Network Working Group M. Chandra (Editor)
Internet-Draft A. Roy (Editor)
Intended status: Standards Track Cisco Systems
Expires: August 2008 February 2008
Extensions to OSPF to Support Mobile Ad Hoc Networking
draft-ietf-ospf-manet-or-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes extensions to OSPF to support mobile ad hoc
networking. Specifically, the document specifies a mechanism for
link-local signaling, a OSPF-MANET interface, a simple technique to
reduce the size of Hello packets by only transmitting incremental
state changes, and a method for optimized flooding of routing
updates.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Problem Statement . . . . . . . . . . . . . . . . . . . . 3
1.2 Motivation for extending OSPF to support MANETs . . . . . 4
2. Specification of Requirements . . . . . . . . . . . . . . . 4
3. Proposed Enhancements . . . . . . . . . . . . . . . . . . . 4
3.1 OSPF-MANET Interface . . . . . . . . . . . . . . . . . . . 5
3.1.1 Interface Operation . . . . . . . . . . . . . . . . . 6
3.1.2 LSA Formats and Examples . . . . . . . . . . . . . . . 6
3.2 Incremental OSPF-MANET Hellos . . . . . . . . . . . . . . 10
3.2.1 The I Option Bit . . . . . . . . . . . . . . . . . . . 10
3.2.2 State Check Sequence TLV . . . . . . . . . . . . . . . 11
3.2.3 Neighbor Drop TLV . . . . . . . . . . . . . . . . . . 12
3.2.4 'Request From' TLV (RF-TLV) . . . . . . . . . . . . . 12
3.2.5 Full State For TLV (FSF-TLV) . . . . . . . . . . . . . 12
3.2.6 Neighbor Adjacencies . . . . . . . . . . . . . . . . . 13
3.2.7 Sending Hellos . . . . . . . . . . . . . . . . . . . . 14
3.2.8 Receiving Hellos . . . . . . . . . . . . . . . . . . . 15
3.2.9 Interoperability . . . . . . . . . . . . . . . . . . . 17
3.2.10 Support for OSPF Graceful Restart . . . . . . . . . 17
3.3 Optimized Flooding (Overlapping Relays) . . . . . . . . . 17
3.3.1 Operation Overview . . . . . . . . . . . . . . . . . . 18
3.3.2 Determination of Overlapping Relays . . . . . . . . . 18
3.3.3 Terminology . . . . . . . . . . . . . . . . . . . . . 19
3.3.4 Overlapping Relay Discovery Process . . . . . . . . . 19
3.3.5 The F Option Bit . . . . . . . . . . . . . . . . . . . 20
3.3.6 Active Overlapping Relay Extension . . . . . . . . . . 20
3.3.7 Willingness TLV . . . . . . . . . . . . . . . . . . . 22
3.3.8 Flooding and Relay Decisions . . . . . . . . . . . . . 22
3.3.9 Intelligent Transmission of Link State
Acknowledgements . . . . . . . . . . . . . . . . . . . 24
3.3.10 Important Timers . . . . . . . . . . . . . . . . . . 24
3.3.11 Miscellaneous Protocol Considerations . . . . . . . 25
3.3.12 Interoperability . . . . . . . . . . . . . . . . . . 26
3.4 New Bits in OSPFv3 Option Field . . . . . . . . . . . . . 26
3.5 Smart Peering . . . . . . . . . . . . . . . . . . . . . . . 26
3.5.1 Introduction . . . . . . . . . . . .. . . . . . . . . . . 26
3.5.2 Previous Related Work . . . . . . .. . . . . . . . . . . 27
3.5.3 Proposed Solution . . . . . . . . .. . . . . . . . . . . 27
4. Security Considerations . . . . . . . . . . . . . . . . . . 30
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 31
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
References . . . . . . . . . . . . . . . . . . . . . . . . . 32
Normative References . . . . . . . . . . . . . . . . . . 32
Informative References . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . 34
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1. Introduction
Mobile Ad Hoc Networks have been an area of study for some time,
within various working groups and areas within the IETF, various
Military branches, and various government agencies. Recently,
networks with mobile ad hoc requirements are being proposed and
seriously considered for deployment in the near term, which means the
concepts and research now needs to be applied to deployed networks.
Towards that end, this draft applies many of the principles and
concepts learned through prior work to [OSPFv3], along with new
concepts based on current requirements.
1.1 Problem Statement
MANETs are synonymous with packet radio networks, which have been
around since the 1960s in a limited military capacity. With the boom
of mobile devices and wireless communications, MANETs are finding
scope in commercial and military environments. The aim of these
networks is to support robust and efficient communication in a mobile
wireless network by incorporating routing functionality into mobile
nodes.
A MANET is an autonomous set of nodes distributed over a wide
geographical area that communicate over bandwidth-constrained
wireless links. Each node may represent a transmitter, receiver, or
relay station with varying physical capabilities. Packets may
traverse through several intermediate (relay) nodes before reaching
their destination. These networks typically lack infrastructure:
nodes are mobile, there is no central hub or controller, and thus
there is no fixed network topology. Moreover, MANETs must contend
with a difficult and variable communication environment. Packet
transmissions are plagued by the usual problems of radio
communication, which include propagation path loss, signal multipath
and fading, and thermal noise. These effects vary with terminal
movement, which also induces Doppler spreading in the frequency of
the transmitted signal. Finally, transmissions from neighboring
terminals, known as multi-access interference, hostile jammers, and
impulsive interference, e.g., ignition systems, generators, and other
non-similar in-band communications, may contribute additional
interference.
Given this nature of MANETs, the existence of a communication link
between a pair of nodes is a function of their variable link quality,
including signal strength and bandwidth. Thus, routing paths vary
based on environment, and the resulting network topology. In such
networks, the topology may be stable for periods of time, and then
suddenly become unpredictable. Since MANETs are typically
decentralized systems, there are no central controllers or specially
designated routers to determine the routing paths as the topology
changes. All of the routing decisions and forwarding (relaying) of
packets must be done by the nodes themselves, and communication is on
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a peer-to-peer basis.
1.2 Motivation for extending OSPF to support MANETs
The motivation to extend a standard protocol, OSPF (described in
[OSPF] and [OSPFv3]) to operate on MANET networks is twofold. The
primary reason is for interoperability--MANET devices need to be able
to work when plugged into a wireline network in as many cases as
possible. The junction point between a MANET and wire-line network
should also be as fluid as possible, allowing a MANET network to
"plug in" to just about any location within a wire-line network, and
find connectivity, etc., as needed.
While routes could be redistributed between two routing protocols,
one designed just for wire-line networks, and the other just for
MANET networks, this adds complexity and overhead to the MANET/
wireline interface, increases the odds of an error being introduced
between the two domains, and decreases flexibility.
The second motivation is that OSPF is a well understand and widely
deployed routing protocol. This provides a strong basis of
experience and skills from which to work. A protocol which is known
to work can be extended, rather than developing a new protocol, which
must then be completely troubleshoot, tested, and modified over a
number of years. Working with a well known protocol allows
development effort to be placed in a narrowly focused area, rather
than rebuilding, from scratch, many things which are already known to
work.
