Internet DRAFT - draft-yourtchenko-6man-dad-issues
draft-yourtchenko-6man-dad-issues
Network Working Group A. Yourtchenko
Internet-Draft cisco
Intended status: Informational E. Nordmark
Expires: September 1, 2015 Arista Networks
February 28, 2015
A survey of issues related to IPv6 Duplicate Address Detection
draft-yourtchenko-6man-dad-issues-01
Abstract
This document enumerates the practical issues observed with respect
to Duplicate Address Detection.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Robustness: Interaction with delay in forwarding . . . . . 3
2.2. Robustness: Behavior on links with unreliable multicast . 4
2.3. Robustness: Partition-join tolerance . . . . . . . . . . . 4
2.4. Robustness: Behavior on collision . . . . . . . . . . . . 4
2.5. Energy Efficiency . . . . . . . . . . . . . . . . . . . . 5
2.6. Wake-up and L2 events . . . . . . . . . . . . . . . . . . 5
3. Solved Issues . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Interaction with looped interfaces . . . . . . . . . . . . 5
3.2. Delays before an address can be used . . . . . . . . . . . 6
4. Observations . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Duplicate L2 address detection . . . . . . . . . . . . . . 6
4.2. Usage of DAD to create state . . . . . . . . . . . . . . . 6
4.3. No support of multi-link subnets . . . . . . . . . . . . . 7
4.4. Anycast Addresses and Duplicate Address Detection . . . . 7
4.5. Implementations doing DAD once per IID . . . . . . . . . . 7
4.6. Backwards compatibility and presence of the DAD proxies . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
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 [RFC2119].
Duplicate Address Detection (DAD) is a procedure in IPv6 performed on
an address before it can be assigned to an interface [RFC2462]. By
default it consists of sending a single multicast Neighbor
Soliciation message and waiting for a response for one second. If no
response is received, the address is declared to not be a duplicate.
Once the address has been tested once, there is no further attempts
to check for duplicates (unless the interface is re-initialized).
On one hand, it is mandatory for all addresses. On the other hand,
it is a "best effort" activity. These somewhat counter-intuitive
properties result in some issues that arise related to DAD. They are
listed below. The issues have been grouped to facilitate discussing
them.
2. Open Issues
Whether it is due to the assumptions made in 1995, or changes in how
networks are built or deployed, there are many reasons why DAD would
fail to detect a duplicate even when one exists. From a historical
perspective it is important to keep in mind that when DAD was
designed we had two forms of IPv6 addresses; those derived from
EUI-64 and statically assigned. Since the IETF has developed
additional methods for address assignment like DHCPv6 and addresses
that improve privacy by reducing linkability.
2.1. Robustness: Interaction with delay in forwarding
The DAD makes an assumption that if a link layer is up, the traffic
can be immediately forwarded, which is frequently not the case in
modern networks. Two prominent cases include the switches running
Spanning Tree Protocol (STP), and bridging modems.
When a port on an STP-enabled switch comes up, it goes through three
phases of Listening then Learning then Forwarding. The default is to
keep it for 15 seconds in Listening and 15 seconds in Learning
states. During this time no user traffic is forwarded by the switch
from and to this port. Therefore, if a DAD process happens during
this period it is guaranteed to not detect any duplicates. This
results in DAD being ineffective for link-local and otherwise pre
configured addresses.
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Similarly, a modem-like device whose line status is invisible to IP
stack either within the modem or to a host connected on the Ethernet
side, also renders the DAD ineffective - the delay before the
connectivity is established can be much longer than any DAD wait.
Some of the link types, notably cable modems, have link-specific
standards to address this issue by requiring a new DAD each time the
RF-side interface bounces, as well as bouncing the LAN interface
triggered by the bounce of the RF interface.
Note that [I-D.ietf-6man-resilient-rs] makes the router solicitation
resilent to the above cases, but there is no counterpart to make DAD
robust.
2.2. Robustness: Behavior on links with unreliable multicast
DAD requires two multicast messages to pass through - the NS and NA.
Thus it shows a noticeable failure rate on links that do not pass
multicast reliably e.g. the 802.11a/b/g/n series of technologies.
See [I-D.vyncke-6man-mcast-not-efficient] for more information.
The author's ad-hoc experimentation at IETF90 revealed the success
rate of detecting the duplicate address on the IETF WiFi network
being about 4 in 5. This may violate the assumptions that other
protocols make.
