SAVI J. Bi
Internet-Draft Tsinghua Univ.
Intended status: Standards Track G. Yao
Expires: January 20, 2016 Baidu
J. Halpern
Newbridge
E. Levy-Abegnoli, Ed.
Cisco
July 19, 2015
SAVI for Mixed Address Assignment Methods Scenario
draft-ietf-savi-mix-09
Abstract
In case that multiple IP address assignment methods are allowed in a
network, the corresponding SAVI methods should be enabled to prevent
spoofing in the network. This document reviews how multiple SAVI
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 20, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Recommendations for preventing collisions . . . . . . . . . . 5
6. Resolving binding collisions . . . . . . . . . . . . . . . . 5
6.1. Same Address on Different Binding Anchors . . . . . . . . 5
6.1.1. Basic preference . . . . . . . . . . . . . . . . . . 6
6.1.2. Overwritten preference . . . . . . . . . . . . . . . 6
6.1.3. Multiple SAVI Device Scenario . . . . . . . . . . . . 7
6.2. Same Address on the Same Binding Anchor . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
There are currently several SAVI documents ([RFC6620], [RFC7513] and
[RFC7219]) that describe the different methods by which a switch can
discover and record bindings between a node's IP 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 method
deals with address collisions (same address, different binding
anchor).
While multiple IP assignment methods can be used in the same layer-2
domain, a SAVI device might have to deal with a mix of SAVI 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.
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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
Three different IP address assignment methods have been analyzed for
SAVI:
1. StateLess Address AutoConfiguration (SLAAC) - analyzed in SAVI-
FCFS[RFC6620]
2. Dynamic Host Control Protocol address assignment (DHCP) -
analyzed in SAVI-DHCP[RFC7513]
3. Secure Neighbor Discovery (SeND) address assignment, analyzed in
SAVI-SEND[RFC7219]
In addition, there is a fourth method for managing (i.e., creation,
management, deletion) a binding on the switch, referred to as
"manual". It is based on manual binding configuration and is
analyzed in [RFC6620] and [RFC7039].
All combinations of address assignment methods can coexist within a
layer-2 domain. A SAVI device MUST implement the corresponding
binding setup methods (referred to as a "SAVI method") to enable
Source Address Validation. If more than one SAVI method is enabled
on a SAVI device, the method is referred to as "mix address
assignment method" in this document.
SAVI methods are independent from each other, each one handling its
own entries. In the absence of reconciliation, each method will
reject packets sourced with an address it did not discovered. To
prevent addresses discovered by one method 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 methods to avoid conflicts,
and resolve conflicts if and when they happen. Collisions happening
within a given method are outside the scope of this document.
4. Architecture
A SAVI device may enable multiple SAVI methods. This mechanism,
called SAVI-MIX, is proposed as a arbiter of the binding generation
algorithms, generating the final binding entries as illustrated in
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Figure 1. Once a SAVI method generates a candidate binding, it will
request SAVI-MIX to set up a corresponding entry in the binding
table. Then SAVI-MIX will check if there is any conflict in the
binding table. A new binding will be generated if there is no
conflict. If there is a conflict, SAVI-MIX will determine whether to
replace the existing binding or reject the candidate binding based on
the policies specified in Section 6.
The packet filtering will not be performed by each SAVI method
separately. Instead, SAVI-MIX will perform filtering based on the
entries in the binding table.
+--------------------------------------------------------+
| |
| SAVI Device |
| |
| |
| +------+ +------+ +------+ |
| | SAVI | | SAVI | | SAVI | |
| | | | | | | |
| | FCFS | | DHCP | | SEND | |
| +------+ +------+ +------+ |
| | | | Binding |
| | | | setup |
| v v v requests |
| +------------------------------+ |
| | | |
| | SAVI-MIX | |
| | | |
| +------------------------------+ |
| | |
| v Final Binding |
| +--------------+ |
| | Binding | |
| | | |
| | Table | |
| +--------------+ |
| |
+--------------------------------------------------------+
Figure 1: SAVI-Mix Architecture
Each entry in the binding table will contain the following fields:
1. IP source address
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2. Binding anchor
3. Lifetime
4. Creation time
5. Binding methods: the methods which request the binding setup.
5. 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[RFC6085], in response to SeND/non-SeND Router
Solicit.
6. Resolving 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 methods.
2. The same address is bound on the same binding anchor by different
SAVI methods.
