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INFORMATIONAL
Internet Engineering Task Force (IETF) J. Wu
Request for Comments: 7039 J. Bi
Category: Informational Tsinghua Univ.
ISSN: 2070-1721 M. Bagnulo
UC3M
F. Baker
Cisco
C. Vogt, Ed.
October 2013
Source Address Validation Improvement (SAVI) Framework
Abstract
Source Address Validation Improvement (SAVI) methods were developed
to prevent nodes attached to the same IP link from spoofing each
other's IP addresses, so as to complement ingress filtering with
finer-grained, standardized IP source address validation. This
document is a framework document that describes and motivates the
design of the SAVI methods. Particular SAVI methods are described in
other documents.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
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/rfc7039.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Deployment Options . . . . . . . . . . . . . . . . . . . . . 5
3.1. IP Address Assignment Methods . . . . . . . . . . . . . . 6
3.2. Binding Anchors . . . . . . . . . . . . . . . . . . . . . 6
4. Scalability Optimizations . . . . . . . . . . . . . . . . . . 7
5. Reliability Optimizations . . . . . . . . . . . . . . . . . . 9
6. Scenario with Multiple Assignment Methods . . . . . . . . . . 10
7. Prefix Configuration . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
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1. Introduction
Since IP source addresses are used by hosts and network entities to
determine the origin of a packet and as a destination for return
data, spoofing of IP source addresses can enable impersonation,
concealment, and malicious traffic redirection. Unfortunately, the
Internet architecture does not prevent IP source address spoofing
[RFC6959]. Since the IP source address of a packet generally takes
no role in forwarding the packet, it can be selected arbitrarily by
the sending host without jeopardizing packet delivery. Extra methods
are necessary for IP source address validation to augment packet
forwarding with an explicit check of whether a given packet's IP
source address is legitimate.
IP source address validation can happen at different granularity.
Ingress filtering [BCP38] [BCP84], a widely deployed standard for IP
source address validation, functions at the coarse granularity of
networks. It verifies that the prefix of an IP source address routes
to the network from which the packet was received. An advantage of
ingress filtering is simplicity: the decision of whether to accept or
to reject an IP source address can be made solely based on the
information available from routing protocols. However, the
simplicity comes at the cost of not being able to validate IP source
addresses at a finer granularity, due to the aggregated nature of the
information available from routing protocols. Finer-grained IP
source address validation would ensure that source address
information is accurate, reduce the ability to launch denial-of-
service attacks, and help with localizing hosts and identifying
misbehaving hosts. Partial solutions [BA2007] exist for finer-
grained IP source address validation but are proprietary and hence
often unsuitable for corporate procurement.
The Source Address Validation Improvement (SAVI) method was developed
to complement ingress filtering with standardized IP source address
validation at the maximally fine granularity of individual IP
addresses. It prevents hosts attached to the same link from spoofing
each other's IP addresses. To facilitate deployment in networks of
various kinds, the SAVI method was designed to be modular and
extensible. This document describes and motivates the design of the
SAVI method.
Note that SAVI raises a number of important privacy considerations
that are discussed more fully in [RFC6959]. SAVI implementers must
take those privacy considerations into account when designing
solutions that match this framework and follow the recommendations
given in [RFC6959].
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2. Model
To enable network operators to deploy fine-grained IP source address
validation without a dependency on supportive functionality on hosts,
the SAVI method was designed to be purely network based. A SAVI
instance enforces the hosts' use of legitimate IP source addresses
according to the following three-step model:
1. Identify which IP source addresses are legitimate for a host,
based on monitoring packets exchanged by the host.
2. Bind a legitimate IP address to a link-layer property of the
host's network attachment. This property, called a "binding
anchor", must be verifiable in every packet that the host sends
and harder to spoof than the host's IP source address itself.
3. Enforce that the IP source addresses in packets match the binding
anchors to which they were bound.
