Keywords: SAVI-DHCP, FCFS SAVI, SEND SAVI
Internet Engineering Task Force (IETF) J. Bi
Request for Comments: 8074 Tsinghua University
Category: Standards Track G. Yao
ISSN: 2070-1721 Tsinghua University/Baidu
J. Halpern
Ericsson
E. Levy-Abegnoli, Ed.
Cisco
February 2017
Source Address Validation Improvement (SAVI)
for Mixed Address Assignment Methods Scenario
Abstract
In networks that use multiple techniques for address assignment, the
spoofing of addresses assigned by each technique can be prevented
using the appropriate Source Address Validation Improvement (SAVI)
methods. This document reviews how multiple SAVI methods can coexist
in a single SAVI device and how collisions are resolved when the same
binding entry is discovered by two or more methods.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8074.
Bi, et al. Standards Track [Page 1]
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Copyright Notice
Copyright (c) 2017 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Recommendations for Assignment Separation . . . . . . . . . . 6
6. Resolving Binding Collisions . . . . . . . . . . . . . . . . 6
6.1. Same Address on Different Binding Anchors . . . . . . . . 6
6.1.1. Basic Preference . . . . . . . . . . . . . . . . . . 7
6.1.2. Exceptions . . . . . . . . . . . . . . . . . . . . . 7
6.1.3. Multiple SAVI Device Scenario . . . . . . . . . . . . 8
6.2. Same Address on the Same Binding Anchor . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
There are currently several Source Address Validation Improvement
(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
technique used for their assignment: StateLess Address
Autoconfiguration (SLAAC), the Dynamic Host Control Protocol (DHCP),
and Secure Neighbor Discovery (SEND), respectively. Each of these
documents describes separately how one particular SAVI method deals
with address collisions (same address but different binding anchor).
While multiple IP assignment techniques can be used in the same layer
2 domain, this means that a single SAVI device might have to deal
with a combination or mix of SAVI methods. The purpose of this
document is to provide recommendations to avoid collisions and to
review collision 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
Three different IP address assignment techniques have been analyzed
for SAVI:
1. StateLess Address Autoconfiguration (SLAAC) -- analyzed in FCFS
SAVI (First-Come, First-Served) [RFC6620]
2. Dynamic Host Control Protocol address assignment (DHCP) --
analyzed in SAVI-DHCP [RFC7513]
3. Secure Neighbor Discovery (SEND) address assignment -- analyzed
in SEND SAVI [RFC7219]
In addition, there is a fourth technique for managing (i.e.,
creation, management, and deletion) a binding on the switch, referred
to as "manual". It is based on manual binding configuration. How to
manage manual bindings is determined by operators, so there is not a
new SAVI method for manual addresses.
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All combinations of address assignment techniques can coexist within
a layer 2 domain. A SAVI device MUST implement the corresponding
binding setup methods (referred to as "SAVI methods") for each such
technique that is in use if it is to provide source address
validation.
SAVI methods are normally viewed as independent from each other, each
one handling its own entries. If multiple methods are used in the
same device without coordination, each method will attempt to reject
packets sourced with any addresses that method did not discover. To
prevent addresses discovered by one SAVI method from being filtered
out by another method, the SAVI binding table SHOULD be shared by all
the SAVI methods in use in the device. This in turn could create
some conflict when the same entry is discovered by two different
methods. The purpose of this document is twofold: to provide
recommendations and methods to avoid conflicts and to resolve
conflicts when they happen. Collisions happening within a given
method are outside the scope of this document.
4. Architecture
A SAVI device may implement and use multiple SAVI methods. This
mechanism, called "SAVI-MIX", is proposed as an arbiter of the
binding generation algorithms from these multiple methods, generating
the final binding entries as illustrated in Figure 1. Once a SAVI
method generates a candidate binding, it will request that SAVI-MIX
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.
As a result of this, the packet filtering in the SAVI device will not
be performed by each SAVI method separately. Instead, the table
resulting from applying SAVI-MIX will be used to perform filtering.
Thus, the filtering is based on the combined results of the different
SAVI mechanisms. It is beyond the scope of this document to describe
the details of the filtering mechanism and its use of the combined
SAVI binding table.
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+--------------------------------------------------------+
| |
| 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
2. Binding anchor [RFC7039]
3. Lifetime
4. Creation time
5. Binding methods: the SAVI method used for this entry
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5. Recommendations for Assignment Separation
If each address assignment technique uses a separate portion of the
IP address space, collisions won't happen. Using non-overlapping
address space across address assignment techniques, and thus across
SAVI methods, is therefore recommended. To that end, one should:
1. DHCP and SLAAC: use a non-overlapping prefix for DHCP and SLAAC.
Set the A bit in the Prefix Information option of the Router
Advertisement for the SLAAC prefix, and set the M bit in the
Router Advertisement for the DHCP prefix. For detailed
explanations of these bits, refer to [RFC4861] and [RFC4862].
