RFC5820: Extensions to OSPF to Support Mobile Ad Hoc Networking
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Related keywords:
(link-local signaling)
(lls)
(MANET)
(open shortest path first)
(ospf-manet)
(ospf-or)
(ospf-overlapping relay)
Internet Engineering Task Force (IETF) A. Roy, Ed.
Request for Comments: 5820 Cisco Systems
Category: Experimental M. Chandra, Ed.
ISSN: 2070-1721 March 2010
Extensions to OSPF to Support Mobile Ad Hoc Networking
Abstract
This document describes extensions to OSPF to support mobile ad hoc
networks (MANETs). The extensions, called OSPF-OR (OSPF-Overlapping
Relay), include mechanisms for link-local signaling (LLS), an 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. OSPF-OR also provides a
means to reduce unnecessary adjacencies to support larger MANETs.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5820.
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Table of Contents
1. Introduction ....................................................4
1.1. Problem Statement ..........................................4
1.2. Motivation for Extending OSPF to Support MANETs ............5
2. Requirements Notation ...........................................5
3. Proposed Enhancements ...........................................5
3.1. OSPF-MANET Interface .......................................7
3.1.1. Interface Operation .................................8
3.1.2. LSA Formats and Examples ............................8
3.2. Incremental OSPF-MANET Hellos .............................12
3.2.1. The I Option Bit ...................................12
3.2.2. State Check Sequence TLV (SCS TLV) .................12
3.2.3. Neighbor Drop TLV ..................................13
3.2.4. Request From TLV (RF TLV) ..........................14
3.2.5. Full State For TLV (FSF TLV) .......................14
3.2.6. Neighbor Adjacencies ...............................15
3.2.7. Sending Hellos .....................................16
3.2.8. Receiving Hellos ...................................17
3.2.9. Interoperability ...................................19
3.2.10. Support for OSPF Graceful Restart .................19
3.3. Optimized Flooding (Overlapping Relays) ...................20
3.3.1. Operation Overview .................................20
3.3.2. Determination of Overlapping Relays ................21
3.3.3. Terminology ........................................21
3.3.4. Overlapping Relay Discovery Process ................22
3.3.5. The F Option Bit ...................................23
3.3.6. Active Overlapping Relay TLV (AOR TLV) .............23
3.3.7. Willingness TLV ....................................24
3.3.8. Flooding and Relay Decisions .......................25
3.3.9. Intelligent Transmission of Link State
Acknowledgments ....................................26
3.3.10. Important Timers ..................................27
3.3.11. Miscellaneous Protocol Considerations .............28
3.3.12. Interoperability ..................................28
3.4. New Bits in LLS Type 1 Extended Options and Flags .........29
3.5. Smart Peering .............................................29
3.5.1. Rationale for Smart Peering ........................29
3.5.2. Previous Related Work ..............................30
3.5.3. Smart Peering Solution .............................30
3.5.4. Advertising 2-Way Links in Router-LSAs .............33
4. Security Considerations ........................................36
5. IANA Considerations ............................................38
6. Contributors ...................................................39
7. Acknowledgments ................................................39
8. References .....................................................39
8.1. Normative References ......................................39
8.2. Informative References ....................................40
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1. Introduction
Mobile ad hoc networks (MANETs) 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 have been proposed and are
being seriously considered for deployment in the near term, which
means the concepts and research now need to be applied to deployed
networks. Towards that end, this document 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
in 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, and there is no central hub or controller; 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
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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
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 MANETs 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 to "plug in" to
just about any location within a wire-line network, and also 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
MANETs, 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-understood and widely
deployed routing protocol. This provides a strong basis of
experience and skills from which to work. A protocol that is known
to work can be extended, rather than developing a new protocol that
must then be completely troubleshot, 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 that 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 RFC 2119 [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
mechanisms defined in Sections 3.2 and 3.3 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
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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.
