Network Working Group J. Bi
Internet-Draft CERNET
Intended status: Standards Track G. Yao
Expires: November 7, 2011 Tsinghua University
J. Halpern
Newbridge Networks Inc
E. Levy-Abegnoli, Ed.
Cisco Systems
May 6, 2011
SAVI for Mixed Address Assignment Methods Scenario
draft-ietf-savi-mix-00
Abstract
This document reviews how multiple address discovery methods can
coexist in a single SAVI device and collisions are resolved when the
same binding entry is discovered by two or more methods.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommendations for preventing collisions . . . . . . . . . . . 4
4. Handing binding collisions . . . . . . . . . . . . . . . . . . 4
4.1. Same Address on Different Binding Anchors . . . . . . . . . 4
4.1.1. Basic preference . . . . . . . . . . . . . . . . . . . 5
4.1.2. Overwritten preference . . . . . . . . . . . . . . . . 5
4.1.3. Multiple SAVI Device Scenario . . . . . . . . . . . . . 5
4.2. Same Address on the Same Binding Anchor . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Contributors and Acknowledgments . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
There are currently several documents [I-D.ietf-savi-fcfs],
[I-D.ietf-savi-dhcp], [I-D.ietf-savi-send] that describe the
different methods by which a switch can discover and record bindings
between a node's layer3 address and a binding anchor and use that
binding to perform Source Address Validation.
The method used by nodes to assign the address drove the break down
into these multiple documents, whether StateLess Autoconfiguration
(SLAAC), Dynamic Host Control Protocol (DHCP), Secure Neighbor
Discovery (SeND) or manual. Each of these documents describes
separately how one particular discovery method deals with address
collisions (same address, different anchor).
While multiple assignment methods can be used in the same layer2
domain, a SAVI device might have to deal with a mix of binding
discovery methods. The purpose of this document is to provide
recommendations to avoid collisions and to review collisions handling
when two or more such methods come up with competing bindings.
2. Problem Scope
There are three address assignment methods identified and reviewed in
one of the SAVI document:
1. StateLess Address AutoConfiguration (SLAAC) - reviewed in
[I-D.ietf-savi-fcfs]
2. Dynamic Host Control Protocol address assignment (DHCP) -
reviewed in [I-D.ietf-savi-dhcp]
3. Secure Neighbor Discovery (SeND) address assignment, reviewed in
[I-D.ietf-savi-send]
Each address assignment method corresponds to a binding discovery
method: SAVI-FCFS, SAVI-DHCP and SAVI-SeND. In addition, there is a
fourth method for installing a bindings on the switch, referred to as
"manual". It is based on manual (address or prefix) binding
configuration and is reviewed in [I-D.ietf-savi-fcfs] and
[I-D.ietf-savi-framework]
All combinations of address assignment methods can coexist within a
layer2 domain. A SAVI device will have to implement the
corresponding SAVI discovery methods (referred to as a "SAVI
solution") to enable Source Address Validation. If more than one
SAVI solution is enabled on a SAVI device, the method is referred to
as "mix address assignment method" in this document.
SAVI solutions are independent from each other, each one handling its
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own entries. In the absence of reconciliation, each solution will
reject packets sourced with an address it did not discovered. To
prevent addresses discovered by one solution to be filtered out by
another, the binding table should be shared by all the solutions.
However this could create some conflict when the same entry is
discovered by two different methods: the purpose of this document is
of two folds: provide recommendations to avoid conflicts, and resolve
conflicts if and when they happen. Collisions happening within a
given solution are outside the scope of this document.
3. Recommendations for preventing collisions
If each solution has a dedicated address space, collisions won't
happen. Thus, in order to avoid overlap in the address space across
SAVI solutions enabled on any particular SAVI device, it is
recommended to
1. DHCP/SLAAC: separate the prefix scope of DHCP and SLAAC. Set the
A bit in Prefix information option of Router Advertisement for
SLAAC prefix. And set the M bit in Router Advertisement for DHCP
prefix. [RFC4861] [RFC4862].
2. SeND/non-SeND: avoid mixed environment (where SeND and non-SeND
nodes are deployed) or separate the prefixes announced to SeND
and non-SenD nodes. One way to separate the prefixes is to have
the router()s announcing different (non-overlapping) prefixes to
SeND and to non-SeND nodes, using unicast Router Advertisements,
in response to SeND/non-SeND Router Solicit.
4. Handing binding collisions
In situations where collisions could not be avoided, two cases should
be considered:
1. The same address is bound on two different binding anchors by
different SAVI solutions.
2. The same address is bound on the same binding anchor by different
SAVI solutions.
