 29]RFC 5614 MANET Extension of OSPF August 2009
The (Backup) Parent is advertised in the (Backup) DR field of each
Hello sent on the interface. As specified in Section 7.2, each
router forms an adjacency with its Parent and Backup Parent if it
exists and is a neighboring MDR/BMDR.
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
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.
(The latter design choice was made because it results in slightly
better performance than choosing no Backup Parent.) If the router
has selected itself as a BMDR, then the Backup Parent is equal to the
router itself.
If the calculating 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. Note that the Parent can be a non-MDR neighbor temporarily
when no MDR neighbor exists. (This design choice was also made for
performance reasons.)
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 Parent selected in the previous
paragraph, if such a neighbor exists. If no such adjacent 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), if such a neighbor exists. Otherwise, the
Backup Parent is null.
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).
Ogier & Spagnolo Experimental [Page 30]RFC 5614 MANET Extension of OSPF August 2009
(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
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 [RFC2328], 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).
Ogier & Spagnolo Experimental [Page 31]RFC 5614 MANET Extension of OSPF August 2009
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 [RFC2328], but have modified
action descriptions because MDRs are selected instead of DRs. The
state transition in [RFC2328] 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 are unchanged from [RFC2328].
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. 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) 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],
Ogier & Spagnolo Experimental [Page 32]RFC 5614 MANET Extension of OSPF August 2009
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.
If full-topology adjacencies are used (AdjConnectivity = 0), the
router forms an adjacency with each bidirectional neighbor. If
adjacency reduction is used (AdjConnectivity is 1 or 2), the router
forms adjacencies with a subset of its neighbors, according to the
rules specified in Section 7.2.
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?.
(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) Parent.
(4) The neighbor selects the router to be its (Backup) 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.
Ogier & Spagnolo Experimental [Page 33]RFC 5614 MANET Extension of OSPF August 2009
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
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-DD 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.
Ogier & Spagnolo Experimental [Page 34]RFC 5614 MANET Extension of OSPF August 2009
If adjacency reduction is not used (AdjConnectivity = 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 neighbor is a (Backup) MDR and is the router's (Backup)
Parent.
(3) The router is a (Backup) MDR and the neighbor is a child.
(4) The neighbor's A-bit is 1, indicating that the neighbor is using
full-topology adjacencies.
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 existing
adjacency should be eliminated. An existing adjacency is maintained
if any of the following is true:
(1) The router is an MDR or Backup MDR.
(2) The neighbor is an MDR or Backup MDR.
(3) The neighbor's A-bit is 1, indicating that the neighbor is using
full-topology adjacencies.
Otherwise, the adjacency MAY be eliminated.
7.4. Sending Database Description Packets
Sending a DD packet on a MANET interface is the same as [RFC5340],
Section 4.2.1.2, 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-DD TLV appended using LLS, and the L bit is set
in the DD packet's option field. The format for the MDR-DD TLV is
specified in Section A.2.4. The DR and Backup DR fields of the MDR-
DD TLV are set exactly the same as the DR and Backup DR fields of a
Hello sent on the same interface.
Ogier & Spagnolo Experimental [Page 35]RFC 5614 MANET Extension of OSPF August 2009
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-DD
TLV is appended, then the MDR-DD TLV is processed as follows.
(1) If the DR field is equal to the neighbor'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.
(3) Else:
(a) Set the MDR Level of the neighbor to MDR Other.
(b) Set the neighbor's Dependent Neighbor variable to 0.
(4) If the DR or Backup DR field is equal to the router's own
Router ID, set the neighbor's Child variable to 1; otherwise,
set it to 0.
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?.
Ogier & Spagnolo Experimental [Page 36]RFC 5614 MANET Extension of OSPF August 2009
In addition, the Database Exchange optimization described in
[RFC5243] SHOULD be performed as follows. If the router accepts a
received DD packet as the next in sequence, the following additional
step should be performed for each LSA listed in the DD packet
(whether the router is master or slave). If the Database summary
list contains an instance of the LSA that is the same as or less
recent than the listed LSA, the LSA is removed from the Database
summary list. This avoids listing 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 [RFC2328], 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 List exists for
the 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 [RFC2328].