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEY].
3. Proposed Enhancements
This document proposes modifications to [OSPFv3] to support mobile ad
hoc networks (MANETs). Note that it is possible to use the mechanism
defined in Section "Incremental OSPF-MANET Hellos" and the mechanism
defined in Section "Optimized Flooding (Overlapping Relays)"
independently of one another.
The challenges with deploying standard [OSPFv3] in a MANET
environment fit into two categories. First, traditional link-state
routing protocols are designed for a statically configured
environment. As a result, most of the configuration is done manually
when a new router is placed in the network. Thus, OSPF will not
function in an environment where routers interconnect and disconnect
in somewhat random topologies and combinations. There are
modifications that must be made in order for routers running the same
protocol to communicate in a heterogeneous and dynamic environment.
Second, a MANET network must transmit more state information to
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maintain reachability. Therefore, OSPF will need scalability
enhancements to support MANETs.
Currently there is no defined interface type that describes a
wireless network. Wireless links have characteristics of both multi-
access and point-to-multipoint links. Treating wireless links as
multi-access does not take into account that not all nodes on the
same Layer 2 link have bi-directional connectivity. However, any
transmission on a link will reach nodes that are within transmission
range. In this way, the link is multi-access due to the fact that
two simultaneous transmissions may collide. A new interface type
needs to be defined in order to accurately describe this behavior.
The second category of challenges fall into is scalability. While
some flooding optimizations are present in OSPF, such as designated
router (DR) election, many of these were built under the assumption
of a true multi-access network. Wireless networks are not true
multi-access because it cannot be assumed that there is two-way
connectivity between everyone on the same Layer 2 link. Therefore,
optimizations such as DR election will not perform correctly in MANET
networks. Without any further optimizations in link-state flooding,
current OSPF would not be able to operate in a highly dynamic
environment in which links are constantly being formed and broken.
The amount of information that would need to be flooded would
overload the network.
Another scalability issue is the periodic transmission of Hello
messages. Currently, even if there are no changes in a router's
neighbor list, the Hello messages still list all the neighbors on a
particular link. For a MANET router, where saving bandwidth and
transmission power is a critical issue, the transmission of
potentially large Hello messages is particularly wasteful.
Finally, current routing protocols will form a neighbor relationship
with any router on a Layer 2 link that is correctly configured. For
MANET routers in a wireless network, this may lead to an excessive
number of parallel links between two routers if communication is
achieved via multiple interfaces. In a statically configured
network, this is not a problem, since the physical topology can be
built to prevent excessive redundancy. However, in a dynamic
network, there must exist additional mechanisms to prevent too many
redundant links. (Note that links between two nodes on different
radio types, different antenna, different channels, etc., are
considered different links and not redundant links.) In scalability
tests, it has been demonstrated that the presence of too many
redundant links will increase both the size of routing updates and
cause extra flooding resulting in even relatively small networks not
converging.
3.1 OSPF-MANET Interface
Interfaces are defined as the connection between a router and one of
its attached networks [OSPF]. Four types of interfaces have been
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defined and supported in [OSPF] and [OSPFv3]: broadcast, NBMA, point-
to-point, and point-to-multipoint.
Point-to-multipoint model has been chosen to represent MANET
interfaces. (The features designed in this document MAY be included
on other interface types as appropriate.) The MANET interface allows
the following:
o OSPF treats all router-to-router connections over the MANET
interface as if they were point-to-point links.
o Link metric can be set on a per neighbor basis.
o Broadcast and multicast can be accomplished through Layer-2
broadcast or Layer-2 pseudo-broadcast.
* The MANET interface supports Layer-2 broadcast if it is able to
address a single physical message to all of the attached
neighbors. One such example is 802.11.
* The MANET interface supports Layer-2 pseudo-broadcast if it is
able to pick up a packet from the broadcast queue, replicate
the packet, and send a copy over each point-to-point link. One
such example is Frame Relay.
o An API must be provided for Layer-3 to determine the layer-2
broadcast capability. Based on the return of the API, OSPF
classifies the MANET interfaces into the following three types:
MANET broadcast, MANET pseudo-broadcast, and MANET non- broadcast.
o Multicast SHOULD be used for OSPF packets. When the MANET
interface supports Layer-2 broadcast or pseudo-broadcast, the
multicast process is transparent to OSPF. Otherwise, OSPF MUST
replicate multicast packets by itself.
3.1.1 Interface Operation
A MANET node has at least one MANET interface. MANET nodes can
communicate with each other through MANET interfaces. MANET nodes
can communicate with non-MANET routers only through normal
interfaces, such as Ethernet, ATM and etc.
For scalability reasons, it is not required to configure IPv6 global
unicast addresses on MANET interfaces. Instead, a management
loopback interface, with an IPv6 global unicast address, MAY be
configured on each MANET node.
The LSAs associated with a MANET interface SHOULD have the DC-bit set
in the OSPFv3 Options Field and the DoNotAge bit set in the LS Age
field as described in [OSPFv3].
3.1.2 LSA Formats and Examples
LSA formats are specified in [OSPFv3].
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In order to display example LSAs, a network map is included below.
Router names are prefixed with the letters RT, network names with the
letter N, and router interface names with the letter I.
o Four MANET nodes, RT1, RT2, RT3, and RT4, reside in area 2.
o RT1 has one MANET interface I11. Through the interface, RT1 is
full adjacent to RT2, RT3, and RT4.
o RT2 has two MANET interfaces, I21 and I22, and one Ethernet
interface I23. RT2 is full adjacent to RT1 and RT4 through the
interface I21, and full adjacent to RT4 through the interface I22.
Stub network N1 is attached with RT2 through the interface I23.
o RT3 has one MANET interface I31, and is full adjacent to RT1
through the interface.
o RT4 has two MANET interfaces, I41 and I42. It is full adjacent to
RT2 through the interface I41, and full adjacent to RT1 and RT2
through the interface I42.
o Moreover, each MANET node is configured with a management loop-
back interface.
+---+I11 I21+---+I23 |
|RT1|-+----------+-|RT2|------|N1
+---+ | | +---+ |
| | VI22
| | +
| | |
| | |
| | |
| | |
| | +
| | ^I41
+---+ | +---+
|RT3|-+ +-|RT4|
+---+I31 I42+---+
The assignment of IPv6 global unicast prefixes to network links is
shown below. (Note: No IPv6 global unicast addresses are configured
on the MANET interfaces)
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Node Interface IPv6 global unicast prefix
-----------------------------------------------------------
RT1 LOOPBACK 5f00:0001::/64
I11 n/a
RT2 LOOPBACK 5f00:0002::/64
I21 n/a
I22 n/a
I23 5f00:0012::/60
RT3 LOOPBACK 5f00:0003::/64
I31 n/a
RT4 LOOPBACK 5f00:0004::/64
I41 n/a
I42 n/a
The OSPF interface IDs and the link local addresses for the router
interfaces in the network illustrated above below. EUIxy represents
the 64-bit interface identifier of the interface Ixy, in Modified
EUI-64 format [IPV6ADD].
