Network Working Group Jianping Wu
Internet-Draft Jun Bi
Intended status: Informational CERNET
Expires: April 28, 2011 Marcelo Bagnulo
UC3M
Fred Baker
Cisco
Christian Vogt, Ed.
Ericsson
October 25, 2010
Source Address Validation Improvement Framework
draft-ietf-savi-framework-01
Abstract
The Source Address Validation Improvement method was developed to
complement ingress filtering with finer-grained, standardized IP
source address validation. This document describes and motivates the
design of the SAVI method.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Deployment Options . . . . . . . . . . . . . . . . . . . . . . 5
4. Scalability Optimizations . . . . . . . . . . . . . . . . . . . 6
5. Reliability Optimizations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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.
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], 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 be helpful to enable IP-source-
address-based authentication, authorization, and host localization,
as well as to efficiently identify 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 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.
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 is located on the path of hosts' packets, enforcing the
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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 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 hosts, 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 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 attaching
to the host.
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 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
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addresses are being validated.
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 sub-sections explain this impact, and describe how the
SAVI method accommodates this.
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 co-exist
on a given link. The SAVI method hence comes in multiple variants:
for links with Stateless Address Autoconfiguration, for links with
DHCP, for links with Secure Neighbor Discovery, and for links that
use any combination of IP address assignment methods.
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 methods or, equivalently from the
perspective of the SAVI method, separate IP address assignment
methods into 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.
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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 PPPoE session identifier, or an L2TP
session identifier in a DSL network.
o A tunnel that connects to a single host, such as an IP-in-IP
tunnel, a GRE tunnel, or an MPLS label-switched path.
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. And 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 co-exist in order to support
large links. Although the model of the SAVI method is independent of
the number of SAVI instances per link, co-existence 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
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if multiple SAVI instances co-exist 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.
..............
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
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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 of figure 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 respectively
other protection perimeter. 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
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.
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
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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
instances 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.
6. Acknowledgment
The author 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.
This document was generated using the xml2rfc tool.
7. References
[BA2007] Baker, F., "Cisco IP Version 4 Source Guard", IETF Internet
draft (work in progress), November 2007.
[BCP38] Paul, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2827, BCP 38, May 2000.
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Authors' Addresses
Jianping Wu
CERNET
Computer Science, Tsinghua University
Beijing 100084
China
Email: jianping@cernet.edu.cn
Jun Bi
CERNET
Network Research Center, Tsinghua University
Beijing 100084
China
Email: junbi@cernet.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)
Ericsson
200 Holger Way
San Jose, CA 95134
United States
Email: christian.vogt@ericsson.com
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