One document matched: draft-ogier-manet-ospf-extension-04.txt
Differences from draft-ogier-manet-ospf-extension-03.txt
OSPF/MANET Working Groups R. Ogier
Internet-Draft SRI International
Expires: January 18, 2006 P. Spagnolo
Boeing
July 18, 2005
MANET Extension of OSPF using CDS Flooding
draft-ogier-manet-ospf-extension-04.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
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This Internet-Draft will expire on January 18, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies an extension of OSPF for IPv6 to support
mobile ad hoc networks (MANETs). The extension, called OSPF-MDR, is
designed as a new OSPF interface type for MANETs. OSPF-MDR is based
on the selection of a subset of MANET routers, consisting of MANET
Designated Routers (MDRs) and Backup MDRs. The MDRs form a connected
dominating set (CDS), and the MDRs and Backup MDRs together form a
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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 LSAs back out the
receiving interface; reliable flooding is ensured by retransmitting
LSAs along adjacencies. Second, adjacencies are formed only between
(Backup) MDRs and a subset of their neighbors, allowing for much
better scaling in dense networks. The CDS is constructed using 2-hop
neighbor information provided in a Hello protocol extension. The
Hello protocol is further optimized by allowing differential Hellos
that report only changes in neighbor states. Options are specified
for originating router-LSAs that provide full or partial topology
information, allowing overhead to be reduced by advertising less
topology information.
Table of Contents
1. Introduction ................................................. 4
2. Overview of OSPF-MDR ......................................... 5
3. Interface and Neighbor Data Structures ....................... 9
3.1. Changes to Interface Data Structure ........................ 9
3.2. New Configurable Interface Parameters ..................... 11
3.3. Changes to Neighbor Data Structure ........................ 12
4. Hello Protocol .............................................. 13
4.1 Sending Hello Packets ...................................... 13
4.2 Receiving Hello Packets .................................... 15
5. MDR Selection Algorithm ..................................... 18
5.1. Phase 1: Creating the Neighbor Connectivity Matrix ........ 19
5.2. Phase 2: MDR Selection .................................... 20
5.3. Phase 3: Backup MDR Selection ............................. 20
5.4. Phase 4: Selection of the (Backup) MDR Parent ............. 21
5.5. Requirements for Compliance ............................... 22
6. Interface State Machine ..................................... 23
6.1. Interface states .......................................... 23
6.2. Events that cause interface state changes ................. 24
6.3. Changes to Interface State Machine ........................ 24
7. Adjacency Maintenance ....................................... 26
7.1. Changes to Neighbor State Machine ......................... 26
7.2. Whether to Become Adjacent ................................ 27
7.3. Whether to Eliminate an Adjacency ......................... 28
7.4. Sending Database Description Packets ...................... 28
7.5. Receiving Database Description Packets .................... 29
8. Flooding Procedure .......................................... 30
8.1. LSA Forwarding Procedure .................................. 31
8.2. Sending Link State Acknowledgments ........................ 34
8.3. Retransmitting LSAs ....................................... 35
8.4. Receiving Link State Acknowledgments ...................... 35
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9. Originating LSAs ............................................ 36
9.1. Routable Neighbors ........................................ 36
9.2. Partial and Full Topology LSAs ............................ 37
10. Calculating the Routing Table .............................. 39
11. Draft Modifications ........................................ 39
References ...................................................... 40
A. Packet Formats .............................................. 40
A.1. Options Field ............................................. 40
A.2. Link-Local Signaling ...................................... 41
A.3. Hello Packet DR and Backup DR Fields ...................... 45
A.4. LSA Formats and Examples .................................. 46
B. Pseudocode For MDR Selection Algorithm ...................... 49
C. Min-Cost LSA Algorithm ...................................... 52
D. Non-Ackable LSAs for Periodic Flooding ...................... 53
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1. Introduction
This document specifies an extension of OSPF for IPv6 [RFC2328,
RFC2740], to support a new interface type for mobile ad hoc networks
(MANETs), i.e., for broadcast-capable, multihop wireless networks in
which routers and hosts can be mobile. Existing OSPF interface types
do not perform adequately in such an environment, due to scaling
issues regarding the flooding protocol operation, inability of the
Designated Router election protocol to converge in all scenarios, and
large numbers of adjacencies when using a Point-to-Multipoint
interface type.
An OSPF implementation that is extended with this MANET interface
type does not preclude the use of any existing interface types, and
is fully compatible with a legacy OSPF implementation. MANET
networks are represented externally as Point-to-Multipoint networks,
although the design borrows concepts used by the OSPF broadcast
interface type.
The approach taken is to generalize the concept of an OSPF Designated
Router (DR) and Backup DR to multihop wireless networks, in order to
reduce overhead by reducing the number of routers that must flood new
LSAs and reducing the number of adjacencies. The generalized
(Backup) Designated Routers are called (Backup) MANET Designated
Routers (MDRs). The MDRs form a connected dominating set (CDS), and
the MDRs and Backup MDRs together form a biconnected CDS for
robustness. By definition, all routers in the MANET either belong to
the CDS or are 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 LLS TLVs, 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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This document is organized as follows. Section 2 presents an overview
of OSPF-MDR, Section 3 presents the new interface and neighbor data
items that are required for the extension, Section 4 describes the
Hello protocol, including procedures for maintaining the 2-hop
neighbor information, Section 5 describes the MDR selection
algorithm, Section 6 describes changes to the Interface state
machine, section 7 describes the procedures for forming adjacencies
and deciding which neighbors should become adjacent, Section 8
describes the flooding procedure, Section 9 specifies the
requirements and options for what to include in router-LSAs, and
Section 10 describes changes in the calculation of the routing table.
The appendix specifies packet formats, detailed pseudocode for the
MDR selection algorithm, an optional algorithm for the selection of
neighbors to include in router-LSAs in order to provide min-cost
routing, and a proposed option that uses "non-ackable" LSAs to
provide periodic flooding that reduces overhead in highly mobile
networks.
2. Overview of OSPF-MDR
Two aspects of mobile wireless networks cause scalability problems
with traditional OSPF interface types. The first is that the
standard OSPF flooding procedure of indiscriminately flooding new
LSAs through all interfaces leads to excessive overhead, dominating
the amount of routing overhead generated in many scenarios. The
second is that mobile routers may be within radio range of many
neighbors, leading to scalability problems due to too many
adjacencies.
The core concept of OSPF-MDR is to select a subset of nodes in the
network as flooding relays. The set of relays should be sufficiently
large to reach all nodes in the network, with some level of
redundancy for robustness due to node mobility. A connected
dominating set (CDS) is one such efficient set of relays. OSPF-MDR
uses a distributed algorithm to select a biconnected CDS that serves
as a flooding backbone for the network. The selection algorithm
should include heuristics that favor stability (permanence) of the
set of CDS nodes, in the face of mobility. The algorithm requires
that routers obtain current 2-hop neighbor information via some
mechanism; in this design, routers provide the requisite information
to their neighbors using link-local signaling (LLS) extensions to the
Hello protocol.
This set of flooding relays can also be exploited to reduce the
number of adjacencies in the network and the amount of topology
advertised in router-LSAs, without significantly compromising path
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lengths computed by the SPF algorithm. In particular, adjacencies
are only necessary between (Backup) MDRs and a subset of their
neighbors, analogous to how the standard OSPF (Backup) Designated
Routers are used to suppress unnecessary adjacencies.
The following subsections provide an overview of each of the main
features of OSPF-MDR.
2.1. Modified Hello Protocol
Hellos are used both for neighbor discovery and for advertising the
set of bidirectional neighbors (in state 2-Way or greater), to be
used by neighbors to learn 2-hop neighbor information. Differential
Hellos are sent every HelloInterval seconds, except when full Hellos
are sent, which happens every 2HopRefresh Hellos. The default values
for HelloInterval and 2HopRefresh are 2 seconds and 3 Hellos,
respectively. Differential Hellos are used to reduce overhead and to
allow Hellos to be sent more frequently, for faster reaction to
topology changes. Full Hellos are sent less frequently to ensure
that all neighbors have current 2-hop neighbor information. The use
of differential Hellos allows HelloInterval to be smaller (e.g. 1
second) while making 2HopRefresh larger (e.g. every 6th Hello),
without a significant increase in overhead, allowing faster response
to topology changes in a highly mobile network.
Each Hello contains a sequence number, which is incremented each time
a Hello is sent on a given interface. As in OSPF, the state of a
neighbor transitions to Down if no Hello is heard for
RouterDeadInterval. In addition, the state of a neighbor transitions
to Init if HelloRepeatCount Hellos are missed, based on the Hello
sequence number.
Both differential and full Hellos may contain a list of Heard
Neighbors (in state Init) and a list of Reported Neighbors (in state
2-Way or greater). In addition, differential Hellos may contain a
list of Lost Neighbors (which recently transitioned to the Down
state). A neighbor that transitions to a different one of these three
categories is included in the appropriate list for the next
HelloRepeatCount Hellos. This ensures that the neighbor will either
learn the new state within HelloRepeatCount Hellos, or will declare
the neighbor to be Down or Init.
2.2. Selection of MDRs and Backup MDRs
To optimize the flooding procedure, rather than have every router
flood each received new LSA, each router decides, based on 2-hop
neighbor information, whether it belongs to a CDS that is responsible
for forwarding/flooding each new LSA.
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The CDS consists of MANET Designated Routers (MDRs) and Backup MDRs.
The MDRs by themselves form a CDS, and the MDRs and Backup MDRs
together form a biconnected CDS to provide redundancy and robustness.
The purpose of (Backup) MDRs in a MANET is similar to the purpose of
the (Backup) DR in an OSPF broadcast network: to reduce the number of
routers that must flood each LSA, and to reduce the number of
adjacencies.
By running the MDR selection algorithm, each router decides whether
it is an MDR, Backup MDR, or MDR Other (neither MDR nor Backup MDR)
based on 2-hop neighbor information which is learned from Hellos.
The algorithm gives priority first to routers with larger MDR Level
(for persistence and stability of MDRs), then to routers with larger
Router Priority, and finally to routers with larger Router ID (to
break ties). This is similar to OSPF's DR election algorithm for
broadcast networks, which gives priority to a router that is already
a DR. In fact, the MDR selection algorithm is a generalization of the
DR election algorithm, in that both algorithms will select the same
two routers as DR/MDR and Backup DR/MDR, in a fully connected
(single-hop) network. (The MDR selection algorithm will also select a
second Backup MDR, so that the subgraph consisting of (Backup) MDRs
forms a biconnected backbone.)
Each (Backup) MDR also selects a subset of "dependent" neighbors, and
each MDR Other also selects two (Backup) MDR neighbors called
"parents". These are used to decide which neighbors to become
adjacent with, as described below.
