One document matched: draft-ietf-softwire-security-requirements-00.txt
Network Working Group S. Yamamoto
Internet-Draft C. Williams
Expires: December 21, 2006 KDDI R&D Labs
F. Parent
Independent Consultant
H. Yokota
KDDI R&D Labs
June 19, 2006
Softwire Security Analysis and Requirements
draft-ietf-softwire-security-requirements-00
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Copyright (C) The Internet Society (2006).
Abstract
This document discusses the security requirements of softwire.
Authentication, integrity, confidentiality and replay protection of
the softwire control and data packets are discussed. Both hub and
spokes and mesh solutions are discussed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Softwire Model . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Hub and Spokes . . . . . . . . . . . . . . . . . . . . . . 3
3.2 Mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Trust Relationship . . . . . . . . . . . . . . . . . . . . . . 6
5. Softwire Security Threat Scenarios . . . . . . . . . . . . . . 6
6. Softwire Security Requirements . . . . . . . . . . . . . . . . 8
7. Guideline for IPSec usage . . . . . . . . . . . . . . . . . . 9
7.1 Softwire Security Protocol . . . . . . . . . . . . . . . . 9
7.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 10
7.2.1 PPP authentication . . . . . . . . . . . . . . . . . . 10
7.2.2 L2TPv2 authentication . . . . . . . . . . . . . . . . 10
7.2.3 IPsec authentication . . . . . . . . . . . . . . . . . 10
7.3 Inter-operability guidelines . . . . . . . . . . . . . . . 11
7.4 IPsec filtering details . . . . . . . . . . . . . . . . . 11
7.5 IPsec SPD entries example . . . . . . . . . . . . . . . . 11
7.5.1 IPv6 over IPv4 Softwire with L2TPv2 example . . . . . 11
7.5.2 IPv4 over IPv6 Softwire with example . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
The Softwire Working Group is developing methods for discovery,
control and encapsulation methods for connecting IPv4 networks across
IPv6 networks and IPv6 networks across IPv4 networks. Such IP based
protocols were defined as softwire. This document discusses the
security requirements based on the softwire problem statement by
analyzing security threat scenarios for both "Hubs and Spokes" and
"Mesh" [I-D.softwire-problem-statement].
As softwire protocol, L2TP was adopted by the Softwire Working Group.
But L2TP does not define tunnel protection mechanism and vulnerable
for potential security attacks. Thus, the use of IPsec was discussed
to provide for tunnel authentication, confidentiality, integrity
checking and replay protection.[RFC 3193] However the usage of IPsec
is not always straightforward. This document provides the guidelines
for IPsec usage in terms of the softwire network scenarios.
2. Terminology
The terminology is based on the softwire problem statement document
[I-D.softwire-problem-statement]. The following terms are essential
for this document.
Softwire Concentrator (SC): The node terminating the softwire in the
service provider network.
Softwire Initiator (SI): The node initiating the softwire within the
customer network.
3. Softwire Model
3.1 Hub and Spokes
The Softwire initiator always resides in the customer network. The
node, in which the softwire initiator resides, can be the CPE access
device, another dedicated CPE router behind the original CPE access
device or any kind of host device such as PC, appliance, sensor etc.
The host device may not always have direct access to its home carrier
network, to which the user has subscribed. For example, the softwire
initiator in the laptop PC can attach to various access networks such
as Wi-Fi hot-spots, visited office network. Therefore, as shown in
Figure 1, the following three use cases should be considered:
Case 1: The softwire initiator connects to the softwire concentrator
that belongs to the home network service provider via the home
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network access provider. The IP address of the host may be changed
periodically due to the home network service provider's policy.
Case 2: The softwire initiator connects to the softwire concentrator
that belongs to the home network service provider via the visited
network access provider. This is typical of nomadic access use case.
The host does not subscribe to the visited network access provider,
but this provider has some roaming agreement with the home network
service provider of the host. The IP address of the host may be
changed periodically due to the home network service provider's
policy.
Case 3: The softwire initiator connects to the softwire concentrator
that belongs to the visited network service provider via the visited
network access provider. This is also typical of nomadic access use
case. The host does not subscribe to the visited network service
provider, but this provider has some roaming agreement with the home
network service provider of the host. If this is the case, the IP
address of the host is determined by the visited network service
provider's policy.
The trust relationship for these three cases will also be different.