This model allows SAVI functionality (a SAVI instance) to be located
anywhere on the link to which the hosts attach, hence enabling
different locations for a SAVI instance. One way to locate a SAVI
instance is in the hosts' default router. IP source addresses are
then validated in packets traversing the default router, yet the IP
source addresses in packets exchanged locally on the link may bypass
validation. Another way to locate a SAVI instance is in a switch
between the hosts and their default router. Thus, packets may
undergo IP source address validation even if exchanged locally on the
link.
The closer a SAVI instance is located to the host, the more effective
the SAVI method is. This is because each of the three steps of the
SAVI model can best be accomplished in a position close to the host:
o Identifying a host's legitimate IP source addresses is most
efficient close to the host because the likelihood that the host's
packets bypass a SAVI instance, and hence cannot be monitored,
increases with the topological distance between the SAVI instance
and the host.
o Selecting a binding anchor for a host's IP source address is
easiest close to the host because many link-layer properties are
unique for a given host only on a link segment directly attached
to the host.
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o Enforcing a host's use of a legitimate IP source address is most
reliable when pursued close to the host because the likelihood
that the host's packets bypass a SAVI instance, and hence do not
undergo IP source address validation, increases with the
topological distance between the SAVI instance and the host.
The preferred location of SAVI instances is therefore close to hosts,
such as in switches that directly attach to the hosts whose IP source
addresses are being validated.
Nevertheless, it is useful for SAVI mechanisms to be able to handle
situations where hosts are not directly attached to the SAVI-capable
device. For instance, deployments with both SAVI-capable and legacy
switches could be supported. In general, a SAVI solution needs to
specify how it deals with a number of deployment scenarios and
exceptional situations, including interaction with legacy devices,
hosts moving between wireless attachment points, network partitions,
and so on.
Besides, in the case of legacy switches, the security level is lower,
as there is no full protection for the hosts connected to the legacy
server.
3. Deployment Options
The model of the SAVI method, as explained in Section 2, is
deployment specific in two ways:
o The identification of legitimate IP source addresses is dependent
on the IP address assignment method in use on a link, since it is
through assignment that a host becomes the legitimate user of an
IP source address.
o Binding anchors are dependent on the technology used to build the
link on which they are used, as binding anchors are link-layer
properties of a host's network attachment.
To facilitate the deployment of the SAVI method in networks of
various kinds, the SAVI method is designed to support different IP
address assignment methods and to function with different binding
anchors. Naturally, both the IP address assignment methods in use on
a link and the available binding anchors have an impact on the
functioning and the strength of IP source address validation. The
following two subsections explain this impact and describe how the
SAVI method accommodates this.
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3.1. IP Address Assignment Methods
Since the SAVI method traces IP address assignment packets, it
necessarily needs to incorporate logic that is specific to particular
IP address assignment methods. However, developing SAVI method
variants for each IP address assignment method is alone not
sufficient since multiple IP address assignment methods may coexist
on a given link. The SAVI method hence comes in multiple variants,
e.g., for links with DHCP [RFC2131] [RFC3315], Stateless Address
Autoconfiguration (SLAAC) [RFC4862] with or without Secure Neighbor
Discovery (SEND) [RFC3971], Internet Key Exchange Protocol Version 2
(IKEv2) [RFC5996] [RFC5739] [RFC5026], and combinations thereof.
The reason to develop SAVI method variants for each single IP address
configuration method, in addition to the variant that handles all IP
address assignment methods, is to minimize the complexity of the
common case. Many link deployments today either are constrained to a
single IP address assignment method or, equivalently from the
perspective of the SAVI method, use different IP address assignment
methods within different IP address prefixes. The SAVI method for
such links can be simpler than the SAVI method for links with
multiple IP address assignment methods per IP address prefix.
3.2. Binding Anchors
The SAVI method supports a range of binding anchors:
o The IEEE extended unique identifier, EUI-48 or EUI-64, of a host's
interface.
o The port on an Ethernet switch to which a host attaches.
o The security association between a host and the base station on
wireless links.
o The combination of a host interface's link-layer address and a
customer relationship in cable modem networks.
o An ATM virtual channel, a PPP session identifier, or a Layer 2
Tunneling Protocol (L2TP) session identifier in a DSL network.
o A tunnel that connects to a single host, such as an IP-in-IP
tunnel, a Generic Routing Encapsulation (GRE) tunnel, or an MPLS
label-switched path.
