V6OPS B. Liu
Internet-Draft S. Jiang
Intended status: Informational Huawei Technologies
Expires: April 30, 2015 R. Bonica
Juniper Networks
X. Gong
W. Wang
BUPT University
October 27, 2014
DHCPv6/SLAAC Address Configuration Interaction Problem Statement
draft-ietf-v6ops-dhcpv6-slaac-problem-03
Abstract
The IPv6 Neighbor Discovery (ND) Protocol includes an ICMPv6 Router
Advertisement (RA) message. The RA message contains three flags,
indicating which autoconfiguration mechanisms are available to on-
link hosts. These are the M, O and A flags. The M and O flags are
advisory, not prescriptive.
This document describes divergent host behaviors observed in popular
operating systems. It also describes operational problems that
divergent behaviors cause.
Status of This Memo
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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
2. The M, O and A Flags . . . . . . . . . . . . . . . . . . . . 3
2.1. M (Managed) Flag . . . . . . . . . . . . . . . . . . . . 3
2.2. O (Otherconfig) Flag . . . . . . . . . . . . . . . . . . 4
2.3. A (Autonomous) Flag . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Divergent Host Behaviors . . . . . . . . . . . . . . . . 4
3.2. Operational Problems . . . . . . . . . . . . . . . . . . 5
3.2.1. Inappropriate Sources . . . . . . . . . . . . . . . . 5
3.2.2. Renumbering . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Test Results . . . . . . . . . . . . . . . . . . . . 7
A.1. Test Environment . . . . . . . . . . . . . . . . . . . . 7
A.2. Host Behavior in the Initial State . . . . . . . . . . . 8
A.3. Host Behavior in State Transitions . . . . . . . . . . . 10
Appendix B. Analysis of the Ambiguities . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
IPv6 [RFC2460] hosts invoke Neighbor Discovery (ND) [RFC4861]
procedures in order to discover which autoconfiguration mechanisms
are available to them. The following is a list of autoconfiguration
mechanisms:
o DHCPv6 [RFC3315]
o Stateless Address Autoconfiguration (SLAAC) [RFC4862]
ND specifies an ICMPv6 [RFC4443] Router Advertisement (RA) message.
Routers periodically broadcast the RA message to all on-link nodes.
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They also unicast RA messages in response to solicitations. The RA
message contains:
o an M (Managed) flag
o an O (OtherConfig) flag
o zero or more Prefix Information (PI) Options
The M flag indicates that addresses are available from DHCPv6. The O
flag indicates that other configuration information (e.g., DNS-
related information) is available from DHCPv6. The PI Option
includes a prefix, an A (Autonomous) flag and other fields. The A
flag indicates that the prefix can be used for SLAAC. The M and O
flags are advisory, not prescriptive (although A flag is also
advisory in definition in standard, it is quite prescriptive in
implementations). For example, the M flag indicates that addresses
are available from DHCPv6. It does not indicate that hosts are
required to acquire addresses from DHCPv6. Similar statements can be
made about the O flag.
In most cases, the M, O and A flags elicit identical behaviors from
most popular operating systems. However, in several cases, the M, O
and A flags elicit divergent behaviors. For example, when a router
changes the settings of the M, O, and A flag from one RA message to
the next, it is likely to elicit one behavior from hosts running one
operating system and another behavior from hosts running a different
operating system.
This document describes divergent host behaviors observed in popular
operating systems. It also describes operational problems that
divergent behaviors cause.
2. The M, O and A Flags
This section briefly reviews how the M, O and A flags are defined in
[RFC4861].
2.1. M (Managed) Flag
The M flag indicates that addresses are available from DHCPv6. If
the M flag is set, the O flag is redundant and can be ignored because
DHCPv6 will return all available configuration information.
M and A flag semantics are independent of one another. The M flag
indicates that addresses are available from DHCPv6, regardless of the
A flag setting. The following settings are all allowed:
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o M=0 A=0
o M=0 A=1
o M=1 A=0
o M=1 A=1
2.2. O (Otherconfig) Flag
The O flag indicates that other configuration information (e.g., DNS-
related information) is available from DHCPv6. If the M flag is set,
the O flag is redundant and can be ignored because DHCPv6 will return
all available configuration information.
