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IETF RFC 7527
Last modified on Saturday, April 25th, 2015
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Internet Engineering Task Force (IETF) R. Asati
Request for Comments: 7527 H. Singh
Updates: 4429, 4861, 4862 W. Beebee
Category: Standards Track C. Pignataro
ISSN: 2070-1721 Cisco Systems, Inc.
E. Dart
Lawrence Berkeley National Laboratory
W. George
Time Warner Cable
April 2015
Enhanced Duplicate Address Detection
 Abstract
IPv6 Loopback Suppression and Duplicate Address Detection (DAD) are
discussed in Appendix A of RFC 4862. That specification mentions a
hardware-assisted mechanism to detect looped back DAD messages. If
hardware cannot suppress looped back DAD messages, a software
solution is required. Several service provider communities have
expressed a need for automated detection of looped back Neighbor
Discovery (ND) messages used by DAD. This document includes
mitigation techniques and outlines the Enhanced DAD algorithm to
automate the detection of looped back IPv6 ND messages used by DAD.
For network loopback tests, the Enhanced DAD algorithm allows IPv6 to
self-heal after a loopback is placed and removed. Further, for
certain access networks, this document automates resolving a specific
duplicate address conflict. This document updates RFCs 4429, 4861,
and 4862.
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/RFC 7527.
Asati, et al. Standards Track PAGE 1 
RFC 7527 Enhanced DAD April 2015
Copyright Notice
Copyright (c) 2015 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Operational Mitigation Options . . . . . . . . . . . . . . . 4
3.1. Disable DAD on an Interface . . . . . . . . . . . . . . . 4
3.2. Dynamic Disable/Enable of DAD Using Layer 2 Protocol . . 5
3.3. Operational Considerations . . . . . . . . . . . . . . . 5
4. The Enhanced DAD Algorithm . . . . . . . . . . . . . . . . . 6
4.1. Processing Rules for Senders . . . . . . . . . . . . . . 6
4.2. Processing Rules for Receivers . . . . . . . . . . . . . 7
4.3. Changes to RFC 4861 . . . . . . . . . . . . . . . . . . . 7
5. Action to Perform on Detecting a Genuine Duplicate . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
IPv6 Loopback Suppression and Duplicate Address Detection (DAD) are
discussed in Appendix A of [RFC 4862]. That specification mentions a
hardware-assisted mechanism to detect looped back DAD messages. If
hardware cannot suppress looped back DAD messages, a software
solution is required. One specific DAD message is the Neighbor
Solicitation (NS), specified in [RFC 4861]. The NS is issued by the
network interface of an IPv6 node for DAD. Another message involved
in DAD is the Neighbor Advertisement (NA). The Enhanced DAD
algorithm specified in this document focuses on detecting an NS
looped back to the transmitting interface during the DAD operation.
Detecting a looped back NA does not solve the looped back DAD
Asati, et al. Standards Track PAGE 2
RFC 7527 Enhanced DAD April 2015
problem. Detection of any other looped back ND messages during the
DAD operation is outside the scope of this document. This document
also includes a section on mitigation that discusses means already
available to mitigate the DAD loopback problem. This document
updates RFCs 4429, 4861, and 4862. It updates RFCs 4429 and 4862 to
use the Enhanced DAD algorithm to detect looped back DAD probes, and
it updates RFC 4861 as described in Section 4.3 below.
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].
1.2. Terminology
o DAD-failed state - Duplication Address Detection failure as
specified in [RFC 4862]. Note even Optimistic DAD as specified in
[RFC 4429] can fail due to a looped back DAD probe. This document
covers looped back detection for Optimistic DAD as well.
o Looped back message - also referred to as a reflected message.
The message sent by the sender is received by the sender due to
the network or an upper-layer protocol on the sender looping the
message back.
o Loopback - A function in which the router's Layer 3 interface (or
the circuit to which the router's interface is connected) is
looped back or connected to itself. Loopback causes packets sent
by the interface to be received by the interface and results in
interface unavailability for regular data traffic forwarding. See
more details in Section 9.1 of [RFC 2328]. The Loopback function
is commonly used in an interface context to gain information on
the quality of the interface, by employing mechanisms such as
ICMPv6 pings and bit-error tests. In a circuit context, this
function is used in wide-area environments including optical Dense
Wavelength Division Multiplexing (DWDM) and Synchronous Optical
Network / Synchronous Digital Hierarchy (SONET/SDH) for fault
isolation (e.g., by placing a loopback at different geographic
locations along the path of a wide-area circuit to help locate a
circuit fault). The Loopback function may be employed locally or
remotely.
o NS(DAD) - shorthand notation to denote a Neighbor Solicitation
(NS) (as specified in [RFC 4861]) that has an unspecified IPv6
source address and was issued during DAD.