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 involves scalability. A MANET must
transmit more state information to maintain reachability. Therefore,
OSPF will need scalability enhancements to support MANETs. 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 networks, because it cannot be assumed that there is
2-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
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radio types, different antennae, 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 both increase 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
defined and supported in [OSPF] and [OSPFv3]: broadcast, Non-
Broadcast Multi-Access (NBMA), point-to-point, and point-to-
multipoint.
The 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.
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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, 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 link state advertisements (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].
Demand Circuits are an optional feature; hence, the DC-bit setting
recommendation level is SHOULD.
3.1.2. LSA Formats and Examples
LSA formats are specified in [OSPFv3].
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 loopback
interface.
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+---+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).
-----------------------------------------------------------
RT1 LOOPBACK 2001:DB8:0001::/64
I11 n/a
RT2 LOOPBACK 2001:DB8:0002::/64
I21 n/a
I22 n/a
I23 2001:DB8:0012::/60
RT3 LOOPBACK 2001:DB8:0003::/64
I31 n/a
RT4 LOOPBACK 2001:DB8:0004::/64
I41 n/a
I42 n/a
The OSPF interface IDs and the link-local addresses for the router
interfaces in the network are shown below. EUIxy represents the
64-bit interface identifier of the interface Ixy, in Modified EUI-64
format [IPV6ADD].
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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.
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
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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,
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 = 2001:DB8:0002::
PrefixLength = 60 ;prefix on I23
PrefixOptions = 0
Metric = 10 ;cost of I23
Address Prefix = 2001:DB8:0012::
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.
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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
associated 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 State Check Sequence
TLV (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 LLS Type 1 Extended Options and Flags
field. The bit is defined for Hello packets and indicates that only
incremental information is present. See Section 5 for placement of
the I-bit.
3.2.2. State Check Sequence TLV (SCS TLV)
A new TLV is defined that indicates the current state, which is
represented by a 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: 6
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.
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o R: Request bit. If set, this is a request for current state. The
list of routers that should respond to this request is indicated
in the Request From TLV (RF TLV) (defined below). If the RF 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 (FSF TLV)
(defined below) are concerned. If the FSF 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, this
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 SCS TLV appended and with the R-bit set will be
referred to as a Hello request.
3.2.3. Neighbor Drop TLV
A new TLV is defined in this document that 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: 7
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.
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3.2.4. Request From TLV (RF TLV)
A new TLV is defined in this document that 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: 8
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
Hello state is being requested.
3.2.5. Full State For TLV (FSF TLV)
A new TLV is defined in this document that indicates neighbor(s) to
which the transmitting node is responding with full state.
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: 9
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.
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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 that 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],
Appendix A.3.2.
o Increment the router's SCS number that it will use in its next
Hello (indicated in the SCS TLV).
o Remove the neighbor from the neighbor list described in [OSPFv3],
Appendix A.3.2, when the neighbor has reached the Exchange state
(as described in [OSPF], Section 10.1).
o Remove the neighbor from the neighbor list described in [OSPFv3],
Appendix A.3.2, if the neighbor is not a DR or backup designated
router (BDR) on an OSPF broadcast link, and if the neighbor is
advertised as connected in the network-LSA advertised by the DR.
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.
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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 by being
brought within range of another device, powering the device on,
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:
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
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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. "Wrap around"
is a condition when the last known SCS number is MAX_SCS (65535) and
the new SCS number is 1. 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], Appendix 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 Section 3.2.6.2. 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 Section 3.2.6.2. 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
documents referring to this RFC. 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,
send a request for current state (by setting the 'R'-bit) 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 SCS 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 receipt 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 SCS TLV included and the R-bit
set, it looks for the RF TLV. If its router ID is listed in the RF
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], Appendix A.3.2.
o An SCS 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 documents referencing this RFC,
indicating the current state of the local system.
o The neighbor ID of all the neighbors who have requested current
state, in the FSF TLV.