Ogier & Spagnolo Experimental [Page 37]RFC 5614 MANET Extension of OSPF August 2009
8.1. LSA Forwarding Procedure
When a new LSA is received, Steps 1 through 5 of [RFC2328], Section
13.3, are performed without modification for each eligible (outgoing)
interface that is not of type MANET. This section specifies the
modified steps that must be performed for each eligible MANET
interface. The eligible interfaces depend on the LSA's flooding
scope as described in [RFC5340], Section 4.5.2. Whenever an LSA is
flooded out a MANET interface, it is included in an LSU packet that
is sent to the multicast address AllSPFRouters. (Retransmitted LSAs
are always unicast, as specified in Section 8.3.)
Step 1 of [RFC2328], Section 13.3, is performed for each eligible
MANET interface with the following modification, so that the new LSA
is placed on the Link State retransmission list for each appropriate
adjacent neighbor. Step 1c 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.)
The following steps must be performed for each eligible MANET
interface, to determine whether the new LSA should be forwarded on
the interface.
(2) If every bidirectional neighbor on the interface satisfies at
least one of the following three conditions, examine the next
interface (the LSA is not flooded out this interface).
Ogier & Spagnolo Experimental [Page 38]RFC 5614 MANET Extension of OSPF August 2009
(a) The LSA was received from the neighbor.
(b) The LSA was received on a MANET or broadcast interface and the
neighbor is covered (defined above).
(c) An Ack for the LSA has been received from the neighbor.
Condition (c) MAY be omitted (thus ignoring Acks) in order to
simplify this step. Note that the above 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 does not satisfy
any of the conditions 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 yet 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) and the next interface is examined.
(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.
(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 and the next interface is examined.
(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
Ogier & Spagnolo Experimental [Page 39]RFC 5614 MANET Extension of OSPF August 2009
Neighbor List is created for each MANET interface, but only
one BackupWait Timer is associated with the LSA. Examine the
next interface (the LSA is not yet flooded out this
interface).
(7) If this step is reached, the LSA is flooded out the interface.
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 that 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 1 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
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 to occur
after RxmtInterval seconds.
(2) The BackupWait Neighbor List is then deleted (whether or not the
LSA is flooded).
Ogier & Spagnolo Experimental [Page 40]RFC 5614 MANET Extension of OSPF August 2009
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
[RFC2328] 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. The interface parameter
AckInterval is the interval between LS Ack packets when only delayed
Acks need to be sent. A delayed Ack SHOULD be delayed by at least
(RxmtInterval - AckInterval - 0.5) seconds and at most (RxmtInterval
- 0.5) seconds after the LSA instance being acknowledged was first
received. If AckInterval and RxmtInterval are equal to their default
values of 1 and 7 seconds, respectively, this reduces Ack traffic by
increasing the chance that a new instance of the LSA will be received
before the delayed Ack is sent. An immediate Ack is sent immediately
in a multicast LS Ack packet, which may also include delayed Acks
that were scheduled to be sent.
The decision whether to send a delayed or immediate Ack 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.
(b) Otherwise, a delayed Ack is sent out the receiving interface.
Ogier & Spagnolo Experimental [Page 41]RFC 5614 MANET Extension of OSPF August 2009
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 [RFC2328]. 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 unicast
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 [RFC2328], 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
that the router itself has NOT yet received. This is necessary
because (unlike [RFC2328]) 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 that is less recent than the
one being acknowledged, the LS Ack is added to the Acked LSA List
for the sending neighbor.
Ogier & Spagnolo Experimental [Page 42]RFC 5614 MANET Extension of OSPF August 2009
(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. Router-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 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.
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
used as next hops in the SPF calculation.
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9.1. Routable Neighbors
If adjacency reduction is used, a bidirectional MANET neighbor
becomes routable if the SPF calculation has found a route to the
neighbor and the neighbor satisfies the routable neighbor quality
condition (defined below). Since only routable and Full 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, 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.
