Internet DRAFT - draft-liu-bonica-v6ops-dhcpv6-slaac-problem
draft-liu-bonica-v6ops-dhcpv6-slaac-problem
Network Working Group B. Liu
Internet Draft Huawei Technologies
Intended status: Proposed Standard R. Bonica
Expires: April 24, 2014 Juniper Networks
X. Gong
W. Wang
BUPT University
October 21, 2013
DHCPv6/SLAAC Address Configuration Interaction Problem Statement
draft-liu-bonica-v6ops-dhcpv6-slaac-problem-00.txt
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Abstract
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This document analyzes the host behavior of DHCPv6/SLAAC interaction
issue. It reviews the standard definition of the host behaviors and
provides the test results of current mainstream implementations. Some
potential operational gaps of the interaction are also described.
Table of Contents
1. Introduction ................................................. 3
2. Host Behavior of DHCPv6/SLAAC Interaction .................... 3
2.1. Relevant RA Flags Defined in Standards .................. 4
2.1.1. A (Autonomous) Flag ................................ 4
2.1.2. M (Managed) Flag ................................... 4
2.1.3. O (Otherconfig) Flag ............................... 4
2.2. Behaviors of Current Implementations .................... 5
2.2.1. A flag ............................................. 5
2.2.2. M flag ............................................. 5
2.2.3. O flag ............................................. 6
3. Possible Operational Gaps of DHCPv6/SLAAC Interaction ........ 6
3.1. Renumbering ............................................. 6
3.2. Cold Start Problems ..................................... 6
3.3. Strong Management ....................................... 7
4. Conclusions .................................................. 7
5. Security Considerations ...................................... 7
6. IANA Considerations .......................................... 7
7. References ................................................... 7
7.1. Normative References .................................... 7
7.2. Informative References .................................. 8
8. Acknowledgments .............................................. 8
Appendix A. Test Details of Host Behaviors ...................... 9
A.1 Host Initialing Behavior ................................ 10
A.2 Host Transition Behavior ................................ 11
A.3 Host Stateful/Stateless DHCPv6 Behavior ................. 11
Authors' Addresses ............................................. 12
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1. Introduction
In IPv6, both of the DHCPv6 [RFC3315] and Neighbor Discovery [RFC4861]
protocols can provide automatic IP address configuration for the
hosts. They are known as stateful address auto-configuration and
SLAAC (stateless address auto-configuration)[RFC4862], and are
suitable for different scenarios respectively. Sometimes the two
address configuration modes may be both available in one network.
In ND protocol, there is a M (ManagedFlag) flag defined in RA message,
indicating the hosts there is DHCPv6 service in the network if the
flag is set. And there is an O "OtherConfigFlag", if set, indicating
configure information other than addresses (e.g. DNS, Route .etc) is
available through DHCPv6 configuration. Moreover, there's another A
(Autonomous) flag defined in ND, which indicating the hosts to do
SLAAC, may also influent the behavior of hosts.
So with the A/M/O flags, the two separated address configuration
modes are somehow correlated. But for some reason, the ND protocol
didn't define the flags as prescriptive but only advisory. This
ambiguous definition may vary the behavior of hosts when interpreting
the flags. In section 2, we provided a brief test result to identify
different host operating systems have taken different approaches.
This would add additional complexity for both the hosts and the
network management.
This draft reviews the standard definition of the above mentioned
flags, and provides a test result of several major desktop operating
systems' behavior. And then identifies potential requirement/gaps of
DHCPv6/SLAAC interaction.
2. Host Behavior of DHCPv6/SLAAC Interaction
In this section, we analyzed A/M/O flags definition, and provide the
test result of host behavior of interpreting these flags in
mainstream operating systems implementations.
Please note that, A flag has no direct relationship with DHCPv6, but
it is somewhat correlated with M/O flags.
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2.1. Relevant RA Flags Defined in Standards
2.1.1. A (Autonomous) Flag
In ND Prefix Information Option, the autonomous address-configuration
flag (A flag). When set indicates that this prefix can be used for
stateless address configuration as specified in SLAAC.
For the host behavior, there is an explicit rule in the SLAAC
specification [RFC4862]: "If the Autonomous flag is not set, silently
ignore the Prefix Information option."