Node Interface Interface ID Link Local address
-----------------------------------------------------------
RT1 LOOPBACK 1 n/a
I11 2 fe80:0002::EUI11
RT2 LOOPBACK 1 n/a
I21 2 fe80:0002::EUI21
I22 3 fe80:0003::EUI22
I23 4 fe80:0004::EUI23
RT3 LOOPBACK 1 n/a
I31 2 fe80:0002::EUI31
RT4 LOOPBACK 1 n/a
I41 2 fe80:0002::EUI41
I42 3 fe80:0003::EUI42
3.1.2.1 Router LSAs
As an example, consider the router LSAs that node RT2 would
originate. Two MANET interfaces, consisting of 3 point-to-point
links, are presented.
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RT2's router-LSA
LS age = DoNotAge+0 ;newly originated
LS type = 0x2001 ;router-LSA
Link State ID = 0 ;first fragment
Advertising Router = 192.0.2.2 ;RT2's Router ID
bit E = 0 ;not an AS boundary router
bit B = 0 ;not an area border router
Options = (V6-bit|E-bit|R-bit)
Type = 1 ;p2p link to RT1 over I21
Metric = 10 ;cost to RT1
Interface ID = 2 ;Interface ID of I21
Neighbor Interface ID = 2 ;Interface ID of I11
Neighbor Router ID = 192.0.2.1 ;RT1's Router ID
Type = 1 ;p2p link to RT4 over I21
Metric = 25 ;cost to RT4
Interface ID = 2 ;Interface ID of I21
Neighbor Interface ID = 3 ;Interface ID of I42
Neighbor Router ID = 192.0.2.4 ;RT4's Router ID
Type = 1 ;p2p link to RT4 over I22
Metric = 15 ;cost to RT4
Interface ID = 3 ;Interface ID of I22
Neighbor Interface ID = 2 ;Interface ID of I41
Neighbor Router ID = 192.0.2.4 ;RT4's Router ID
3.1.2.2 Link LSAs
A MANET node originates a separate Link-LSA for each attached
interface. As an example, consider the Link-LSA that RT3 will build
for its MANET interface I31.
RT3's Link-LSA for MANET interface I31
LS age = DoNotAge+0 ;newly originated
LS type = 0x0008 ;Link-LSA
Link State ID = 2 ;Interface ID of I31
Advertising Router = 192.0.2.3 ;RT3's Router ID
Rtr Pri = 1 ;default priority
Options = (V6-bit|E-bit|R-bit)
Link-local Interface Address = fe80:0002::EUI31
# prefixes = 0 ;no global unicast address
3.1.2.3 Intra Area Prefix LSAs
A MANET node originates an intra area prefix LSA to advertise its own
prefixes, and those of its attached stub links. As an example,
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consider the intra area prefix LSA that RT2 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 else
Advertising Router = 192.0.2.2 ;RT2'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 = 192.0.2.2
;RT2's Router ID
PrefixLength = 64 ;prefix on RT2's LOOPBACK
PrefixOptions = 0
Metric = 0 ;cost of RT2's LOOPBACK
Address Prefix = 5f00:0002::
PrefixLength = 60 ;prefix on I23
PrefixOptions = 0
Metric = 10 ;cost of I23
Address Prefix = 5f00:0012:: ;pad to 64-bit
Note: MANET nodes may originate Intra-Area-Prefix-LSAs for attached
transit (broadcast/NBMA) networks. This is normal behavior defined
in [OSPFv3], which is irrelevant to MANET interfaces. Please consult
[OSPFv3] for details.
3.2 Incremental OSPF-MANET Hellos
In MANETs, reducing the size of periodically transmitted packets can
be very important in decreasing the total amount of overhead associ-
ated with routing. Towards this end, removing the list of neighbors
from Hello packets unless that information changes can reduce routing
protocol overhead. While the reduction for each hello packet is
small, over time it will be significant.
A new option bit is defined in this document to facilitate the
operation of incremental Hello packets. A new SCS-TLV and Neighbor
Drop TLV are also defined, transmitted using LLS [LLS].
3.2.1 The I Option Bit
A new I bit is defined in the OSPFv3 option field. The bit is
defined for Hello packets and indicates that only incremental
information is present. See Section "New Bits in OSPFv3 Options
Field" for placement of the I bit within the OSPFv3 options field.
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3.2.2 State Check Sequence TLV
A new TLV is defined that indicates the current state, which is
represented by an State Check Sequence (SCS) number of the
transmitting router.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCS Number |R|FS|N | Reserved |
+-----------------------------------------------------------------+
o Type: Type, set to 2.
o Length: Set to 4.
o SCS Number: A circular two octet unsigned integer indicating the
current state of the transmitting device. Note that when the
incremental Hello mechanism is invoked (or re-started), an initial
SCS value of '1' SHOULD be used for the first incremental Hello
packet. This sequence number is referred to as InitialSCS. Note
that InitialSCS also implies a full state.
o R: Request bit. If set, this is a request for current state. The
list of routers who should respond to this request are listed in
the 'Request From' TLV defined below. If the 'Request From' TLV
is not present, it is assumed that the request is meant for all
nodes.
o FS: Full State bit. If set, the Hello packet contains full state
as far as the neighbor(s) in the 'Full State For' TLV (defined
below) are concerned. If the 'Full State For' TLV is not present,
the Hello packet contains full state for all neighbors.
o N: InComplete bit. If NOT set, the complete state associated with
the SCS number is included in the Hello packet. If set, indicates
that the appended TLVs are being sent 'persistently', and that
there is more state associated with the SCS number that was sent
originally, but is not included in this Hello packet. This bit
allows any desired TLVs to be sent 'persistently' for a number of
Hellos with the same SCS number without requiring all of the TLVs
associated with that SCS number to be transmitted. The first time
an SCS number is sent, the entire state associated with that SCS
number is transmitted, and the 'N' bit MUST NOT be set.
o Reserved: Set to 0. Reserved for future use.
A Hello with the State Check Sequence TLV appended with the R bit set
will be referred to as a Hello request.
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3.2.3 Neighbor Drop TLV
A new TLV is defined in this document which indicates neighbor(s)
that have been removed from the list of known neighbors.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dropped Neighbor(s) |
+---------------------------------------------------------------+
| ....
+--------------------
o Type: Type, set to 3.
o Length: Set to the number of dropped neighbors included in the TLV
multiplied by 4.
o Dropped Neighbor(s) - Router ID of the neighbor being dropped.
3.2.4 'Request From' TLV (RF-TLV)
A new TLV is defined in this document which indicates neighbor(s)
from which the latest Hello state is being requested.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request From Neighbor(s) |
+---------------------------------------------------------------+
| ....
+--------------------
o Type: set to 4
o Length: Set to the number of neighbors included in the TLV
multiplied by 4.
o Request From Neighbor(s) - Router ID of the neighbor(s) from which
o Hello state is being requested.
3.2.5 Full State For TLV (FSF-TLV)
A new TLV is defined in this document which indicates neighbor(s) to
which the transmitting node is responding with full state.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Full State For Neighbor(s) |
+---------------------------------------------------------------+
| ....