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RFC 7527 Enhanced DAD April 2015
2. Problem Statement
Service providers have reported a problem with DAD that arises in a
few scenarios. In the first scenario, loopback testing for
troubleshooting purposes is underway on a circuit connected to an
IPv6-enabled interface on a router. The interface issues an NS for
the IPv6 link-local address DAD. The NS is reflected back to the
router interface due to the loopback condition of the circuit, and
the router interface enters a DAD-failed state. After the loopback
condition is removed, IPv4 will return to operation without further
manual intervention. However, IPv6 will remain in DAD-failed state
until manual intervention on the router restores IPv6 to operation.
In the second scenario, two broadband modems are served by the same
service provider and terminate to the same Layer 3 interface on an
IPv6-enabled access concentrator. In this case, the two modems'
Ethernet interfaces are also connected to a common local network
(collision domain). The access concentrator serving the modems is
the first-hop IPv6 router for the modems and issues a NS(DAD) message
for the IPv6 link-local address of its Layer 3 interface. The NS
message reaches one modem first, and this modem sends the message to
the local network, where the second modem receives the message and
then forwards it back to the access concentrator. The looped back NS
message causes the network interface on the access concentrator to be
in a DAD-failed state. Such a network interface typically serves
thousands of broadband modems, and all would have their IPv6
connectivity affected until the DAD-failed state is cleared.
Additionally, it may be difficult for the user of the access
concentrator to determine the source of the looped back DAD message.
Thus, in order to avoid IPv6 outages that can potentially affect
multiple users, there is a need for automated detection of looped
back NS messages during DAD operations by a node.
Note: In both examples above, the IPv6 link-local address DAD
operation fails due to a looped back DAD probe. However, the problem
of a looped back DAD probe exists for any IPv6 address type including
global addresses.
3. Operational Mitigation Options
Two mitigation options are described below that do not require any
change to existing implementations.
3.1. Disable DAD on an Interface
One can disable DAD on an interface so that there are no NS(DAD)
messages issued. While this mitigation may be the simplest, the
mitigation has three drawbacks: 1) care is needed when making such
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RFC 7527 Enhanced DAD April 2015
configuration changes on point-to-point interfaces, 2) this is a one-
time manual configuration on each interface, and 3) genuine
duplicates on the link will not be detected.
A service provider router, such as an access concentrator, or network
core router, SHOULD support the DAD deactivation per interface.
3.2. Dynamic Disable/Enable of DAD Using Layer 2 Protocol
Some Layer 2 protocols include provisions to detect the existence of
a loopback on an interface circuit, usually by comparing protocol
data sent and received. For example, the Point-to-Point Protocol
(PPP) uses a magic number (Section 6.4 of [RFC 1661]) to detect a
loopback on an interface.
When a Layer 2 protocol detects that a loopback is present on an
interface circuit, the device MUST temporarily disable DAD on the
interface. When the protocol detects that a loopback is no longer
present (or the interface state has changed), the device MUST
(re-)enable DAD on that interface.
This mitigation has several benefits. It leverages the Layer 2
protocol's built-in hardware loopback detection capability, if
available. Being a hardware solution, it scales better than the
software solution proposed in this document. This mitigation also
scales better since it relies on an event-driven model that requires
no additional state or timer. This may be significant on devices
with hundreds or thousands of interfaces that may be in loopback for
long periods of time (e.g., awaiting turn-up).
Detecting looped back DAD messages using a Layer 2 protocol SHOULD be
enabled by default, and it MUST be a configurable option if the Layer
2 technology provides means for detecting loopback messages on an
interface circuit.
3.3. Operational Considerations
The mitigation options discussed above do not require the devices on
both ends of the circuit to support the mitigation functionality
simultaneously and do not propose any capability negotiation. They
are effective for unidirectional circuit or interface loopback (i.e.
the loopback is placed in one direction on the circuit, rendering the
other direction nonoperational), but they may not be effective for a
bidirectional loopback (i.e., the loopback is placed in both
directions of the circuit interface, so as to identify the faulty
segment). This is because, unless both ends followed a mitigation
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RFC 7527 Enhanced DAD April 2015
option specified in this document, the noncompliant device would
follow current behavior and disable IPv6 on that interface due to DAD
until manual intervention restores it.
