Network Working Group B. Liu
Internet Draft Huawei Technologies
Intended status: Informational R. Bonica
Expires: June 28, 2014 Juniper Networks
T. Yang
China Mobile
December 25, 2013
DHCPv6/SLAAC Interaction Operational Guidance
draft-liu-v6ops-dhcpv6-slaac-guidance-00.txt
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Abstract
ND and DHCPv6 protocols could have some interaction on address
provisioning with the A, M, and O flags defined in ND protocol. But
the relevant standard definitions of the flags contain ambiguity, so
that current implementations in operating systems have varied on
interpreting the flags. The variation might impact real network
operations, so this document aims to provide some operational
guidance to eliminate the impact as much as possible.
Table of Contents
1. Introduction ................................................. 3
2. Problems Summary ............................................. 3
3. Operational Guidance.......................................... 4
3.1. General Guidelines ...................................... 4
3.2. Guidance for DHCPv6-only Deployment ..................... 5
3.3. Guidance for SLAAC-only Deployment ...................... 5
3.4. Guidance for DHCPv6/SLAAC Co-exist Deployment ........... 6
4. Security Considerations ...................................... 6
5. IANA Considerations .......................................... 6
6. References ................................................... 7
6.1. Normative References .................................... 7
6.2. Informative References .................................. 7
7. Acknowledgments .............................................. 7
Authors' Addresses .............................................. 8
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1. Introduction
In draft [DHCPV6-SLAAC-PS], the DHCPv6/SLAAC interaction issues on
host were reported. More specifically, the interaction is regarding
with the A, M, and O flags which are defined in ND protocol. Test
results have identified that current implementations in operating
systems have varied on interpreting the flags. The variation might
cause some operational problems as described in the document.
Given the fact that the above mentioned issues might impact real
network operations, this document aims to provide some operational
guidance to eliminate the impact as much as possible.
Note that, as similar to the [DHCPV6-SLAAC-PS], this document mainly
focuses on address provisioning problems and doesn't intent to cover
other provisions such as routes, DNS etc.
This document either doesn't intent to cover the topic of selection
between RA and DHCPv6 for the overlapped functions. There always are
arguments about what should be done through RA options or through
DHCPv6 options. For this general issue, draft [RA-DHCPv6-COMP] could
be referred.
2. Problems Summary
The main problem described in [DHCPV6-SLAAC-PS] is standard
definition ambiguity which means, on interpreting the same messages,
different hosts might behave differently. Thus it could be un-
controlled or un-predictable for administrators on some operations.
The ambiguity is summarized as the following aspects.
#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 Advisory VS Prescriptive
Some platforms interpret the flags as advisory while others interpret
them prescriptive. At initialing state, all the platforms we tested
just treated the flags as prescriptive. But when flags are in
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transition, e.g. the host is already SLAAC-configured, then M flag
transition from 0 to 1, or the host is already DHCPv6-configured,
then A flag transitions from 0 to 1, the behavior of different
platforms varied. Some still treated the flags as prescriptive while
others just treated them as advisory and did nothing.
#3 "Address Configuring Method" VS "Address Lifetime"
When method changes, should the hosts immediately release the
addresses or just wait them expired? It is not clearly specified in
standards.
#4 Dependencies between the flags
The semantics of the flags seems not totally independent, but the
standards didn't clearly clarify it. For example, when M=1 & O=1,
should the host initial one stateful DHCPv6 session to get both
address and info-configuration or initial two independent sessions of
which one is dedicated for address provisioning and the other is for
information provision? When A=0 & M=0 & O=1, should the host initiate
a stand-alone stateless DHCPv6 session?
3. Operational Guidance
3.1. General Guidelines
- Always Turn RAs On
Currently, turning RAs on is actually a basic requirement for running
IPv6 network since only RA could advertise default route(s) for the
end nodes. And if the nodes want to communicate with each other on
the same link via DHCPv6-configured addresses, they also need to be
advertised with L flag set in RAs. So for current networks, an IPv6
network could not run without RAs, unless the network only demands a
communication via link-local addresses.
- SLAAC Could be considered as a Bottom Line for Address Provisioning
SLAAC is a basic requirement for an IPv6 node as specified in
[RFC6434]. So every node could always get a prefix if RAs with PIO
Prefix Information Option are available. If the administrators
need to management nodes which might include multiple operation
systems, and they want to make sure every node could at least get one
global prefix, SLAAC is the bottom line. Then A flag always needs to
be turned on in this case.