6.1. Same Address on Different Binding Anchors
This would typically occur in case assignment address spaces could
not be separated. 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,
and SAVI-DHCP will generate a candidate binding entry, anchored to
"anchor-Y".
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6.1.1. 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, i.e., first-come.
1. SLAAC: s5.4.5 of [RFC4862]
2. DHCPv4: s3.1-p5 of [RFC2131]
3. DHCPv6: s18.1.8 of [RFC3315]
4. SeND: s8 of [RFC3971]
In the absence of any configuration or protocol hint (see
Section 6.1.2) the SAVI device should choose the first-come binding
anchor, whether it was learnt from SLAAC, SeND or DHCP.
6.1.2. 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.
6.1.2.1. CGA preference
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 "First-Come, First-
Served" (FCFS) principle.
6.1.2.2. configuration preference
For configuration driven exceptions, the SAVI device may allow the
configuration of a triplet ("prefix", "anchor", "method") or
("address", "anchor", "method"), where at least one of ("anchor",
"method") should be specified. Later, if a DAD message is received
with the following conditions verified:
1. The target in the DAD message does not exist in the binding table
2. The target is within configured "prefix" (or equal to "address")
3. The anchor bound to target is different from the configured
anchor, when specified
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4. The configured method, if any, is different from SAVI-FCFS
the switch should defend the address by responding to the DAD
message, with a NA message or an ARP response, on behalf of the
target node. Plain NA will be replied to SeND DAD. SeND accepts
plain NA for the first time (see s8 of [RFC3971]). The probability
for a SeND node to generate a colliding address more than one time is
trivial in practice, thus the response can effectively protect an
existing binding.
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. 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.
6.1.3. 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.
6.2. Same Address on the Same Binding Anchor
A binding may be set up on the same binding anchor by multiple
methods, typically SAVI-FCFS and SAVI-DHCP. If the binding lifetimes
obtained from the two methods are different, priority should be given
to 1) Manual configuration 2) SAVI-DHCP 3) SAVI-FCFS as the least
authoritative. The binding will be removed when the prioritized
lifetime expires, even if a less authoritative method had a longer
lifetime.
7. Security Considerations
SAVI MIX does not eliminate the security problems of each SAVI
method. Thus, the potential attacks, e.g., the DoS attack against
the SAVI device resource, can still happen. In deployment, the
security threats from each enabled SAVI methods should be prevented
by the corresponding proposed solutions in each document.
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SAVI MIX is only a binding setup/removal arbitration mechanism. It
does not introduce additional security threats only if the principle
of decision is reasonable. However, there is a slight problem. SAVI
MIX is more tolerant about binding establish than each SAVI method
alone. As long as one of the enabled SAVI method generates a
binding, the binding will be applied. As a result, the allowed
binding number limitation or allowed binding setup rate limitation
will be the sum of all the enabled SAVI methods. In deployment,
whether a SAVI device is capable to support that resource requirement
should be evaluated.
8. IANA Considerations
This memo asks the IANA for no new parameters.
9. Acknowledgment
Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and
Jari Arkko for their valuable contributions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
DOI 10.17487/RFC3971, March 2005,
<http://www.rfc-editor.org/info/rfc3971>.
[RFC6085] Gundavelli, S., Townsley, M., Troan, O., and W. Dec,
"Address Mapping of IPv6 Multicast Packets on Ethernet",
RFC 6085, DOI 10.17487/RFC6085, January 2011,
<http://www.rfc-editor.org/info/rfc6085>.
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[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, DOI 10.17487/RFC6620, May 2012,
<http://www.rfc-editor.org/info/rfc6620>.
[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, DOI 10.17487/RFC7039, October 2013,
<http://www.rfc-editor.org/info/rfc7039>.
[RFC7219] Bagnulo, M. and A. Garcia-Martinez, "SEcure Neighbor
Discovery (SEND) Source Address Validation Improvement
(SAVI)", RFC 7219, DOI 10.17487/RFC7219, May 2014,
<http://www.rfc-editor.org/info/rfc7219>.
[RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
Validation Improvement (SAVI) Solution for DHCP",
RFC 7513, DOI 10.17487/RFC7513, May 2015,
<http://www.rfc-editor.org/info/rfc7513>.
10.2. Informative References
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
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
Baidu
Baidu Science and Technology Park, Building 1
Beijing 100193
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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