2.3. Adjacencies
Rather than have each router form adjacencies with all of its
neighbors, each (Backup) MDR becomes adjacent with each dependent
neighbor that is a (Backup) MDR, to form a biconnected backbone.
Each MDR Other becomes adjacent with two selected (Backup) MDR
neighbors called "parents", thus providing a biconnected subgraph of
adjacencies. The parent selection is persistent, i.e., a router
updates its parents only when necessary. The two parents are
indicated in the DR and Backup DR fields of each Hello. The
persistence of the (Backup) MDRs, combined with the persistence of
the parent selection, maximizes the stability (lifetime) of the
adjacencies.
Once two neighbors become adjacent, they remain adjacent as long as
they remain bidirectional and at least one of them is an MDR or
Backup MDR. Since this condition is weaker than the condition for
forming an adjacency, it provides hysteresis for additional
stability.
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To reduce the overhead of forming adjacencies, a database exchange
optimization is used in which a router (master or slave) performing
database exchange does not include an LSA header in its DD packets if
it knows the neighbor has the same or newer instance of the LSA
(based on DD packets received from the neighbor). This reduces the
overhead due to DD packets by approximately 50% in large networks.
An option is provided to ensure that the adjacencies form a subgraph
that is (uni)connected, but not necessarily biconnected, in order to
reduce overhead and allow scalability to larger networks. This
option (obtained by setting AdjConnectivity to 1) typically results
in a slightly lower delivery ratio, due to some loss of robustness.
2.4. Flooding via MDRs and Backup MDRs
Only (Backup) MDRs flood a new LSA back out the receiving MANET
interface. Each MDR floods a new LSA the first time the LSA is
received (unless it can be determined that such flooding is
unnecessary). Each Backup MDR waits a short interval
(BackupWaitInterval), and then floods the LSA only if there exists an
adjacent or dependent neighbor from which an (explicit or implicit)
ACK has not been received, and which is not covered by another
neighbor from which the LSA has been received.
MDR Other routers never flood LSAs back out the receiving MANET
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.
2.5. Link State Acknowledgments
All Link State ACKs are multicast. An LSA received as a multicast is
acknowledged only the first time it is received. An LSA that is
flooded back out the same interface is treated as an implicit ACK.
Link State ACKs may be delayed up to AckInterval seconds to allow
coalescing multiple ACKs in the same packet. The only exception is
that (Backup) MDRs send a multicast ACK immediately when a duplicate
LSA is received as a unicast (to prevent additional retransmissions).
Only ACKs from adjacent neighbors are processed, and retransmitted
LSAs are sent (via unicast) only to adjacent neighbors.
2.6. Partial and Full Topology 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 connections in its
router-LSA. The choice of which neighbors to advertise is flexible,
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and is determined by the configurable parameter LSAFullness.
As a minimum requirement, each router must advertise all of its fully
adjacent neighbors in its router-LSA. This minimum choice corresponds
to LSAFullness = 0. This choice results in the minimum amount of LSA
flooding overhead, but does not provide routing along shortest paths.
Setting LSAFullness = 1 provides min-cost routing under certain
assumptions (see Section 9). Each router decides which neighbors to
include in its LSA by looking at the LSAs originated by its
neighbors, and including in its LSA the minimum set of neighbors
necessary to provide a minimum cost path (in each direction) between
each pair of neighbors that are not neighbors of each other.
If LSAFullness = 2, then each (Backup) MDR originates a full LSA (as
described below), while each MDR Other originates minimal LSAs. This
choice provides routing along nearly min-cost paths, and typically
results in less flooding overhead than LSAFullness = 1.
If LSAFullness = 3, then each router originates a full LSA, which
includes all "routable" neighbors. A neighbor is considered to be
routable if the SPF calculation produces a path to the neighbor. Note
that a routable neighbor need not be adjacent. However, the
routability condition implies the existence of a path to the neighbor
via full adjacencies, thus providing some assurance of
synchronization.
2.7. Shortest-Path Tree Calculation
The SPF calculation differs from RFC 2328 in that it allows any
routable neighbor to be a next hop to a destination. We note,
however, that RFC 2328 also allows a non-adjacent neighbor to be a
next hop, if both routers are fully adjacent to the DR of a broadcast
network. Allowing any routable neighbor to be a next hop is a
generalization of this condition to multihop wireless networks.
3. Interface and Neighbor Data Structures
3.1. Changes to Interface Data Structure
The following modified or new data items are required for the
Interface Data Structure of a MANET interface:
Type
A router that implements this extension can have one or more
interfaces of type MANET, in addition to the OSPF interface types
defined in RFC 2328.
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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.
MDR Parent
The router selected as the MDR Parent, as described in Section 5.
This replaces the Designated Router data item, when the interface
type is MANET. If the router is itself an MDR, then the MDR
Parent is the router itself. Otherwise, it is a neighoring router
that is an MDR. The MDR Parent is initialized to 0.0.0.0,
indicating the lack of an MDR Parent. The Router ID of the MDR
Parent is included in the DR field of each Hello sent on the
interface.
Backup MDR Parent
The router selected as Backup MDR Parent, as described in Section
5.4. This replaces the Backup Designated Router data item, when
the interface type is MANET. If the router is itself a Backup
MDR, then the Backup MDR Parent is the router itself. Otherwise,
it is a neighboring router that is an MDR or Backup MDR. The
Backup MDR Parent is initialized to 0.0.0.0, indicating the lack
of a Backup MDR Parent. The Router ID of the Backup MDR Parent is
included 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.
Hello Sequence Number (HSN)
The 16-bit sequence number carried by the Hello Sequence TLV. The
HSN is incremented by 1 every time a (differential or full) Hello
is sent on the interface.
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Lost Neighbor List (LNL)
A list of the Router IDs of neighbors whose states have recently
changed to Down. These Router IDs are included in the Lost
Neighbor List TLV of Hello packets sent on the interface.
3.2. New Configurable Interface Parameters
The following new configurable interface parameters are required for
a MANET interface. The default values for HelloInterval and
RouterDeadInterval for a MANET interface are 2 seconds and 6 seconds,
respectively.
2HopRefresh
Full neighbor state must be included in one of every 2HopRefresh
Hello packets. Other Hellos include only differential state
information. Default value is 3.
HelloRepeatCount
The number of consecutive Hellos in which a neighbor must be
included when its state changes. Default value is 3.
AckInterval
The maximum number of seconds that an acknowledgment may be held
before it is multicast so that acknowledgments may be coalesced.
Default value is 1.8 seconds.
BackupWaitInterval
The number of seconds that a Backup MDR must wait after receiving
a new LSA, before it decides whether to flood the LSA. Default
value is 2 seconds.
AdjConnectivity
If equal to the default value of 2, then the set of adjacencies
form a biconnected graph. If equal to the optional value of 1,
then the set of adjacencies form a (uni)connected graph.
MDRConstraint
A parameter of the MDR selection algorithm, which affects the
number of MDRs selected. The default value of 3 results in nearly
the minimum number of MDRs. The optional value 2 results in a
larger number of MDRs.
LSAFullness
Determines which neighbors a router should advertise in its
router-LSA. The value 0 results in minimal LSAs that include only
fully adjacent neighbors. The value 1 results in partial-topology
LSAs that provide min-cost routing under certain assumptions. The
value 2 results in (Backup) MDRs originating full LSAs and other
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routers originating minimal LSAs. The value 3 results in all
routers originating full LSAs. The default value is 2.
3.3. Changes to Neighbor Data Structure
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.
Reported Neighbor List (RNL)
The Reported Neighbor List for the neighbor, which is updated when
a Hello is received from the neighbor that contains an RNL TLV.
The Reported Neighbor Lists for all neighbors represent the 2-hop
neighbor information.
Report2Hop
A single-bit variable equal to 1 if a full Hello (which contains a
full Reported Neighbor List) has been received from the neighbor.
Neighbor's MDR Level
The MDR Level of the neighbor, computed from the neighbor's MDR
Parent and Backup MDR Parent. The MDR Level of a neighbor is 2 if
the neighbor is an MDR, 1 if the neighbor is a Backup MDR, and 0
otherwise.
Neighbor's MDR Parent
The neighbor's choice for MDR Parent, obtained from the DR field
of the last Hello packet received from the neighbor or from the
MDR TLV in a DD packet received from the neighbor. If this is the
neighbor itself, then the neighbor is an MDR. This replaces the
Neighbor's Designated Router data item, when the interface type is
MANET.
Neighbor's Backup MDR Parent
The neighbor's choice for Backup MDR Parent, obtained from the
Backup DR field of the last Hello received from the neighbor or
from the MDR TLV in a DD packet received from the neighbor. If
this is the neighbor itself, then the neighbor is a Backup MDR.
This replaces the Neighbor's Backup Designated Router data item,
when the interface type is MANET.
Child
A single-bit variable equal to 1 if the neighbor is a child, i.e.,
if the neighbor has selected the router as a (Backup) MDR Parent.
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Dependent
A single-bit variable equal to 1 if the neighbor is a Dependent
Neighbor, which is decided by the MDR selection algorithm.
Backup Dependent
A single-bit variable equal to 1 if the neighbor is a Backup
Dependent Neighbor, which is decided by the MDR selection
algorithm.
Dependent Selector
A single-bit variable equal to 1 if the neighbor has selected the
router to be (Backup) Dependent. If a DD packet with an MDR TLV is
received from a neighbor that is a (Backup) MDR, then that
neighbor becomes a Dependent Selector, and remains a Dependent
Selector as long as the neighbor is a (Backup) MDR and has state
2-Way or greater.
Routable
A single-bit variable equal to 1 if the neighbor is routable. A
neighbor is routable if either its state is FULL, or the routing
table includes a route to the neighbor. Only routable neighbors
are included in the router-LSA and are allowed as next hops in the
routing table.
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 state or differential
Hellos. Differential Hellos are used to reduce overhead, and they
allow Hellos to be sent more frequently (for faster reaction to
topology changes). If differential Hellos are used, full Hellos are
sent less frequently to ensure that all neighbors have current 2-hop
neighbor information.
4.1. Sending Hello Packets
Hello packets are sent according to [RFC2740] Section 3.2.1.1 and
[RFC2328] Section 9.5 with the following MANET specific
specifications beginning after paragraph 3 of Section 9.5. The Hello
packet format is defined in [RFC2740] A.3.2.
There are no changes to the Hello packet format. However, the Hello
packet reports the router's current choice for MDR Parent and Backup
MDR Parent in the Designated Router and Backup Designated Router
fields, respectively. If the router is an MDR, then its MDR Parent
is the router itself, and if the router is a Backup MDR, then its
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Backup MDR Parent is the router itself. The Hello packet's Neighbor
Router ID list is not used on the MANET interface.
Hellos are sent every HelloInterval seconds. Full state 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 contains full neighbor state information. If
2HopRefresh is set to 1, then all Hellos are full state. The first
Hello sent by a neighbor should be a full state Hello.