4. The Enhanced DAD Algorithm
The Enhanced DAD algorithm covers detection of a looped back NS(DAD)
message. This document proposes use of a random number in the Nonce
Option specified in SEcure Neighbor Discovery (SEND) [RFC 3971]. Note
[RFC 3971] does not provide a recommendation for pseudorandom
functions. Pseudorandom functions are covered in [RFC 4086]. Since a
nonce is used only once, the NS(DAD) for each IPv6 address of an
interface uses a different nonce. Additional details of the
algorithm are included in Section 4.1.
If there is a collision because two nodes used the same Target
Address in their NS(DAD) and generated the same random nonce, then
the algorithm will incorrectly detect a looped back NS(DAD) when a
genuine address collision has occurred. Since each looped back
NS(DAD) event is logged to system management, the administrator of
the network will have access to the information necessary to
intervene manually. Also, because the nodes will have detected what
appear to be looped back NS(DAD) messages, they will continue to
probe, and it is unlikely that they will choose the same nonce the
second time (assuming quality random number generators).
The algorithm is capable of detecting any ND solicitation (NS and
Router Solicitation) or advertisement (NA and Router Advertisement)
that is looped back. However, there may be increased implementation
complexity and memory usage for the sender node to store a nonce and
nonce-related state for all ND messages. Therefore, this document
does not recommend using the algorithm outside of the DAD operation
by an interface on a node.
4.1. Processing Rules for Senders
If a node has been configured to use the Enhanced DAD algorithm, when
sending an NS(DAD) for a tentative or optimistic interface address,
the sender MUST generate a random nonce associated with the interface
address, MUST store the nonce internally, and MUST include the nonce
in the Nonce option included in the NS(DAD). If the interface does
not receive any DAD failure indications within RetransTimer
milliseconds (see [RFC 4861]) after having sent DupAddrDetectTransmits
Neighbor Solicitations, the interface moves the Target Address to the
assigned state.
Asati, et al. Standards Track PAGE 6
RFC 7527 Enhanced DAD April 2015
If any probe is looped back within RetransTimer milliseconds after
having sent DupAddrDetectTransmits NS(DAD) messages, the interface
continues with another MAX_MULTICAST_SOLICIT number of NS(DAD)
messages transmitted RetransTimer milliseconds apart. Section 2 of
[RFC 3971] defines a single-use nonce, so each Enhanced DAD probe uses
a different nonce. If no probe is looped back within RetransTimer
milliseconds after MAX_MULTICAST_SOLICIT NS(DAD) messages are sent,
the probing stops. The probing MAY be stopped via manual
intervention. When probing is stopped, the interface moves the
Target Address to the assigned state.
4.2. Processing Rules for Receivers
If the node has been configured to use the Enhanced DAD algorithm and
an interface on the node receives any NS(DAD) message where the
Target Address matches the interface address (in tentative or
optimistic state), the receiver compares the nonce included in the
message, with any stored nonce on the receiving interface. If a
match is found, the node SHOULD log a system management message,
SHOULD update any statistics counter, and MUST drop the received
message. If the received NS(DAD) message includes a nonce and no
match is found with any stored nonce, the node SHOULD log a system
management message for a DAD-failed state and SHOULD update any
statistics counter.
4.3. Changes to RFC 4861
The following text is appended to the Source Address definition in
Section 4.3 of [RFC 4861]:
If a node has been configured to use the Enhanced DAD algorithm, an
NS with an unspecified source address adds the Nonce option to the
message and implements the state machine of the Enhanced DAD
algorithm.
The following text is appended to the RetransTimer variable
description in Section 6.3.2 of [RFC 4861]:
The RetransTimer MAY be overridden by a link-specific document if a
node supports the Enhanced DAD algorithm.
5. Action to Perform on Detecting a Genuine Duplicate
As described in the paragraphs above, the nonce can also serve to
detect genuine duplicates even when the network has potential for
looping back ND messages. When a genuine duplicate is detected, the
node follows the manual intervention specified in Section 5.4.5 of
[RFC 4862]. However, in certain cases, if the genuine duplicate
Asati, et al. Standards Track PAGE 7
RFC 7527 Enhanced DAD April 2015
matches the tentative or optimistic IPv6 address of a network
interface of the access concentrator, additional automated action is
recommended.
