Network Working Group J. Bi
Internet-Draft CERNET
Intended status: Standards Track G. Yao
Expires: May 20, 2011 Tsinghua University
J. Halpern
Newbridge Networks Inc
E. Levy-Abegnoli
Cisco Systems
November 16, 2010
SAVI for Mixed Address Assignment Methods Scenario
<draft-bi-savi-mix-03.txt>
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Mixed Address Assignment Methods Scenario . . . . . . . . . . . 3
4. Basic Structure . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Problem Scope, Statement and Solution . . . . . . . . . . . . . 4
5.1. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . 4
5.2. Recommendations for preventing collisions . . . . . . . . . 4
5.3. Binding on the Same Address . . . . . . . . . . . . . . . . 4
5.3.1. Same Address on Different Binding Anchors . . . . . . . 5
5.3.1.1. Basic preference . . . . . . . . . . . . . . . . . 5
5.3.1.2. Issues in Multiple SAVI Device Scenario . . . . . . 6
5.3.1.3. Conflict Announcement . . . . . . . . . . . . . . . 7
5.3.2. Same Address on the Same Binding Anchor . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Contributors and Acknowledgments . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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
(SLACC), 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.
While multiple assignment methods can be used in the same layer2
domain, a savi-switch 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. Terminology
3. Mixed Address Assignment Methods Scenario
Currently, there are four SAVI solutions which cover different types
of address assignment methods:
1. SAVI-FCFS: SLAAC
2. SAVI-DHCP: stateful DHCP, static DHCP
3. SAVI-SeND: CGA with certificate, CGA without certificate
4. Manually configuration: static address manually configured by
administrator on SAVI device.
Any combination of address assignment methods can be potentially
found within a layer2 domain, and a savi device will have to
implement the corresponding savi discovery methods (savi solutions)
to prevent packets from valid sources to be filtered out. 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.
4. Basic Structure
Different savi solutions are independent from each other, each one
handling its own entries. In the absence of a reconciliation, each
solution will reject packets sourced with an address it did not
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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 main purpose of
this document is to resolve such conflicts if and when they happen.
5. Problem Scope, Statement and Solution
5.1. Problem Scope
This document reviews the case of collisions between different SAVI
solutions. Collision happening within a given solution is not in the
scope of this document.
5.2. Recommendations for preventing collisions
If each solution has a dedicated address space, collisions won't
happen. Thus, it is recommended to avoid overlap in the address
space across SAVI solutions enabled on any particular savi switch.
More specifically:
1. DHCP/Static: exclude the static address from the DHCP pool.
2. 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].
3. SLAAC/Static: separate the prefix scope of SLAAC and Static. It
may be impossible in practice. SAVI device can perform DAD proxy
for static address to hold the address from SLAAC node.
4. SEND/non-SEND: In an environment where SeND is deployed, the only
way to avoid collisions in the SAVI devices is to have SeND-only
nodes. In a mixed environment, two nodes, SeND and non-SeND,
could configure the same address and the SAVI-device will have to
deal with a collision.
5.3. Binding on the Same Address
In situation 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.
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5.3.1. Same Address on Different Binding Anchors
5.3.1.1. 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. For example an address could be initially
bound to a binding anchor by SAVI-FCFS solution. If another host is
assigned the same address from DHCP and the DAD procedure is not
performed, the same address will also be bound to the new binding
anchor. Both bindings are legitimate in the corresponding solution.
Though it is possible that the hosts and network can still work in
such scenario, the uniqueness of address is not assured. The SAVI
device must decide whom the address should be bound with. A binding
preference level based solution is proposed here.
To determine a proper preference level, following evidences are used:
1. "Duplicate Address Detection MUST be performed on all unicast
addresses prior to assigning them to an interface, regardless of
whether they are obtained through stateless autoconfiguration,
DHCPv6, or manual configuration,..." [RFC4862]
2. "A tentative address that is determined to be a duplicate as
described above MUST NOT be assigned to an interface,..."
[RFC4862]
3. "The client SHOULD perform duplicate address detection on each of
the addresses in any IAs it receives in the Reply message before
using that address for traffic." [RFC3315]
4. "A SEND node that uses the CGA authorization method to protect
Neighbor Solicitations SHOULD perform Duplicate Address Detection
as follows. If Duplicate Address Detection indicates that the
tentative address is already in use, the node generates a new
tentative CGA. If after three consecutive attempts no non-unique
address is generated, it logs a system error and gives up
attempting to generate an address for that interface.