MANET Hellos require the use of the HNL TLV, RNL TLV, LNL TLV, and HS
TLV (see Appendix A.2.2). Depending on the need, each of these TLVs
are appended to the Hello packet with LLS (see Appendix A.2 for link-
local signaling).
4.1.1. Full State Hello Packets
The full state Hello requires the HS TLV and may include the HNL TLV
and RNL TLV appended with LLS. The L bit is set in the Hello's
option field to indicate LLS.
The HS TLV is built by populating the Sequence Number field with the
interface's Hello Sequence Number (HSN). The HSN is then
incremented.
If the router has neighbor(s) in state Init, the HNL TLV is built by
including a list of all neighbors in state Init.
If the router has neighbor(s) in state 2-Way or greater, the RNL TLV
is built by including a list of all neighbors in state 2-Way or
greater.
4.1.2. Differential Hello Packets
The differential Hello requires the HS TLV and may include the HNL
TLV, RNL TLV, and LNL TLV based on need. The D and L bits are set in
the Hello's option field to indicate differential Hellos and link-
local signaling.
The HS TLV is built by populating the Hello Sequence Number field
with the interface's HSN. The HSN is then incremented.
The HNL TLV is built by including a list of all neighbors that have
transitioned to state Init within the last HelloRepeatCount Hellos.
If none exist, the HNL TLV is not appended.
The RNL TLV is built by including a list of all neighbors that have
transitioned from Init to state 2-Way or greater within the last
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HelloRepeatCount Hellos, and all neighbors in state 2-Way or greater
such that the router is not in the neighbor's Reported Neighbor List.
If none exist, the RNL TLV is not appended.
The LNL TLV is built by including a list of all neighbors that have
transitioned to state Down within the last HelloRepeatCount Hellos.
These neighbors are found in the Lost Neighbor List. If none exist,
the LNL TLV is not appended. Neighbors that have been in the Lost
Neighbor List longer than HelloRepeatCount Hellos should be removed
from the list and not included in the LNL TLV.
4.2. Receiving Hello Packets
Hello packets are received according to [RFC2740] Section 3.2.2.1 and
[RFC2328] Section 10.5 with the following MANET specific
specifications beginning after paragraph 3 of Section 10.5. The
Hello packet format is defined in [RFC2740] A.3.2.
On a MANET interface, the source of a Hello packet is identified by
the neighbor's Router ID, and the neighbor is identified by its
Router ID.
Now the rest of the Hello Packet is examined, generating events to be
given to the neighbor and interface state machines. These state
machines are specified either to be executed or scheduled (see
[RFC2328] Section 4.4 "Tasking support"). For example, by specifying
below that the neighbor state machine be executed in line, several
neighbor state transitions may be affected by a single received
Hello.
o If the L bit is set in the options field, then there are TLVs to
be processed.
o If the LLS contains an HS TLV, the neighbor state machine is
executed with the event HelloReceived. Otherwise, an error has
occurred and the Hello should be discarded.
o The Hello Sequence Number in the HS TLV should be stored in the
neighbor's data structure.
o The DR and Backup DR fields should be processed as follows.
(1) If the DR field is equal to the neighbor's Router ID,
set the MDR Level of the neighbor to MDR.
(2) Else if the Backup DR field is equal to the neighbor's
Router ID, set the MDR Level of the neighbor to Backup MDR.
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(3) Else, set the MDR Level of the neighbor to MDR Other.
(4) If the DR or Backup DR field is equal to the router's own
Router ID, the neighbor's Child variable is set to one,
otherwise it is zero.
Further processing of the TLV depends on whether the Hello is full
state or differential, which is indicated by the value of the D
option bit.
4.2.1. Full State Hello Packets
o If the router itself appears in the HNL TLV neighbor list, or if
the router itself appears in the RNL TLV neighbor list, the
neighbor state machine should be executed with the event
2-WayReceived. Otherwise, the neighbor state machine should be
executed with the event 1-WayReceived.
o Report2Hop should be set to one.
o If the neighbor list in the RNL TLV differs from the Reported
Neighbor List for the neighbor, the receiving interface's state
machine is scheduled with the event MDRNeighborChange.
o The Reported Neighbor List for the neighbor should be replaced
with the RNL TLV neighbor list.
4.2.2. Differential Hello Packets
o If an LNL TLV exists, then perform the following steps.
(1) If the router itself appears in the LNL TLV neighbor list,
(a) The neighbor state machine should be executed with the
event 1-WayReceived.
(b) Remove the router from the Reported Neighbor List (for
the neighbor) if it is in the list.
(2) If a Router ID in the LNL TLV neighbor list is in the
Reported Neighbor List,
(a) Remove the Router ID from the Reported Neighbor List.
(b) Schedule the receiving interface's state machine
with the event MDRNeighborChange.
o If an HNL TLV exists, then perform the following steps.
(1) If the router itself appears in the HNL TLV neighbor list
and did not appear in the LNL TLV neighbor list,
(a) The neighbor state machine should be executed with the
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event 2-WayReceived.
(b) Remove the router from the Reported Neighbor List if it
is in the list.
(2) If a Router ID in the HNL TLV neighbor list is in the
Reported Neighbor List,
(a) Remove the Router ID from the Reported Neighbor List.
(b) Schedule the receiving interface's state machine
with the event MDRNeighborChange.
o If an RNL TLV exists, then perform the following steps.
(1) If the router itself appears in the RNL TLV neighbor list
and did not appear in the LNL or HNL TLV neighbor list,
(a) The neighbor state machine should be executed with the
event 2-WayReceived.
(b) Add the router itself to the Reported Neighbor List if
it does not belong.
(2) If a Router ID in the RNL TLV neighbor list is not
in the Reported Neighbor List,
(a) Add the Router ID to the Reported Neighbor List.
(b) Schedule the receiving interface's state machine
with the event MDRNeighborChange.
o If the router itself did not appear in any of the TLV neighbor
lists, 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
should be executed with the event 2-WayReceived (the state does
not change).
o If 2-WayReceived or 1-WayReceived was not executed, then the
neighbor state machine should be executed with the event
1-WayReceived.
The following applies to both full state and differential Hellos.
o If a change in the neighbor's Router Priority field was noted, the
receiving interface's state machine is scheduled with the event
MDRNeighborChange.
o If the neighbor is bidirectional and its MDR Level has changed,
then the receiving interface's state machine is scheduled with the
event MDRNeighborChange, and the neighbor state machine is
scheduled with the event AdjOK?.
o If the neighbor's Child status has changed from 0 to 1, the
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neighbor state machine is scheduled with the event AdjOK?.
o If the neighbor's state changed from less than 2-Way to 2-Way or
greater, the receiving interface's state machine is scheduled with
the event MDRNeighborChange and the neighbor state machine is
scheduled with the event AdjOK?. Else if the neighbor's state
changed from 2-Way or greater to below 2-Way, the receiving
interface's state machine is scheduled with the event
MDRNeighborChange.
5. MDR Selection Algorithm
This section describes the MDR selection algorithm, which determines
whether the router is an MDR, Backup MDR, or MDR Other on a given
interface. The algorithm also selects the (Backup) Dependent
Neighbors and the (Backup) MDR Parent, which are used to decide which
neighbors should become adjacent (see Section 7).
The MDR selection algorithm is invoked by the interface event
MDRNeighborChange as described in Section 6. After running the MDR
selection algorithm, the AdjOK? event may be invoked for some or all
neighbors as specified in Section 7.
The purpose of the MDRs is to provide a minimal set of relays for
flooding LSAs, and the purpose of the Backup MDRs is to provide
backup relays to flood LSAs when flooding by MDRs does not succeed.
The set of MDRs forms a CDS, and the set of (Backup) MDRs forms a
biconnected CDS. Note that there may be fewer Backup MDRs than MDRs,
since the MDRs themselves may already provide some redundancy.
Each MDR will become adjacent with each Dependent Neighbor that is an
MDR, forming a connected backbone network. If AdjConnectivity = 2
(the default value), then each (Backup) MDR will become adjacent with
each (Backup) Dependent Neighbor that is a (Backup) MDR, forming a
biconnected backbone network. The (Backup) MDR Parents that are
selected (as described below) will then connect each MDR Other router
with this biconnected backbone, via two adjacencies. This ensures
that the set of adjacencies forms a biconnected subgraph that spans
all routers.
The MDR selection algorithm is a distributed CDS algorithm that uses
2-hop neighbor information obtained from Hellos. More specifically,
it uses as inputs the set of bidirectional neighbors (in state 2-Way
or greater), the triplet (MDR Level, Router Priority, Router ID) for
each such neighbor and for the router itself, and the neighbor
variables Reported Neighbor List (RNL) and Report2Hop for each such
neighbor. The MDR selection algorithm can be implemented in O(d^2)
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time, where d is the number of neighbors.
The above triplet will be abbreviated as (MDR Level, RtrPri, RID).
The triplet (MDR Level, RtrPri, 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 four phases. Phase 1 creates
the neighbor connectivity matrix, which determines which pairs of
neighbors are neighbors of each other. Phase 2 decides whether the
calculating router is an MDR, and which neighbors are Dependent.
Phase 3 decides whether the calculating router is a Backup MDR, and
which neighbors are Backup Dependent. Finally, Phase 4 selects the
MDR Parent and Backup MDR Parent.
The second phase 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.
For convenience, in the following description, the term "neighbor"
will refer to a neighbor on the MANET interface that is bidirectional
(in state 2-Way or greater).
5.1. Phase 1: Creating the Neighbor Connectivity Matrix
The neighbor connectivity matrix (NCM) assigns a value of 0 or 1 for
each pair of (bidirectional) neighbors, depending on the Reported
Neighbor List (RNL) and the value of Report2Hop for each neighbor.
NCM is a symmetric matrix that defines a topology graph for the set
of neighbors (not including the router itself). A value of 1 for a
given pair of neighbors indicates that the neighbors are assumed to
be neighbors of each other in the MDR selection algorithm. The value
of the matrix is set as follows for each pair of neighbors j and k.
(1.1) If Report2Hop is 1 for both neighbors j and k: NCM(j,k) =
NCM(k,j) is 1 only if j belongs to the RNL of neighbor k and k
belongs to the RNL of neighbor j.
(1.2) If Report2Hop 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 RNL of
neighbor j.
(1.3) If Report2Hop is 0 for both neighbors j and k: NCM(j,k) =
NCM(k,j) = 0.
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In step 1.1 above, two neighbors are considered to be neighbors of
each other only if they both agree that the other router is a
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 RNL, so neighbor j is believed in deciding
whether j and k are neighbors of each other. As Step 1.3 indicates,
two neighbors are assumed not to be neighbors of each other if
neither neighbor has reported its full RNL.
5.2. Phase 2: MDR Selection
(2.1) The set of Dependent Neighbors is initialized to be empty.
