SAVI J. Bi
Internet-Draft G. Yao
Intended status: Standards Track Tsinghua Univ.
Expires: October 29, 2012 J. Halpern
Newbridge
E. Levy-Abegnoli, Ed.
Cisco
April 27, 2012
SAVI for Mixed Address Assignment Methods Scenario
draft-ietf-savi-mix-02
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Recommendations for preventing collisions . . . . . . . . . . . 4
5. Handing binding collisions . . . . . . . . . . . . . . . . . . 4
5.1. Same Address on Different Binding Anchors . . . . . . . . . 5
5.1.1. Basic preference . . . . . . . . . . . . . . . . . . . 5
5.1.2. Overwritten preference . . . . . . . . . . . . . . . . 5
5.1.3. Multiple SAVI Device Scenario . . . . . . . . . . . . . 6
5.2. Same Address on the Same Binding Anchor . . . . . . . . . . 6
6. Disscusion on Assumption Conflict . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Informative References . . . . . . . . . . . . . . . . . . 8
10.2. Normative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
There are currently several documents [savi-fcfs], [savi-dhcp] and
[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. Each of these documents specifies how to learn on-link
addresses, based on the method used for their assignment,
respectively: StateLess Autoconfiguration (SLAAC), Dynamic Host
Control Protocol (DHCP) and Secure Neighbor Discovery (SeND). 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. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [rfc2119].
3. Problem Scope
There are three address assignment methods identified and reviewed in
one of the SAVI document:
1. StateLess Address AutoConfiguration (SLAAC) - reviewed in
[savi-fcfs]
2. Dynamic Host Control Protocol address assignment (DHCP) -
reviewed in [savi-dhcp]
3. Secure Neighbor Discovery (SeND) address assignment, reviewed in
[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 [savi-fcfs] and [savi-framework].
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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
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 and method to avoid conflicts,
and resolve conflicts if and when they happen. Collisions happening
within a given solution are outside the scope of this document.
4. Recommendations for preventing collisions
If each solution has a dedicated address space, collisions won't
happen. Using non overlapping address space across SAVI solutions is
therefore recommended. To that end, one should:
1. DHCP/SLAAC: use non-overlapping prefix for 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. For detail explanations on these bits, refer to
[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.
5. 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.
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2. The same address is bound on the same binding anchor by different
SAVI solutions.
5.1. Same Address on Different Binding Anchors
This would typically occur in case assignment address spaces could
not be separated. For instance,overl 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".
5.1.1. Basic preference
The SAVI device must decide whom the address should be bound with
(anchor-X or anchor-Y in this example). 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 5.1.2) the SAVI device should choose the
first-come entry, whether it was learnt from SLACC, SeND or DHCP.
5.1.2. 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. Note that if an attacker was
trying to replay CGA credentials, he would then compete on the
base of fcfs (first-come, first-serve).
2. The SAVI device should allow the configuration of a triplet
("prefix", "anchor", "method") or ("address", "anchor",
"method"). Later, if a DAD message is received for a target
within "prefix" (or equal "address") bound to "anchor1"
(different from "anchor"), or via a discovery method different
from "method", the switch should defend the address by responding
to the DAD message. It should not at this point install the
entry into the binding table. It will simply prevent the node to
assign the address, and will de-facto prioritize the configured
anchor or configured assignment method for that address. 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.
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5.1.3. 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
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.
5.2. Same Address on the Same Binding Anchor
A binding may be set up on the same binding anchor by multiple
solutions. For example, if SAVI-FCFS and SAVI-DHCP are both enabled
on one SAVI device, a DHCP address be bound by both SAVI instances.
There is no conflict if the binding is valid in all the solutions.
However, the binding lifetimes of different solutions can be
different. If one SAVI instance changes the state of a binding to
invalid on lifetime expires, conflict will happen.
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. Disscusion on Assumption Conflict
Different assumptions are made as the basis of solutions. The
assumptions of each solution specified which entity is the origin of
the trust. Indeed, the binding between address and binding anchor is
actually the derivative of the assumptions based on the principles of
binding set up. The conflict in identifier field of address is
specified in the above sections. This section specifies the conflict
in prefix field from different assumptions.
SAVI FCFS and SAVI DHCP trust routers to get the legitimate prefixes
for local link; however, only RADV validated by SEND is trusted by
SAVI SEND. In this solution, if any SAVI solution regards a prefix
to be valid, the prefix is valid for the whole mechanism.
7. Security Considerations
As described in [savi-framework], this solution cannot strictly
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prevent spoofing. There are two scenarios in which spoofing can
still happen:
1. The binding anchor is spoofable. if the binding anchor is
spoofable, e.g., plain MAC address, an attacker can use forged
binding anchor to send packet which will not be regarded as
spoofing by SAVI device. Indeed, using binding anchor that can
be easily spoofed is dangerous. An attacker can use the binding
anchor of another host to perform a lot of DHCP procedures, and
the SAVI device will refuse to set up new binding for the host
whenever the binding number limitation has been reached. Thus,
it is RECOMMENDED to use strong enough binding anchor, e.g.,
switch port, secure association in 802.11ae/af and 802.11i.
2. The binding anchor is shared by more than one host. If the
binding anchor is shared by more than one host, they can spoof
the addresses of each other. For example, a number of hosts can
attach to the same switch port of a SAVI device through a hub.
The SAVI device cannot distinguish packets from different hosts
and thus the spoofing between them will not be detected. This
problem can be solved through not sharing binding anchor between
hosts.
8. IANA Considerations
This memo asks the IANA for no new parameters.
Note to RFC Editor: This section will have served its purpose if it
correctly tells IANA that no new assignments or registries are
required, or if those assignments or registries are created during
the RFC publication process. From the authors' perspective, it may
therefore be removed upon publication as an RFC at the RFC Editor's
discretion.
9. Acknowledgment
Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and
Jari Arkko for their valuable contributions.
This document was generated using the xml2rfc tool.
10. References
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10.1. Informative References
[rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, Match 1997.
10.2. 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.
[savi-dhcp]
Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
DHCP", draft-ietf-savi-dhcp-12 (work in progress),
February 2012.
[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-14 (work
in progress), February 2012.
[savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-06 (work in progress),
December 2011.
[savi-send]
Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
Address Validation Implementation",
draft-ietf-savi-send-06 (work in progress), October 2011.
Authors' Addresses
Jun Bi
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
Email: junbi@tsinghua.edu.cn
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Guang Yao
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
Email: yaoguang@cernet.edu.cn
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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