When performing Duplicate Address Detection for the first
tentative address, the node accepts both secured and unsecured
Neighbor Advertisements and Solicitations received in response to
the Neighbor Solicitations. When performing Duplicate Address
Detection for the second or third tentative address, it ignores
unsecured Neighbor Advertisements and Solicitations." [RFC3971]
5. "The node MAY have a configuration option whereby it ignores
unsecured advertisements, even when performing Duplicate Address
Detection for the first tentative address. This configuration
option SHOULD be disabled by default. This is a recovery
mechanism for cases in which attacks against the first address
become common." [RFC3971]
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From the above materials, FCFS is found to be a universal principle
with only one exception: SEND node may use a duplicate address if the
DAD NA is only from non-SEND node. And Duplicate Address Detection
is enforced to detect the uniqueness of address (though in [RFC3315],
"SHOULD" is used but not "MUST"). The static address is not covered
in any document, as we believe the "manual configuration" in
[RFC4862] means address configured on host by user, but not static
address must be protected for servers and special purpose.
The following preference level can be inferred from listed materials
and above analysis:
1. SLAAC, DHCP and manually configured address by user have the same
priority.
2. SEND can have higher priority because it may configure an address
bound by non-SEND node.
3. Static address should have the highest priority to ensure
administrator having the right to manage the usage of address.
Combined solution preference with binding sequence, there will be 16
scenarios (Denote solutions by FCFS, DHCP, SEND, and Admin
correspondingly):
Existing Candidate PREFERENCE
FCFS FCFS In the scope of SAVI-SLAAC
FCFS DHCP FCFS
FCFS SEND SEND
FCFS Admin Admin
DHCP FCFS DHCP
DHCP DHCP In the scope of SAVI-DHCP
DHCP SEND SEND
DHCP Admin Admin
SEND FCFS SEND
SEND DHCP SEND
SEND SEND In the scope of SAVI-SEND
SEND Admin Admin
Admin FCFS Admin
Admin DHCP Admin
Admin SEND Admin
Admin Admin Candidate binding
5.3.1.2. Issues in Multiple SAVI Device Scenario
A single SAVI device doesn't have the information of all bound
addresses on the perimeter. Therefore a collision may not be
explicit based only on local bindings. To make the perimeter-scope
collision explicit to each SAVI device requires:
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1. A SAVI device must have the ability to know whether a local
binding setup request violate a binding on other SAVI devices or
not.
2. A SAVI device must have the ability to know whether a local
binding should be removed because the address is bound on another
SAVI device by solution with higher priority.
The first requirement is relatively easy to meet, as DAD must have
been performed on address bound by SAVI-SLAAC and SAVI-SEND, and
there is no need to check if a static address violates an existing
binding. However DAD is not required by SAVI-DHCP, and static
addresses must be prevented from being grabbed by other solutions.
Thus, following mechanisms MUST be enforced:
1. SAVI device MUST perform DAD procedure on DHCP address or track
if DAD performed by DHCP client itself is successful before
binding a DHCP address. Only if the DAD succeeds, the DHCP
address can be bound.
2. SAVI device MUST perform DAD proxy for static address. Or all
the other SAVI devices MUST be configured to deny static address
bound on other SAVI devices, in condition that SAVI-SEND is
enabled and it may bind a static address.
3. The second requirement is relatively hard to satisfy. Whenever
SAVI-SEND decides to bind an address even it is used by a non-
SEND node, and a bound address is bound manually to another
binding anchor, the SAVI device with the existing binding must
get noticed and delete the binding. Following mechanisms MUST be
enforced:
1. If the SAVI-SEND solution decides to bind an address despite
that the binding collides with an existing FCFS/DHCP address,
a SEND NA MUST be sent by the SAVI device.
2. If a SAVI device receives a SEND NA targeting at a local
bound address by FCFS and DHCP, it MUST remove the binding,
and announce the conflict to the host with the binding.
3. If a static address bound manually collides with any exiting
binding, the existing binding MUST be removed manually by
administrator, and the conflict MUST be announced to the host
with existing binding.
5.3.1.3. Conflict Announcement
If a host is prohibited from using a bound address, the violation
MUST be announced to it, through delivering one (or more) Neighbor
Advertisement message to the host.
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5.3.2. 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.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References
[I-D.ietf-savi-dhcp]
Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
DHCP", draft-ietf-savi-dhcp-06 (work in progress),
September 2010.
[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 Addresses", draft-ietf-savi-fcfs-05 (work
in progress), October 2010.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-01 (work in progress),
October 2010.
[I-D.ietf-savi-send]
Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
Address Validation Implementation",
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draft-ietf-savi-send-04 (work in progress), October 2010.
[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
Guang Yao
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
Email: yaog@netarchlab.tsinghua.edu.cn
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Joel M. Halpern
Newbridge Networks Inc
Email: jmh@joelhalpern.com
Eric Levy-Abegnoli
Cisco Systems
Village d'Entreprises Green Side - 400, Avenue Roumanille
Biot-Sophia Antipolis - 06410
France
Email: elevyabe@cisco.com
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