(2.2) If the router has a larger value of (MDR Level, RtrPri, RID)
than all of its neighbors, then the router selects itself as an
MDR, and selects all of its neighbors as Dependent Neighbors.
Else, proceed to Step 2.3.
(2.3) Let Rmax be the neighbor that has the largest value of (MDR
Level, RtrPri, RID).
(2.4) Using NCM to determine the connectivity of neighbors, compute
the minimum number of hops, denoted hops(u), from Rmax to each
other neighbor u, using only intermediate nodes that are
neighbors with a larger value of (MDR Level, RtrPri, 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.)
(2.5) If hops(u) is at most MDRConstraint for each neighbor u, then
the router does not select itself as an MDR, and selects no
Dependent Neighbors.
(2.6) Else, the router selects itself as an MDR, and selects the
following neighbors as Dependent Neighbors: Rmax, and each
neighbor u such that hops(u) is greater than MDRConstraint.
Step 2.4 can be implemented using a breadth-first-search (BFS)
algorithm to compute min-hop paths from node Rmax to all other
neighbors, modified to allow a node as an intermediate node only if
its value of (MDR Level, RtrPri, 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 the Appendix.
5.3. Phase 3: Backup MDR Selection
(3.1) The set of Backup Dependent Neighbors is initialized to be
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empty.
(3.2) Using NCM to determine the connectivity of neighbors, determine
whether or not there exist two node-disjoint paths from Rmax to
each other neighbor u, using only intermediate nodes that are
neighbors with a larger value of (MDR Level, RtrPri, 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 neighbor u, then the router does not select itself as a
Backup MDR, and selects no Backup Dependent Neighbors.
(3.4) Else, the router selects itself as a Backup MDR (unless it
already selected itself as an MDR in Phase 2), and selects each
of the following neighbors as a Backup Dependent Neighbor
(unless the neighbor has already been selected as a Dependent
Neighbor): Rmax, and each neighbor u such that step 3.2 did not
find two node-disjoint paths from Rmax to u.
Step 3.2 can be implemented using a modification of the algorithm
[Suurballe] to find the node-disjoint paths. A detailed description
of this algorithm, which runs in O(d^2) time, is given in the
Appendix. The Appendix also describes an alternative algorithm for
Step 3.2, which is simpler but results in a larger number of Backup
MDRs.
5.4. Phase 4: Selection of the (Backup) MDR Parent
Each router will select (for each MANET interface) an MDR Parent,
which will be the router itself if the router is an MDR, and will
otherwise be a neighboring MDR if one exists. Each router will also
select a Backup MDR Parent, which will be the router itself if the
router is a Backup MDR, and will otherwise be a neighboring MDR or
Backup MDR if one exists that is not the MDR Parent.
For a given MANET interface, let Rmax (respectively Rmax2) denote the
router with the lexicographically largest (respectively second
largest), value of (MDR Level, RtrPri, RID) among all neighbors in
state 2-Way or greater. Rmax is null if there are no neighbors, and
Rmax2 is null if there is only one neighbor on the interface.
If the calculating router has selected itself as an MDR, then its MDR
Parent is equal to the router itself, and its Backup MDR Parent is
Rmax. If the calculating router has selected itself as a Backup MDR,
then its MDR Parent is Rmax, and its Backup MDR Parent is the router
itself.
If the calculating router has selected itself as an MDR Other, then
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the following algorithm is used to select the MDR Parent and Backup
MDR Parent. In the following algorithm, Old (Backup) Parent and New
(Backup) Parent denote the current and new (Backup) MDR Parent,
respectively. To maximize the lifetime of parents, the algorithm
selects the parents persistently, i.e., it does not change its MDR
Parent as long as it is still a bidirectional neighbor and an MDR,
and it does not change its Backup MDR Parent as long as it is still
bidirectional and a (Backup) MDR.
(4.1) If the Old Parent is not null, is still a bidirectional
neighbor, and is an MDR, then the New Parent is equal to the
Old Parent.
(4.2) Else, if the Old Backup Parent is not null, is still a
bidirectional neighbor, and is an MDR, then the New Parent is
equal to the Old Backup Parent.
(4.3) Else, the New Parent is equal to Rmax (defined above).
(4.4) If AdjConnectivity is 1, then the New Backup Parent is null.
(The Backup MDR Parent is always null if the option of
uniconnected adjacencies is used.)
(4.5) Else, if the Old Backup Parent is not null, is still a
bidirectional neighbor, is not equal to the New Parent, and is
either an MDR or Backup MDR, then the New Backup Parent is
equal to the Old Backup Parent.
(4.6) Else, if the Old Parent is not null, is still a bidirectional
neighbor, is a Backup MDR, and the New Parent is not equal to
the Old Parent, then the New Backup Parent is equal to the Old
Parent.
(4.7) Else, if the New Parent is not equal to Rmax, then the New
Backup Parent is equal to Rmax.
(4.8) Else, the New Backup Parent is equal to Rmax2 (which can be
null).
5.5. Requirements for Compliance
A router may use another MDR selection algorithm while still being
compliant with this document. However, the MDR selection algorithm
described above SHOULD be used to minimize overhead. Specifically,
an MDR selection algorithm is compliant with this document if it
satisfies the following conditions, where MDRSA-INF denotes the above
algorithm with MDRConstraint equal to infinity:
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o A router MUST select itself as an MDR if it would do so with
MDRSA-INF (given the same input).
o A router MUST select itself as an MDR or Backup MDR if it would
select itself as Backup MDR with MDRSA-INF.
o A router MUST select a neighbor to be Dependent if it would do so
with MDRSA-INF.
o A router MUST select a neighbor to be Dependent or Backup
Dependent, if it would select the neighbor as Backup Dependent
with MDRSA-INF.
In particular, a router may use the MDR selection algorithm with
MDRConstraint equal to 2 or 3, since this algorithm satisfies the
above conditions. (Setting MDRConstraint equal to infinity results in
the smallest number of MDRs.) In addition, the alternative algorithm
for Step 3.2 presented in Appendix B.3 is compliant with this
document. The above conditions also allow each (Backup) MDR to
select all bidirectional neighbors to be (Backup) Dependent, and thus
to form an adjacency with each neighbor that is a (Backup) MDR;
however, this will result in a larger number of adjacencies,
especially in sparse networks. All MDR selection algorithms that
satisfy the above conditions are interoperable with each other;
therefore, it is not necessary for all routers to use the same MDR
selection algorithm.
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
(after the Wait Timer expires in RouterDeadInterval seconds).
This prevents unnecessary changes in the MDR selection resulting
from incomplete neighbor information.
DR Other
The router has run the MDR selection algorithm and determined that
it is not a (Backup) MDR. The router forms adjacencies with its
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MDR Parent and Backup MDR Parent (if they exist).
Backup
The router has selected itself as a Backup MDR. The router
establishes adjacencies with all Dependent Neighbors that are
(Backup) MDRs, and with its children, i.e., neighbors that
selected the router as (Backup) MDR Parent.
DR
The router has selected itself as an MDR. The router establishes
adjacencies with all Dependent Neighbors that are (Backup) MDRs,
and with its children, i.e., neighbors that selected the router as
(Backup) MDR Parent.
6.2. Events that cause interface state changes
All interface events defined in RFC 2328, Section 9.2 apply to MANET
interfaces, except for BackupSeen and NeighborChange. BackupSeen is
never invoked for a MANET interface (since seeing a Backup MDR does
not imply that the router itself cannot also be an MDR or Backup
MDR). The event NeighborChange is replaced with the new event
MDRNeighborChange, defined as follows.
MDRNeighborChange
There has been a change in neighbor information that requires the
MDR selection algorithm to be run. The following neighbor changes
lead to the MDRNeighborChange event:
o The state of a neighbor changes from Init or lower to 2-Way or
greater, or vice versa.
o The MDR Level of a bidirectional neighbor has changed, as
detected via Hello packets from the neighbor.
o The advertised Router Priority of a bidirectional neighbor has
changed, as detected via Hello packets from the neighbor.
o The MDR Level or Router Priority of the router itself has
changed.
o The Reported Neighbor List or Report2Hop has changed for a
bidirectional neighbor, as detected via Hello packets from the
neighbor.
6.3. Changes to Interface State Machine
This section describes the changes to the interface state machine for
a MANET interface. The first two state transitions are for state-
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event pairs that are described in RFC 2328, but have modified action
descriptions because MDRs are selected instead of DRs. The third
state transition describes the action taken when the event
MDRNeighborChange is invoked, and replaces the corresponding state
transition in RFC 2328 for the event NeighborChange. 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 RFC 2328.
State: Down
Event: InterfaceUp
New state: Depends on action routine.
Action: Start the interval Hello Timer, enabling the periodic
sending of Hello packets out the interface. If the router
is not eligible to become an MDR (Router Priority is 0),
the state transitions to DR Other. Otherwise, the state
transitions to Waiting and the single shot Wait Timer is
started.
State: Waiting
Event: WaitTimer
New state: Depends on action routine.
Action: Run the MDR selection algorithm, which may result in a
change to the router's MDR Level, Dependent Neighbors,
and (Backup) MDR Parent. As a result of this calculation,
the new interface state will be DR Other, Backup, or DR.
As a result of these changes, the AdjOK? neighbor event
may be invoked for some or all neighbors. (See
Section 7.)
State(s): DR Other, Backup or DR
Event: MDRNeighborChange
New state: Depends on action routine.
Action: Run the MDR selection algorithm, which may result in a
change to the router's MDR Level, Dependent Neighbors,
and (Backup) MDR Parent. As a result of this calculation,
the new interface state will be DR Other, Backup, or DR.
As a result of these changes, the AdjOK? neighbor event
may be invoked for one or more neighbors. (See
Section 7.) To limit the amount of processing, the router
may delay running the MDR selection algorithm for up to
HelloInterval seconds. (For example, a router may wait
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until just before the next Hello is sent, allowing the
updated MDR Parents to be included in the next Hello.)
7. Adjacency Maintenance
Adjacency forming and eliminating on non-MANET interfaces remain
unchanged. Adjacency maintenance on a MANET interface requires
changes to transitions in the neighbor state machine ([RFC2328]
Section 10.3), to deciding whether to become adjacent ([RFC2328]
Section 10.4), sending of DD packets ([RFC2328] Section 10.8), and
receiving of DD packets ([OSPF] Section 10.6). The specification
below relates to the MANET interface only.
Adjacencies are established with some subset of the router's
neighbors. Each (Backup) MDR forms adjacencies with a subset of its
(Backup) MDR neighbors to form a biconnected backbone, and each MDR
Other forms adjacencies with two selected (Backup) MDR neighbors
called "parents", thus providing a biconnected subgraph of
adjacencies.
An adjacency maintenance decision is made when any of the following
four events occur between a router and its neighbor. The decision is
made by executing the neighbor event AdjOK?.
