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Internet Engineering Task Force (IETF) F. Costa
Request for Comments: 6957 J-M. Combes, Ed.
Category: Standards Track X. Pougnard
ISSN: 2070-1721 France Telecom Orange
H. Li
Huawei Technologies
June 2013
Duplicate Address Detection Proxy
Abstract
The document describes a proxy-based mechanism allowing the use of
Duplicate Address Detection (DAD) by IPv6 nodes in a point-to-
multipoint architecture with a "split-horizon" forwarding scheme,
primarily deployed for Digital Subscriber Line (DSL) and Fiber access
architectures. Based on the DAD signaling, the first-hop router
stores in a Binding Table all known IPv6 addresses used on a point-
to-multipoint domain (e.g., VLAN). When a node performs DAD for an
address already used by another node, the first-hop router defends
the address rather than the device using the address.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc6957.
Costa, et al. Standards Track [Page 1]
RFC 6957 DAD-Proxy June 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Why Existing IETF Solutions Are Not Sufficient . . . . . . . 4
3.1. Duplicate Address Detection . . . . . . . . . . . . . . . 4
3.2. Neighbor Discovery Proxy . . . . . . . . . . . . . . . . 5
3.3. 6LoWPAN Neighbor Discovery . . . . . . . . . . . . . . . 5
3.4. IPv6 Mobility Manager . . . . . . . . . . . . . . . . . . 6
4. Duplicate Address Detection Proxy (DAD-Proxy) Specifications 6
4.1. DAD-Proxy Data Structure . . . . . . . . . . . . . . . . 6
4.2. DAD-Proxy Mechanism . . . . . . . . . . . . . . . . . . . 7
4.2.1. No Entry Exists for the Tentative Address . . . . . . 7
4.2.2. An Entry Already Exists for the Tentative Address . . 7
4.2.3. Confirmation of Reachability to Check the Validity of
the Conflict . . . . . . . . . . . . . . . . . . . . 9
5. Manageability Considerations . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. Interoperability with SEND . . . . . . . . . . . . . . . 11
6.2. Protection against IP Source Address Spoofing . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. DAD-Proxy State Machine . . . . . . . . . . . . . . 14
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1. Introduction
This document specifies a function called Duplicate Address Detection
(DAD) proxy allowing the use of DAD by the nodes on the same point-
to-multipoint domain with a "split-horizon" forwarding scheme,
primarily deployed for Digital Subscriber Line (DSL) and Fiber access
architectures [TR-101]. It only impacts the first-hop router and it
doesn't need modifications on the other IPv6 nodes. This mechanism
is fully effective if all the nodes of a point-to-multipoint domain
(except the DAD proxy itself) perform DAD.
This document explains also why the DAD mechanism [RFC4862] without a
proxy cannot be used in a point-to-multipoint architecture with a
"split-horizon" forwarding scheme (IPv6 over PPP [RFC5072] is not
affected). One of the main reasons is that, because of this
forwarding scheme, IPv6 nodes on the same point-to-multipoint domain
cannot have direct communication: any communication between them must
go through the first-hop router of the same domain.
It is assumed in this document that link-layer addresses on a point-
to-multipoint domain are unique from the first-hop router's point of
view (e.g., in an untrusted Ethernet architecture, this assumption
can be guaranteed thanks to mechanisms such as Media Access Control
(MAC) address translation performed by an aggregation device between
IPv6 nodes and the first-hop router).
1.1. Requirements Language
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 [RFC2119].
2. Background
Terminology in this document follows that in "Neighbor Discovery for
IP version 6 (IPv6)" [RFC4861] and "IPv6 Stateless Address
Autoconfiguration" [RFC4862]. In addition, this section defines
additional terms related to DSL and Fiber access architectures, which
are an important case where the solution described in this document
can be used:
Customer Premises Equipment (CPE)
The first IPv6 node in a customer's network.
Access Node (AN)
The first aggregation point in the public access network. It
is considered as an L2 bridge in this document.
Costa, et al. Standards Track [Page 3]
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Broadband Network Gateway (BNG)
The first-hop router from the CPE's point of view.
VLAN N:1 architecture
A point-to-multipoint architecture where many CPEs are
connected to the same VLAN. The CPEs may be connected on the
same or different Access Nodes.
split-horizon model
A forwarding scheme where CPEs cannot have direct layer 2
communications between them (i.e., IP flows must be forwarded
through the BNG via routing).
The following figure shows where the different entities are, as
defined above.