2.3. Robustness: Partition-join tolerance
[RFC4862] explicitly mentions this problem: "Note that the method for
detecting duplicates is not completely reliable, and it is possible
that duplicate addresses will still exist (e.g., if the link was
partitioned while Duplicate Address Detection was performed)."
In contrast, IPv4 stacks typically implement the Address Conflict
Detection (ACD) from [RFC5227]. This disparity results in a less
robust operation of IPv6 compared to IPv4 and is undesirable.
Note that solutions along the lines of ACD, while improving
robustness, might result in more resource usage in on the links and
nodes by multicasting more ND packets.
2.4. Robustness: Behavior on collision
[RFC4862] in its section "5.4.5. When Duplicate Address Detection
Fails" is much more prescriptive than [RFC2462] that it superceeds.
However, it has been observed that some implementations may simply
reset the network interface and attempt the DAD process again. This
behavior, while being more resilient in case the DAD failure is
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happening erroneously, is different from what is recommended in the
standard.
TBD: Do the other RFCs for address allocation require some retry
behavior?
2.5. Energy Efficiency
The use of multicast messages for DAD results in some inefficiencies
for both the network, in particular when multicast uses more layer 2
resources than unicast, and also has efficiency implications for
hosts. Potential techniques for making DAD reliably detect and
recover from duplicates might result in reduced efficiency. The
impact for WiFi is shown in
[I-D.desmouceaux-ipv6-mcast-wifi-power-usage].
If a node wants to "defend" its address using DAD, it has to be awake
and listening on the solicited node multicast address in order to
receive the DAD NS. In the low-power environments this may
significantly impact the battery life of the devices.
2.6. Wake-up and L2 events
In mobile environments, node may roam in different parts of the
network and also take "naps". The specification in [RFC4862] does
not explicitly discuss this scenario, nor does DNA [RFC6059], so
there is a room for ambiguity in implementation. This may either
result in less robust DAD coverage (if the node does not perform the
DAD again when an L2 event happens), or an excessive amount of
multicast packets (when a node performs the dad every time L2 event
happens and there is a lot of them moving within a segment).
Thus this item could be categorized as being either in the robustness
or efficiency group of items.
3. Solved Issues
Some issues have been or are in the process of being solved.
3.1. Interaction with looped interfaces
[RFC4862] explicitly defines that the case of a physically looped
back interface is not a failure: "If the solicitation is from the
node itself (because the node loops back multicast packets), the
solicitation does not indicate the presence of a duplicate address."
However, the practical experiences show that the measures described
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in [RFC4862] are either incomplete or incorrectly implemented: a
loopback on the interface causes DAD failure.
[I-D.ietf-6man-enhanced-dad] provides the solution to this issue.
3.2. Delays before an address can be used
Section "5.4. Duplicate Address Detection" of [RFC4862] specifies
that until the DAD procedure completes, the address remains in
Tentative state. In this state, any traffic to this address other
than that related to DAD-related is dropped. This introduces delay
between the interface getting connected to the network and an address
on this interface becoming usable. For fast-moving nodes it may be a
problem.
[RFC4429] introduces "Optimistic DAD" process, which addresses this.
That document has some notes about potentially causing TCP RST when
there is a duplicate, which can reset an existing TCP connection for
the existing user of the IPv6 address. That has some overall impact
on the robustness of the network and implicitly assumes that all
application protocols will always retry in order to handle such an
event.
4. Observations
Some issues we can't do much about in that they are more observations
of what can be done.
4.1. Duplicate L2 address detection
DAD does not detect duplicate L2 addresses in all cases. Depending
on the medium, it may be impossible to detect a duplicate L2 address
- e.g. if this address itself is used as a determinant in order to
establish the L2 connection.
4.2. Usage of DAD to create state
[RFC4862] in section "5.4. Duplicate Address Detection" states that
DAD must be performed on all addresses. Given the potentially
decentralized nature of address assignment in IPv6, this property is
being used to prebuild the state in the network about the host's
addresses - e.g. for "First Come First Served" security as described
in section "3.2.3. Processing of Local Traffic" of [RFC6620].
If the delivery of the DAD_NS packets is unreliable or there are
nodes on the segment which use the Optimistic DAD mechanism, state
created purely on DAD_NS packets might be also unreliable. The
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specific case of [RFC6620] solves the issue by triggering the
recreation of state based on data packets as well, however it might
not be possible in some scenarios.