(1) The neighbor state changes from Init to 2-Way.
(2) The MDR level changes for the neighbor or for the router itself.
(3) The neighbor is selected to be the (Backup) MDR Parent.
(4) The neighbor selects the router to be its (Backup) MDR Parent.
7.1. Changes to Neighbor State Machine
The following specifies new transitions in the neighbor state
machine.
State(s): Init
Event: 2-WayReceived
New state: 2-Way
Action: Transition to neighbor state 2-Way.
State(s): 2-Way
Event: AdjOK?
New state: Depends on action routine.
Action: Determine whether an adjacency should be formed with the
neighboring router (see Section 7.2). If not, the
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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 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. If the interface event
MDRNeighborChange is scheduled, it should be executed before
proceeding. The following decisions are different based on whether
uniconnected or biconnected adjacencies are to be formed.
An adjacency is established using biconnected adjacencies if one of
the following is true.
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(1) The router is a (Backup) MDR and the neighbor is a
(Backup) MDR and a (Backup) Dependent neighbor.
(2) The neighbor is a (Backup) MDR and is a Dependent Selector.
(3) The router is a (Backup) MDR and the neighbor is a child.
(4) The neighbor is a (Backup) MDR and is the router's
(Backup) MDR Parent.
An adjacency is established using uniconnected adjacencies if one of
the following is true.
(1) The router is an MDR and the neighbor is an MDR and a
Dependent neighbor.
(2) The neighbor is a (Backup) MDR and is a Dependent Selector.
(3) The router is a (Backup) MDR and the neighbor is a child.
(4) The neighbor is a (Backup) MDR and is the router's MDR Parent.
Otherwise, an adjacency is not established and the neighbor remains
in state 2-Way.
7.3. Whether to Eliminate an Adjacency
The following defines the method to determine if an adjacency should
be eliminated between neighbors in a state above 2-way. If the
interface event MDRNeighborChange is scheduled, it should be executed
before proceeding.
An adjacency is maintained if one of the following is true.
(1) The router is an MDR.
(2) The router is a Backup MDR.
(3) The neighbor is an MDR.
(4) The neighbor is a Backup MDR.
Otherwise, the adjacency is eliminated.
7.4 Sending Database Description Packets
Sending a DD packet on a MANET interface is the same as [RFC2740]
Section 3.2.1.2 and [RFC2328] Section 10.8 with the following
additions to paragraph 3 of Section 10.8.
If the neighbor state is ExStart, the standard initialization packet
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is sent with an MDR TLV appended using LLS, and the L bit is set in
the DD packet's option field. The MDR TLV (see Appendix A) is built
as follows.
(1) If the router is an MDR, then
(a) The MDR Parent field is set to the router's Router ID.
(b) The Backup MDR Parent field is set to the router's parent if
it exists, else it is set to zero.
(2) Else if the router is a Backup MDR, then
(a) The Backup MDR Parent field is set to the router's Router ID.
(b) The MDR Parent field is set to the router's parent if it
exists, else it is set to zero.
(3) Else, the MDR Parent field is set to the MDR Parent and the
Backup MDR Parent field is set to the Backup MDR Parent.
7.5. Receiving Database Description Packets
Processing a DD packet received on a MANET interface is the same as
[RFC2328] Section 10.6, except for the changes described in this
section. The following additional steps are performed before
processing the packet based on neighbor state in paragraph 3 of
Section 10.6.
o If the DD packet's L bit is set in the options field and an MDR
TLV is appended, then the MDR TLV is processed as follows.
(1) If the MDR Parent field is equal to the neighhor's Router ID,
(a) Set the MDR Level of the neighbor to MDR.
(b) Set the neighbor's Dependent Selector variable to one.
(2) Else if the Backup MDR Parent 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 one.
(3) Else,
(a) Set the MDR Level of the neighbor to MDR Other.
(b) Set the neighbor's Dependent Selector variable to zero.
(4) If the MDR Parent or Backup MDR Parent fields are equal to
the router's Router ID, the neighbor's Child variable is set
to one, otherwise it is zero.
o If the neighbor state is Init, the neighbor event 2-WayReceived is
executed.
o If the MDR Level of the neighbor changed, the neighbor state
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machine is scheduled with the event AdjOK?.
o If the neighbor's Child status has changed from 0 to 1, the
neighbor state machine is scheduled with the event AdjOK?.
o If the neighbor's neighbor state changed from less than 2-Way to
2-Way or greater, the neighbor state machine is scheduled with the
event AdjOK?.
In addition, if the router accepts a received DD packet and processes
its contents, then the following action SHOULD be performed for each
LSA listed in the DD packet (whether the router is master or slave).
If the router has an instance of the LSA in the Database summary list
for the neighbor, which is the same or less recent than the LSA
listed in the packet, then the LSA is removed from the Database
summary list. This avoids including the LSA in a DD packet sent to
the neighbor, when the neighbor already has an instance of the LSA
that is the same or more recent. This optimization reduces overhead
due to DD packets by approximately 50% in large networks.
8. Flooding Procedure
This section specifies the changes to RFC 2328, Section 13 for
routers that support OSPF-MDR. The first part of Section 13 (before
Section 13.1) is the same except for the following three changes.
o To exploit the broadcast nature of MANETs, if the Link State
Update (LSU) packet was received on a MANET interface, then the
packet is dropped without further processing only if the sending
neighbor is in a lesser state than 2-Way. Otherwise, the LSU
packet is processed as described in this section.
o If the received LSA is the same instance as the database copy, the
following actions are performed in addition to step 7. For each
MANET interface for which a BackupWait Neighbor 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 RNL 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.
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There are no changes to Sections 13.1, 13.2, or 13.4. The following
subsections describe the changes to Sections 13.3 (Next step in the
flooding procedure), 13.5 (Sending Link State Acknowledgments), 13.6
(Retransmitting LSAs), and 13.7 (Receiving Link State
Acknowledgments) of RFC 2328.
8.1. LSA Forwarding Procedure
Step 1 of [RFC2328], Section 13.3 should be performed, with the
following change, so that the new LSA is placed on the Link State
retransmission list for each appropriate adjacent neighbor. Step
1(c) is replaced with the following action, so that the LSA is not
placed on the retransmission list for a neighbor that has already
acknowledged the LSA.
o If the new LSA was received from this neighbor, or an 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 is defined to be covered if it belongs to the
Reported Neighbor List (RNL) for the neighbor from which the new LSA
was received.
Steps 2 through 5 of [RFC2328], Section 13.3 are unchanged if the
outgoing interface (on which the LSA may be forwarded) is not of type
MANET. If the outgoing interface is of type MANET, then steps 2
through 5 are replaced with the following steps, to determine whether
the LSA should be forwarded on each eligible MANET interface.
(2) If either of the following two conditions is satisfied for every
bidirectional neighbor on the interface that is either adjacent
(in state Exchange or greater) or (backup) dependent, examine the
next interface (the LSA is not flooded out this interface).
(a) The LSA or an ACK for the LSA has been received from the
neighbor (over any interface).
(b) The LSA was received on a MANET interface, and the neighbor
is covered (defined above).
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Note that the above two conditions do not assume the outgoing
interface is the same as the receiving interface.
(3) If the LSA was received on this interface, and the router is an
MDR Other for this interface, examine the next interface (the LSA
is not flooded out this interface).
(4) If the LSA was received on this interface, and the router is a
Backup MDR 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 adjacent and (backup)
dependent neighbor on this interface that fails to satisfy both
conditions (a) and (b) in step 2. A single shot BackupWait Timer
associated with the LSA is started, which is set to expire after
BackupWaitInterval seconds plus a small amount of random jitter.
(The actions performed when the BackupWait Timer expires are
described below.) Examine the next interface (the LSA is not
immediately flooded out this interface).
(5) If the router is an MDR for this interface, or if the LSA was
originated by the router itself, then the LSA is flooded out the
interface (whether or not the LSA was received on this
interface). The LSA is included in an LSU packet which is
multicast out the interface using the destination IP address
AllSPFRouters.
(6) If the LSA was received on a MANET 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 (MDR Level, RtrPri, RID)
on the outgoing interface than every covered neighbor
(defined above) that is a neighbor on BOTH the receiving and
outgoing interfaces (or if no such neighbor exists), then the
LSA is flooded out the interface.
(b) Else, the router waits BackupWaitInterval before deciding
whether to flood the LSA on the interface, by performing the
actions in step 4 for a Backup MDR (whether or not the router
is a Backup MDR on this interface). A separate BackupWait
Neighbor List is created for each interface, but only one
BackupWait Timer is associated with the LSA. Examine the
next interface (the LSA is not immediately flooded out this
interface).
(7) If the optional step 6 is not performed, then the LSA is flooded
out the interface. The LSA is included in an LSU packet which is
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multicast out the interface using the destination IP address
AllSPFRouters.
8.1.1. 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 adjacent or (backup)
dependent, 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 (if
necessary) so that it does not occur within AckInterval plus
propagation delays.
(2) The BackupWait Neighbor list is then deleted (whether or not the
LSA is flooded).
8.1.2. Optional Treatment of Broadcast Network as MANET
In the LSA forwarding procedure described above, a router MAY treat
each of its broadcast interfaces the same as a MANET interface, with
the following substitutions. A DR is treated as an MDR, a Backup DR
is treated as a Backup MDR, and all neighbors on a broadcast
interface are considered to be covered if the LSA was sent by the DR
or Backup DR on the same interface. There are no (backup) dependent
neighbors on a broadcast interface. As in RFC 2328, Section 13.3,
only the DR and Backup DR use the IP address AllSPFRouters to flood
an LSA on a broadcast interface; all other routers use AllDRouters to
flood an LSA on a broadcast interface.
Treating a broadcast network as a MANET can greatly reduce flooding
overhead in some cases. For example, assume the LSA was received from
the DR of a broadcast network that includes 100 routers, and 50 of
the routers (not including the DR) are also attached to a MANET.
Assume that these 50 routers are neighbors of each other in the
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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
treating the broadcast network as a MANET in step 6 of the LSA
forwarding procedure, the number of routers that forward the LSA from
the broadcast network to the MANET is reduced from 50 to just 1
(assuming that at most one of the 50 routers is an MDR).
8.2. Sending Link State Acknowledgments
This section describes the procedure for sending Link State
Acknowledgments (LS ACKs) on MANET interfaces. Section 13.5 of RFC
2328 remains unchanged for non-MANET interfaces, but does not apply
to MANET interfaces. To minimize overhead due to LS ACKs, and to
take advantage of the broadcast nature of MANETs, a method similar to
that of [Chandra] is used for sending LS ACKs on MANET interfaces.
All LS ACK packets sent on a MANET interface are multicast using the
IP address AllSPFRouters.