The security consideration must take them into account. In
particular, to allow cases 2 and 3, AAA interactions between the home
network service provider and visited network access/service provider
should be considered. The details of this scenario are given in
Section Section 4.
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visited network visited network
access provider service provider
+---------------------------------+
| |
+......v......+ +.....................|......+
. . . v .
+------+ . (case 3) . . +------+ +--------+ .
| |=====================.==| | | | .
| SI |__.________ . . | SC |<---->| AAAv | .
| |---------- \ . . | | | | .
+------+ . \\ . . +------+ +--------+ .
. \\ . . ^ .
^ +..........\\.+ +.....................|......+
| \\ |
| (case 2) \\ |
| \\ |
| \\ |
| +............+ \\ +.....................|......+
. . \\. v .
+------+ . . \\__+------+ +--------+ .
| | . (case 1) . ---| | | | .
| SI |=====================.==| SC |<---->| AAAh | .
| | . . . | | | | .
+------+ . . . +------+ +--------+ .
. . . .
+............+ +............................+
home network home network
access provider service provider
Figure 1: Softwire hub and spokes authentication model
3.2 Mesh
The softwire initiator and concentrator model is no more applied to
the Mesh softwire. The Address Family Boundary Routers (AFBRs) are
advertising route (reachability information?) for relatively large
network islands. The individual fine-grained authentication is not
necessary.
Although the softwire for mesh must support authentication, it may be
deactivated in some circumstances. This means that the
authentication is done between AFBRs. However, if a routing protocol
is used to advertise the softwire encapsulation, it must activate
authentication.
In the data plane, the security mechanism must be supported.
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4. Trust Relationship
To perform authentication between the SC and the SI, the AAA servers
need to be involved. One or more AAA servers should reside in the
same administrative domain as the SC to authenticate the SI. When
the SI is mobile, it may roam from the home network service/access
provider network to another (Cases 2 and 3 in Figure 1). In such a
situation, the SI may not always connect to the same SC. From the
SI's viewpoint, the AAA server that is in the same administrative
domain is called the home AAA server and those not in the same
administrative domain are called visited AAA servers. The trust
relationships between those nodes are as follows:
The SC and the AAA server in the same administrative domain must
share a trust relationship, which can be done statically or
dynamically by the network operator. When the SC needs to
authenticate the SI, the SC communicates with the AAA server to
request authentication and/or to obtain security information.
Therefore, the communication between the SC and the AAA server must
be protected. If the SI roams into a network that is not in the same
administrative domain, the visited AAA server (AAAv in Figure 1)
needs to communicate with the home AAA server (AAAh in Figure 1) that
has the SI's security information. The home and visited AAA servers,
therefore, must share a trust relationship and the connection between
them must also be protected.
It can be assumed that the SI and the home AAA server share a trust
relationship. The home AAA server provides security information on
the SI when it is requested by the visited AAA server. The SI and
the visited AAA server do not usually have a trust relationship;
however, if the SI can confirm that the home AAA server is involved
with the authentication of the SI and the visited AAA server does not
alter security information from the home AAA server, the visited AAA
server can be trusted by the SI. The communication between the SI,
the home and visited AAA servers must be protected.
The SI and the SC do not necessarily share a trust relationship
especially when the SI roams into a different administrative domain.
When they are mutually authenticated by means of e.g. AAA servers,
they can start trusting each other. Unless authentication is
successfully performed, the softwire protocol should not be continued
any further.
5. Softwire Security Threat Scenarios
Softwire can be used to connect IPv4 networks across public IPv6
networks and IPv6 networks across public IPv4 networks. The control
and data packets used during the softwire session are vulnerable to
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attack.
A complete threat analysis of softwire requires examination of the
protocols used for the for the softwire setup, the encapsulation
method used to transport the payload, and other protocols used for
configuration (e.g., router advertisements, DHCP).
Threat analysis done for other protocols L2TP using IPsec [RFC3193],
PANA [RFC4016], NSIS [RFC4081], [I-D.rpsec-routing-threats] are
applicable here as well and should be used as reference.
Examples of attacks on softwire include:
1. An adversary may try to discover identities by snooping data
packets.
2. An adversary may try to modify packets (both control and data).
3. An adversary may try to hijack the softwire tunnel.
4. An adversary can launch denial of service attacks by terminating
softwire created tunnels.
5. An adversary may attempt to disrupt the softwire negotiation in
order to weaken or remove confidentiality protection.