(2) 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.
If adjacency reduction is not used (AdjConnectivity = 0), then
routable neighbors are not computed and the set of routable neighbors
remains empty.
Ogier & Spagnolo Experimental [Page 44]RFC 5614 MANET Extension of OSPF August 2009
9.2. Backbone Neighbors
The flexible choice for the router-LSA is made possible by defining
two types of neighbors that are included in the router-LSA: backbone
neighbors and Selected Advertised Neighbors.
If adjacency reduction is used, a bidirectional neighbor is defined
to be a backbone neighbor if and only if it satisfies the condition
for becoming adjacent (see Section 7.2). If adjacency reduction is
not used (AdjConnectivity = 0), a bidirectional neighbor is a
backbone neighbor if and only if the neighbor's A-bit is 0
(indicating that the neighbor is using adjacency reduction). This
definition allows the interoperation between routers that use
adjacency reduction and routers that do not.
If adjacency reduction is used, then a router MUST include in its
router-LSA all Full neighbors and all routable backbone neighbors. A
minimal LSA, corresponding to LSAFullness = 0, includes only these
neighbors. This choice guarantees connectivity, but does not ensure
shortest paths. However, this choice is useful in large networks to
achieve maximum scalability.
9.3. Selected Advertised Neighbors
To allow flexibility while ensuring that router-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 set (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
backbone neighbors (and thus Dependent Neighbors), the SANS and DNS
are disjoint. Both of these neighbor sets are advertised in Hellos.
If LSAFullness is 0 (minimal LSAs), then the SANS is empty. At the
other extreme, if LSAFullness is 4 (full-topology LSAs), the SANS
includes all bidirectional MANET neighbors except backbone neighbors.
In between these two extremes, a router that is using adjacency
reduction may select any subset of bidirectional non-backbone
neighbors as its SANS. The resulting router-LSA is constructed as
specified in Section 9.4.
Since a router that is not using adjacency reduction typically has no
backbone neighbors (unless it has neighbors that are using adjacency
reduction), to ensure connectivity, such a router must choose its
SANS to contain the SANS corresponding to LSAFullness = 1. Thus, if
AdjConnectivity is 0 (no adjacency reduction), then LSAFullness must
be 1, 2, or 4.
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RFC 5614 MANET Extension of OSPF August 2009
If LSAFullness is 1, the router originates min-cost LSAs, which are
partial-topology LSAs that (when flooded) provide each router with
sufficient information to calculate a shortest (minimum-cost) path to
each destination. 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 neighbor's
Bidirectional Neighbor Set (BNS), Dependent Neighbor Set (DNS),
Selected Advertised Neighbor Set (SANS), and the Metric TLV. The
Metric TLV, specified in Section A.2.5, is appended to each Hello
(unless all link costs are 1) to advertise the link cost to each
bidirectional neighbor.
If LSAFullness is 2, the SANS must be selected to be a superset of
the SANS corresponding to LSAFullness = 1. This choice provides
shortest-path routing while allowing the router to advertise
additional neighbors to provide redundant routes.
If LSAFullness is 3, each MDR originates a full-topology LSA (which
includes all Full and routable neighbors), while each non-MDR router
originates a minimal LSA. If the router has multiple MANET
interfaces, the router-LSA includes all Full and routable neighbors
on each interface for which it is an MDR, and advertises only Full
neighbors and routable backbone neighbors on its other interfaces.
This choice provides routing along nearly shortest paths with
relatively low overhead.
Although this document specifies a few choices of the SANS, which
correspond to different values of LSAFullness, it is important to
note that other choices are possible. In addition, 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.
9.4. Originating Router-LSAs
When a new router-LSA is originated, it includes a point-to-point
(type 1) link for each MANET neighbor that is advertised. The set of
neighbors to be advertised is determined as follows. If adjacency
reduction is used, the router advertises all Full neighbors, and
advertises each routable neighbor j that satisfies any of the
following three conditions. If adjacency reduction is not used
(AdjConnectivity = 0), the router advertises each Full neighbor j
that satisfies any of the following three conditions.