+--------------------
o Type: set to 5
o Length: Set to the number of neighbors included in the TLV
multiplied by 4.
o Full State For Neighbor(s) - Router ID of the neighbor(s) should
process this packet.
3.2.6 Neighbor Adjacencies
This section describes building neighbor adjacencies and the failure
of such adjacencies using the incremental Hello signaling.
3.2.6.1 Building Neighbor Adjacencies
Hello packets are sent periodically in accordance with [OSPF] and
[OSPFv3]. An OSPF implementation which supports sending only partial
neighbor information in Hello packets SHOULD always set the I bit in
its transmitted Hello packets, except as described elsewhere in this
document. Hello packets MAY be suppressed from being transmitted
every HelloInterval if other packet transmissions are sent by the
router during that time.
On receiving a Hello packet from a new neighbor (in this context, a
new neighbor is a neighbor in less than Init state as defined in
Section 10.1 [OSPF]), if the Hello has the I bit set, a router will:
o Place the new neighbor in the neighbor list described in [OSPFv3],
A.3.2.
o Increment the router's SCS number that it will use in its next
Hello (indicated in the State Check Sequence TLV).
o When the neighbor has reached the EXCHANGE state, described in
[OSPF], 10.1, it is removed from the list of neighbors described
in [OSPFv3], A.3.2.
o If the neighbor is not a DR or backup designated router (BDR) on
an OSPF broadcast link, and the neighbor is advertised as
connected in the Network LSA advertised by the DR, it is removed
from the list of neighbors described in [OSPFv3], A.3.2.
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3.2.6.2 Adjacency Failure
On discovering an adjacency failure (going to state less than
EXCHANGE), a router using I bit signaling SHOULD:
o Remove the adjacent router from local tables, and take the
appropriate actions for a failed adjacency described in [OSPF] and
[OSPFv3].
o Add the formerly adjacent router to a Neighbor Drop TLV.
o Increment the router's SCS number that it will transmit in its
next Hello.
o Transmit Hellos with this Neighbor Drop TLV - It may be desirable
to send the Neighbor Drop TLV in three consecutive Hellos to
increase the probability of reception. In this case, 'persistent'
Hello packets would be sent with the same SCS number, the Neighbor
Drop TLV, and the 'N' bit set. Thus, the receiver knows that the
Neighbor Drop TLV is being sent persistently and there is more
state associated with the SCS in case it must request missing
state presumably transmitted in a previous Hello.
3.2.7 Sending Hellos
When a device is first attached to a network (whether through being
brought into range of another device, powering the device on, or
enabling the device's radio interface, etc.), it will need to obtain
complete neighbor state from each of its neighbors before it can
utilize the incremental Hello mechanism. Thus, upon initialization,
a device MAY send a multicast Hello request (and omit the 'Request
From' TLV). Neighbors will receive the request and respond with a
Hello with their complete neighbor state.
If a device is in INIT state with a neighbor and receives a Hello
from the neighbor without its router ID listed in the neighbor list,
the device SHOULD request the current state from the neighbor. Note
that this is to avoid a race condition since the received Hello can
either mean that the device is NOT SEEN by the neighbor, or that the
device is adjacent and not listed in the incremental list. Thus, by
receiving a Hello request, the neighbor will respond with its
neighbor state for the neighbor.
The first Hello packet with a particular SCS number MUST contain the
full state associated with that SCS number, i.e., all state changes
since the last SCS number. The 'N' bit MUST NOT be set in the State
Check Sequence TLV.
Incremental Hello packets can be sent persistently (sent in k
successive Hello packets), with flexibility in the actual amount of
information being sent. The three options include:
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o The entire incremental Hello packet is sent persistently. This is
accomplished by simply sending the entire state associated with a
SCS number for k successive Hellos. Since the SCS number remains
the same, the 'N' bit is not set in these incremental Hello
packets.
o Partial information for a particular SCS number is sent
persistently. After the first Hello packet with a particular SCS
number is sent, only the TLVs that are desired to be sent
persistently are sent in subsequent Hellos with the same SCS
number and the 'N' bit set.
o No information is sent persistently. This is simply the default
behavior where an incremental Hello packet with a particular SCS
number is only sent once.
3.2.8 Receiving Hellos
Each OSPF device supporting incremental Hello signaling, as described
in this document, MUST keep the last known SCS number from each
neighbor it has received Hellos from as long as the neighbor
adjacency structure is maintained.
If a device receives a Hello from an adjacent neighbor with an SCS
number less than the last known SCS number from that neighbor, it
MUST first check if the SCS number is a wrap around. If it is NOT a
wrap around, then the device MUST send a Hello request to the
neighbor.
If it is a wrap around or if a device receives a Hello from an
adjacent neighbor with an SCS number one greater than the last known
SCS number from that neighbor, it MUST:
o Examine the neighbor list described in [OSPFv3], A.3.2. If any
neighbors are contained in this list, increment the SCS number
contained in the adjacent neighbor's data structure.
o Examine the Neighbor-Drop TLV as described in the section
Adjacency Failure. If this list contains a neighbor other than
the local router, increment the SCS number contained in the
adjacent neighbor's data structure.
o Examine the Neighbor Drop TLV as described in the section
Adjacency Failure. If the local router identifier is contained in
this list, destroy the transmitting adjacent neighbor's data
structures.
o Examine any other TLVs incrementally signaled, as described in
drafts referring to this document. If there are other state
changes indicated, increment the SCS number contained in the
adjacent neighbor's data structure.
o If no state change information is contained in the received Hello,
a request for current state (by setting the 'R' bit) is sent in
the next Hello.
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If a device receives a Hello from an adjacent neighbor with an SCS
number greater than the last known SCS number + 1 from that neighbor,
it MUST send a Hello request to the neighbor since it may be missing
some neighbor state.
3.2.8.1 Receiving Hellos with the N Bit Set
If a device receives a Hello with the State Check Sequence TLV
included, and the 'N' bit set in this TLV, it MUST verify that it has
already received the SCS number with the 'N' bit NOT set from the
neighbor. If the device determines that this is the first reception
of the SCS number from this neighbor, then it MUST send a Hello
request to the neighbor since it missed the initial Hello packet with
the SCS number, and thus is missing state.
3.2.8.2 Receiving Hellos with the R Bit Set
If a device receives a Hello with the State Check Sequence TLV
included, and the R bit set, it looks for the 'Request From' TLV. If
it's router ID is listed in the 'Request From' TLV or the TLV is not
found, it includes its full state in the next Hello. This MUST
include:
o The neighbor ID of the requesting neighbor(s) in the list of
neighbors described in [OSPFv3], A.3.2.,
o An State Check Sequence TLV with the transmitter's current SCS
number, and the FS bit set. Note that the transmitter's SCS
number is NOT incremented.
o Any other TLVs, defined in other drafts referencing this document,
indicating the current state of the local system.
o The neighbor ID of all the neighbors who have requested current
state, in the 'Full State For' TLV.