6. An adversary may impersonate the softwire concentrator to
intercept traffic ("rogue" softwire concentrator).
7. If ingress filtering is not in place within the access network, a
number of DoS attack can happen:
* A malicious user can impersonate someone else's IPv4 address
during the set-up phase and redirect a tunnel to that IP
address. A then can, for example, start a high bandwidth
multimedia flow across that tunnel and saturate its victim's
uplink.
* A malicious user impersonates a large number of IPv4 addresses
and request tunnel of their behalf. That would quickly
saturate the ISP tunnel server infrastructure.
8. If ingress filtering is in place in the core ISP backbone but not
in the access network, the potential victims of the above
problems will be limited to the ISP's own customers.
9. If specific filtering is not in place in the ISP core network,
another kind of attack can happen. Customers from another ISP
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could start using its tunneling infrastructure to get free IPv6
connectivity, transforming effectively the ISP into a IPv6
transit provider.
It is important to consider the differences between the hub and
spokes and the mesh solutions with respect to security
considerations.
For Hub and Spokes model, the SI needs to discover the softwire
concentrator. This involves sending solicitations (setup protocol).
Once the client has discovered the concentrator, the two will enter
authentication exchange. Once the access is granted, the client will
most likely exchange data with other nodes in the Internet. These
steps are vulnerable to man-in-the-middle (MITM), denial of service
(DoS), and service theft attacks, which are discussed in this
document.
For Mesh model, security threat is not serious because in most cases,
softwire is applied among the trusted domains through AFBRs. When
the softwire is used inside a provider network, "roaming" issues will
not be raised.
6. Softwire Security Requirements
Based on the security threat analysis in other sections in this
document, an enumeration of security requirements are summarized
below:
The tunnel set-up protocol must not introduce any new vulnerability
to the network.
The softwire protocol MUST NOT assume that the discovery process is
protected.
The Softwire MUST BE able to mutually authenticate the initiator and
the concentrator. The softwire protocol MUST be able to establish
keys between the initiator and the concentrator to protect the
Softwire messages.
The Softwire signaling communication between the client and the
concentrator MUST BE protected against eavesdropping and spoofing
attacks.
The Softwire protocol MUST BE able to protect itself against replay
attacks.
The Softwire protocol MUST BE able to protect the device identifier
against spoofing when it is exchanged between the initiator and the
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concentrator.
The Softwire protocol MUST BE able to protect disconnect-type and
revocation-type messages.
The Softwire protocol MUST BE able to securely bind the authenticated
session to the device identifier of the client, to prevent service
theft.
Softwire security MUST meet the key management requirements of the
IPsec protocol suite. IKE SHOULD be supported for authentication,
security association negotiation, and key management
7. Guideline for IPSec usage
Note: this section only covers the h&s softwire model. The mesh
scenario will be covered at a later stage.
The Softwire security requirements state that control and data plane
must be able to provide payload security when desired [softwire-
problem-statement, section 2.11]. [RFC3193] discusses how L2TP can
use IPsec to provide tunnel authentication, privacy protection,
integrity checking and replay protection.
[RFC3193] can be applied in the softwire h&s model context. New
additions to IPsec ([RFC3948],[RFC3947],[RFC4306]) are necessary to
meet the softwire requirements were published after [RFC3193].
The following sections will discuss [RFC3193] as applied in the
softwire hub&spoke model.
In softwire, L2TP is used in a voluntary tunneling model only. The
Softwire Initiator (SI) acts as a LAC and PPP endpoint. The L2TP
tunnel is always initiated from the SI.
The scope of the security is on the L2TP tunnel between the SI and
SC. If end to end security is required, a security protocol should
be used in the payload packets (out of scope of this document for
now).
7.1 Softwire Security Protocol
[RFC3193] section 2.1 defines the security requirement for L2TP. The
same requirements are used for the h&s softwire model. A softwire
security compliant implementation MUST implement the security
protocol requirements as defined in [RFC3193] section 2.1:
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o IPsec ESP [RFC4303] in transport mode to secure both the L2TP
control and data packets.
o IKE MUST be supported for authentication, security association
negotiation and key management for IPsec (this is a SHOULD in
[RFC3193])
o Since softwire must support NAT traversal, UDP encapsulation of
IPsec ESP packets [RFC3948] and negotiation of NAT-traversal in
IKE [RFC3947] MUST be supported.