But when A flag is set, the SLAAC protocol didn't provide a
prescriptive definition.
2.1.2. M (Managed) Flag
In earlier SLAAC specification [RFC2462], the host behavior of
interpreting M flag is as below:
"On receipt of a valid Router Advertisement, a host copies the value
of the advertisement's M bit into ManagedFlag. If the value of
ManagedFlag changes from FALSE to TRUE, and the host is not already
running the stateful address autoconfiguration protocol, the host
should invoke the stateful address auto-configuration protocol,
requesting both address information and other information. If the
value of the ManagedFlag changes from TRUE to FALSE, the host should
continue running the stateful address auto-configuration, i.e., the
change in the value of the ManagedFlag has no effect. If the value
of the flag stays unchanged, no special action takes place. In
particular, a host MUST NOT reinvoke stateful address configuration
if it is already participating in the stateful protocol as a result
of an earlier advertisement."
But in the updated SLAAC specification [RFC4862], the relative
description was removed, the reason was "considering the maturity of
implementations and operational experiences. ManagedFlag and
OtherConfigFlag were removed accordingly. (Note that this change does
not mean the use of these flags is deprecated.)"
2.1.3. O (Otherconfig) Flag
As mentioned above, the situation of O flag is similar with M. In
earlier SLAAC [RFC2462], the host behavior is clear:
"If the value of OtherConfigFlag changes from FALSE to TRUE, the host
should invoke the stateful autoconfiguration protocol, requesting
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information (excluding addresses if ManagedFlag is set to FALSE). If
the value of the OtherConfigFlag changes from TRUE to FALSE, the host
should continue running the stateful address autoconfiguration
protocol, i.e., the change in the value of OtherConfigFlag has no
effect. If the value of the flag stays unchanged, no special action
takes place. In particular, a host MUST NOT reinvoke stateful
configuration if it is already participating in the stateful protocol
as a result of an earlier advertisement."
And there's another description of the relationship of M and O flags
in [RFC2462]:
"In addition, when the value of the ManagedFlag is TRUE, the value of
OtherConfigFlag is implicitely TRUE as well. It is not a valid
configuration for a host to use stateful address autoconfiguration to
request addresses only, without also accepting other configuration
information."
2.2. Behaviors of Current Implementations
We did tests of current mainstream desktop/mobile operating systems
on the behaviors; please refer to the appendix for details. This
section illustrates the important results of the tests.
2.2.1. A flag
A flag is a switch to control whether to do SLAAC, and it is
independent with M/O flags, in another word, A is independent with
DHCPv6.
At the non-SLAAC-config state (either non-configured or DHCPv6-
configured only), all the OSes acted the same with A flag, if A set,
they all configured SLAAC, it is obvious and reasonable. But when
SLAAC-configured, and A changed from 1 to 0, the behaviors varied,
some deprecated SLAAC while some ignored the RA messages.
2.2.2. M flag
M is a key flag to interact ND/DHCPv6, but the host behaviors on M
flag were quite different.
In our test, one OS treats the flag as instruction, it even released
DHCPv6 session when M=0. But the other two just treat the flag as
advisory, when SLAAC was done, it won't care about M=1, and M=0 won't
cause operation for the already configured DHCPv6 addresses. Moreover,
the two OSes even would not initiate DHCPv6 session until they
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receives RA messages with M=1, this behavior has an implication that
DHCPv6 somehow depends on ND.
Please refer to [I-D.liu-6renum-dhcpv6-slaac-switching] for more
details.
2.2.3. O flag
In our tests, when M flag is set, the O flag is implicitly set as
well; in another word, the hosts would not initial stateful DHCPv6
and stateless DHCPv6 respectively. This is a reasonable behavior.
But the O flag is not independent from A flag in some OSes. In our
test, there are two OSes won't initiate stateless DHCPv6 when A flag
is not set, that is to say, it is not applicable to have a ''stateless
DHCPv6 only'' configuration state for some operating systems; it is
also not applicable for these two OSes to switch between stateful
DHCPv6 and stateless DHCPv6 (according to O flag changing from 0 to 1
or verse vice).