(1) The router's SANS (for any interface) includes j.
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RFC 5614 MANET Extension of OSPF August 2009
(2) Neighbor j's SANS includes the router (to ensure symmetry).
(3) Neighbor j is a backbone neighbor.
Note that backbone neighbors and neighbors in the SANS need not be
routable or Full, but only routable and Full 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.
The events that cause a new router-LSA to be originated are the same
as in [RFC2328] and [RFC5340] except that a MANET neighbor changing
to/from the Full state does not always cause a new router-LSA to be
originated. Instead, a new router-LSA is originated whenever a
change occurs that causes any of the following three conditions to
occur:
o There exists a MANET neighbor j that satisfies the above
conditions for inclusion in the router-LSA, but is not included in
the current router-LSA.
o The current router-LSA includes a MANET neighbor that is no longer
bidirectional.
o The link metric has changed for a MANET neighbor that is included
in the current router-LSA.
The above conditions may be checked periodically just before sending
each Hello, instead of checking them every time one of the neighbor
sets changes. The following implementation was found to work well.
Just before sending each Hello, and whenever a bidirectional neighbor
transitions to less than 2-Way, the router runs the MDR selection
algorithm; updates its adjacencies, routable neighbors, and Selected
Advertised Neighbors; then checks the above conditions to see if a
new router-LSA should be originated. In addition, if a neighbor
becomes bidirectional or Full, the router updates its routable
neighbors and checks the above conditions.
10. Calculating the Routing Table
The routing table calculation is the same as specified in [RFC2328],
except for the following changes to Section 16.1 (Calculating the
shortest-path tree for an area). If full-topology adjacencies and
full-topology LSAs are used (AdjConnectivity = 0 and LSAFullness =
4), there is no change to Section 16.1.
If adjacency reduction is used (AdjConnectivity is 1 or 2), then
Section 16.1 is modified as follows. Recall from Section 9 that a
router can use any routable neighbor as a next hop to a destination,
Ogier & Spagnolo Experimental [Page 47]RFC 5614 MANET Extension of OSPF August 2009
whether or not the neighbor is advertised in the router-LSA. This is
accomplished by modifying Step 2 so that the router-LSA associated
with the root vertex is replaced with a dummy router-LSA that
includes links to all Full neighbors and all routable MANET
neighbors. 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. However, Step 2b must still be executed when V is
not the root vertex, to ensure compatibility with OSPFv3.
If LSAFullness is 0 (minimal LSAs), then the calculated paths need
not be shortest paths. In this case, the path actually taken by a
packet can be shorter than the calculated path, since intermediate
routers may have routable neighbors that are not advertised in any
router-LSA.
If full-topology adjacencies and partial-topology LSAs are used, then
Section 16.1 is modified as follows. Step 2 is modified so that the
router-LSA associated with the root vertex is replaced with a dummy
router-LSA that includes links to all Full neighbors. In addition,
Step 2b MUST be skipped when V is the root vertex and W is a Full
MANET neighbor. (This is necessary since the neighbor's router-LSA
need not contain a link back to the router.)
If adjacency reduction is used with partial-topology LSAs, 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 routing 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. If the set of routable neighbors is updated periodically
every HelloInterval seconds, then it is not necessary to update the
set of routable neighbors immediately after the routing table is
updated.
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RFC 5614 MANET Extension of OSPF August 2009
11. Security Considerations
As with OSPFv3 [RFC5340], OSPF-MDR can use the IPv6 Authentication
Header (AH) [RFC4302] and/or the IPv6 Encapsulation Security Payload
(ESP) [RFC4303] to provide authentication, integrity, and/or
confidentiality. The use of AH and ESP for OSPFv3 is described in
[RFC4552].
Generic threats to routing protocols are described and categorized in
[RFC4593]. The mechanisms described in [RFC4552] provide protection
against many of these threats, but not all of them. In particular,
as mentioned in [RFC5340], these mechanisms do not provide protection
against compromised, malfunctioning, or misconfigured routers (also
called Byzantine routers); this is true for both OSPFv3 and OSPF-MDR.