7.2 Authentication
Softwire requirements for authentication vary from no-authentication
to mutual authentication between the SI and SC [I-D.softwire-problem-
statement, section 2.11]. The no-authentication mode SHOULD NOT be
used on public IP networks.
7.2.1 PPP authentication
PPP can provide mutual authentication between the SI and SC using
CHAP [RFC1994] during the connection establishment phase (LCP). PPP
CHAP authentication can be used when the SI and SC are on a trusted,
non-public IP network.
Since CHAP does not provide per-packet authentication, integrity, or
replay protection, PPP CHAP authentication MUST NOT be used on a
public IP network.
7.2.2 L2TPv2 authentication
L2TPv2 provides an optional CHAP-like [RFC1994] tunnel authentication
during the control connection establishment [RFC2661, 5.1.1]. The
same restrictions apply to L2TPv2 authentication and PPP CHAP
authentication.
7.2.3 IPsec authentication
pre-shared key, certificates, and (for IKEv2) an EAP exchange
identity (IP address, DNS name, and X.500 distinguished name)
Main mode vs. aggressive mode
IPsec authentication vs. PPP/L2TPv2 authentication: keep both?
[RFC3193, 5.1], user vs. machine authentication.
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7.3 Inter-operability guidelines
The inter-operability guidelines in [RFC3193] concerning tunnel
teardown, fragmentation and per-packet security checks must be
followed. [Note: nothing specific to softwire.]
7.4 IPsec filtering details
The IPsec filtering details from [RFC3193] section 4 are applicable
to softwire h&s model.
Although the L2TP specification allows the responder (SC in softwire)
to use a new IP address when sending the SCCRP, a softwire
concentrator implementation SHOULD NOT do this ([RFC3193] section 4).
(NOTE: this point should be discussed on the mailing list. This
feature may be needed for "load-balancing" between SC) [Mote: To be
completed]
7.5 IPsec SPD entries example
The SPD examples in [RFC3193] appendix A can be applied to softwire
model. In that case, the initiator is always the client (SI), and
responder is the SC.
7.5.1 IPv6 over IPv4 Softwire with L2TPv2 example
(To be completed)
7.5.2 IPv4 over IPv6 Softwire with example
(To be completed)
8. Security Considerations
This document discusses security issues that should be considered in
the deployment of softwire. The requirements in Section 6, however,
are not fully discussed yet with respect to security in service
discovery, scaling, routing and multicast.
9. References
9.1 Normative References
[I-D.softwire-problem-statement]
Li, X., Durand, A., Ward, D., and S. Dawkins, "Softwire
Problem Statement",
draft-ietf-softwire-problem-statement-01 (work in
progress), February 2006.
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[I-D.v6ops-tunneling-requirements]
Durand, A. and F. Parent, "Requirements for assisted
tunneling",
draft-durand-v6ops-assisted-tunneling-requirements-00
(work in progress), September 2004.
9.2 Informative References
[I-D.bellovin-useipsec]
Bellovin, S., "Guidelines for Mandating the Use of IPsec",
draft-bellovin-useipsec-04 (work in progress),
September 2005.
[I-D.rpsec-routing-threats]
Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", draft-ietf-rpsec-routing-threats-07
(work in progress), October 2005.
[RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,
"Securing L2TP using IPsec", RFC 3193, November 2001.
[RFC4016] Parthasarathy, M., "Protocol for Carrying Authentication
and Network Access (PANA) Threat Analysis and Security
Requirements", RFC 4016, March 2005.
[RFC4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for
Next Steps in Signaling (NSIS)", RFC 4081, June 2005.
Authors' Addresses
Shu Yamamoto
KDDI R&D Labs
2-1-15 Fujimino-shi
Saitama, 356-8502
Japan
Phone: 81 (49) 278-7311
Email: shu@kddilabs.jp
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Carl Williams
KDDI R&D Labs
Palo Alto, CA 94301
USA
Phone: +1.650.279.5903
Email: carlw@mcsr-labs.org
Florent Parent
Independent Consultant
Quebec, QC
Canada
Phone: +1 418 265 7357
Email: Florent.Parent@mac.com
Hidetoshi Yokota
KDDI R&D Labs
2-1-15 Ohara
Fujimino, Saitama 356-8502
Japan
Phone: 81 (49) 278-7894
Email: yokota@kddilabs.jp
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