If the full state is being sent to a large number of existing
neighbors, an implementation could choose to instead generate a full
state for all neighbors and omit the FSF TLV.
3.2.8.3. Receiving Hellos with the FS-bit Set
When a device receives a Hello with the SCS 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:
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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.
o If it is listed in the FSF TLV, or if the FSF TLV is not present,
the device MUST save the SCS number, process the Hello as
described in Section 3.2.8, 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],
Appendix 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], Appendix A.3.2, to determine that an adjacent router has
restarted, and other signaling to determine that 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 that 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, the SCS TLV included with the FS-bit
set, and the InitialSCS value (e.g., SCS of '1'). Subsequent
Hello packets will include only incremental state.
Routers that are neighboring with a restarting router MUST continue
sending their Hello packets with the I-bit set.
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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 receive a particular routing update only once.
However, there must be a trade-off 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
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 (virtual circuit)
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 Section 3.3.8. Because more than one such speaker may exist, the
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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-LSA packets. Every speaker
is then in a position to determine their 2-hop neighborhood, i.e.,
those nodes that are neighbors of the speaker's 1-hop neighbors.
A bidirectional neighbor is considered an overlapping relay for a
speaker if it can reach a node in the 2-hop neighborhood of the
speaker, i.e., if it has 1-hop neighbors (excluding the speaker
itself).
The set of Active Overlapping Relays for a speaker is the minimum set
of direct neighbors such that every node in the 2-hop neighborhood of
the speaker is a neighbor of at 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
Section 3.3.4 and is based on the multipoint relay (MPR) selection
algorithm described in [OLSR]). The behavior of the overlapping
relays MUST follow that specified in Section 3.3.8.
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 3.3.6, unless the
neighbor is the only neighbor to reach a 2-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] and [OSPFv3]. Note
that all neighbor references in this document are assumed to be
FULL neighbors.
o N: N is the set of FULL neighbors of the node.
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o 2-hop FULL neighbors (N2): 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 D(y): The degree of a 1-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 Relay TLV (AOR TLV) as
defined in Section 3.3.6.
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 that are now
covered by a node in the active set.
4. While there exist nodes in N2 that are not covered by at least one
node in the active set:
A. For each node in N, calculate the reachability, i.e., the
number of nodes in N2 that are not yet covered by at least one
node in the active set and that are reachable through this
1-hop neighbor.
B. Select as an Active Overlapping Relay the node with the highest
Willingness value (Section 3.3.7) among the nodes in N with
non-zero reachability. In the case of multiple choices, select
the node that provides reachability to the maximum number of
nodes in N2. In the case of multiple nodes providing the same
amount of reachability, select as active the node 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 that are
now covered by a node in the active set.
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5. As an optimization, process each node, y, in the active set in
increasing order of Willingness value. 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.
3.3.5. The F Option Bit
A single new option bit, the F-bit, is defined in the LLS Type 1
Extended Options and Flags field. The F-bit indicates that the node
supports the optimized flooding mechanism as specified in this
document. See Section 5 for placement of the F-bit.
3.3.6. Active Overlapping Relay TLV (AOR TLV)
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 LLS [LLS].
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: 10
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
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chosen to be an Active Overlapping Relay unless it is the *only*
neighbor that can reach 2-hop neighbor(s). Note that if the node
is selected as an Active Overlapping Relay and the node cannot
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. MUST be set to zero by the
sender, and MUST be ignored upon receipt.
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 listed 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 Section 3.2, 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 message. The
TLV is transmitted using the LLS [LLS].
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: 11
o Length - 4 bytes. It does not include the Type and Length fields.
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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 a bi-directional neighbor, 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 Section 3.3.10)) to decide
whether to transmit. [Jitter is used to try to avoid 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
2-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
Section 3.3.9.
A. 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.
B. 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
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result in a redundant transmission, the node MUST suppress its
transmission. Otherwise, it MUST transmit upon expiration of
PushbackInterval plus jitter.