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The various binding anchors differ significantly in the security they
provide. IEEE extended unique identifiers, for example, fail to
render a secure binding anchor because they can be spoofed with
little effort. Switch ports alone may be insufficient because they
may connect to more than a single host, such as in the case of
concatenated switches.
Given this diversity in the security provided, one could define a set
of possible binding anchors and leave it up to the administrator to
choose one or more of them. Such a selection of binding anchors
would, of course, have to be accompanied by an explanation of the
pros and cons of the different binding anchors. In addition, SAVI
devices may have a default binding anchor depending on the lower
layers. Such a default could be to use switch ports when available
and MAC addresses otherwise or to use MAC addresses and switch ports
in addition if available.
4. Scalability Optimizations
The preference to locate a SAVI instance close to hosts implies that
multiple SAVI instances must be able to coexist in order to support
large links. Although the model of the SAVI method is independent of
the number of SAVI instances per link, coexistence of multiple SAVI
instances without further measures can lead to higher-than-necessary
memory requirements. Since a SAVI instance creates bindings for the
IP source addresses of all hosts on a link, bindings are replicated
if multiple SAVI instances coexist on the link. High memory
requirements, in turn, increase the cost of a SAVI instance. This is
problematic in particular for SAVI instances that are located on a
switch since it may significantly increase the cost of such a switch.
To reduce memory requirements for SAVI instances that are located on
a switch, the SAVI method enables the suppression of binding
replication on links with multiple SAVI instances. This requires
manual disabling of IP source address validation on switch ports that
connect to other switches running a SAVI instance. Each SAVI
instance is then responsible for validating IP source addresses only
on those ports to which hosts attach either directly or via switches
without a SAVI instance. On ports towards other switches running a
SAVI instance, IP source addresses are not validated. The switches
running SAVI instances thus form a "protection perimeter". The IP
source addresses in packets passing the protection perimeter are
validated by the ingress SAVI instance, but no further validation
takes place as long as the packets remain within or leave the
protection perimeter.
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..............
protection perimeter --> : +--------+ :
+---+ +---+ : | SAVI | :
| A | | B | <-- hosts : | switch | :
+---+ +---+ : +--------+ :
...|......|.............................: | :
: +--------+ +--------+ +--------+ :
: | SAVI |----------| legacy | | SAVI | :
: | switch | | switch |----------| switch | :
: +--------+ +--------+ +--------+ :
: | ...............................|........:
: +--------+ : +--------+
: | SAVI | : | legacy |
: | switch | : | switch |
: +--------+ : +--------+
:............: | |
+---+ +---+
hosts --> | C | | D |
+---+ +---+
Figure 1: Protection Perimeter Concept
Figure 1 illustrates the concept of the protection perimeter. The
figure shows a link with six switches, of which four, denoted "SAVI
switch", run a SAVI instance. The protection perimeter created by
the four SAVI instances is shown as a dotted line in the figure. IP
source address validation is enabled on all switch ports on the
protection perimeter, and it is disabled on all other switch ports.
Four hosts, denoted A through D in the figure, attach to the
protection perimeter.
In the example in Figure 1, the protection perimeter encompasses one
of the legacy switches, located in the middle of the depicted link
topology. This enables a single, unpartitioned protection perimeter.
A single protection perimeter minimizes memory requirements for the
SAVI instances because every binding is kept only once, namely, by
the SAVI instance that attaches to the host being validated.
Excluding the legacy switch from the protection perimeter would
result in two smaller protection perimeters to the left and to the
right of the depicted link topology. The memory requirements for the
SAVI instances would then be higher: since IP source address
validation would be activated on the two ports connecting to the
legacy switch, the SAVI instances adjacent to the legacy switch would
replicate all bindings from the other protection perimeter,
respectively. The reason why it is possible to include the legacy
switch in the protection perimeter is because the depicted link
topology guarantees that packets cannot enter the protection
perimeter via this legacy switch. Without this guarantee, the legacy
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switch would have to be excluded from the protection perimeter in
order to ensure that packets entering the protection perimeter
undergo IP source address validation.