O and A flag semantics are independent of one another. The O flag
indicates that other configuration is available from DHCPv6,
regardless of the A flag setting. The following settings are all
allowed:
o O=0 A=0
o O=0 A=1
o O=1 A=0
o O=1 A=1
2.3. A (Autonomous) Flag
The A flag indicates that the prefix that is also carried by the PI
option can be used for SLAAC. A flag semantics are independent of M
and O flag semantics. The A flag indicates that the prefix can be
used by SLAAC, regardless of the M and O flag settings.
3. Problem Statement
3.1. Divergent Host Behaviors
The authors tested several popular operating systems in order to
determine what behaviors the M, O and A flag elicit. In some cases,
the M, A and O flags elicit identical behaviors from most popular
operating systems. However, in several cases, the M, O and A flags
elicit divergent behaviors. The table below characterizes those
cases:
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Host State Input Behavior
Host has not No RA Some popular operating systems acquire
acquired any addresses from DHCPv6. Others do not.
addresses
Host has not RA Some popular operating systems acquire
acquired any with other information from DHCPv6, regardless
addresses M=0, of the A flag setting. Others do so, but
O=1 only if A=1
Host has acquired RA Some operating systems release DHCPv6
addresses from with M addresses immediately. Some release DHCPv6
DHCPv6 only (M = =0 addresses when they expire.
1)
Host has acquired RA Some operating systems acquire DHCPv6
addresses from with M addresses immediately. Others do so only
SLAAC only (A=1) = 1 if their SLAAC addresses expire and cannot
be refreshed.
3.2. Operational Problems
This section describes operational issues caused by the divergent
behaviors, described above.
3.2.1. Inappropriate Sources
Some operating systems base their decision to acquire configuration
information upon inappropriate sources. For example, some operating
systems acquire other configuration information if M = 0, O = 1, and
A = 1, but not if M = 0, 0 = 1 and A = 0. In other words, on some
operating systems, it is impossible to acquire other information from
DHCPv6 unless addresses are acquired from either DHCPv6 or SLAAC.
3.2.2. Renumbering
According to [RFC6879] a renumbering exercise can include the
following steps:
o Causing hosts that have acquired addresses from one
autoconfiguration mechanism to release those addresses and acquire
new addresses from another autoconfiguration mechanism
o Causing hosts that have acquired addresses from one
autoconfiguration mechanism to release those addresses and acquire
new addresses from the same autoconfiguration mechanism
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o Causing hosts that have acquired addresses from one
autoconfiguration mechanism to retain those addresses and acquire
new addresses from another autoconfiguration mechanism
Ideally, these steps could be initiated by broadcasting RA message
onto the subnetwork that is being renumbered. Sadly, this is not
possible, because the RA message may elicit a different behavior from
each host. According to Section 3.1, renumbering operations would
have the following limitations:
o When M flag is turned off, some operating systems release DHCPv6
acquired addresses immediately, while other will retain then until
they expire. This means a flash switch from DHCPv6 to SLAAC would
happen on some hosts. Normally, the "make-before-break" approach
proposed in [RFC4192] is considered better than flash renumbering.
o On some operating systems, if a host has acquired addresses from
SLAAC, it is impossible to acquire additional addresses from
DHCPv6. This may be required as part of a renumbering operation.
4. Security Considerations
As this memo does not introduce any new protocols or procedures, it
does not introduce any new security vulnerabilities.
5. IANA Considerations
This draft does not request any IANA action.
6. Acknowledgements
The authors wish to acknowledge BNRC-BUPT (Broad Network Research
Centre in Beijing University of Posts and Telecommunications) for
their testing efforts. Special thanks to Xudong Shi, Longyun Yuan
and Xiaojian Xue for their extraordinary effort.