4.3. No support of multi-link subnets
DAD doesn't support multi-link subnets: a multicast DAD_NS sent on
one link will not be seen on the other.
[RFC6275] specifically provides one way to construct a multi-link
subnet (consisting of a broadcast link and a collection of point to
point tunnels). It explicitly defines the procedures for making DAD
work in that topology.
[RFC4903] discusses the issues related to multi-link subnets - and
given the multi-link subnets might be created in many ways, it might
be prudent to keep enhancements to DAD whose sole purpose is related
to multi-link subnets, to be out of scope.
One may also argue that since [RFC4861] defers the clarifications on
IPv6 operation on NBMA networks to [RFC2491], it is unreasonable to
expect [RFC4862] describe the operation of DAD on NBMA type links,
and it is up to a link-specific document to describe such operation.
(An example is cable industry, where the cable standards define it).
However, it is then unclear where to address the frequently used
scenario of WiFi with blocked direct communication between the
stations - whether it is supposed to be an IEEE document or IETF
document ? And is there enough fundamental differences between the
different NBMA models to warrant the link-specific approaches to DAD
?
4.4. Anycast Addresses and Duplicate Address Detection
Section 5.4 "Duplicate Address Detection" of [RFC4862] specifies that
Duplicate Address Detection MUST NOT be performed on anycast
addresses. This, stems from the fact that the anycast addresses are
syntactically indistinguishable from unicast addresses. One can
argue that this allows for misconfiguration if an address deemed to
be anycast already exist on the network.
4.5. Implementations doing DAD once per IID
Section 5.4 of [RFC4862] mentions the implementations performing a
single DAD per interface identifier, and discourages that
"optimization". As the practice is emerging in the industry is to
move away from the fixed interface identifiers anyhow, the necessity
to perform a DAD on a per-address basis might be useful to elevate to
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a requirement status.
4.6. Backwards compatibility and presence of the DAD proxies
While not being an issue as such, this is a reminder that the
operation of DAD has to remain backwards compatible, both to remain
cooperative with the existing hosts, and the potentially present DAD
proxies as described in [RFC6957].
There are also various forms of sleep proxies [ECMA-393]
[http://en.wikipedia.org/wiki/Bonjour_Sleep_Proxy] which perform
handoffs of Neighbor Discovery protocol processing that need to be
considered.
5. Acknowledgements
Thanks to Ole Troan for creating and curating the original list.
Thanks a lot to Lorenzo Colitti, Suresh Krishnan, Hemant Singh,
Hesham Soliman, Eric Vyncke, and James Woodyatt for the reviews and
useful suggestions.
6. IANA Considerations
None.
7. Security Considerations
There are no additional security considerations as this document only
outlines the issues observed with the current Duplicate Address
Detection protocol.
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.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
over Non-Broadcast Multiple Access (NBMA) networks",
RFC 2491, January 1999.
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[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
[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.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
June 2007.
[RFC5227] Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
July 2008.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
November 2010.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011.
[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.
[RFC6957] Costa, F., Combes, J-M., Pougnard, X., and H. Li,
"Duplicate Address Detection Proxy", RFC 6957, June 2013.
8.2. Informative References
[I-D.desmouceaux-ipv6-mcast-wifi-power-usage]
Desmouceaux, Y., "Power consumption due to IPv6 multicast
on WiFi devices",
draft-desmouceaux-ipv6-mcast-wifi-power-usage-01 (work in
progress), August 2014.
[I-D.ietf-6man-enhanced-dad]
Asati, R., Singh, H., Beebee, W., Pignataro, C., Dart, E.,
and W. George, "Enhanced Duplicate Address Detection",
draft-ietf-6man-enhanced-dad-13 (work in progress),
February 2015.
[I-D.ietf-6man-resilient-rs]
Krishnan, S., Anipko, D., and D. Thaler, "Packet loss
resiliency for Router Solicitations",
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draft-ietf-6man-resilient-rs-04 (work in progress),
October 2014.
[I-D.vyncke-6man-mcast-not-efficient]
Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
Yourtchenko, "Why Network-Layer Multicast is Not Always
Efficient At Datalink Layer",
draft-vyncke-6man-mcast-not-efficient-01 (work in
progress), February 2014.
Authors' Addresses
Andrew Yourtchenko
cisco
6b de Kleetlaan
Diegem 1831
Belgium
Email: ayourtch@cisco.com
Erik Nordmark
Arista Networks
Santa Clara, CA
USA
Email: nordmark@arista.com
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