2. SEND and non-SEND: avoid mixed environments (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 cannot be avoided by assignment
separation, 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 if assignment address spaces could not be
separated. For instance, an address is assigned by SLAAC on node X,
installed in the binding table using FCFS SAVI, and 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
If there is any manually configured binding, the SAVI device SHOULD
choose the manually configured binding anchor.
For an address not covered by any manual bindings, the SAVI device
must decide to which binding anchor the address should be bound
(anchor-X or anchor-Y in this example). Current standard documents
of address assignment methods have implied the prioritization
relationship based on order in time, i.e., First-Come, First-Served.
o SLAAC: Section 5.4.5 of [RFC4862]
o DHCPv4: Section 3.1, Point 5 of [RFC2131]
o DHCPv6: Section 18.1.8 of [RFC3315]
o SEND: Section 8 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 learned from SLAAC, SEND, or DHCP.
6.1.2. Exceptions
There are two identified exceptions to the general prioritization
model, one being Cryptographically Generated Addresses (CGA)
[RFC3971] and the other 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 the "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"). The "prefix" or "address"
represents the address or address prefix to which this preference
entry applies. The "anchor" is the value of a known binding anchor
that this device expects to see using this address or addresses from
this prefix. The "method" is the SAVI method that this device
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expects to use in validating address binding entries from the address
or prefix. At least one of "anchor" and "method" MUST be specified.
Later, if a Duplicate Address Detection (DAD) message [RFC4861] 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 the configured "prefix" (or equal to
"address"),
3. The anchor bound to the target is different from the configured
anchor, when specified, and
4. The configured method, if any, is different from FCFS SAVI,
then the switch SHOULD defend the address by responding to the DAD
message, with a Neighbor Advertisement (NA) message, on behalf of the
target node. It SHOULD NOT install the entry into the binding table.
The DAD message SHOULD be discarded and not forwarded. Forwarding it
may cause other SAVI devices to send additional defense NAs. SEND
nodes in the network MUST disable the option to ignore unsecured
advertisements (see Section 8 of [RFC3971]). If the option is
enabled, the case is outside the scope of this document. It is
suggested to limit the rate of defense NAs to reduce security threats
to the switch. Otherwise, a malicious host could consume the
resource of the switch heavily with flooding DAD messages.
This will simply prevent the node from assigning the address and will
de facto prioritize the configured anchor. It is especially useful
to protect well-known bindings (such as a static address of a server)
against any other host, even when the server is down. It is also a
way to give priority to a binding learned from SAVI-DHCP over a
binding for the same address, learned from FCFS SAVI.
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 look up
local bindings to identify a collision. However, assuming DAD is
performed throughout the security perimeter for all addresses
regardless of the assignment method, then the DAD response will
inform all SAVI devices about any collision. In that case, "First-
Come, First-Served" will apply the same way as in a single switch
scenario. If the admin configured a prefix (or a single static
binding) on one of the switches to defend, the DAD response generated
by this switch will also prevent the binding from being installed on
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other switches on the perimeter. The SAVI-MIX preferences of all the
SAVI devices in the same layer 2 domain should be consistent.
Inconsistent configurations may cause network breaks.
6.2. Same Address on the Same Binding Anchor
A binding may be set up on the same binding anchor by multiple
methods, typically FCFS SAVI 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) and FCFS SAVI 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
Combining SAVI methods (as in SAVI-MIX) does not improve or eliminate
the security considerations associated with each individual SAVI
method. Therefore, security considerations for each enabled SAVI
method should be addressed as described in that method's associated
RFC. Moreover, combining methods (as in SAVI-MIX) has two additional
implications for security. First, it may increase susceptibility to
DoS attacks, because the SAVI binding setup rate will be the sum of
the rates of all enabled SAVI methods. Implementers must take these
added resource requirements into account. Second, because SAVI-MIX
supports multiple binding mechanisms, it potentially reduces the
security level to that of the weakest supported method, unless
additional steps (e.g., requiring non-overlapping address spaces for
different methods) are taken.
8. Privacy Considerations
When implementing multiple SAVI methods, privacy considerations of
all methods apply cumulatively.
9. IANA Considerations
This document does not require any IANA registrations.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
[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>.
[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>.
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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>.
[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>.
Acknowledgments
Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun, David
Lamparter, Scott G. Kelly, and Jari Arkko for their valuable
contributions.
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Authors' Addresses
Jun Bi
Tsinghua University
Institute for Network Sciences and Cyberspace, Tsinghua University
Beijing 100084
China
Email: junbi@tsinghua.edu.cn
Guang Yao
Tsinghua University/Baidu
Baidu Science and Technology Park, Building 1
Beijing 100193
China
Email: yaoguang.china@gmail.com
Joel M. Halpern
Ericsson
Email: joel.halpern@ericsson.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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