A survey of issues related to IPv6 Duplicate Address Detection
draft-yourtchenko-6man-dad-issues-00

Abstract

This document enumerates the practical issues observed with respect to Duplicate Address Detection.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 30, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Duplicate L2 address detection
• 3. Interaction with delay in forwarding on the link
• 4. Behavior on links with unreliable multicast
• 5. Interaction with looped interfaces
• 6. Delays before an address can be used
• 7. Partition-join tolerance
• 8. Behavior on collision
• 9. Energy efficiency
• 10. Wake-up and L2 events
• 11. Usage of DAD to create state
• 12. Support of multi-link subnets
• 13. Anycast Addresses and Duplicate Address Detection
• 14. Implementations doing DAD once per IID
• 15. Backwards compatibility and presence of the DAD proxies
• 16. Acknowledgements
• 17. IANA Considerations
• 18. Security Considerations
• 19. References
• 19.1. Informative References
• 19.2. Normative References
• Author's Address

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 is a procedure in IPv6 performed before any address can be assigned to the interface. 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.

2. 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.

3. Interaction with delay in forwarding on the link

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.

Similarly, a DSL or cable modem 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.

4. 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). Author's ad-hoc experimentation at IETF90 revealed the success rate of detecting the duplicate address being about 4 in 5. This may violate the assumptions that other protocols make.

5. 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 in [RFC4862] are either incomplete or incorrectly implemented: a loopback on the interface causes DAD failure.

[I-D.ietf-6man-enhanced-dad] discusses the solution to this issue.

6. 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.

7. 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.

8. 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 happening erroneously, is different from what is recommended in the standard.

9. Energy efficiency

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.

10. 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, 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).

11. 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 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.

12. 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.

13. 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.

14. 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 anywhere, the necessity to perform a DAD on a per-address basis might be useful to elevate to a requirement status.

15. 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].

16. Acknowledgements

Thanks to Ole Troan for creating and curating the original list. Thanks a lot to Suresh Krishnan and Erik Nordmark for the reviews and useful suggestions.

17. IANA Considerations

None.

18. Security Considerations

There are no additional security considerations as this document only outlines the issues observed with the current Duplicate Address Detection protocol.

19. References

19.1. Informative References

19.2. Normative References

Author's Address