The extension of OSPFv3 to include MANET routers does not introduce
any new security threats. However, the use of a wireless medium and
lack of infrastructure, inherent with MANET routers, may render some
of the attacks described in [RFC4593] easier to mount. Depending on
the network context, these increased vulnerabilities may increase the
need to provide authentication, integrity, and/or confidentiality, as
well as anti-replay service.
For example, sniffing of routing information and traffic analysis are
easier tasks with wireless routers than with wired routers, since the
attacker only needs to be within the radio range of a router. The
use of confidentiality (encryption) provides protection against
sniffing but not traffic analysis.
Similarly, interference attacks are also easier to mount against
MANET routers due to their wireless nature. Such attacks can be
mounted even if OSPF packets are protected by authentication and
confidentiality, e.g., by transmitting noise or replaying outdated
OSPF packets. As discussed below, an anti-replay service (provided
by both ESP and AH) can be used to protect against the latter attack.
The following threat actions are also easier with MANET routers:
spoofing (assuming the identify of a legitimate router),
falsification (sending false routing information), and overloading
(sending or triggering an excessive amount of routing updates).
These attacks are only possible if authentication is not used, or the
attacker takes control of a router or is able to forge legitimacy
(e.g., by discovering the cryptographic key).
[RFC4552] mandates the use of manual keying when current IPsec
protocols are used with OSPFv3. Routers are required to use manually
configured keys with the same security association (SA) parameters
for both inbound and outbound traffic. For MANET routers, this
Ogier & Spagnolo Experimental [Page 49]
RFC 5614 MANET Extension of OSPF August 2009
implies that all routers attached to the same MANET must use the same
key for multicasting packets. This is required in order to achieve
scalability and feasibility, as explained in [RFC4552]. Future
specifications can explore the use of automated key management
protocols that may be suitable for MANETs.
As discussed in [RFC4552], the use of manual keys can increase
vulnerability. For example, manual keys are usually long lived, thus
giving an attacker more time to discover the keys. In addition, the
use of the same key on all routers attached to the same MANET leaves
all routers insecure against impersonation attacks if any one of the
routers is compromised.
Although [RFC4302] and [RFC4303] state that implementations of AH and
ESP SHOULD NOT provide anti-replay service in conjunction with SAs
that are manually keyed, it is important to note that such service is
allowed if the sequence number counter at the sender is correctly
maintained across local reboots until the key is replaced.
Therefore, it may be possible for MANET routers to make use of the
anti-replay service provided by AH and ESP.
When an OSPF routing domain includes both MANET networks and fixed
networks, the frequency of OSPF updates either due to actual topology
changes or malfeasance could result in instability in the fixed
networks. In situations where this is a concern, it is recommended
that the border routers segregate the MANET networks from the fixed
networks with either separate OSPF areas or, in cases where legacy
routers are very sensitive to OSPF update frequency, separate OSPF
instances. With separate OSPF areas, the 5-second MinLSInterval will
dampen the frequency of changes originated in the MANET networks.
Additionally, OSPF ranges can be configured to aggregate prefixes for
the areas supporting MANET networks. With separate OSPF instances,
more conservative local policies can be employed to limit the volume
of updates emanating from the MANET networks.
12. IANA Considerations
This document defines three new LLS TLV types: MDR-Hello TLV (14),
MDR-Metric TLV (16), and MDR-DD TLV (15) (see Section A.2).
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RFC 5614 MANET Extension of OSPF August 2009
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, Acee Lindem, and Emmanuel Baccelli for helpful discussions
and comments.
14. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, June 2006.
[RFC5243] Ogier, R., "OSPF Database Exchange Summary List
Optimization", RFC 5243, May 2008.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Yeung, "OSPF Link-Local Signaling", RFC 5613, August
2009.
15. Informative References
[Lawler] Lawler, E., "Combinatorial Optimization: Networks and
Matroids", Holt, Rinehart, and Winston, New York, 1976.