If the full state is being sent to a large number of existing
neighbors, and implementation could choose to instead generate a full
state for all neighbors and omit the 'Full State For' TLV.
3.2.8.3 Receiving Hellos with the FS Bit Set
When a device receives a Hello with the State Check Sequence TLV
included, and the FS bit set, the Hello packet contains the
neighbor's full state for the device. The packet SHOULD be processed
as follows:
o If the received SCS number is equal to the last known SCS number,
the packet SHOULD be ignored since the device already has the
latest state information.
o If the received SCS number is different than the last known SCS
number, this Hello has new information and MUST be parsed.
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o If it is listed in the 'Full State For' TLV or if the 'Full State
For' TLV is not present, the device MUST save the SCS number,
process the Hello as described in the Receiving Hellos section,
and process any other appended TLVs.
3.2.9 Interoperability
On receiving a Hello packet from a new neighbor without the I bit
set, the local router will continue to place that router's identifier
in transmitted Hellos on this link as described in [OSPFv3], A.3.2.
3.2.10 Support for OSPF Graceful Restart
OSPF graceful restart, as described in [OSPFREST] and [OSPFGR],
relies on the lack of neighbors in the list of neighbors described in
[OSPFv3], A.3.2, to determine that an adjacent router has restarted,
and other signaling to determine the adjacency should not be torn
down. If all Hello packets transmitted by a given router have an
empty Hello list, reliance on an empty Hello packet to signal a
restart (or to reliably tear down an OSPF adjacency) is no longer
possible. Hence, this signaling must be slightly altered.
When a router would like to tear down all adjacencies, or signal it
has restarted:
o On initially restarting, during the first RouterDeadInterval after
restart, the router will transmit Hello packets with an empty
neighbor list, and the I bit cleared. Any normal restart or other
signaling may be included in these initial Hello packets.
o As adjacencies are learned, these newly learned adjacent routers
are included in the multicast Hellos transmitted on the link.
o After one RouterDeadInterval has passed, the incremental Hello
mechanism is invoked. An incremental Hello packet with full state
is sent with the 'I' bit set, and the SCS TLV included with the
'A' bit set and the InitialSCS value, e.g., SCS of '1'.
Subsequent Hello packets will include only incremental state.
Routers which are neighboring with a restarting router MUST continue
sending their Hello packets with the I bit set.
3.3 Optimized Flooding (Overlapping Relays)
A component that may influence the scalability and convergence
characteristics of OSPF [OSPF],[OSPFv3] in a MANET environment is how
much information needs to be flooded. The ideal solution is that a
router will only receive a particular routing update only once.
However, there must be a tradeoff between protocol complexity and
ensuring that every speaker in the network receives all of the
information. Note that a speaker refers to any node in the network
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that is running the routing protocol and transmitting routing updates
and Hello messages.
Controlling the amount of information on the link has increased
importance in a MANET environment due to the potential transmission
costs and resource availability in general.
In some environments, a group of speakers that share the same logical
segment may not be directly visible to each other; some of the
possible causes are the following: low signal strength, long distance
separation, environmental disruptions, partial VC meshing, etc. In
these networks, a logical segment refers to the local flooding domain
dynamically determined by transmission radius. In these situations,
some speakers (the ones not able to directly reach the sender) may
never be able to synchronize their databases. To solve the
synchronization issues encountered in these environments, a mechanism
is needed through which all the nodes on the same logical segment can
receive the routing information, regardless of the state of their
adjacency to the source.
3.3.1 Operation Overview
The optimized flooding operation relies on the ability of a speaker
to advertise all of its locally connected neighbors. In OSPF, this
ability is realized through the use of link state advertisements
(LSA)s [OSPF],[OSPFv3].
A speaker receives router LSAs from its adjacent neighbors. A
speaker's router LSA conveys the list of the adjacent speakers of the
originator ("neighbor list"). The local speaker can compare the
neighbor list reported by each speaker to its own neighbor list. If
the local neighbor list contains adjacent speakers that the
originator cannot reach directly (i.e. those speakers that are not in
the originator's neighbor list), then these speakers are locally
known as non-overlapping neighbors for the originator.
The local speaker should relay any routing information to non-
overlapping neighbors of the sender based on the algorithm outlined
in the Flooding and Relay Decisions section. Because more than one
such speaker may exist, the mechanism is called "overlapping relays."
The algorithm, however, does select the set of overlapping relays
that should transmit first. This set is known as the active set of
overlapping relays for a speaker.
3.3.2 Determination of Overlapping Relays
The first step in the process is for each speaker to build and
propagate their neighbor lists in router LSAs packets. Every speaker
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is then in a position to determine their two-hop neighborhood, i.e.,
those nodes that are neighbors of the speaker's one-hop neighbors. A
neighbor is considered an overlapping relay for a speaker if it can
reach a node in the two-hop neighborhood of the neighbor, i.e., if it
has one-hop neighbors.
The set of active overlapping relays for a speaker is the minimum set
of direct neighbors such that every node in the two-hop neighborhood
of the speaker is a neighbor of a least one overlapping relay in the
active set. Each speaker SHOULD select a set of active overlapping
relays based on a selection algorithm (one such algorithm is
suggested in the "Overlapping Relay Discovery Process" Section and is
based on the MPR selection algorithm described in [OLSR]). Note that
a speaker MUST NOT choose a neighbor to serve as an active
overlapping relay if that neighbor set the 'N' bit in its Active
Overlapping Relay TLV as defined in Section "Active Overlapping Relay
Extension", unless the neighbor is the only neighbor to reach a two-
hop neighbor. Election of active overlapping relays is done across
interfaces, and thus, it is node-based and not link-based.
3.3.3 Terminology
The following heuristic and terminology for active overlapping relay
selection is largely taken from [OLSR]:
o FULL: Neighbor state FULL as defined in [OSPF] & [OSPFv3]. Note
that all neighbor references in this document are assumed to be
FULL neighbors.
o 2-hop FULL neighbors: The list of 2-hop neighbors of the node that
are FULL and that can be reached from direct neighbors, excluding
any directly connected neighbors.
o Active Set: A (sub)set of the neighbors selected such that through
these selected nodes, all 2-hop FULL neighbors are reachable.
o N: N is the set of FULL neighbors of the node.
o N2: A subset of 2-hop FULL neighbors excluding the nodes only
reachable by members of N.
o D(y): The degree of a one hop neighbor node y (where y is a member
of N), is defined as the number of FULL neighbors of node y,
EXCLUDING all the members of N and EXCLUDING the node performing
the computation.
3.3.4 Overlapping Relay Discovery Process
A possible algorithm for discovering overlapping relays is the
following:
1. Start with an active set made of all members of N that have set
the 'A' bit in their Active Overlapping Relays TLV as defined in
Section "Active Overlapping Relays Extension".
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2. Calculate D(y), where y is a member of N, for all nodes in N.
3. Add to the active set those nodes in N, which are the *only*
nodes to provide reachability to a node in N2, i.e., if node-b in
N2 can be reached only through a symmetric link to node-a in N,
then add node-a to the active set. Remove the nodes from N2
which are now covered by a node in the active set.