C. 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 action 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 acknowledgment mechanism
outlined in Section 3.3.9 MUST be followed, and any timeout
mechanism for unicast LSAs MAY be followed.
Note that a node can determine whether further flooding an LSA will
only result in a redundant transmission by already having heard link
state acknowledgments (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
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 Acknowledgments
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
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either explicit or implicit acknowledgments 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 acknowledgment to the sender. If no information is
being relayed, then 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 were introduced in Sections
3.3.8 and 3.3.9.
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
receipt 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 acknowledgment packet. This wait reduces the number of packet
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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 re-flooded properly to all the other
routers attached to the broadcast network. Such designated 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.
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
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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 3.2 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 LLS Type 1 Extended Options and Flags
Two new option bits are defined in the "LLS Type 1 Extended Options
and Flags" Field [LLS] as follows:
o I-bit - defined in Section 3.2.1: The I-bit is only defined for
Hello packets and indicates that only incremental information is
present.
o F-bit - defined in Section 3.3.5: The F-bit indicates that the
node supports the optimized flooding mechanism as specified in
this document.
3.5. Smart Peering
There is significant overhead in OSPF when a router has to establish
adjacencies with every peer with whom it can verify 2-way
connectivity. OSPF supports the broadcast network type for these
scenarios, where you only have to peer with the designated router
(DR). However, a full mesh of connectivity is required for proper
operation, and this doesn't help in networks with overlapping partial
meshes of connectivity. This document proposes a technique to reduce
the number of adjacencies based on shortest path tree (SPT)
reachability information.
3.5.1. Rationale for Smart Peering
In OSPF ([OSPF], [OSPFv3]), nodes establish an adjacency by first
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 LSAs. As a result, the peers are
maintained in the link state database and are included in all SPF
(Shortest Path First) calculations. During the reliable flooding
process, a node must ensure that each peer has indeed received the
flooded routing update via an acknowledgment and retransmission
mechanism.
Consequently, maintaining an adjacency for a particular peer is a
trade-off between the added redundancy in routing paths and network
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reachability versus the associated 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 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 discovery (2-way
relationship) and database synchronization. To prevent 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 routing information is learned over time. An example of this
approach is:
o 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 in this specification 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 backward compatible.
3.5.3. Smart Peering Solution
Two routers are defined as synchronized when they have identical link
state databases. To limit the number of neighbors that are formed,
an algorithm is needed to select which neighbors with whom to peer.
The algorithm MUST provide reachability to every possible destination
in the network, just as when normal adjacency formation processes are
used. We should always peer with a neighbor if it provides our only
path to currently unreachable destinations.
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3.5.3.1. SPT Reachability Heuristics
The peering decision is really a local matter to a router. If a
router can ensure that reachability to other nodes is available
without bringing up a new adjacency, it can choose not to bring up
the new adjacency.
We propose an algorithm that uses the 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 form an
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:
o 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, then bringing up adjacency with this
neighbor does not provide reachability to any new destinations.
Let's 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 suppress adjacency
formation.
It's worth noting that as the number of links and redundancy in the
network is reduced, the likelihood of suboptimal routing increases.
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 (Step (3)
below), beyond the SPT reachability, to decide whether or not it
considers a new adjacency to be of value.
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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
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3.5.4. Advertising 2-Way Links in Router-LSAs
The technique described in Section 3.5.3 minimizes the number of
adjacencies in highly meshed environments. This is especially useful
when the network is in motion and the average adjacency lifetime is
small.
However, it suffers from an undesirable side effect of limiting the
number of transit links available 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
state remains 2-way, we don't incur control plane (database sync and
flooding) overhead. However, advertising the link in the router-LSA
makes the link available to the data plane.
This can be safely done if the neighbor is reachable in a special SPT
constructed by ignoring any other 2-way links in the network. This
optional optimization is described below.