[Suurballe] Suurballe, J.W. and R.E. Tarjan, "A Quick Method for
Finding Shortest Pairs of Disjoint Paths", Networks, Vol.
14, pp. 325-336, 1984.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
Ogier & Spagnolo Experimental [Page 51]RFC 5614 MANET Extension of OSPF August 2009
Appendix A. Packet Formats
A.1. Options Field
The L bit of the OSPF options field is used for link-local signaling,
as described in [RFC5613]. Routers set the L bit in Hello and DD
packets to indicate that the packet contains an LLS data block.
Routers set the L bit in a self-originated router-LSA to indicate
that the LSA is non-ackable.
A.2. Link-Local Signaling
OSPF-MDR uses link-local signaling [RFC5613] to append the MDR-Hello
TLV and MDR-Metric TLV to Hello packets, and to append the MDR-DD TLV
to Database Description packets. Link-local signaling is an
extension of OSPFv2 and OSPFv3 that allows the exchange of arbitrary
data using existing 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 OSPF packet length field does not include the
length of the LLS data block, but the IPv6 packet length does include
this length.
A.2.1. LLS Data Block
The data block used for link-local signaling is formatted as
described below in Figure A.1.
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.1: Format of LLS Data Block
The Checksum field contains the standard IP checksum of the entire
contents of the LLS block.
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.
Ogier & Spagnolo Experimental [Page 52]RFC 5614 MANET Extension of OSPF August 2009
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 the LLS data block are constructed using TLVs. See
Figure A.2 for the TLV format.
The Type field contains the TLV ID, which is unique for each type of
TLV. 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.2: Format of LLS TLVs
Note that TLVs are always padded to a 32-bit boundary, but padding
bytes are not included in the TLV Length field (though they are
included in the LLS Data Length field of the LLS block header). All
unknown TLVs MUST be silently ignored.
A.2.3. MDR-Hello TLV
The MDR-Hello TLV is appended to each MANET Hello using LLS. 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 differential (via the D-bit), and whether the router is
using full-topology adjacencies (via the A-bit).
Ogier & Spagnolo Experimental [Page 53]RFC 5614 MANET Extension of OSPF August 2009
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 is incremented by 1 (modulo 2^16) 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 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-DD TLV
When a Database Description packet is sent to a neighbor in state
ExStart, an MDR-DD TLV is appended to the packet using LLS. It
includes the same two Router IDs that are included in the DR and
Backup DR fields of a Hello sent by the router, and is used to
indicate the router's MDR Level and Parent(s).
Ogier & Spagnolo Experimental [Page 54]RFC 5614 MANET Extension of OSPF August 2009
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
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. MDR-Metric TLV
If LSAFullness is 1 or 2, an MDR-Metric TLV must be appended to each
MANET Hello packet using LLS, unless all link metrics are 1. This
TLV advertises 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.
Ogier & Spagnolo Experimental [Page 55]RFC 5614 MANET Extension of OSPF August 2009
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 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.
Ogier & Spagnolo Experimental [Page 56]RFC 5614 MANET Extension of OSPF August 2009
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 Parent, or is 0.0.0.0 if the
Parent is null. The Parent of an MDR is always the router's own
RID.
o Backup DR: This field is the router's Backup Parent, or is
0.0.0.0 if the Backup Parent is null. The Backup Parent of a BMDR
is always the router's own RID.
A.4. LSA Formats and Examples
LSA formats are specified in [RFC5340], Section 4.4. Figure A.3
below gives an example network map for a MANET in a single area.
o Four MANET routers 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.
Ogier & Spagnolo Experimental [Page 57]RFC 5614 MANET Extension of OSPF August 2009
..........................................
. Area 1.
. + .
. | .
. | 2+---+1 1+---+
. N1 |---|RT1|----+ +---|RT4|----
. | +---+ |\ / +---+
. | | \ / .
. + | \ N3 / .
. | \ / .
. + | \ / .
. | | \ / .
. | 2+---+1 | \ / .
. N2 |---|RT2|----+-------+ .