When a router receives an LSA, it must decide whether to send a
delayed ACK, an immediate ACK, or no ACK. (However, a non-ackable
LSA is never acknowledged, as described in Appendix D.) A delayed
ACK may be delayed for up to AckInterval seconds, and allows several
LS ACKs to be grouped into a single multicast LS ACK packet. An
immediate ACK is also sent in a multicast LS ACK packet, and may
include other LS ACKs that were scheduled to be sent as delayed ACKs.
The decision depends on whether the received LSA is new (i.e., is
more recent than the database copy) or a duplicate (the same instance
as the database copy), and on whether the LSA was received as a
multicast or a unicast (which indicates a retransmitted LSA). The
following rules are used to make this decision.
(1) If the received LSA is new, a delayed ACK is sent on each
MANET interface associated with the area, unless the LSA is
flooded out the interface.
(2) If the LSA is a duplicate and was received as a multicast,
the LSA is not acknowledged.
(3) If the LSA is a duplicate and was received as a unicast:
(a) If the router is a (Backup) MDR, an immediate ACK is
sent out the receiving interface.
(b) If the router is an MDR Other, a delayed ACK is sent
out the receiving interface.
The reason that (Backup) MDRs send an immediate ACK when a
retransmitted LSA is received, is to try to prevent other adjacent
neighbors from retransmitting the LSA, since (Backup) MDRs usually
have a large number of adjacent neighbors. MDR Other routers do not
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send an immediate ACK because they have fewer adjacent neighbors, and
so the potential benefit does not justify the additional overhead
resulting from sending immediate ACKs.
8.3. Retransmitting LSAs
LSAs are retransmitted according to Section 13.6 of RFC 2328. Thus,
LSAs are retransmitted only to adjacent routers. Therefore, since
OSPF-MDR does not allow an adjacency to be formed between two MDR
Other routers, an MDR Other never retransmits an LSA to another MDR
Other, only to its parents, which are (Backup) MDRs.
Retransmitted LSAs are included in LSU packets that are sent directly
to an adjacent neighbor that did not acknowledge the LSA (explicitly
or implicitly). The length of time between retransmissions is given
by the configurable interface parameter RxmtInterval, whose default
is 5 seconds for a MANET interface. To reduce overhead, several
retransmitted LSAs should be included in a single LSU packet whenever
possible.
8.4. Receiving Link State Acknowledgments
A Link State Acknowledgment (LS ACK) packet that is received from an
adjacent neighbor (in state Exchange or greater) is processed as
described in Section 13.7 of RFC 2328, with the additional steps
described in this section. An LS ACK packet that is received from a
neighbor in a lesser state than Exchange is discarded.
Each router maintains an Acked LSA List for each adjacent neighbor,
to keep track of any LSA instances the neighbor has acknowledged, but
which the router itself has NOT yet received. This is necessary
because (unlike RFC 2328) each router acknowledges an LSA only the
first time it is received as a multicast.
If the neighbor from which the LS ACK packet was received is in state
Exchange or greater, then the following steps are performed for each
ACK in the received LS ACK packet:
(1) If the router does not have a database copy of the LSA being
acknowledged, or has a database copy which is less recent than
the one being acknowledged, the LS ACK is added to the Acked LSA
List for the sending neighbor.
(2) If the router has a database copy of the LSA being acknowledged,
which is the same as the instance being acknowledged, then the
following action is performed. For each MANET interface for which
a BackupWait Neighbor List exists for the LSA (see Section 8.1),
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remove the sending neighbor from the BackupWait Neighbor list if
it belongs to the list.
9. Originating LSAs
Unlike the DR of an OSPF broadcast network, an MDR does not originate
a network-LSA, since a network-LSA cannot be used to describe the
general topology of a MANET. Instead, each router advertises a
subset of its MANET neighbors as point-to-point links in its router-
LSA. The choice of which neighbors to advertise is flexible, and is
determined by the configurable parameter LSAFullness.
As a minimum requirement, each router must advertise all of its fully
adjacent 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.
9.1. Routable Neighbors
A neighbor is defined to be routable if either it is in state FULL,
or it is bidirectional (in state 2-Way or greater) and the SPF
calculation has produced a route to the neighbor. This condition
implies that there exists (or recently existed) a path of full
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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.
The single-bit neighbor variable Routable indicates whether the
neighbor is routable. This variable is initially 0, and is updated as
follows when the state of the neighbor changes or the SPF calculation
finds a route to the neighbor:
(1) If the state of the neighbor changes from less than 2-Way to
2-Way or greater, and there exists a route to the neighbor,
Routable is set to 1 for the neighbor.
(2) If the state of the neighbor changes to FULL, Routable is set to
1 for the neighbor.
(3) If a route to the neighbor has been calculated, and the state of
the neighbor is 2-Way or greater, Routable is set to 1 for the
neighbor.
(4) If the state of the neighbor changes from 2-Way or greater, to
less than 2-Way, Routable is set to 0 for the neighbor.
9.2. Partial and Full Topology LSAs
The choice of which MANET neighbors to include in the router-LSA is
flexible, subject only to the following requirements:
(1) A router MUST include all FULL neighbors in its router-LSA. (2)
A router MUST NOT include any non-routable neighbors in its LSA.
Thus, a minimum LSA includes only FULL neighbors, corresponding to
LSAFullness = 0. At the other extreme, a router may include all
routable neighbors in its router-LSA, corresponding to LSAFullness =
3 (full-topology LSAs). Between these two extremes, a router may
include any subset of routable neighbors in its router-LSA, as long
as all FULL neighbors are included. It is not necessary for
different routers to make the same choice; the different choices are
interoperable because each router-LSA must include all FULL
neighbors, which allows the SPF calculation to find routes to all
reachable routers.
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A new router-LSA is originated whenever an event occurs that causes
the contents of the LSA to change (which depends on the choice of the
LSA contents). However, as stated in RFC 2328, Section 12.4, two
instances of the same LSA may not be originated within the time
period MinLSInterval. This may require that the generation of the
next instance be delayed by up to MinLSInterval. When a new LSA is
originated, it is installed in the database as described in Section
13.2 of RFC 2328, which may cause the routing table to be
recalculated. The new LSA is also flooded as described in Section 8
of this document.
This document specifies two additional choices for partial-topology
LSAs, which provide shorter paths than minimal LSAs, but generate
substantially less overhead than full-topology LSAs.
9.2.1. Min-Cost LSAs (LSAFullness = 1)
Each router decides which MANET neighbors to include in its router-
LSA by looking at the router-LSAs originated by its neighbors, and
including in its router-LSA the minimum set of neighbors necessary to
provide a 2-hop path (in both directions) between each pair of
neighbors that are not neighbors of each other. If another neighbor
is already providing such a path between a given pair of neighbors,
then the router includes the pair of neighbors in its LSA only if it
can provide a lower cost path. The details of this algorithm are
given in Appendix C.
If all routers originate min-cost LSAs, then the shortest paths
calculated by each router (from its database) will have minimum cost
in the following sense. Assuming the metric that each router
advertises in its router-LSA is the same for all neighbor connections
included in the LSA (e.g., is equal to the configured cost for the
interface), then the calculated shortest paths will be such that the
sum of these metrics over all intermediate routers is minimized. For
example, if the interface cost is configured to be smaller for high
bandwidth routers than for low bandwidth routers, then the calculated
route will use high bandwidth routers whenever possible.
9.2.2. MDR Full LSAs (LSAFullness = 2)
Each (Backup) MDR originates a full LSA (which includes all routable
neighbors), while each MDR Other originates a minimum LSA (which
includes only FULL neighbors). If a router has multiple MANET
interfaces, its LSA includes all routable neighbors on the interfaces
for which it is a (Backup) MDR, and includes only FULL neighbors on
its other interfaces. When a router changes its MDR Level from MDR
Other to (Backup) MDR on a given interface, it originates a new LSA.
This choice provides routing along nearly min-cost paths, and
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typically results in less flooding overhead than min-cost LSAs.
A variation of MDR Full LSAs is possible, in which some MDR Other
routers also select themselves to originate full LSAs, based on 2-hop
neighbor information. A heuristic can be used for such a selection
that results in routes that are arbitrarily close to min-cost on
average. Such a heuristic may be described in a future version of
this draft.
10. Calculating the Routing Table
The routing table calculation is the same as specified in RFC 2328,
except for the following change to Section 16.1 (Calculating the
shortest-path tree for an area).
Recall from Section 9 that a router can use any routable neighbor as
a next hop to a destination. However, unless LSAFullness = 3 (full
topology LSAs), the router-LSA originated by the router usually does
not include all routable neighbors. Therefore, the shortest-path
tree calculation described in Section 16.1 of RFC 2328 must be
modified to allow any routable neighbor on a MANET interface to be
used as a next hop. This is accomplished simply by modifying step 2
so that the router-LSA associated with the root vertex (i.e., the
router doing the calculation) is augmented to include all routable
neighbors on each MANET interface. That is, the router-LSA used in
the SPF calculation is the one that the router would originate if
LSAFullness were equal to 3 (even if LSAFullness is actually less
than 3).
Note that, if LSAFullness is less than 3, 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
routing table must be recalculated even if the router-LSA does not
change.
11. Draft Modifications
The main changes from version 03 to version 04 of this draft are as
follows:
o The draft has been rewritten to specify complete details.
o Packet formats are now specified.
o The term MANET Designated Router (MDR) is now used instead of
Designated Router (DR) for MANET interfaces.
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o Only a single parametrized MDR selection algorithm is now
specified (previously called the MPN CDS algorithm), which
includes the Essential CDS algorithm as a special case. This
algorithm runs in O(d^2) time, where d is the number of neighbors.
o The optional ANP CDS algorithm has been omitted from the draft.
o A procedure for selecting the MDR Parent and Backup MDR Parent has
been added as Phase 4 of the MDR selection algorithm.
o The term "synchronized neighbor" has been changed to "routable
neighbor", to reflect that such a neighbor is not perfectly
synchronized, but is sufficiently synchronized to be advertised in
router-LSAs and used as a next hop.
o A new option for partial-topology LSAs, called min-cost LSAs, has
been added, which provides minimum cost routes under certain
assumptions.
References
[RFC2328] J. Moy. "OSPF Version 2", RFC 2328, April 1998.
[RFC2740] R. Coltun, D. Ferguson, and J. Moy. "OSPF for IPv6", RFC
2740, December 1999.
[LLS] Zinin, A., Friedman, B., Roy, A., Nguyen, L., and D. Yeung,
"OSPF Link-local Signaling", draft-nguyen-ospf-lls-05.txt (work
in progress), March 2005.
[Chandra] M. Chandra. "Extensions to OSPF to Support Mobile Ad Hoc
Networking", draft-chandra-ospf-manet-ext-03.txt (work in
progress), April 2005.