The authors also wish to acknowledge Brian E Carpenter, Ran Atkinson,
Mikael Abrahamsson, Tatuya Jinmei, Mark Andrews and Mark Smith for
their helpful comments.
7. References
7.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 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.
7.2. Informative References
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
September 2005.
[RFC6879] Jiang, S., Liu, B., and B. Carpenter, "IPv6 Enterprise
Network Renumbering Scenarios, Considerations, and
Methods", RFC 6879, February 2013.
Appendix A. Test Results
A.1. Test Environment
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/-----\
+---------+ // \\
| DHCPv6 | | Router |
| server | \\ //
+----+----+ \--+--/
| |
| |
| |
----+--+----------+----------+---+-----
| | |
| | |
| | |
+----+---+ +----+---+ +----+---+
| | | | | |
| Host 1 | | Host 2 | | Host 3 |
+--------+ +--------+ +--------+
Figure 1: Test Environment
The test environment depicted Figure 1 in was replicated on a single
server using VMware. For simplicity of operation, only one host was
run at a time. Network elements were as follows:
o Router: Quagga 0.99-19 soft router installed on Ubuntu 11.04
virtual host
o DHCPv6 Server: Dibbler-server installed on Ubuntu 11.04 virtual
host
o Host 1: Window 7 / Window 8.1 Virtual Host
o Host 2: Ubuntu 14.04 (Linux Kernel 3.12.0) Virtual Host
o Host 3: Mac OS X v10.9 Virtual Host
o Host 4: IOS 8.0 (model: Apple iPhone 5S, connected via wifi)
A.2. Host Behavior in the Initial State
The bullet list below describes host behavior in the initial state,
when the host has not yet acquired any autoconfiguration information.
Each bullet item represents an input and the behavior elicited by
that input.
o A=0, M=0, O=0
* Windows 8.1 acquired addresses and other information from
DHCPv6.
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* All other hosts acquired no configuration information.
o A=0, M=0, O=1
* Windows 8.1 acquired addresses and other information from
DHCPv6.
* Windows 7, OSX 10.9 and IOS 8.0 acquired other information from
DHCPv6.
* Ubuntu 14.04 acquired no configuration information.
o A=0, M=1, O=0
* All hosts acquired addresses and other information from DHCPv6.
o A=0, M=1, O=1
* All hosts acquired addresses and other information from DHCPv6.
o A=1, M=0, O=0
* Windows 8.1 acquired addresses from SLAAC and DHCPv6. It also
acquired non-address information from DHCPv6.
* All the other host acquired addresses from SLAAC
o A=1, M=0, O=1
* Windows 8.1 acquired addresses from SLAAC and DHCPv6. It also
acquired other information from DHCPv6.
* All the other hosts acquired addresses from SLAAC and other
information from DHCPv6.
o A=1, M=1, O=0
* All hosts acquired addresses from SLAAC and DHCPv6. They also
acquired other information from DHCPv6.
o A=1, M=1, O=1
* All hosts acquired addresses from SLAAC and DHCPv6. They also
acquired other information from DHCPv6.
As showed above, four inputs result in divergent behaviors.
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A.3. Host Behavior in State Transitions
The bullet list below describes behavior elicited during state
transitions. The value x can represents both 0 and 1.
o Old state (M = x, O = x, A = 1) , New state (M = x, O = x, A = 0)
(This means a SLAAC-configured host, which is regardless of DHCPv6
configured or not, receiving A transitioning from 1 to 0. )
* All the hosts retain SLAAC addresses until they expire
o Old state (M = 0, O = x, A = 1), New state (M = 1, O = x, A = 1)
(This means a SLAAC-only host receiving M transitioning from 0 to
1.)
* Windows 7 acquires addresses from DHCPv6, immediately.
* Ubuntu 14.04/OSX 10.9/IOS 8.0 acquires addresses from DHCPv6
only if the SLAAC addresses are allowed to expire
* Windows 8.1 was not tested because it always acquire addresses
from DHCPv6 regardless of the M flag setting.
o Old state (M = 1, O = x, A = x), New state (M = 0, O = x, A = x)
(This means a DHCPv6-configured host receiving M transitioning
from 1 to 0.)