. | +---+ |1 .
. | +---+ .
. | |RT3|----------------
. + +---+ .
. |2 .
. +------------+ .
. |1 N4 .
. +---+ .
. |RT5| .
. +---+ .
..........................................
Figure A.3: 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
Ogier & Spagnolo Experimental [Page 58]RFC 5614 MANET Extension of OSPF August 2009
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
Ogier & Spagnolo Experimental [Page 59]RFC 5614 MANET Extension of OSPF August 2009
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.
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 = 1 ;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 = 1 ;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 = 1 ;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)
Ogier & Spagnolo Experimental [Page 60]RFC 5614 MANET Extension of OSPF August 2009
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
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
Ogier & Spagnolo Experimental [Page 61]RFC 5614 MANET Extension of OSPF August 2009
Appendix 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
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.
Ogier & Spagnolo Experimental [Page 62]RFC 5614 MANET Extension of OSPF August 2009
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] that 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:
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.
Ogier & Spagnolo Experimental [Page 63]
RFC 5614 MANET Extension of OSPF August 2009
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.
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.
Ogier & Spagnolo Experimental [Page 64]RFC 5614 MANET Extension of OSPF August 2009
Appendix 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. 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. 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 construct the router-LSA as described
in Section 9.4. The min-cost LSA algorithm must be run to update the
SANS (and possibly originate a new router-LSA) either periodically
just before sending each Hello, or 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, Parent(s),
or link metrics.
o An LSA originated by a non-MANET neighbor is received.
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
Ogier & Spagnolo Experimental [Page 65]RFC 5614 MANET Extension of OSPF August 2009
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
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
Ogier & Spagnolo Experimental [Page 66]RFC 5614 MANET Extension of OSPF August 2009
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) 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), RID(u)) is
lexicographically greater than (SANM(j,i), SANM(i,j),
RtrPri(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.
Ogier & Spagnolo Experimental [Page 67]RFC 5614 MANET Extension of OSPF August 2009
(7) With the SANS updated, a new router-LSA may need to be originated
as described in Section 9.4.
The lexicographical comparison of Step 5d 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.
Appendix 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 (see Section A.1). 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
can be used 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). For this option to work,
RxmtInterval must be larger than MinLSInterval 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.
Ogier & Spagnolo Experimental [Page 68]RFC 5614 MANET Extension of OSPF August 2009
Appendix E. Simulation Results
This section presents simulation results that predict the performance
of OSPF-MDR for up to 160 nodes with min-cost LSAs and up to 200
nodes with minimal LSAs. The results were obtained using the GTNetS
simulator with OSPF-MDR version 1.01, available at
http://hipserver.mct.phantomworks.org/ietf/ospf.
The following scenario parameter values were used: radio range = 200
m and 250 m, grid length = 500 m, wireless alpha = 0.5, (maximum)
velocity = 10 m/s, pause time = 0, packet rate = 10 pkts/s, packet
size = 40 bytes, random seed = 8, start time (for gathering
statistics) = 1800 s. The stop time was 3600 s for up to 80 nodes
and 2700 s for more than 80 nodes. The source and destination are
selected randomly for each generated UDP packet. The simulated MAC
protocol is 802.11b.
Tables 4 and 6 show the results for the default configuration of
OSPF-MDR, except that differential Hellos were used (2HopRefresh = 3)
since they are recommended when the number of neighbors is large.
Tables 5 and 7 show the results for the same configuration except
that minimal LSAs were used instead of min-cost LSAs. The tables
show the results for total OSPF overhead in kb/s, the total number of
OSPF packets per second, the delivery ratio for UDP packets, and the
average number of hops traveled by UDP packets that reach their
destination.
Tables 5 and 7 for minimal LSAs also show the following statistics:
the average number of bidirectional neighbors per node, the average
number of fully adjacent neighbors per node, the number of changes in
the set of bidirectional neighbors per node per second, and the
number of changes in the set of fully adjacent neighbors per node per
second. These statistics do not change significantly when min-cost
LSAs are used instead of minimal LSAs.