3. Possible Operational Gaps of DHCPv6/SLAAC Interaction
According to the abovementioned tests, there are possible operational
issues as the following.
3.1. Renumbering
During IPv6 renumbering, the SLAAC-configured hosts can reconfigure
IP addresses by receiving ND Router Advertisement (RA) messages
containing new prefix information. The DHCPv6-configured hosts can
reconfigure addresses by initialing RENEW sessions when the current
addresses' lease time is expired or receiving the reconfiguration
messages initialed by the DHCPv6 servers.
The above mechanisms have an implicit assumption that SLAAC-
configured hosts will remain SLAAC while DHCPv6-managed hosts will
remain DHCPv6-managed. But in some situations, SLAAC-configured hosts
may need to switch to DHCPv6-managed, or verse vice. In [RFC6879], it
described several renumbering scenarios in enterprise network for
this requirement; for example, the network may split, merge, relocate
or reorganize. But due to current implementations, this requirement
is not applicable and has been identified as a gap in [RFC7010].
3.2. Cold Start Problems
If all nodes, or many nodes, restart at the same time after a power
cut, the results might not consistent.
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3.3. Strong Management
Since the host behavior of address configuration is somehow un-
controlled by the network side, it might cause gaps to the networks
that need strong management (for example, the enterprise networks and
the ISP CPE networks). Examples are:
- the network wants the hosts to do DHCPv6-only configuration, it is
not applicable for some operating systems due to current
implementation unless manually configure the hosts to DHCPv6-only
model
- the hosts have been SLAAC-configured, then the network need the
hosts to do DHCPv6 simultaneously (e.g. for multihoming)
- the network wants the hosts to do statelss DHCPV6-only; for example,
the hosts are configured with self-generated addresses (e.g. ULA),
and they also need to contact the DHCPv6 server for info-
configuration
4. Conclusions
- The host behavior of SLAAC/DHCPv6 interaction is ambiguous in
standard.
- The implementations have been varied on this issue. In [RFC4862] it
is said "Removed the text regarding the M and O flags, considering
the maturity of implementations and operational experiences." The
description seems not true anymore.
- It is foreseeable that the un-uniformed host behavior can cause
operational gaps, e.g. in renumbering and strong management.
5. Security Considerations
No more security considerations than the Neighbor Discovery protocol
[RFC4861].
6. IANA Considerations
None.
7. References
7.1. Normative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
7.2. Informative References
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC3315] R. Droms, 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.
[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, May 2010.
[RFC7010] Liu, B., Jiang, S., Carpenter, B., Venaas, S., and W.
George, "IPv6 Site Renumbering Gap Analysis", RFC 7010,
September 2013.
[RFC6879] Jiang, S., Liu, B., and B. Carpenter, "IPv6 Enterprise
Network Renumbering Scenarios, Considerations, and Methods",
RFC 6879, February 2013.
8. Acknowledgments
The test was done by our research partner BNRC-BUPT (Broad Network
Research Centre in Beijing University of Posts and
Telecommunications). Thanks for the hard efficient work of student
Xudong Shi and Longyun Yuan.
Valuable comment was received from Sheng Jiang and Brian E Carpenter
to improve the draft.
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. Test Details of Host Behaviors
/-----\
+---------+ // \\
| DHCPv6 | | Router |
| server | \\ //
+----+----+ \--+--/
| |
| |
| |
----+--+----------+----------+---+-----
| | |
| | |
| | |
+----+---+ +----+---+ +----+---+
| | | | | |
| Host1 | | Host2 | | Host3 |
+--------+ +--------+ +--------+
Figure 1 Test Environment
The 5 elements were all created in Vmware in one computer, for ease
of operation.