3.5.4.1. Unsynchronized Adjacencies
If the new neighbor is already reachable in the SPT, there is no
urgency in doing a full database sync with it. These are the steps
we need to perform when a neighbor has reached 2-way state.
Note that when we say "SPT" in this section, we mean the special SPT
constructed based on rules in Section 3.5.4.2.
o After a 2-WayReceived event, check if the neighbor is reachable in
the SPT. If yes, mark the neighbor as FULL with respect to link
advertisement.
o This means that the router-LSA or network-LSA link corresponding
to the neighbor is advertised as if the neighbor is FULL.
o The adjacency information is constructed with the U-bit (see
below).
o Database synchronization is postponed:
* By a configured amount of time -OR-
* Until the time it's absolutely "necessary"
In either case, if a database sync is currently pending, it is
started as soon as we detect that the neighbor is no longer reachable
in the SPT. The database sync can be done by Out-of-Band Sync [OOB],
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which maintains the current adjacency and does the sync in the
background. A normal resync can alternately be done with the
drawback of adjacency flap.
In standard OSPF, we first bring up adjacency and then announce a
transit link. The approach described above allows the link to be
used as a forwarding path very quickly and still allows the database
to be synchronized in a timely fashion when the alternate flooding
path has recently been broken.
There is a circular dependency issue that also needs to be resolved.
Once you start announcing the link, the shortest path will likely be
via this very link. So it's non-trivial to detect when the alternate
dependent path is gone. We would like to be able to detect that the
neighbor is reachable via a path that doesn't traverse an
unsynchronized path.
We have generally solved this class of problems by running an SPF and
pretending that the link in question doesn't exist. It doesn't
require a full SPF, but just enough to see if ANY other path is
available to reach the neighbor. The worst case is when the
alternate path is really gone, which we find that out by building a
full SPT. This needs to be done every time the link state database
changes, and for EACH link that has SPT dependence for its viability.
This approach has scalability concerns and is not considered further
here.
We can achieve the same results with just ONE additional SPF that is
capable of ignoring these Unsynchronized links. The result from this
SPT can be used to satisfy the reachability condition for ANY number
of Unsynchronized Adjacencies. This basically requires that we can
actually tell the difference between a normal FULL adjacency and this
new Unsynchronized Adjacency. We can do this in one of two ways:
(A) Defining LD Options and using a bit in it, as shown below:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | LD Options | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link Description in a Router-LSA
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LD Options
Link Description options. Used to specify some special
capability or state of a link.
+-+-+-+-+-+-+-+-+
| | | | | | | |U|
+-+-+-+-+-+-+-+-+
LD Options
U-bit
The "Unsynchronized" bit. This is set if the adjacency is
being announced before databases are fully synchronized.
This approach is backward compatible, because the only routers
looking at this bit are those that support the mechanisms
specified in this document.
(B) Introducing a new link type in router-LSA.
This is a much more complex solution, with backward compatibility
concerns, due to the fact that unknown link type handling is not
defined in the OSPF standard [OSPF]. Hence, this solution isn't
considered further.
3.5.4.2. Unsynchronized SPT
Whenever link state changes happen, we need to run ONE additional SPF
by ignoring all links with the U-bit set. This SPT is then consulted
to see if any of our Unsynchronized Adjacencies need to start
database sync. This SPT is also consulted when a new neighbor goes
into 2-way state to decide if we should form the adjacency
immediately or defer it for later.
3.5.4.3. Flooding Considerations
One of the main goals in trying to delay the database synchronization
is to be able to reduce unnecessary OSPF packets traversing these
links. Since the unsynchronized Adjacencies remain in 2-way state,
OSPF updates will not be flooded over the corresponding interfaces,
resulting in additional savings.
An option is provided to enable or disable flooding over these
Unsynchronized Adjacencies. The advantage of allowing flooding is
being able to use more links for control plane purposes. We will
still have the savings of not having to form the adjacency.