4. While there exist nodes in N2 which are not covered by at least
one node in the active set:
1. For each node in N, calculate the reachability, i.e., the
number of nodes in N2 which are not yet covered by at least
one node in the active set, and which are reachable through
this one hop neighbor;
2. Select as an active overlapping relay the node with highest
Willingness among the nodes in N with non-zero reachability.
In case of multiple choice select the node which provides
reachability to the maximum number of nodes in N2. In case
of multiple nodes providing the same amount of reachability,
select the node as active whose D(y) is greater. As a final
tie breaker, the node with the highest router ID should be
chosen. Remove the nodes from N2 which are now covered by a
node in the active set.
5. As an optimization, process each node, y, in the active set in
increasing order of Willingness. If all nodes in N2 are still
covered by at least one node in the active set excluding node y,
and if Willingness of node y is smaller than MAX_WILLINGNESS,
then node y should be removed from the active set.
Note that the active overlapping relays selection algorithm is
implementation specific, and the above is simply a suggested
algorithm. However, the behavior of the overlapping relays MUST
follow that specified in the "Flooding and Relay Decisions" Section.
Moreover, the same selection algorithm MUST be used by all nodes
within an area.
3.3.5 The F Option Bit
A single new option bit, the F bit, is defined in the OSPFv3 option
field. The F bit indicates that the node supports the Optimized
Flooding mechanism as specified in this draft. See Section "New Bits
in OSPFv3 Options Field" for placement of the F bit.
3.3.6 Active Overlapping Relay Extension
A new TLV is defined so that each speaker can convey its set of
active overlapping relays in the Hello messages. The TLV is
transmitted using the Link Local Signaling method described in "Link
Local Signaling" Section.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Relays Added |A|N| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID(s) of Active Overlapping Relay(s) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Type, set to 4.
o Length - variable; Length of TLV in bytes NOT including Type and
Length.
o Relays Added - variable; Number of active overlapping relays that
are being added. Note that the number of active overlapping
relays that are being dropped is then given by: [(Length - 4)/4 -
Relays Added].
o A bit - If this bit is set, the node is specifying that it will
always flood routing updates that it receives, regardless of
whether it is selected as an Active Overlapping Relay.
o N bit - If this bit is set, the node is specifying that it most
likely will not flood routing updates. The node SHOULD NOT be
chosen to be an active overlapping relay unless it is the *only*
neighbor that can reach two-hop neighbor(s). Note that if the
node is selected as an Active Overlapping relay and the node can
not perform the required duties, network behavior is not
compromised since it results in the same behavior as if the node
was not chosen as an active overlapping relay.
o Reserved - Reserved for future use and MUST be ignored upon
reception.
o Router ID(s) of Active Overlapping Relay(s) - The router ID(s) of
neighbor(s) that are either chosen to serve as an active
overlapping relay, or removed from serving as an active
overlapping relay. The active overlapping relays that are being
added MUST be listed first, and the number of such relays MUST
equal Add Length. The remaining list of relays are being dropped
as active overlapping relays, and the number of such relays MUST
equal [(Length - 4)/4 - Relays Added].
Note that the 'A' bit and 'N' bit are independent of any particular
selection algorithm to determine the set of Active Overlapping
Relays. However, the bits SHOULD be considered as input into the
selection algorithm.
If a node is selected as an active overlapping relay and it does not
support the Incremental Hello mechanism defined in the "Incremental
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OSPF-MANET Hellos" Section, then it SHOULD always be included as an
Active Overlapping Relay in the TLV. Note that while a node needs to
know whether it is an active overlapping relay, it does not
necessarily have to know the identities of the other active
overlapping relays.
3.3.7 Willingness TLV
A new TLV is defined so that each speaker can convey its willingness
to serve as an active overlapping relay in the Hello meassage. The
TLV is transmitted using the Link Local Signaling method described in
the "Link Local Signaling" Section.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Willingness | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: Type, set to 5.
o Length - 4 bytes. It does not include the Type and Length fields.
o Willingness - 1 byte to indicate the willingness of the node to
serve as an active overlapping relay for its neighbors.
* 0: MIN_WILLINGNESS
* 128: DEFAULT_WILLINGNESS
* 255: MAX_WILLINGNESS
The TLV is optional and MUST be silently ignored if not understood.
If the Willingness TLV is not included in the Hello packet, the
Willingness value SHOULD be taken as DEFAULT_WILLINGNESS.
3.3.8 Flooding and Relay Decisions
The decision whether to relay any received LSAs and when to relay
such information, is made depending on the topology and whether the
node is part of the set of active overlapping relays.
Upon receiving an LSA from an adjacent speaker, a node makes flooding
decisions based on the following algorithm:
1. If the node is an active overlapping relay for the adjacent
speaker, then the router SHOULD immediately relay any information
received from the adjacent speaker.
2. If the node is a non-active overlapping relay for the adjacent
speaker, then the router SHOULD wait a specified amount of time
(PushbackInterval plus jitter, see "Important Timers" Section) to
decide whether to transmit. [Jitter is used to try to avoid
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several non-active overlapping relays from propagating redundant
information.] Note that a node with the 'N' bit set in the
'Active Overlapping Relays' Extension will not be chosen as an
active overlapping relay unless it is the only node to provide
reachability to a two-hop neighbor. However, it MUST perform the
duties of a non-active overlapping relay as required. Non-active
overlapping relays MUST follow the acknowledgment mechanism
outlined in the section 'Intelligent Transmission of Link State
Acknowledgements.'
1. During this time, if the node determines that flooding the
LSA will only result in a redundant transmission, the node
MUST suppress its transmission. Otherwise, it MUST transmit
upon expiration of PushbackInterval plus jitter.
2. If a non-active overlapping relay hears a re-flood from
another node that covers its non-overlapping neighbors before
its timer to transmit expires, it SHOULD reset its
PushbackInterval plus jitter timer. (Note that the
implementation should take care to avoid resetting the
PushbackInterval timer based on transmissions from active
overlapping relays.) During this time, if the node
determines that flooding the update will only result in a
redundant transmission, the node MUST suppress its
transmission. Otherwise, it MUST transmit upon expiration of
PushbackInterval + jitter.
3. If a non-active overlapping relay hears an old instance of
the LSA during this time, it SHOULD ignore the LSA and it
SHOULD NOT send a unicast packet to the neighbor with the
most recent LSA as specified in [OSPFv3].
3. For LSAs that are received unicast because of retransmission by
the originator, the node MUST determine whether it has already
received the LSA from another speaker. If it already has the
current instance of the LSA in its database, it MUST do nothing
further in terms of flooding the LSA (since it would have taken
appropriate behavior when it initially received the LSA.)
However, if it does not have the current instance of the LSA in
its database, it MUST take action according to the rules above,
just as if it received the multicast LSA. The acknowledgement
mechanism outlined in the section 'Intelligent Transmission of
Link State Acknowledgements' MUST be followed, and any timeout
mechanism for unicast LSAs MAY be followed.