The results show that OSPF-MDR achieves good performance for up to at
least 160 nodes when min-cost LSAs are used, and up to at least 200
nodes when minimal LSAs are used. Also, the results for the number
of hops show that the routes obtained with minimal LSAs are only 2.3%
to 4.5% longer than with min-cost LSAs when the range is 250 m, and
3.5% to 7.4% longer when the range is 200 m.
The results also show that the number of adjacencies per node and the
number of adjacency changes per node per second do not increase as
the number of nodes increases, and are dramatically smaller than the
number of neighbors per node and the number of neighbor changes per
node per second, respectively. These factors contribute to the low
overhead achieved by OSPF-MDR. For example, the results in Table 5
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RFC 5614 MANET Extension of OSPF August 2009
imply that with 200 nodes and range 250 m, there are 2.136/.039 = 55
times as many adjacency formations with full-topology adjacencies as
with uniconnected adjacencies. Additional simulation results for
OSPF-MDR can be found at http://www.manet-routing.org.
Number of nodes
20 40 60 80 100 120 160
------------------------------------------------------------------
OSPF kb/s 27.1 74.2 175.3 248.6 354.6 479.2 795.7
OSPF pkts/s 29.9 69.2 122.9 163.7 210.3 257.2 357.7
Delivery ratio .970 .968 .954 .958 .957 .956 .953
Avg no. hops 1.433 1.348 1.389 1.368 1.411 1.361 1.386
Table 4: Results for range 250 m with min-cost LSAs
Number of nodes
20 40 60 80 120 160 200
------------------------------------------------------------------
OSPF kb/s 15.5 41.6 91.0 132.9 246.3 419.0 637.4
OSPF pkts/sec 18.8 42.5 78.6 102.8 166.8 245.6 321.0
Delivery ratio .968 .968 .951 .953 .962 .956 .951
Avg no. hops 1.466 1.387 1.433 1.412 1.407 1.430 1.411
Avg no. nbrs/node 11.38 25.82 36.30 50.13 75.87 98.65 125.59
Avg no. adjs/node 2.60 2.32 2.38 2.26 2.25 2.32 2.13
Nbr changes/node/s .173 .372 .575 .752 1.223 1.654 2.136
Adj changes/node/s .035 .036 .046 .040 .032 .035 .039
Table 5: Results for range 250 m with minimal LSAs
Number of nodes
20 40 60 80 100 120 160
------------------------------------------------------------------
OSPF kb/s 40.5 123.4 286.5 415.7 597.5 788.9 1309.8
OSPF pkts/s 37.6 83.9 135.1 168.6 205.4 247.7 352.3
Delivery ratio .926 .919 .897 .900 .898 .895 .892
Avg no. hops 1.790 1.628 1.666 1.632 1.683 1.608 1.641
Table 6: Results for range 200 m with min-cost LSAs
Ogier & Spagnolo Experimental [Page 70]RFC 5614 MANET Extension of OSPF August 2009
Number of nodes
20 40 60 80 120 160 200
------------------------------------------------------------------
OSPF kb/s 24.0 63.6 140.6 195.2 346.9 573.2 824.6
OSPF pkts/sec 26.4 58.8 108.3 138.8 215.2 311.3 401.3
Delivery ratio .930 .927 .897 .907 .907 .904 .902
Avg no. hops 1.853 1.714 1.771 1.743 1.727 1.758 1.747
Avg no. nbrs/node 7.64 18.12 25.27 35.29 52.99 68.13 86.74
Avg no. adjs/node 2.78 2.60 2.70 2.50 2.39 2.36 2.24
Nbr changes/node/s .199 .482 .702 .959 1.525 2.017 2.611
Adj changes/node/s .068 .069 .081 .068 .055 .058 .057
Table 7: Results for range 200 m with minimal LSAs
Authors' Addresses
Richard G. Ogier
SRI International
EMail: rich.ogier@earthlink.net or rich.ogier@gmail.com
Phil Spagnolo
Boeing Phantom Works
EMail: phillipspagnolo@gmail.com
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