[Suurballe] J.W. Suurballe and R.E. Tarjan. "A Quick Method for
Finding Shortest Pairs of Disjoint Paths", Networks, Vol. 14,
pp. 325-336, 1984.
A. Packet Formats
A.1. Options Field
A new bit, called L (for LLS) is introduced to OSPFv3 Options field
(see Figure A.1). The mask for the bit is 0x200. Routers set the L
bit in Hello and DD packets to indicate that the packet contains LLS
data block. Routers set the L bit in a self-originated router-LSA to
indicate that the LSA is non-ackable.
A new D bit is defined in the OSPFv3 option field. The bit is
defined for Hello packets and indicates that only differential
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information is present. The mask for the bit is 0x400.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
| | | | | | | | | | | | | |D|L|AF|*|*|DC| R| N|MC| E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
Figure A.1: The Options field
A.2. Link-Local Signaling
Link-local signaling (LLS) describes a modification to [OSPF] which
allows the exchange of arbitrary data using existing, standard [OSPF]
packet types.
The proposal for extending [OSPF] can be found in [LLS]. Here we use
the LLS method in [OSPFv3], as is done in [Chandra].
LLS is accomplished by adding an LLS data block at the end of the
OSPFv3 packet.
The IPv6 header length includes the total length of the OSPFv3
header, OSPFv3 data, and LLS data, but the OSPFv3 header does not
contain the LLS data length in its length field. The IPv6 packet
format is depicted in Figure A.2 below.
+---------------------+ --
| IPv6 Header | ^
| Length = HL+X+Y | | Header Length = HL
| | v
+---------------------+ --
| OSPFv3 Header | ^
| Length = X | |
|.....................| | X
| | |
| OSPFv3 Data | |
| | v
+---------------------+ --
| | ^
| LLS Data | | Y
| | v
+---------------------+ --
Figure A.2: Attaching LLS Data Block
The LLS data block may be attached to OSPFv3 Hello and Database
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Description (DD) packets. The data included in the LLS block
attached to a Hello packet may be used for dynamic signaling, since
Hello packets may be sent at any moment in time. However, delivery of
LLS data in Hello packets is not guaranteed. The data sent with DD
packets is guaranteed to be delivered as part of the adjacency
forming process.
A.2.1 LLS Data Block
The data block used for link-local signaling is formatted as
described below (see Figure A.3 for illustration).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | LLS Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| LLS TLVs |
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure A.3: Format of LLS Data Block
The Checksum field contains the standard IP checksum of the entire
contents of the LLS block.
The 16-bit LLS Data Length field contains the length (in 32-bit
words) of the LLS block including the header and payload.
Implementations should not use the Length field in the IPv6 packet
header to determine the length of the LLS data block.
The rest of the block contains a set of Type/Length/Value (TLV)
triplets as described in the following section. All TLVs must be
32-bit aligned (with padding if necessary).
A.2.2 LLS TLVs
The contents of LLS data block is constructed using TLVs. See Figure
A.4 for the TLV format.
The type field contains the TLV ID which is unique for each type of
TLVs. The Length field contains the length of the Value field (in
bytes) that is variable and contains arbitrary data.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Value .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure A.4: Format of LLS TLVs
Note that TLVs are always padded to 32-bit boundary, but padding
bytes are not included in TLV Length field (though it is included in
the LLS Data Length field of the LLS block header). All unknown TLVs
MUST be silently ignored.
A.2.2.1 Heard Neighbor List TLV
A new TLV is defined in this document which indicates neighbor(s)
that are in state Init (or recently changed to Init if the Hello is
differential). This TLV is used in conjunction with a Hello packet.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Heard Neighbor(s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
+--------------------
o Type: Type, set to 11.
o Length: Set to the number of heard neighbors included in
the TLV multiplied by 4.
o Heard Neighbor(s) - Router ID of the heard neighbor.
A.2.2.2 Reported Neighbor List TLV
A new TLV is defined in this document which indicates neighbor(s)
that are in state 2-Way or greater (or recently changed to 2-Way or
greater if the Hello is differential). This TLV is used in
conjunction with a Hello packet.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reported Neighbor(s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
+--------------------
o Type: Type, set to 12.
o Length: Set to the number of reported neighbors included in
the TLV multiplied by 4.
o Reported Neighbor(s) - Router ID of the reported neighbor.
A.2.2.3 Lost Neighbor List TLV
A new TLV is defined in this document which indicates neighbor(s)
that have recently been lost by the sender. This TLV is used in
conjunction with a Hello packet.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lost Neighbor(s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ....
+--------------------
o Type: Type, set to 13.
o Length: Set to the number of lost neighbors included in
the TLV multiplied by 4.
o Lost Neighbor(s) - Router ID of the reported neighbor.
A.2.2.4 Hello Sequence TLV
A new TLV is defined that indicates the current Hello sequence number
(HSN) for the transmitting interface. This TLV is used in
conjunction with a Hello packet.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Hello Sequence Number | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Type, set to 14.
o Length: Set to 4.
o Hello Sequence Number: A circular two octet unsigned integer
indicating the current HSN for the transmitting interface. The
HSN for the interface MUST be incremented by 1 every time a
(differential or full) Hello is sent on the interface.
o Reserved: Set to 0. Reserved for future use.
A.2.2.5 MDR TLV
A new TLV is defined which reports the router's MDR Parent and Backup
MDR Parent. This TLV is used in conjunction with a Database
Description packet.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| MDR Parent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup MDR Parent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Type, set to 15.
o Length: Set to 8.
o MDR Parent: The 32-bit Router ID of the sender's MDR Parent.
This value is zero if the sender does not have an MDR Parent.
o Backup MDR Parent: The 32-bit Router ID of the sender's Backup
MDR Parent. This value is zero if the sender does not have a
Backup MDR Parent.
The two Router IDs included in the MDR TLV are the same IDs included
in the DR and Backup DR fields of a Hello.
A.3. Hello Packet DR and Backup DR Fields
The Designated Router (DR) and Backup DR fields of a Hello packet are
used to report the router's MDR Parent and Backup MDR Parent,
respectively. If the router is an MDR, then its MDR Parent is the
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router itself, and if the router is a Backup MDR, then its Backup MDR
Parent is the router itself.
A.4. LSA Formats and Examples
LSA formats are specified in [OSPFv3] Section 3.4.3. Figure A.5
below gives an example network map for a MANET in a single area.
o Four MANET nodes RT1, RT2, RT3, and RT4 are in area 1.
o RT1's MANET interface has links to RT2 and RT3's MANET interfaces.
o RT2's MANET interface has links to RT1 and RT3's MANET interfaces.
o RT3's MANET interface has links to RT1, RT2, and RT3's MANET
interfaces.
o RT4's MANET interface has a link to RT3's MANET interface.
o RT1 and RT2 have stub networks attached on broadcast interfaces.
o RT3 has a transit network attached on a broadcast interface.
..........................................
. Area 1.
. + .
. | .
. | 2+---+1 1+---+
. N1 |--|RT1|-----+ +---|RT4|----
. | +---+ | / +---+
. | | / .
. + | N3 / .
. | / .
. + | / .
. | | / .
. | 2+---+1 | / .
. N2 |--|RT2|-----+-------+ .
. | +---+ |1 .
. | +---+ .
. | |RT3|----------------
. + +---+ .
. |2 .
. +------------+ .
. |1 N4 .
. +---+ .
. |RT5| .
. +---+ .
..........................................
Figure A.5: Area 1 with IP addresses shown
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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
Router interface Interface ID IPv6 global unicast prefix
-----------------------------------------------------------
RT1 LOOPBACK 0 5f00:0001::/64
to N3 1 n/a
to N1 2 5f00:0000:c001:0200::RT1/56
RT2 LOOPBACK 0 5f00:0002::/64
to N3 1 n/a
to N2 2 5f00:0000:c001:0300::RT2/56
RT3 LOOPBACK 0 5f00:0003::/64
to N3 1 n/a
to N4 2 5f00:0000:c001:0400::RT3/56
RT4 LOOPBACK 0 5f00:0004::/64
to N3 1 n/a
RT5 to N4 1 5f00:0000:c001:0400::RT5/56
Table 2: IPv6 link prefixes for sample network
Router interface Interface ID link-local address
-------------------------------------------------------
RT1 LOOPBACK 0 n/a
to N1 1 fe80:0001::RT1
to N3 2 fe80:0002::RT1
RT2 LOOPBACK 0 n/a
to N2 1 fe80:0001::RT2
to N3 2 fe80:0002::RT2
RT3 LOOPBACK 0 n/a
to N3 1 fe80:0001::RT3
to N4 2 fe80:0002::RT3
RT4 LOOPBACK 0 n/a
to N3 1 fe80:0001::RT4
RT5 to N4 1 fe80:0002::RT5
Table 3: OSPF Interface IDs and link-local addresses
A.4.1 Router-LSAs
As an example, consider the router-LSA that node RT3 would originate.
The node consists of one MANET, one broadcast, and one loopback
interface.
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RT3's router-LSA
LS age = DoNotAge+0 ;newly originated
LS type = 0x2001 ;router-LSA
Link State ID = 0 ;first fragment
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
Options = (V6-bit|E-bit|R-bit)
Type = 1 ;p2p link to RT1
Metric = 11 ;cost to RT1
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.1 ;RT1's Router ID
Type = 1 ;p2p link to RT2
Metric = 12 ;cost to RT2
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.2 ;RT2's Router ID
Type = 1 ;p2p link to RT4
Metric = 13 ;cost to RT4
Interface ID = 1 ;Interface ID
Neighbor Interface ID = 1 ;Interface ID
Neighbor Router ID = 192.1.1.4 ;RT4's Router ID
Type = 2 ;connects to N4
Metric = 1 ;cost to N4
Interface ID = 2 ;RT3's Interface ID
Neighbor Interface ID = 1 ;RT5's Interface ID (elected DR)
Neighbor Router ID = 192.1.1.5 ;RT5's Router ID (elected DR)
A.4.2 Link-LSAs
Consider the link-LSA that RT3 would originate for its MANET
interface.
RT3's Link-LSA for its MANET interface
LS age = DoNotAge+0 ;newly originated
LS type = 0x0008 ;Link-LSA
Link State ID = 1 ;Interface ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
Rtr Pri = 1 ;default priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0001::RT3
# prefixes = 0 ;no global unicast address
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A.4.3 Intra-Area-Prefix-LSAs
A MANET node originates an intra-area-prefix-LSA to advertise its own
prefixes, and those of its attached networks or stub links. As an
example, consider the intra-area-prefix-LSA that RT3 will build.