- Router quagga 0.99-19 soft router installed on Ubuntu 11.04
virtual host
- DHCPv6 Server: dibbler-server installed on Ubuntu 11.04 virtual
host
- Host A Window 7 Virtual Host
- Host B Ubuntu 12.10 Virtual Host
- Host C Mac OS X v10.7 Virtual Host
Another test was done dedicated for the mobile phone operating
systems. The environment is similar (not in VMware, all are real PC
and mobile phones):
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- Router quagga 0.99-17 soft router installed on Ubuntu 12.10
- DHCPv6 Server: dibbler-server installed on Ubuntu 12.10
- Host D Android 4.0.4 (kernel: 3.0.16-gfa98030; device: HTC
Incredible S)
- Host E IOS 6.1.3 (model: iPod Touch 4)
(Note: The tested Android version didn't support DHCPv6 well, so the
following results don't include Android.)A.1 Host Initialing Behavior
Host from non-configured to configured, we tested different A/M/O
combinations in each OS platform. The states are enumerated as the
following, 3 operation systems respectively:
o Window 7/Apple IOS
- A=0&M=O&O=0, non-config
- A=1&M=0&O=0, SLAAC only
- A=1&M=0&O=1, SLAAC + Stateless DHCPv6
- A=1&M=1&O=0, SLAAC + DHCPv6
- A=1&M=1&O=1, SLAAC + DHCPv6
- A=0&M=1&O=0, DHCPv6 only (A=0 or Non-PIO)
- A=0&M=1&O=1, DHCPv6 only (A=0 or Non-PIO)
- A=0&M=0&O=1, Stateless DHCPv6 only
o Linux/MAC OS X
- A=0&M=O&O=0, non-config
- A=1&M=0&O=0, SLAAC only
- A=1&M=0&O=1, SLAAC + Stateless DHCPv6
- A=1&M=1&O=0, SLAAC + DHCPv6
- A=1&M=1&O=1, SLAAC + DHCPv6
- A=0&M=1&O=0, DHCPv6 only (A=0 or Non-PIO)
- A=0&M=1&O=1, DHCPv6 only (A=0 or Non-PIO)
- A=0&M=0&O=1, non-config
As showed above, Linux and MAC OSX acted the same way, but differated
from Windows 7 and Apple IOS. The only difference is when A=0&M=0&O=1,
Windows 7/Apple IOS did stateless DHCPv6 while Linux/MAC OSX did
nothing.
Result summary:
- A is interpreted as prescript in each OS at the initial state
- M is interpreted as prescript in each OS at the initial state
- O is interpreted as prescript in Windows 7
- A and M are independent in each OS at the initial state
- A and O are not totally independent in Linux and Mac, A=1 is
required for O=1 triggering DHCPv6 info-request
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- M and O are not totally independent in each OS. M=1 has the
implication O=1
A.2 Host Transition Behavior
o SLAAC-only host receiving A=0&M=1
- Window 7 would deprecate SLAAC and initiate DHCPv6
- Linux/MAC/IOS would keep SLAAC and don't initiate DHCPv6 unless
SLAAC is expired and no continuous RA
o DHCPv6-only host receiving A=1&M=0
- Window 7 would release DHCPv6 and do SLAAC
- Linux/MAC/IOS would keep DHCPv6 and do SLAAC
When the host has been configured, either by SLAAC or DHCPv6, the
operating systems interpreting the M flag quite differently. Windows
7 treats the flag as instruction, it even released DHCPv6 session
when M=0. Linux and OS X were likely to treat the flag as advisory,
when SLAAC was done, it won't care about M=1, and M=0 won't cause
operation for the already configured DHCPv6 addresses.
Please refer to [I-D.liu-6renum-dhcpv6-slaac-switching] for more
details.
A.3 Host Stateful/Stateless DHCPv6 Behavior
o Stateless DHCPv6-configured host receiving M=1 (while keeping O=1)
- Window 7 would initiate stateful DHCPv6, configuring address as
well as re-configuring other information
- Linux/MAC/IOS no action
o Statefull DHCPv6-configured host receiving M=0 (while keeping O=1)
- Window 7 would release all DHCPv6 configurations including
address and other information, and initiate stateless DHCPv6
- Linux/MAC/IOS no action
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Authors' Addresses
Bing Liu
Q14-4-A Building
Huawei Technologies Co., Ltd
Zhong-Guan-Cun Environment Protection Park, No.156 Beiqing Rd.
Hai-Dian District, Beijing
P.R. China
Email: leo.liubing@huawei.com
Ron Bonica
Juniper Networks
Sterling, Virginia 20164
USA
Email: rbonica@juniper.net
Xiangyang Gong
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
Wendong Wang
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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