+------+ +----+
| CPE3 |---------| AN |
+------+ +----+
|
|
+------+ +----+
| CPE2 |---------| AN |---+
+------+ +----+ |
+------+ | |
| CPE1 |------------+ |
+------+ +-----+
| BNG |--- Internet
+-----+
Figure 1: DSL and Fiber Access Architecture
3. Why Existing IETF Solutions Are Not Sufficient
In a DSL or Fiber access architecture depicted in Figure 1, CPE1,
CPE2, CPE3, and the BNG are IPv6 nodes, while AN is an L2 bridge
providing connectivity between the BNG and each CPE. The AN enforces
a split-horizon model so that CPEs can only send and receive frames
(e.g., Ethernet frames) to and from the BNG but not to each other.
That said, the BNG is on the same link with all CPEs, but a given CPE
is not on the same link with any other CPE.
3.1. Duplicate Address Detection
Duplicate Address Detection (DAD) [RFC4862] is performed when an IPv6
node verifies the uniqueness of a tentative IPv6 address. This node
sends a Neighbor Solicitation (NS) message with the IP destination
set to the solicited-node multicast address of the tentative address.
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This NS message is multicasted to other nodes on the same link. When
the tentative address is already used on the link by another node,
this last one replies with a Neighbor Advertisement (NA) message to
inform the first node. So, when performing DAD, a node expects the
NS messages to be received by any node currently using the tentative
address.
However, in a point-to-multipoint network with a split-horizon
forwarding scheme implemented in the AN, the CPEs are prevented from
talking to each other directly. All packets sent out from a CPE are
forwarded by the AN only to the BNG but not to any other CPE. NS
messages sent by a certain CPE will be received only by the BNG and
will not reach other CPEs. So, other CPEs have no idea that a
certain IPv6 address is used by another CPE. That means, in a
network with split-horizon, DAD, as defined in [RFC4862], can't work
properly without additional help.
3.2. Neighbor Discovery Proxy
Neighbor Discovery (ND) Proxy [RFC4389] is designed for forwarding ND
messages between different IP links where the subnet prefix is the
same. An ND Proxy function on a bridge ensures that packets between
nodes on different segments can be received by this function and have
the correct link-layer address type on each segment. When the ND
Proxy receives a multicast ND message, it forwards it to all other
interfaces on a same link.
In DSL or Fiber networks, when the AN, acting as an ND Proxy,
receives an ND message from a CPE, it will forward it to the BNG but
none of the other CPEs, as only the BNG is on the same link with the
CPE. Hence, implementing ND Proxy on the AN would not help a CPE
acknowledge link-local addresses used by other CPEs.
As the BNG must not forward link-local scoped messages sent from a
CPE to other CPEs, ND Proxy cannot be implemented in the BNG.
3.3. 6LoWPAN Neighbor Discovery
[RFC6775] defines an optional modification of DAD for IPv6 over Low-
Power Wireless Personal Area Networks (6LoWPAN). When a 6LoWPAN node
wants to configure an IPv6 address, it registers that address with
one or more of its default routers using the Address Registration
Option (ARO). If this address is already owned by another node, the
router informs the 6LoWPAN node that this address cannot be
configured.
This mechanism requires modifications in all hosts in order to
support the ARO.
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3.4. IPv6 Mobility Manager
According to [RFC6275], a home agent acts as a proxy for mobile nodes
when they are away from the home network: the home agent defends a
mobile node's home address by replying to NS messages with NA
messages.
There is a problem for this mechanism if it is applied in a DSL or
Fiber public access network. Operators of such networks require that
an NA message is only received by the sender of the corresponding NS
message, for security and scalability reasons. However, the home
agent per [RFC6275] multicasts NA messages on the home link and all
nodes on this link will receive these NA messages. This shortcoming
prevents this mechanism from being deployed in DSL or Fiber access
networks directly.
4. Duplicate Address Detection Proxy (DAD-Proxy) Specifications
First, it is important to note that, as this mechanism is strongly
based on DAD [RFC4862], it is not completely reliable, and the goal
of this document is not to fix DAD.
4.1. DAD-Proxy Data Structure
A BNG needs to store in a Binding Table information related to the
IPv6 addresses generated by any CPE. This Binding Table can be
distinct from the Neighbor Cache. This must be done per point-to-
multipoint domain (e.g., per Ethernet VLAN). Each entry in this
Binding Table MUST contain the following fields:
o IPv6 Address
o Link-layer Address
For security or performances reasons, it must be possible to limit
the number of IPv6 addresses per link-layer address (possibly, but
not necessarily, to 1).