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3.5.4.4. Overlapping Relay (OR) Election Impact
The overlapping relay election algorithm uses the 2-hop neighborhood
it gleans from our neighbor's router-LSAs. The introduction of
Unsynchronized Adjacencies needs to be considered in the relay
election algorithm.
If flooding is enabled on unsynchronized Adjacencies, no change is
needed in the relay election algorithm. If flooding is disabled,
then the relay election algorithm needs to prune neighbors that are
connected via an Unsynchronized Adjacency from our 1-hop and 2-hop
neighbor lists.
4. Security Considerations
In a MANET, 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 at Layer 2 will be relegated to Layer 2 security
mechanisms, such as 802.1x, and others. Controlling the ability of
attached devices to transmit traffic will require some type of
security system (outside the scope of this document) that can
authenticate, and provide authorization for, individual members of
the routing domain.
Additional security considerations are similar to any MANET protocol
extension. The following text is from [MDR]:
As with OSPFv3 [OSPFv3], OSPF-OR can use the IPv6 Authentication
Header (AH) [AH] and/or the IPv6 Encapsulation Security Payload (ESP)
[ESP] to provide authentication, integrity, and/or confidentiality.
The use of AH and ESP for OSPFv3 is described in [OSPFv3-SEC].
Generic threats to routing protocols are described and categorized in
[THREATS]. The mechanisms described in [OSPFv3-SEC] provide
protection against many of these threats, but not all of them. In
particular, as mentioned in [OSPFv3], these mechanisms do not provide
protection against compromised, malfunctioning, or misconfigured
routers (also called Byzantine routers); this is true for both OSPFv3
and OSPF-OR.
The extension of OSPFv3 to include MANET routers does not introduce
any new security threats. However, the use of a wireless medium and
lack of infrastructure, inherent with MANET routers, may render some
of the attacks described in [THREATS] easier to mount. Depending on
the network context, these increased vulnerabilities may increase the
need to provide authentication, integrity, and/or confidentiality, as
well as anti-replay service.
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For example, sniffing of routing information and traffic analysis are
easier tasks with wireless routers than with wired routers, since the
attacker only needs to be within the radio range of a router. The
use of confidentiality (encryption) provides protection against
sniffing but not traffic analysis.
Similarly, interference attacks are also easier to mount against
MANET routers due to their wireless nature. Such attacks can be
mounted even if OSPF packets are protected by authentication and
confidentiality, e.g., by transmitting noise or replaying outdated
OSPF packets. As discussed below, an anti-replay service (provided
by both ESP and AH) can be used to protect against the latter attack.
The following threat actions are also easier with MANET routers:
spoofing (assuming the identity of a legitimate router),
falsification (sending false routing information), and overloading
(sending or triggering an excessive amount of routing updates).
These attacks are only possible if authentication is not used, or the
attacker takes control of a router or is able to forge legitimacy
(e.g., by discovering the cryptographic key).
[OSPFv3-SEC] mandates the use of manual keying when current IPsec
protocols are used with OSPFv3. Routers are required to use manually
configured keys with the same security association (SA) parameters
for both inbound and outbound traffic. For MANET routers, this
implies that all routers attached to the same MANET must use the same
key for multicasting packets. This is required in order to achieve
scalability and feasibility, as explained in [OSPFv3-SEC]. Future
specifications can explore the use of automated key management
protocols that may be suitable for MANETs.
As discussed in [OSPFv3-SEC], the use of manual keys can increase
vulnerability. For example, manual keys are usually long lived, thus
giving an attacker more time to discover the keys. In addition, the
use of the same key on all routers attached to the same MANET leaves
all routers insecure against impersonation attacks if any one of the
routers is compromised.
Although [AH] and [ESP] state that implementations of AH and ESP
SHOULD NOT provide anti-replay service in conjunction with SAs that
are manually keyed, it is important to note that such service is
allowed if the sequence number counter at the sender is correctly
maintained across local reboots until the key is replaced.