- Avoid Flags Transition as Possible
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As described in problem #3, the behavior would be unpredictable/un-
controlled when flags are in transition. So the administrators need
to carefully plan the network and try to avoid host address
configuration method switch as possible, since it might not be
applicable by transitioning the flags thus significant operation
burden might be involved.
3.2. Guidance for DHCPv6-only Deployment
In IPv4, there is only one method (DHCPv4) of automatically
configuring the hosts. Many network operations/mechanisms, especially
in enterprise networks, are built around this central-managed model.
So it is reasonable for people who are accustomed to DHCPv4-only
deployment still prefer DHCPv6-only in IPv6 networks.
So in this case, the administrators could always force SLAAC off
(A=0), indicate DHCPv6 is available (M=1), and install DHCPv6 relays
on all routers. Instead of forcing the A flag off, simply not
including any PIO in RAs could also make the same effect. But before
doing this, the administrators need to be clear of the fact that
every node in their intended management scope is supposed to support
DHCPv6.
- Use Cases Might NOT be Supported
Per problem #1, DHCPv6-only configuration without RAs is not
applicable for some hosts. But this is an invalid use case so far, as
analyzed above. Never the less, it might become a problem when
mechanism like [DHCPv6-ROUTE] are available in the future.
Per problem #2, if the administrators want to switch the DHCPv6-only
configured hosts to SLAAC-only, it might be not applicable for some
hosts without manually changing the hosts' configuration or using
additional management tools.
Per problem #4, for some platforms, the A flag and O flag might not
be independent, when SLAAC is off, a stand-alone stateless DHCPv6
session would not be applicable. However, this might not be a common
use case.
3.3. Guidance for SLAAC-only Deployment
In contrast with DHCPv6-only, some scenarios might be suitable for
SLAAC-only which allows minimal administration burden and node
capability requirement, e.g. an isolated network.
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The administrators MUST turn the A flag on, and should turn M flag
off. Note that some platforms (e.g. Window 8) might still initiate
DHCPv6 session regardless of M flag off. But since there is no DHCPv6
service available, the only problem is that there would be some
unnecessary traffic.
- Use Cases Might NOT be Supported
Per problem #2, if the administrators want to switch the SLAAC-only
configured hosts to DHCPv6-only, it might be not applicable for some
hosts without manually changing the hosts' configuration or using
additional management tools.
3.4. Guidance for DHCPv6/SLAAC Co-exist Deployment
If the administrators want all nodes at least could configure a
global scope address, then they could turn A flag and M flag both on
in case some nodes only support one of the mechanisms due to
capability limitation (SLAAC-only) or specific configuration manually
or mistakenly (DHCPv6-only).
If the two mechanisms would bring two prefixes for the nodes
respectively, then the administrators need to make sure the SLAAC and
DHCPv6 are available simultaneously (through RA with M=1) before
nodes get online since once the nodes were configured with one prefix,
per problem #2, later they might not care about the other newly added
prefix.
Per problem #3, when administrators want to deprecate a SLAAC/DHCPv6
prefix/address, it's better NOT simply turning the A/M flag off since
some platforms might immediately release the addresses. It might be
harmful in some situations, especially in make-before-break
renumbering.
4. Security Considerations
No more security considerations than the Neighbor Discovery protocol
[RFC4861].
5. IANA Considerations
None.
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6. References
6.1. Normative References
[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.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
6.2. 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.
[DHCPV6-SLAAC-PS]
Liu, B., Bonica, R., Gong, X., and W. Wang, "DHCPv6/SLAAC
Address Configuration Interaction Problem Statement", Work
in Progress, November 2013
[RA-DHCPv6-COMP]
A. Yourtchenko, "A comparison between the DHCPv6 and RA
based host configuration", Work in Progress, November 2013
[DHCPv6-ROUTE]
Dec, W., Mrugalski, T., Sun, T., Sarikaya, B., and A.
Matsumoto, " DHCPv6 Route Options ", Work in Progress,
August 2012
7. Acknowledgments
Valuable comment was received from Sheng Jiang and Brian E Carpenter
to initiate the draft.
This document was prepared using 2-Word-v2.0.template.dot.
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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
Tianle Yang
China Mobile
32, Xuanwumenxi Ave.
Xicheng District, Beijing 100053
China
Email: yangtianle@chinamobile.com