Note that if such configuration is used, care must be taken as any
hosts on subnets attached to non-enforcing ports will be able to use
spoofed source addresses.
5. Reliability Optimizations
The explicit storage of legitimate IP addresses in the form of
bindings implies that failure to create a binding, or the premature
removal of bindings, can lead to loss of legitimate packets. There
are three situations in which this can happen:
o Legitimate IP address configuration packets, which should trigger
the creation of a binding in a SAVI instance, are lost before
reaching the SAVI instance.
o A SAVI instance loses a binding, for example, due to a restart.
o The link topology changes, resulting in hosts to communicate
through SAVI instances that do not have a binding for those hosts'
IP addresses.
To limit the disruption that missing bindings for legitimate IP
addresses can have, the SAVI method includes a mechanism for reactive
binding creation based on regular packets. This mechanism
supplements the proactive binding creation based on IP address
configuration packets. Reactive binding creation occurs when a SAVI
instance recognizes excessive drops of regular packets originating
from the same IP address. The SAVI instance then verifies whether
said IP address is unique on the link. How the verification is
carried out depends on the IP address configuration method that the
SAVI instance supports. The SAVI method variant for Stateless
Address Autoconfiguration and for Secure Neighbor Discovery verifies
an IP address through the Duplicate Address Detection procedure. The
SAVI method variant for DHCP verifies an IP address through a DHCP
Lease Query message exchange with the DHCP server. If verification
indicates that the IP address is unique on the link, the SAVI
instance creates a binding for the IP address. Otherwise, no binding
is created, and packets sent from the IP address continue to be
dropped. These reliability issues should be addressed in all the
SAVI protocols describing particular SAVI methods.
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6. Scenario with Multiple Assignment Methods
While multiple assignment methods can be used on the same link, the
SAVI device may have to deal with a mix of binding discovery methods.
If the address prefix used for each assignment method is different,
the "mix scenario" behaves the same as with the scenario with only
one assignment method. If different address assignment methods are
used to assign addresses from the same prefix, additional
considerations are needed because one binding mechanism may create a
binding violating an existing binding from another binding mechanism,
e.g., binding from First-Come, First-Served (FCFS) SAVI [RFC6620] may
violate a binding from SAVI-DHCP [SAVI-DHCP]. Thus, the collision
between different SAVI mechanisms in the mix scenario must be handled
in case more than one address assignment method is used to assign
addresses from the same prefix.
The prioritization of relationships between different address
assignment methods is used as the basis to solve possible collisions.
Current standard documents of address assignment methods (DHCP
[RFC2131], DHCPv6 [RFC3315], SLAAC [RFC4862], and SEND [RFC3971])
have implied the prioritization relationship in general cases.
However, in some scenarios, the default prioritization level may not
be quite suitable. A configurable prioritization level should be
supported in the SAVI solution for the mix scenario [SAVI-MIX].
7. Prefix Configuration
Before setting up a host-level granularity binding, it is important
to configure correct prefixes on the SAVI device. This document
suggests a set of 3 prefix configuration mechanisms at a SAVI device:
o Manual Prefix Configuration: The allowed prefix scope of IPv4
addresses, IPv6 static addresses, IPv6 addresses assigned by
Stateless Address Autoconfiguration (SLAAC), and IPv6 addresses
assigned by DHCPv6 can be set manually at the SAVI device.
FE80::/64 is always a feasible prefix in IPv6.
o Prefix Configuration by Router Advertisement (RA) Snooping: The
allowed prefix scope of IPv6 static addresses and IPv6 addresses
assigned by SLAAC can be set at the SAVI device through snooping
an RA message at the SAVI device.
o Prefix Configuration by DHCP Prefix Delegation (DHCP-PD) Snooping:
The allowed prefix scope of IPv6 static addresses, IPv6 addresses
assigned by SLAAC, and IPv6 addresses assigned by DHCPv6 can be
set through snooping a DHCP-PD message at the SAVI device.