Therefore, it may be possible for MANET routers to make use of the
anti-replay service provided by AH and ESP.
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When an OSPF routing domain includes both MANETs and fixed networks,
the frequency of OSPF updates either due to actual topology changes
or malfeasance could result in instability in the fixed networks. In
situations where this is a concern, it is recommended that the border
routers segregate the MANETs from the fixed networks with either
separate OSPF areas or, in cases where legacy routers are very
sensitive to OSPF update frequency, separate OSPF instances. With
separate OSPF areas, the 5-second MinLSInterval will dampen the
frequency of changes originated in the MANETs. Additionally, OSPF
ranges can be configured to aggregate prefixes for the areas
supporting MANETs. With separate OSPF instances, more conservative
local policies can be employed to limit the volume of updates
emanating from the MANETs.
5. IANA Considerations
IANA has made the assignments as explained below using the policies
outlined in [IANA].
o I-bit and F-bit from "LLS Type 1 Extended Options and Flags"
registry as defined below:
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
| * | * | * | * | * | * | * |...| * | * | * | * | F | I | RS| LR|
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
Bits in Extended Options and Flags TLV
o New TLV types from the "Link Local Signalling TLV Identifiers (LLS
Types)" registry as defined below:
TLV Name TLV Type
-------- --------
State Check Sequence TLV 6
Neighbor Drop TLV 7
Request From TLV 8
Full State For TLV 9
Active Overlapping Relay TLV 10
Willingness TLV 11
o A new registry has been defined for LD Options as defined in
Section 3.5.4.1. The U-bit is allocated by this document.
All future additions to LD Options are subject to OSPF WG review
and require IETF Review.
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6. Contributors
The following persons are contributing authors to the document:
Fred Baker Dave Cook
Cisco Systems Cisco Systems
1121 Via Del Rey 7025-4 Kit Creek Road
Santa Barbara, CA 93117 Research Triangle Park, NC 27709
USA USA
EMail: fred@cisco.com EMail: dacook@cisco.com
Alvaro Retana Yi Yang
Cisco Systems Cisco Systems
7025-4 Kit Creek Road 7025-4 Kit Creek Road
Research Triangle Park, NC 27709 Research Triangle Park, NC 27709
USA USA
EMail: aretana@cisco.com EMail: yiya@cisco.com
Russ White
Cisco Systems
7025-4 Kit Creek Road
Research Triangle Park, NC 27709
USA
EMail: riw@cisco.com
7. Acknowledgments
The authors and contributors would like to thank Pratap Pellakuru and
Stan Ratliff for their feedback and implementation of the document.
Thanks to Richard Ogier and John Avery for doing a final review.
8. References
8.1. Normative References
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
April 1998.
[OSPFv3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
[LLS] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and
D. Yeung, "OSPF Link-Local Signaling", RFC 5613,
August 2009.
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[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[KEY] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. 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] Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart
Signaling", RFC 4812, March 2007.
[OOB] Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band
Link State Database (LSDB) Resynchronization",
RFC 4811, March 2007.
[OLSR] Clausen, T., Ed., and P. Jacquet, Ed., "Optimized Link
State Routing Protocol (OLSR)", RFC 3626,
October 2003.
[WINTF] Ahrenholz J., et al., "OSPFv2 Wireless Interface
Type", Work in Progress, May 2004.
[MDR] Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network
(MANET) Extension of OSPF Using Connected Dominating
Set (CDS) Flooding", RFC 5614, August 2009.
[AH] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[OSPFv3-SEC] Gupta, M. and N. Melam,
"Authentication/Confidentiality for OSPFv3", RFC 4552,
June 2006.
[THREATS] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats
to Routing Protocols", RFC 4593, October 2006.
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Authors' Addresses
Abhay Roy (Editor)
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Madhavi W. Chandra (Editor)
113 Holmhurst Court
Cary, NC 27519
EMail: mw.chandra@gmail.com
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