* Windows 7 immediately released the DHCPv6 address
* Windows 8.1/Ubuntu 14.04/OSX 10.9/IOS 8.0 keep the DHCPv6
addresses until they expire
o Old state (M = 1, O = x, A = 0), New state (M = 1, O = x, A = 1)
(This means a DHCPv6-only host receiving A transitioning from 0 to
1.)
* All host acquire addresses from SLAAC
o Old state (M = 0, O = 1, A = x), New state (M = 1, O = 1, A = x)
(This means a Stateless DHCPv6-configured host [RFC3736], which is
regardless of SLAAC configured or not, receiving M transitioning
from 0 to 1 with keeping O=1 )
* Windows 7 acquires addresses and refreshes other information
from DHCPv6
* Ubuntu 14.04/OSX 10.9/IOS 8.0 does nothing
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* Windows 8.1 was not tested because it always acquire addresses
from DHCPv6 regardless of the M flag setting.
o Old state (M = 1, O = 1, A = x), New state (M = 0, O = 1, A = x)
(This means a Stateful DHCPv6-configured host, which is regardless
of SLAAC configured or not, receiving M transitioning from 0 to 1
with keeping O=1 )
* Windows 7 released all DHCPv6 addresses and refreshes all
DHCPv6 other information.
* Windows 8.1/Ubuntu 14.04/OSX 10.9/IOS 8.0 does nothing
Appendix B. Analysis of the Ambiguities
Following is a comprehensive analysis of the ambiguities as defined
in the standards. In theory, all the ambiguities might cause
divergent host behavior. Some of the divergence has been identified
by the tests while some haven't. It is worth to document all the
ambiguities.
1. Dependency between DHCPv6 and RA
In standards, behavior of DHCPv6 and Neighbor Discovery protocols
is specified respectively. But it is not clear that whether there
should be any dependency between them. More specifically, is RA
(with M=1) required to trigger DHCPv6? If there are no RAs at
all, should hosts initiate DHCPv6 by themselves?
2. Behaviors of Flag Transition
When flags are in transition, e.g. the host is already SLAAC-
configured, then M flag changes from FALSE to TRUE, it is not
clear whether the host should start DHCPv6 or not; or vise versa,
the host is already both SLAAC/DHCPv6 configured, then M flag
change from TRUE to FALSE, it is also not clear whether the host
should turn DHCPv6 off or not.
3. Distinction between "Address Configuring Method" and "Address
Lifetime"
When one address configuration method is off, that is, the A flag
or M flag changes from TRUE to FALSE, it is not clear whether the
host should immediately release the corresponding address(es) or
just retain it(them) until expired.
4. Dependencies between the flags
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The semantics of the flags seems not totally independent, but the
standards didn't clearly clarify it. For example, when both M and
O flags are TRUE, it is not clear whether the host should initiate
one stateful DHCPv6 session to get both address and info-
configuration or initiate two independent sessions of which one is
dedicated for address provisioning and the other is for
information provision. When A and M flags are FALSE and O flag is
TRUE, it is not clear whether the host should initiate a stand-
alone stateless DHCPv6 session.
Authors' Addresses
Bing Liu
Huawei Technologies
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
Email: leo.liubing@huawei.com
Sheng Jiang
Huawei Technologies
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
Email: jiangsheng@huawei.com
Ron Bonica
Juniper Networks
Sterling, Virginia
20164
USA
Email: rbonica@juniper.net
Xiangyang Gong
BUPT University
No.3 Teaching Building
Beijing University of Posts and Telecommunications (BUPT)
No.10 Xi-Tu-Cheng Rd.
Hai-Dian District, Beijing
P.R. China
Email: xygong@bupt.edu.cn
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Wendong Wang
BUPT University
No.3 Teaching Building
Beijing University of Posts and Telecommunications (BUPT)
No.10 Xi-Tu-Cheng Rd.
Hai-Dian District, Beijing
P.R. China
Email: wdwang@bupt.edu.cn
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