Some networks follow a trust model where a trusted router serves
untrusted IPv6 host nodes. Operators of such networks have a desire
to take automated action if a network interface of the trusted router
has a tentative or optimistic address duplicated by a host. One
example of a type of access network is cable broadband deployment
where the access concentrator is the first-hop IPv6 router to
multiple broadband modems and supports proxying of DAD messages. The
network interface on the access concentrator initiates DAD for an
IPv6 address and detects a genuine duplicate due to receiving an
NS(DAD) or an NA message. On detecting such a duplicate, the access
concentrator SHOULD log a system management message, drop the
received ND message, and block the modem on whose Layer 2 service
identifier the duplicate NS(DAD) or NA message was received. Any
other network that follows the same trust model MAY use the automated
action proposed in this section.
6. Security Considerations
This document does not improve or reduce the security posture of
[RFC 4862]. The nonce can be exploited by a rogue deliberately
changing the nonce to fail the looped back detection specified by the
Enhanced DAD algorithm. SEND is recommended to circumvent this
exploit. Additionally, the nonce does not protect against the DoS
caused by a rogue node replying by a fake NA to all DAD probes. SEND
is recommended to circumvent this exploit also. Disabling DAD has an
obvious security issue before a remote node on the link can issue
reflected NS(DAD) messages. Again, SEND is recommended for this
exploit. Source Address Validation Improvement (SAVI) [RFC 6620] also
protects against various attacks by on-link rogues.
7. Normative References
[RFC 1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994,
<http://www.rfc-editor.org/info/RFC 1661>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998,
<http://www.rfc-editor.org/info/RFC 2328>.
Asati, et al. Standards Track PAGE 8
RFC 7527 Enhanced DAD April 2015
[RFC 3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005,
<http://www.rfc-editor.org/info/RFC 3971>.
[RFC 4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005, <http://www.rfc-editor.org/info/RFC 4086>.
[RFC 4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006,
<http://www.rfc-editor.org/info/RFC 4429>.
[RFC 4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007, <http://www.rfc-editor.org/info/RFC 4861>.
[RFC 4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007,
<http://www.rfc-editor.org/info/RFC 4862>.
[RFC 6620] 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, <http://www.rfc-editor.org/info/RFC 6620>.
Acknowledgements
Thanks (in alphabetical order by first name) to Adrian Farrel, Benoit
Claise, Bernie Volz, Brian Haberman, Dmitry Anipko, Eric Levy-
Abegnoli, Eric Vyncke, Erik Nordmark, Fred Templin, Hilarie Orman,
Jouni Korhonen, Michael Sinatra, Ole Troan, Pascal Thubert, Ray
Hunter, Suresh Krishnan, Tassos Chatzithomaoglou, and Tim Chown for
their guidance and review of the document. Thanks to Thomas Narten
for encouraging this work. Thanks to Steinar Haug and Scott Beuker
for describing some of the use cases.
Asati, et al. Standards Track PAGE 9
RFC 7527 Enhanced DAD April 2015
Authors' Addresses
Rajiv Asati
Cisco Systems, Inc.
7025 Kit Creek road
Research Triangle Park, NC 27709-4987
United States
EMail: rajiva@cisco.com
URI: http://www.cisco.com/
Hemant Singh
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
United States
Phone: +1 978 936 1622
EMail: shemant@cisco.com
URI: http://www.cisco.com/
Wes Beebee
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
United States
Phone: +1 978 936 2030
EMail: wbeebee@cisco.com
URI: http://www.cisco.com/
Carlos Pignataro
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
United States
EMail: cpignata@cisco.com
URI: http://www.cisco.com/
Asati, et al. Standards Track PAGE 10
RFC 7527 Enhanced DAD April 2015
Eli Dart
Lawrence Berkeley National Laboratory
1 Cyclotron Road, Berkeley, CA 94720
United States
EMail: dart@es.net
URI: http://www.es.net/
Wesley George
Time Warner Cable
13820 Sunrise Valley Drive
Herndon, VA 20171
United States
EMail: wesley.george@twcable.com
Asati, et al. Standards Track PAGE 11
RFC TOTAL SIZE: 24528 bytes
PUBLICATION DATE: Saturday, April 25th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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