RT2's intra-area-prefix-LSA for its own prefixes
LS age = DoNotAge+0 ;newly originated
LS type = 0x2009 ;intra-area-prefix-LSA
Link State ID = 177 ;or something
Advertising Router = 192.1.1.3 ;RT3's Router ID
# prefixes = 2
Referenced LS type = 0x2001 ;router-LSA reference
Referenced Link State ID = 0 ;always 0 for router-LSA reference
Referenced Advertising Router = 192.1.1.3 ;RT2's Router ID
PrefixLength = 64 ;prefix on RT3's LOOPBACK
PrefixOptions = 0
Metric = 0 ;cost of RT3's LOOPBACK
Address Prefix = 5f00:0003::/64
PrefixLength = 56 ;prefix on RT3's interface 2
PrefixOptions = 0
Metric = 1 ;cost of RT3's interface 2
Address Prefix = 5f00:0000:c001:0400::RT3/56 ;pad
B. Pseudocode For MDR Selection Algorithm
This section gives detailed pseudocode for Phase 2 (MDR Selection)
and Phase 3 (Backup MDR Selection) of the MDR selection algorithm
described in Section 5. The pseudocode uses a breadth-first search
(BFS) algorithm for Step 2.4 of Phase 2, and uses a variation of the
Suurballe-Tarjan algorithm [Suurballe] for finding pairs of node-
disjoint paths in Step 3.2 of Phase 3. Both algorithms run in O(d^2)
time, where d is the number of neighbors. An alternative algorithm
for Phase 3, which is simpler but results in a larger number of
Backup MDRs, is given at the end of this section.
For convenience, in the following description, the term "neighbor"
will refer to a neighbor on the MANET interface that is bidirectional
(in state 2-Way or greater). Also, node i denotes the router
performing the calculation.
The following pseudocode 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 neighbor with the (lexicographically)
largest value of (MDR Level, RtrPri, RID). The BFS algorithm uses a
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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 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 neighbors of node i and that have a larger value of
(MDR Level, RtrPri, RID) than node i. Also, parent(u) will be equal
to the parent of node u on the BFS tree, which is used in Step 3.2.
B.1. Pseudocode for Step 2.4 of the MDR Selection Algorithm
(a) Compute a matrix of link costs c(u,v) for each pair of
neighbors u and v as follows: If node u has a larger value
of (MDR Level, RtrPri, 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 neighbors u other than Rmax,
and set hops(s) = 0. Initially, parent(u) is undefined
for each neighbor u. 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 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 parent(v) = u, and add node v to the tail of the queue.
The following pseudocode performs Step 3.2 of the MDR selection
algorithm, and assumes that Phases 1 and 2 have been performed. When
the BFS algorithm terminates, hops2(u), for each neighbor node u of
node i, will be finite if and only if there exist two node-disjoint
paths from Rmax to node u, using only intermediate nodes that are
neighbors of node i and that have a larger value of (MDR Level,
RtrPri, RID) than node i.
B.2. Pseudocode for Step 3.2 of the MDR Selection Algorithm
(a) Compute a matrix of link costs c2(u,v) for each pair of
neighbors u and v as follows: If c(u,v) is infinity,
then set c2(u,v) to infinity. Otherwise set
c2(u,v) = 1 + hops(u) - hops(v).
(b) Set hops2(u) = infinity for all neighbors u other than s, and
set hop2(s) = 0. Initially, all neighbors u are unlabeled.
(c) Label node s. This divides the BFS tree (defined by the
parents selected in Phase 1) into smaller unlabeled
subtrees, one for each child of node s. For each pair
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u, v of nodes belonging to different subtrees:
If hops2(v) > c2(u,v), then set hops2(v) = c2(u,v).
(d) While there exists an unlabeled node with a finite value
of hops2:
o Let node k be the unlabeled node with the minimum value of
hops2, and label node k. This divides the unlabeled subtree
containing k into smaller unlabeled subtrees, one subtree
(called the parent subtree) containing the parent of k if
it exists and is unlabeled, and one subtree (called a child
subtree) for each unlabeled child of node k. If the parent
of k does not exist or is labeled, then continue with the
next iteration of step (d).
o For each node u in the parent subtree:
If hops2(u) > hops2(k) + c2(k,u), set
hop2(u) = hops2(k) + c2(k,u).
For each node v in one of the child subtrees:
If hop2(v) > hops2(k) + c2(u,v), set
hop2(v) = hops2(k) + c2(u,v).
If hop2(u) > hops2(k) + c2(v,u), set
hop2(u) = hops2(k) + c2(v,u).
When the above algorithm terminates, hops2(u), if finite, will be
equal to the total number of hops in both disjoint paths from Rmax to
u, minus 2 * hops(u). Thus, if hops2(u) = 0, then both disjoint paths
have the same length, hops(u). We do not give the procedure for
constructing the disjoint paths themselves, since this is not
required for the MDR selection algorithm.
We note that in step (d), the nodes of each unlabeled subtree can be
found using a depth-first search (DFS), starting from the root of the
subtree, and using labeled nodes to define the boundary of the
subtree. The tree structure is defined by the values of parent(u)
computed in Step 2.4, which can be used to define a list of children
for each node. The algorithm runs in O(d^2) time, since each pair of
nodes (u,v) is considered only once in step (d).
We next describe an alternative algorithm for Step 3.2 of Phase 3,
which is simpler but typically results in a larger number of Backup
MDRs, since it imposes a more restrictive condition on the disjoint
paths, i.e., the second path is not allowed to use any intermediate
nodes of the BFS tree computed in Phase 2.
B.3. Alternative Algorithm for Step 3.2
(a) Compute a matrix of link costs c2(u,v) for each pair of
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neighbors u and v as follows: If c(u,v) is infinity, or if
u is an intermediate node of the BFS tree computed in Phase 2
(i.e., is not Rmax and is the parent of some other node),
then set c2(u,v) to infinity. Otherwise set c2(u,v) = 1.
(b) Run BFS to compute min-hop paths from node Rmax to the other
neighbors of node i, using the link costs c2(u,v). Let
hops2(u) equal the number of hops in the resulting min-hop
path from s to u, or infinity if no finite cost path exists.
(c) Note that step (b) does not compute disjoint paths to
neighbors of node Rmax. For each neighbor u of node i that is
a neighbor of node Rmax: If there exists another neighbor v of
node i that is a neighbor of both nodes Rmax and u, and has a
larger value of (MDR Level, RtrPri, RID) than node i, then set
hops2(u) = 2; else set hops2(u) = infinity.
If hops2(u) is finite for all neighbors u, then in Step 3.3 of Phase
3, node i does not select itself as a Backup MDR, and does not select
any Backup Dependent Neighbors. Otherwise, in Step 3.4, node i
selects itself as a Backup MDR (unless it already selected itself as
an MDR in Phase 2), and selects each of the following neighbors as a
Backup Dependent Neighbor (unless the neighbor has already been
selected as a Dependent Neighbor): Rmax, and each neighbor u such
that hops2(u) equals infinity.
C. Min-Cost LSA Algorithm
This section describes the algorithm for determining which neighbors
to include in the router-LSA when LSAFullness = 1 (min-cost LSAs).
The algorithm is described for a single MANET interface, but is
easily extended to multiple interfaces. The input to this algorithm
is the set of routable neighbors, the Reported Neighbor List (RNL)
for each bidirectional neighbor, and the router-LSA originated by
each bidirectional neighbor. The output of the algorithm is the set
of advertised neighbors to be included in the router-LSA. The min-
cost LSA algorithm must be run to possibly originate a new router-LSA
whenever any of the following events occurs:
o The set of routable neighbors changes.
o The Reported Neighbor List or Report2Hop changes for a neighbor.
o A new router-LSA originated by a neighbor is received.
For convenience, in the following description, the term "neighbor"
will refer to a neighbor on the MANET interface that is bidirectional
(in state 2-Way or greater). To perform the min-cost LSA algorithm,
the following steps are performed.
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(1) Create the neighbor connectivity matrix NCM as in Section 5.1.
(2) Create the LSA cost matrix LCM as follows. Initialize LCM(j,k) to
LSInfinity for each pair of neighbors j and k. For each neighbor
j:
(a) Find the router-LSA originated by neighbor j. If the LSA does
not exist in the database, examine the next neighbor.
(b) For each point-to-point connection described in the router-
LSA, set LCM(j,k) to the metric for the connection, where k
is the neighbor advertised for the connection.
(3) Initialize the set of advertised neighbors to include all
neighbors in the FULL state. Let metric(j) denote the router's
own metric to each neighbor j.
(4) For each pair j, k of routable neighbors such that NCM(j,k) = 0,
(j and k are not neighbors of each other):
(a) Find the (bidirectional) neighbor u with the minimum value of
LCM(u,j) + LCM(u,k). If multiple neighbors achieve this
minimum value, choose the one that maximizes (MDR Level,
RID).
(b) If the router itself is currently advertising both neighbors
j and k in its router-LSA: If either metric(j) + metric(k) <
LCM(u,j) + LCM(u,k), or metric(j) + metric(k) = LCM(u,j) +
LCM(u,k) and the router itself has a larger value of (MDR
Level, RID) than neighbor u, add both j and k to the set of
advertised neighbors (j and k will continue to be
advertised).
(c) Else (the router is not currently advertising both j and k):
If metric(j) + metric(k) < LCM(u,j) + LCM(u,k), add both j
and k to the set of advertised neighbors
D. Non-Ackable LSAs for Periodic Flooding
In a highly mobile network, it is possible that a router almost
always originates a new router-LSA every MinLSInterval seconds. In
this case, it should not be necessary to send ACKs for such an LSA,
or to retransmit such an LSA as a unicast, or to describe such an LSA
in a DD packet. In this case, the originator of an LSA MAY indicate
that the router-LSA is "non-ackable" by setting the L bit in the
options field of the LSA. For example, a router can originate non-
ackable LSAs if it determines (e.g., based on an exponential moving
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average) that a new LSA is originated every MinLSInterval seconds at
least 90 percent of the time. (Simulations are needed to determine
the best threshold.)
A non-ackable LSA is never acknowledged, nor is it ever retransmitted
as a unicast or described in a DD packet, thus saving substantial
overhead. However, the originating router must periodically
retransmit the current instance of its router-LSA as a multicast
(until it originates a new LSA, which will usually happen before the
previous instance is retransmitted), and each MDR must periodically
retransmit each non-ackable LSA as a multicast (until it receives a
new instance of the LSA, which will usually happen before the
previous instance is retransmitted). The retransmission interval
should be slightly larger than MinLSInterval (e.g., MinLSInterval +
1) so that a new instance of the LSA is usually received before the
previous one is retransmitted. Note that the reception of a
retransmitted (duplicate) LSA does not result in immediate forwarding
of the LSA; only a new LSA (with a larger sequence number) may be
forwarded immediately, according to the flooding procedure of Section
8.
Authors' Addresses
Richard G. Ogier
SRI International
Email: rich.ogier@earthlink.net, richard.ogier@sri.com
Phil Spagnolo
Boeing Phantom Works
Email: phillip.a.spagnolo@boeing.com
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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