On the reception of an unsolicited NA (e.g., when a CPE wishes to
inform its neighbors of a new link-layer address) for an IPv6 address
already recorded in the Binding Table, each entry associated to this
IPv6 address MUST be updated consequently: the current link-layer
address is replaced by the one included in the unsolicited NA
message.
For security or performances reasons, the Binding Table MUST be large
enough for the deployment in which it is used: if the Binding Table
is distinct from the Neighbor Cache, it MUST be at least the same
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size as this last one. Implementations MUST either state the fixed
size of the Binding Table that they support or make the size
configurable. In the latter case, implementations MUST state the
largest Binding Table size that they support. Additionally,
implementations SHOULD allow an operator to inquire about the current
occupancy level of the Binding Table to determine if it is about to
become full. Implementations encountering a full Binding Table will
likely handle it in a way similar to NS message loss.
It is recommended to apply technical solutions to minimize the risk
that the Binding Table becomes full. These solutions are out of the
scope of this document.
4.2. DAD-Proxy Mechanism
When a CPE performs DAD, as specified in [RFC4862], it sends a
Neighbor Solicitation (NS) message, with the unspecified address as
the source address, in order to check if a tentative address is
already in use on the link. The BNG receives this message and MUST
perform actions specified in the following sections based on the
information in the Binding Table.
4.2.1. No Entry Exists for the Tentative Address
When there is no entry for the tentative address, the BNG MUST create
one with the following information:
o IPv6 Address field set to the tentative address in the NS message.
o Link-layer Address field set to the link-layer source address in
the link-layer header of the NS message.
The BNG MUST NOT reply to the CPE or forward the NS message.
4.2.2. An Entry Already Exists for the Tentative Address
When there is an entry for the tentative address, the BNG MUST check
the following conditions:
o The address in the Target Address field in the NS message is equal
to the address in the IPv6 Address field in the entry.
o The source address of the IPv6 Header in the NS message is equal
to the unspecified address.
When these conditions are met and the source address of the link-
layer header in the NS message is equal to the address in the Link-
layer Address field in the entry, that means the CPE is still
Costa, et al. Standards Track [Page 7]
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performing DAD for this address. The BNG MUST NOT reply to the CPE
or forward the NS message.
When these conditions are met and the source address of the link-
layer header in the NS message is not equal to the address in the
Link-layer Address field in the entry, that means possibly another
CPE is performing DAD for an already owned address. The BNG then has
to verify whether there is a real conflict by checking if the CPE
whose IPv6 address is in the entry is still connected. In the
following text, we will call IPv6-CPE1 the IPv6 address of the
existing entry in the Binding Table, Link-layer-CPE1 the link-layer
address of that entry, and Link-layer-CPE2 the link-layer address of
the CPE that is performing DAD, which is different from Link-layer-
CPE1.
The BNG MUST check if the potential address conflict is real. In
particular:
o If IPv6-CPE1 is in the Neighbor Cache and it is associated with
Link-layer-CPE1, the reachability of IPv6-CPE1 MUST be confirmed
as explained in Section 4.2.3.
o If IPv6-CPE1 is in the Neighbor Cache, but in this cache it is
associated with a link-layer address other than Link-layer-CPE1,
that means that there is possibly a conflict with another CPE, but
that CPE did not perform DAD. This situation is out of the scope
of this document, since one assumption made above is that all the
nodes of a point-to-multipoint domain (except the DAD proxy
itself) perform DAD.
o If IPv6-CPE1 is not in the Neighbor Cache, then the BNG MUST
create a new entry based on the information of the entry in the
Binding Table. This step is necessary in order to trigger the
reachability check as explained in Section 4.2.3. The entry in
the Neighbor Cache MUST be created based on the algorithm defined
in Section 7.3.3 of [RFC4861], in particular by treating this case
as though a packet other than a solicited Neighbor Advertisement
were received from IPv6-CPE1. Thus, the new entry of the Neighbor
Cache MUST contain the following information:
* IPv6 address: IPv6-CPE1
* Link-layer address: Link-layer-CPE1
* State: STALE
The reachability of IPv6-CPE1 MUST be confirmed as soon as
possible following the procedure explained in Section 4.2.3.