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If some of the prefix scopes are set to have no prefix, the
implication is that the corresponding address assignment method is
not allowed in the network.
There is no need to explicitly present these prefix scopes, but these
restrictions should be used as the premier check in binding setup.
When SAVI is partially deployed, binding may fail as RA messages and
DHCP-PD can be spoofed. So, it is recommended that Manual Prefix
Configuration be used unless SAVI gets fully deployed.
8. Acknowledgments
The authors would like to thank the SAVI working group for a thorough
technical discussion on the design and the framework of the SAVI
method as captured in this document, in particular Erik Nordmark,
Guang Yao, Eric Levy-Abegnoli, and Alberto Garcia. Thanks also to
Torben Melsen for reviewing this document.
9. Security Considerations
This document only discusses the possible methods to mitigate the
usage of forged IP addresses. Some such methods may rely on
cryptographic methods, but not all do. As a result, it is generally
not possible to prove address ownership in any strong sense. If a
binding anchor is not exclusive for each IP address, or is without
strong security, addresses can still be forged. Besides, the binding
may not accord with the address management requirement, which can be
more specified for each client. However, given no new protocol is
introduced, the improvements are still acceptable. If strong
security is required when using IP addresses, then cryptographic-
based authentication must be used as it is the only way to provide
strong security.
Section 2 explains how the preferred location of SAVI instances is
close to hosts. However, in some cases, this makes the SAVI
instances themselves vulnerable and may defeat the purpose of
deploying a SAVI solution. For instance, deployments should not
place SAVI functionality in devices that are physically exposed.
Even if the device correctly monitors the source address usage of
hosts, an attacker could replace the device with one that does not
check or hook up to a trusted interface from the device to the rest
of the network. Similarly, deployments where SAVI instances are
distributed across administrative boundaries are not recommended.
For instance, in most cases, it would be undesirable to deploy a
distributed SAVI solution on both sides of a customer-provider
interface if the solution required trusting the correct operation of
the SAVI devices on the other side of the interface.
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10. References
10.1. Normative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[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.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC5026] Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6
Bootstrapping in Split Scenario", RFC 5026, October 2007.
[RFC5739] Eronen, P., Laganier, J., and C. Madson, "IPv6
Configuration in Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 5739, February 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[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, May 2012.
[RFC6959] McPherson, D., Baker, F., and J. Halpern, "Source Address
Validation Improvement (SAVI) Threat Scope", RFC 6959,
May 2013.
10.2. Informative References
[BA2007] Baker, F., "Cisco IP Version 4 Source Guard", Work in
Progress, November 2007.
[BCP38] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP
Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[BCP84] Baker, F. and P. Savola, "Ingress Filtering for
Multihomed Networks", BCP 84, RFC 3704, March 2004.
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[SAVI-DHCP] Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
DHCP", Work in Progress, June 2013.
[SAVI-MIX] Bi, J., Yao, G., Halpern, J., and E. Levy-Abegnoli, "SAVI
for Mixed Address Assignment Methods Scenario", Work in Progress, May
2013.
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Authors' Addresses
Jianping Wu
Tsinghua University
Computer Science, Tsinghua University
Beijing 100084
China
EMail: jianping@cernet.edu.cn
Jun Bi
Tsinghua University
Network Research Center, Tsinghua University
Beijing 100084
China
EMail: junbi@tsinghua.edu.cn
Marcelo Bagnulo
Universidad Carlos III de Madrid
Avenida de la Universidad 30
Leganes, Madrid 28911
Spain
EMail: marcelo@it.uc3m.es
Fred Baker
Cisco Systems
Santa Barbara, CA 93117
United States
EMail: fred@cisco.com
Christian Vogt (editor)
3507 Palmilla Drive
San Jose, CA 95134
United States
EMail: mail@christianvogt.net
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