4.1. Same Address on Different Binding Anchors
This is the very case of collision that could not be prevented by
separating the assignment address spaces. For instance, an address
is assigned by SLAAC on node X, installed in the binding table using
SAVI-FCFS, anchored to "anchor-X". Later, the same address is
assigned by DHCP to node Y, as a potential candidate in the same
binding table, anchored to "anchor-Y".
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4.1.1. Basic preference
Within the SAVI perimeter, one address bound to a binding anchor by
one SAVI solution could also be bound by another SAVI solution to a
different binding anchor. If the DAD procedure is not performed, the
same address will also be bound to the new binding anchor. Both
bindings are legitimate within the corresponding solution.
Though it is possible that the hosts and network can still work in
such scenario, the uniqueness of address is not insured. The SAVI
device must decide whom the address should be bound with. Current
standard documents of address assignment methods have implied the
prioritization relationship (first-come). In the absence of any
configuration or protocol hint (see Section 4.1.2) the SAVI device
should choose the first-come entry, whether it was learnt from SLACC,
SeND or DHCP.
4.1.2. Overwritten preference
There are two identified exceptions to the general prioritization
model, one of them being CGA addresses, another one controlled by the
configuration of the switch:
1. When CGA addresses are used, and a collision is detected,
preference should be given to the anchor that carries the CGA
credentials once they are verified, in particular the CGA
parameters and the RSA options.
2. The SAVI device should allow the configuration of a prefix or a
single address, together with a given anchor or constrained to be
discovered by a particular SAVI solution (see also "Prefix
Configuration" section in [I-D.ietf-savi-framework]. If a DAD
message for a target within a configured prefix (or equal to a
configured single address) is received on the SAVI device from an
anchor, or via a discovery method different from the one
configured, the switch should defend the address by responding to
the DAD message. It should not at this point install an entry
into the binding table. This is especially useful to protect
well known bindings such as a static address of a server over
anybody, even when the server is down. It is also a way to give
priority to a binding learnt from SAVI-DHCP over a binding for
the same address, learnt from SAVI-FCFS.
4.1.3. Multiple SAVI Device Scenario
A single SAVI device doesn't have the information of all bound
addresses on the perimeter. Therefore it is not enough to lookup
local bindings to identify a collision. However, assuming DAD is
performed throughout the security perimeter for all addresses
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regardless of the assignment method, then DAD response will inform
all SAVI devices about any collision. In that case, FCFS will apply
the same way as in a single switch scenario. If the admin configured
on one the switches a prefix (or a single static binding) to defend,
the DAD response generated by this switch will also prevent the
binding to be installed on other switches of the perimeter.
4.2. Same Address on the Same Binding Anchor
A binding may be set up on the same binding anchor by multiple
solutions. Generally, the binding lifetimes of different solutions
are different. Potentially, if one solution requires to remove the
binding, the node using the address may be taken the use right.
For example, a node performs DAD procedure after being assigned an
address from DHCP, then the address will also be bound by SAVI-FCFS.
If the SAVI-FCFS lifetime is shorter than DHCP lifetime, when the
SAVI-FCFS lifetime expires, it will request to remove the binding.
If the binding is removed, the node will not be able to use the
address even the DHCP lease time doesn't expire.
The solution proposed is to keep a binding as long as possible. A
binding is kept until it has been required to be removed by all the
solutions that ever set up it.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
5.2. Informative References
[I-D.ietf-savi-dhcp]
Wu, J., Yao, G., Bi, J., and F. Baker, "SAVI Solution for
DHCP", draft-ietf-savi-dhcp-09 (work in progress),
April 2011.
[I-D.ietf-savi-fcfs]
Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come First-Serve Source-Address Validation for
Locally Assigned IPv6 Addresses", draft-ietf-savi-fcfs-09
(work in progress), April 2011.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
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"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-04 (work in progress),
March 2011.
[I-D.ietf-savi-send]
Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
Address Validation Implementation",
draft-ietf-savi-send-05 (work in progress), April 2011.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[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.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[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.
Appendix A. Contributors and Acknowledgments
Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and
Jari Arkko for their valuable contributions.
Authors' Addresses
Jun Bi
CERNET
Network Research Center, Tsinghua University
Beijing 100084
China
Email: junbi@cernet.edu.cn
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Guang Yao
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
Email: yaoguang.china@gmail.com
Joel M. Halpern
Newbridge Networks Inc
Email: jmh@joelhalpern.com
Eric Levy-Abegnoli (editor)
Cisco Systems
Village d'Entreprises Green Side - 400, Avenue Roumanille
Biot-Sophia Antipolis - 06410
France
Email: elevyabe@cisco.com
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