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4.2.3. Confirmation of Reachability to Check the Validity of the
Conflict
Given that the IPv6-CPE1 is in an entry of the Neighbor Cache, the
reachability of IPv6-CPE1 is checked by using the Neighbor
Unreachability Detection (NUD) mechanism described in Section 7.3.1
of [RFC4861]. This mechanism MUST be triggered as though a packet
had to be sent to IPv6-CPE1. Note that in some cases this mechanism
does not do anything. For instance, if the state of the entry is
REACHABLE and a positive confirmation was received recently that the
forward path to the IPv6-CPE1 was functioning properly (see RFC 4861
for more details), this mechanism does not do anything.
Next, the behavior of the BNG depends on the result of the NUD
process, as explained in the following sections.
4.2.3.1. The Result of the NUD Process is Negative
If the result of the NUD process is negative (i.e., if this process
removes IPv6-CPE1 from the Neighbor Cache), that means that the
potential conflict is not real.
The conflicting entry in the Binding Table (Link-layer-CPE1) is
deleted and it is replaced by a new entry with the same IPv6 address,
but the link-layer address of the CPE is performing DAD (Link-layer-
CPE2), as explained in Section 4.2.1.
4.2.3.2. The Result of the NUD Process is Positive
If the result of the NUD process is positive (i.e., if after this
process the state of IPv6-CPE1 is REACHABLE), that means that the
potential conflict is real.
As shown in Figure 2, the BNG MUST reply to the CPE that is
performing DAD (CPE2 in Figure 1) with an NA message that has the
following format:
Layer 2 Header Fields:
Source Address
The link-layer address of the interface on which the BNG
received the NS message.
Destination Address
The source address in the Layer 2 Header of the NS
message received by the BNG (i.e., Link-layer-CPE2).
Costa, et al. Standards Track [Page 9]
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IPv6 Header Fields:
Source Address
An address assigned to the interface from which the
advertisement is sent.
Destination Address
The all-nodes multicast address.
ICMPv6 Fields:
Target Address
The tentative address already used (i.e., IPv6-CPE1).
Target Link-layer Address
The link-layer address of the interface on which the BNG
received the NS message.
CPE1 CPE2 BNG
| | |
(a)| | |
| | |
(b)|===================>|
| | |(c)
| | |
| (d)| |
| | |
| (e)|=========>|
| | |
| |<=========|(f)
| | |
(a) CPE1 generates a tentative address
(b) CPE1 performs DAD for this one
(c) BNG updates its Binding Table
(d) CPE2 generates a same tentative address
(e) CPE2 performs DAD for this one
(f) BNG informs CPE2 that DAD fails
Figure 2: DAD Failure
The BNG and the CPE MUST support the unicast transmission on the link
layer of IPv6 multicast messages [RFC6085], to be able, respectively,
to generate and to process such a packet format.
Costa, et al. Standards Track [Page 10]
RFC 6957 DAD-Proxy June 2013
5. Manageability Considerations
The BNG SHOULD support a mechanism to log and emit alarms whenever a
duplication of IPv6 addresses is detected by the DAD-Proxy function.
Moreover, the BNG SHOULD implement a function to allow an operator to
access logs and to see the current entries in the Binding Table. The
management of access rights to get this information is out of the
scope of this document.
6. Security Considerations
6.1. Interoperability with SEND
The mechanism described in this document will not interoperate with
SEcure Neighbor Discovery (SEND) [RFC3971]. This is due to the BNG
not owning the private key associated with the Cryptographically
Generated Address (CGA) [RFC3972] needed to correctly sign the
proxied ND messages [RFC5909].
Secure Proxy ND Support for SEND [RFC6496] has been specified to
address this limitation, and it SHOULD be implemented and used on the
BNG and the CPEs.
6.2. Protection against IP Source Address Spoofing
To ensure protection against IP source address spoofing in data
packets, this proposal can be used in combination with Source Address
Validation Improvement (SAVI) mechanisms [RFC6620] [SAVI-SEND]
[SAVI-MIX].
If SAVI mechanisms are used, the SAVI device is the BNG, and the
Binding Anchor for a CPE is its MAC address, which is assumed to be
unique in this document (cf. Section 1).
7. Acknowledgments
The authors would like to thank Alan Kavanagh, Wojciech Dec, Suresh
Krishnan, and Tassos Chatzithomaoglou for their comments. The
authors would like also to thank the IETF 6man WG members and the BBF
community for their support.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC6085] Gundavelli, S., Townsley, M., Troan, O., and W. Dec,
"Address Mapping of IPv6 Multicast Packets on Ethernet",
RFC 6085, January 2011.