Note that a node can determine whether further flooding a LSA will
only result in a redundant transmission by already having heard link
state acknowledgements (ACKs) or floods for the LSA from all of its
neighbors.
Due to the dynamic nature of a network, the set of active overlapping
relays may not be up to date at the time the relay decision is made
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or may not be able to perform the flooding duties, e.g., due to poor
link quality. The non-active overlapping relays prevent this
situation from causing database synchronization issues and thus,
packet loss.
Since the originator of the information, the relay, and the receiver
are all in the same dynamically determined local flooding domain, the
relay MUST NOT change the routing update information. In general,
LSAs SHOULD be sent to a well-known multicast address. In some
cases, routing updates MAY be sent using unicast packets.
3.3.9 Intelligent Transmission of Link State Acknowledgements
In order to optimize the bandwidth utilization on the link, a speaker
MUST follow these recommendations related to ACK transmissions:
1. All ACKs MUST be sent via multicast.
2. Typically, LSAs are acknowledged by all of the adjacent speakers.
In the case of relayed information, the relay MUST only expect
either explicit or implicit acknowledgements from neighbors that
have not previously acknowledged this LSA.
3. Because routing updates are sent via multicast, the set of
overlapping speakers will usually receive the same update more
than once. A speaker SHOULD only acknowledge the first update
received on the link.
4. An active overlapping relay SHOULD NOT explicitly acknowledge
information that it is relaying. The relayed information will
serve as an acknowledgement to the sender. If no information is
being relayed, than an explicit ACK MUST be sent.
5. Several ACKs MAY be bundled into a single packet. The wait
(AckInterval) before sending one such packet reduces the number
of packet transmissions required in acknowledging multiple LSAs.
6. All ACK packets SHOULD reset the RouterDeadInterval at the
receiver. If there is no state waiting to be transmitted in a
Hello packet at the sender, then the HelloInterval at the sender
SHOULD be reset. Note that an ACK serves as a Hello packet with
no state change.
7. Any LSA received via unicast MUST be acknowledged. (Note that
acknowledgment is via multicast as specified in rule (1) above.)
An ACK received from a non-overlapping neighbor should prevent
redundant transmission of the information to it by another
overlapping relay.
3.3.10 Important Timers
This section details the timers that are introduced in the Flooding
and Relay Decisions and Intelligent Transmission of Link State
Acknowledgements sections.
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o PushbackInterval: The length of time in seconds that a non-active
overlapping relay SHOULD wait before further flooding an LSA if
needed. This timer MUST be less than 1/2 of the RxmtInterval
[OSPF],[OSPFv3] minus propagation delays, i.e., (PushbackInterval
+ propagation delay) < RxmtInterval/2. The PushbackInterval is
set by a non-active overlapping relay upon reception of an LSA.
o AckInterval: After a node determines that it must transmit an ACK
and the AckInterval timer is not already set, the node SHOULD set
the AckInterval timer. The AckInterval is the length of time in
seconds that a node should wait in order to transmit many ACKs in
the acknowledgement packet. This wait reduces the number of
packet transmissions required in acknowledging multiple LSAs. The
AckInterval MUST be less than the PushbackInterval minus
propagation delays, i.e., (AckInterval + propagation delay) <
PushbackInterval.
3.3.11 Miscellaneous Protocol Considerations
The mechanism described refers to the operation of relays on a common
media segment. In other words, an LSA is only relayed out the same
interface through which it was received. However, the concept of
information relay may be extended to the flooding of all link state
advertisements received on any interface (and forwarded on any other
interface). OSPF works on the premise that all of the nodes in a
flooding domain will receive all of the routing information. Note
that one of the important properties is that the routing information
is not altered when relayed.
If each speaker advertised all of its adjacent neighbors on all
interfaces, then the overlap check would result in the determination
of which speakers are adjacent to both speakers. As a result, link
state information should only be flooded to non-overlapping neighbors
(taking all of the interfaces into account).
The flooding mechanism in OSPF relies on a designated router to
guarantee that any new LSA received by one router attached to the
broadcast network will be reflooded properly to all the other routers
attached to the broadcast network. Such desginated routers must be
able to reach all of the other speakers on the same subnet. A
designated router SHOULD NOT be elected if overlapping relays are
used.
If such designated routers already exist, then the relays MUST be
capable of differentiating them, and then making the relaying
decisions based on the OSPF's normal operation. As a result, there
may be groups of neighbors to which some information should not be
relayed. This mode of operation is NOT RECOMMENDED as it adds to the
complexity of the system.
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The intent of the overlapping relay mechanism is to optimize flooding
of routing control information. However, other information (such as
data) may also be relayed in some networks using the same mechanism.
3.3.12 Interoperability
On receiving a Hello packet from a new neighbor without the F bit
set, the local router will assume that the new neighbor will flood
normally as described in [OSPFv3]. Thus, the local router SHOULD
include the neighbor in its overlapping relay set since the neighbor
will flood by default. This will allow the local router to more
optimally select its entire overlapping relay set.
If the F bit is set and the I bit as defined in Section "Incremental
OSPF-MANET Hellos" is not set in the neighbor's Hello and the
neighbor is selected as an overlapping relay by the local router, the
local router will continue to include the neighbor's identifier in
its active relay set.
3.4 New Bits in OSPFv3 Option Field
Three new option bits are defined in the OSPFv3 Options Field
(defined in [OSPFv3], A.2) as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
| | | | | | | | | | | | |F|I|L|AF|*|*|DC| R| N|MC| E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+--+--+--+--+--+
o I bit - defined in Section "I Option Bit": The I bit is only
defined for Hello packets and indicates that only incremental
information is present.
o F bit - defined in Section "F Option Bit": The F bit indicates
that the node supports the Optimized Flooding mechanism as
specified in this draft.
3.5 Smart Peering
There is a significant overhead in OSPF if a router has to establish
adjacencies with every peer it can verify 2-way connectivity with.
OSPF supports a broadcast network type for these scenarios, where you
only have to peer with a designated router (DR). But a full mesh of
connectivity is required for proper DR election, so this doesn't help
in networks with overlapping partial meshes of connectivity. This
draft proposes an optional technique to reduce the number of
adjacencies based on the shortest path tree (SPT) reachability
information.
3.5.1 Introduction
In OSPF [OSPF] [OSPFv3], nodes establish an adjacency by first
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verifying 2-way connectivity between them and then synchronizing
their link state databases. Once the peering relationship is complete
and the adjacency is established, the nodes will continue to
advertise each other in their periodic Hellos and LSAs. As a result,
the peers are maintained in the link state database and are included
in all SPF calculations. During the reliable flooding process, a node
must ensure that each peer has indeed received the flooded routing
update via an acknowledgement and retransmission mechanism.
Consequently, maintaining an adjacency for a particular peer is a
tradeoff between the added redundancy in routing paths and network
reachability versus the overhead (memory consumption, SPF
computations, routing overhead, and network convergence).
Consider the possibility of reducing the number of adjacencies that a
node maintains without compromising reachability and redundancy.