8.2. Informative References
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, April 2006.
[RFC5072] Varada, S., Ed., Haskins, D., and E. Allen, "IP Version 6
over PPP", RFC 5072, September 2007.
[RFC5909] Combes, J-M., Krishnan, S., and G. Daley, "Securing
Neighbor Discovery Proxy: Problem Statement", RFC 5909,
July 2010.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.
[RFC6496] Krishnan, S., Laganier, J., Bonola, M., and A. Garcia-
Martinez, "Secure Proxy ND Support for SEcure Neighbor
Discovery (SEND)", RFC 6496, February 2012.
[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses", RFC
6620, May 2012.
Costa, et al. Standards Track [Page 12]
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[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, November 2012.
[SAVI-MIX] Bi, J., Yao, G., Halpern, J., and E. Levy-Abegnoli, Ed.,
"SAVI for Mixed Address Assignment Methods Scenario",
Work in Progress, May 2013.
[SAVI-SEND] Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
Address Validation Implementation", Work in Progress,
April 2013.
[TR-101] The Broadband Forum, "Migration to Ethernet-Based DSL
Aggregation", Issue 2, Technical Report TR-101, July
2011, <http://www.broadband-forum.org/technical/download/
TR-101_Issue-2.pdf>.
Costa, et al. Standards Track [Page 13]
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Appendix A. DAD-Proxy State Machine
This appendix, which is informative, contains a summary (cf. Table 1)
of the actions done by the BNG when it receives a DAD-based NS
(DAD-NS) message. The tentative address in this message is IPv6-CPE1
and the associated link-layer address is Link-layer-CPE2. The
actions are precisely specified in Section 4.2.
+------------+--------------------+--------------------+------------+
| Event | Check | Action | New event |
+------------+--------------------+--------------------+------------+
| DAD-NS | * No entry for | Create an entry | - |
| message | IPv6-CPE1 in the | for IPv6-CPE1 | |
| reception. | Binding Table. | bound to Link- | |
| | | layer-CPE2 in the | |
| | | Binding Table. | |
| | * Entry for | - | Existing |
| | IPv6-CPE1 in the | | entry. |
| | Binding Table. | | |
| | | | |
| Existing | * Link-layer-CPE2 | - | - |
| entry. | bound to IPv6-CPE1 | | |
| | in the Binding | | |
| | Table. | | |
| | * Another link- | - | Conflict? |
| | layer address, | | |
| | Link-layer-CPE1, | | |
| | bound to IPv6-CPE1 | | |
| | in the Binding | | |
| | Table. | | |
| | | | |
| Conflict? | * IPv6-CPE1 | - | Reachable? |
| | associated to | | |
| | Link-layer-CPE1 in | | |
| | the Neighbor | | |
| | Cache. | | |
| | * IPv6-CPE1 | Out of scope. | - |
| | associated to | | |
| | another link-layer | | |
| | address than Link- | | |
| | layer-CPE1 in the | | |
| | Neighbor Cache. | | |
| | * IPv6-CPE1 is not | Create an entry | Reachable? |
| | in the Neighbor | for IPv6-CPE1 | |
| | Cache. | associated to | |
| | | Link-layer-CPE1 in | |
| | | the Neighbor | |
| | | Cache. | |
Costa, et al. Standards Track [Page 14]
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| Reachable? | * NUD process is | IPv6-CPE2 is bound | - |
| | negative. | to Link-layer- | |
| | | CPE2, instead to | |
| | | Link-layer-CPE1, | |
| | | in the Binding | |
| | | Table. | |
| | * NUD process is | A NA message is | - |
| | positive. | sent. | |
+------------+--------------------+--------------------+------------+
Table 1: DAD-Proxy State Machine
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Authors' Addresses
Fabio Costa
France Telecom Orange
61 rue des Archives
75141 Paris Cedex 03
France
EMail: fabio.costa@orange.com
Jean-Michel Combes (editor)
France Telecom Orange
38 rue du General Leclerc
92794 Issy-les-Moulineaux Cedex 9
France
EMail: jeanmichel.combes@orange.com
Xavier Pougnard
France Telecom Orange
2 avenue Pierre Marzin
22300 Lannion
France
EMail: xavier.pougnard@orange.com
Hongyu Li
Huawei Technologies
Huawei Industrial Base
Shenzhen
China
EMail: lihy@huawei.com
Costa, et al. Standards Track [Page 16]