This will have direct implications on network scalability and is
especially attractive in environments where the network topology is
dynamic. For example, in a Mobile Ad-Hoc Network (MANET) where nodes
are mobile and the topology is constantly changing, it seems highly
desirable to 'intelligently' become adjacent with only selected peers
and thus, not establish a peering session with every node that comes
within transmission range. Selective peering can be particularly
useful in avoiding the peering process for unstable nodes, i.e.,
nodes that come in and out of transmission range.
3.5.2 Previous Related Work
The formation of a FULL adjacency requires the discovery (2-way
relationship) and the database synchronization. To prevent from
achieving the FULL state, others have taken the approach of modifying
link state protocols to use periodic advertisements (instead of a
database exchange). The result is that neighbor discovery is still
required, but the routing information is learned over time. An
example of this approach is:
- OSPFv2 Wireless Interface Type [WINTF]
- where the use of periodic advertisements "eliminates the
formation of full adjacencies on wireless interfaces; all
neighbor states beyond 2-Way are not reached,and no database
synchronization is performed".
What we propose below, goes a step further by not requiring the
formation and maintenance of neighbor state (2-way, or other) *and*
without changing the route distribution mechanisms in the link state
protocols. In other words, the mechanism described is completely
backwards compatible.
3.5.3 Proposed Solution
Two routers are called synchronized when they have identical link
state database. To limit the number of neighbors that are formed, an
algorithm is needed to select which neighbors to peer with.
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The algorithm MUST provide reachability to every possible destination
in the network, just like when normal adjacency formation processes
are used. We should always peer with a neighbor if it provides our
only path to currently unreachable destinations.
3.5.3.1 SPT Reachability Heuristics
The peering decision is really a local matter to a router. If a
router can ensure that the reachability to other nodes is available
without bringing up a new adjacency, it can choose to not bring up
the new adjacency.
We propose an algorithm which uses the already existing information
about a new neighbor's reachability in the SPT. If the two routers
can already reach each other in the SPT, it is not necessary to build
up the adjacency between them.
The decision to peer or not, is made when a hello is received. When a
hello is received from a new neighbor or a neighbor in a state lower
than Exchange:
- A check is made in the link state database to see if the peer is
already reachable in the SPT.
- If the peer is either not known in the SPT or is not reachable,
we start the Exchange process.
- If the peer is reachable than bringing up adjacency with this
neighbor does not provide reachability to any new destinations.
Lets take an example of a single OSPF area. This check would look for
the neighbor's Router LSA. If the LSA is present in the database and
is reachable in the SPT, we have a chance to keep this adjacency from
going to Exchange.
It's worth noting that as the number of links and redundancy in the
network is reduced, there may be an increase in suboptimal routing.
3.5.3.2 State Machine
The state machine of a basic implementation of this algorithm is
provided below: An implementation MAY use some heuristics below
(step (3)), beyond the SPT reachability to decide if it considers a
new adjacacency to be of value or not.
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......................
|Receive a hello |
(1) |from a new potential|
|neighbor |
'`''''''''''''''''''''
|
|
|
,''''''''''''''''''''''|
|Check to see if there |
(2) |is a router LSA from |----no--(4)form a
|the new potential | new
|neighbor in the link | neighbor
|state database, which |
|is reachable in SPT |
'`''''''''''''''''''''''
|
|yes
(3) |
,'''''''''''''''''''''''''''''''''''''''''''''''''''''''''|
| (3b)........................ |
|(3a),______________________ |Determine if the | |
| |Determine if the new | |number of redundant | |
| |link cost is better | |paths to the potential| |
| |than the current path| |neighbor is < the | |
| |cost by a configured | |maximum configured | |
| |amount | |value | |
| '`''''''''''''''''''''' '`'''''''''''''''''''''' |
| \ / |
| .....\.........../.... |
| |User configurable | |
| |selection algorithm | |
| '`'''/'''''''\'''''''' |
| / \ |
'`'''''''''''''''''''''/'''''''''''\'''''''''''''''''''''''
/ \
requirements requirements
met not met
/ \
/ \
(4) form a new neighbor (5) do not become
neighbors
3.5.3.3 Announce the 2-way link in Router-LSA
While the technique described above minimizes the number of
adjacencies in highly meshed einvironments, it may lead to the
undesirable result of limiting the number of transit links available
on which to forward traffic.
An implementation may choose to allow some (or even all) of these
2-way state neighbors to be announced in the Router-LSA. Since the
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state remains 2-way, we don't incur control plan (database sync and
flooding) overhead. But at the same time, announcing the link in the
Router-LSA makes the link available to the data plane.
4. Security Considerations
In a MANET network, security is both more difficult and important due
to the wireless nature of the medium. Controlling the ability of
devices to connect to a MANET network at Layer 2 will be relegated to
Layer 2 security mechanisms, such as 802.1x, and others. Controlling
the ability of attached devices to transit traffic will require some
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type of security system (outside the scope of this document) which
can authenticate and provide authorization for individual members of
the routing domain.
5. IANA Considerations
IANA will make assignments as explained below using the policies
outlined in [IANA].
o An option bit value for the I-bit and F-bit as defined in the "New
Bits in OSPFv3 Options Field" section above.
o TLV type codes for SCS-TLV, Neighbor Drop TLV, Request From TLV,
Full State For TLV and Willingness TLV as defined in this
document. This allocation comes from LLS [LLS] TLV types values.
6. Contributors
The following persons are contributing authors to the document:
Fred Baker
Cisco Systems
1121 Via Del Rey
Santa Barbara, CA 93117
USA
Email: fred@cisco.com
Alvaro Retana
Cisco Systems
7025-4 Kit Creek Road
RTP, NC 27709
USA
Email: aretana@cisco.com
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Russ White
Cisco Systems
7025-4 Kit Creek Road
RTP, NC 27709
USA
Email: riw@cisco.com
Dave Cook
Cisco Systems
7025-4 Kit Creek Road
RTP, NC 27709
USA
Email: dacook@cisco.com
Yi Yang
Cisco Systems
7025-4 Kit Creek Road
RTP, NC 27709
USA
Email: yiya@cisco.com
7. Acknowledgements
The authors and contributors would like to thank Pratap Pellakuru and
Stan Ratliff for their feedback and implementation of the document.
References
Normative References
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[OSPFv3] Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
RFC 2740, December 1999.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[KEY] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Informative References
[IPV6ADD] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[OSPFGR] Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful OSPF
Restart", RFC 3623, November 2003.
[OSPFREST] Zinin, A., Roy, A., and L. Nguyen, "OSPF Restart
Signaling", RFC 4812, March 2007.
[LLS] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Yeung, "OSPF Link-local Signaling",
draft-ietf-ospf-lls-03.txt (work in progress),
February 2008.
[OLSR] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol", RFC 3626, October 2003.
[WINTF] Ahrenholz J. et al, "OSPFv2 Wireless Interface Type",
draft-spagnolo-manet-ospf-wireless-interface
(work in progress)
Authors' Addresses
Madhavi W. Chandra (Editor)
Email: mw.chandra@gmail.com
Abhay Roy (Editor)
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: akr@cisco.com
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