| Rfc | 5619 |
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| Title | Softwire Security Analysis and Requirements |
|---|
| Author | S. Yamamoto, C.
Williams, H. Yokota, F. Parent |
|---|
| Date | August 2009 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group S. Yamamoto
Request for Comments: 5619 NICT/KDDI R&D Labs
Category: Standards Track C. Williams
H. Yokota
KDDI R&D Labs
F. Parent
Beon Solutions
August 2009
Softwire Security Analysis and Requirements
Abstract
This document describes security guidelines for the softwire "Hubs
and Spokes" and "Mesh" solutions. Together with discussion of the
softwire deployment scenarios, the vulnerability to security attacks
is analyzed to provide security protection mechanisms such as
authentication, integrity, and confidentiality to the softwire
control and data packets.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 5
3. Hubs and Spokes Security Guidelines . . . . . . . . . . . . . 5
3.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 5
3.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 7
3.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 8
3.4. Softwire Security Guidelines . . . . . . . . . . . . . . . 11
3.4.1. Authentication . . . . . . . . . . . . . . . . . . . . 12
3.4.2. Softwire Security Protocol . . . . . . . . . . . . . . 13
3.5. Guidelines for Usage of IPsec in Softwire . . . . . . . . 13
3.5.1. Authentication Issues . . . . . . . . . . . . . . . . 14
3.5.2. IPsec Pre-Shared Keys for Authentication . . . . . . . 15
3.5.3. Inter-Operability Guidelines . . . . . . . . . . . . . 15
3.5.4. IPsec Filtering Details . . . . . . . . . . . . . . . 16
4. Mesh Security Guidelines . . . . . . . . . . . . . . . . . . . 19
4.1. Deployment Scenario . . . . . . . . . . . . . . . . . . . 19
4.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 20
4.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 20
4.4. Applicability of Security Protection Mechanism . . . . . . 21
4.4.1. Security Protection Mechanism for Control Plane . . . 21
4.4.2. Security Protection Mechanism for Data Plane . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Normative References . . . . . . . . . . . . . . . . . . . 23
7.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 26
A.1. IPv6-over-IPv4 Softwire with L2TPv2 Example for IKE . . . 26
A.2. IPv4-over-IPv6 Softwire with Example for IKE . . . . . . . 26
1. Introduction
The Softwire Working Group specifies the standardization of
discovery, control, and encapsulation methods for connecting IPv4
networks across IPv6 networks and IPv6 networks across IPv4 networks.
The softwire provides connectivity to enable the global reachability
of both address families by reusing or extending existing technology.
The Softwire Working Group is focusing on the two scenarios that
emerged when discussing the traversal of networks composed of
differing address families. This document provides the security
guidelines for two such softwire solution spaces: the "Hubs and
Spokes" and "Mesh" scenarios. The "Hubs and Spokes" and "Mesh"
problems are described in [RFC4925] Sections 2 and 3, respectively.
The protocols selected for softwire connectivity require security
considerations on more specific deployment scenarios for each
solution. The scope of this document provides analysis on the
security vulnerabilities for the deployment scenarios and specifies
the proper usage of the security mechanisms that are applied to the
softwire deployment.
The Layer Two Tunneling Protocol (L2TPv2) is selected as the phase 1
protocol to be deployed in the "Hubs and Spokes" solution space. If
L2TPv2 is used in the unprotected network, it will be vulnerable to
various security attacks and MUST be protected by an appropriate
security protocol, such as IPsec as described in [RFC3193]. The new
implementation SHOULD use IKEv2 (Internet Key Exchange Protocol
version 2) as the key management protocol for IPsec because it is a
more reliable protocol than IKEv1 and integrates the required
protocols into a single platform. This document provides
implementation guidance and specifies the proper usage of IPsec as
the security protection mechanism by considering the security
vulnerabilities in the "Hubs and Spokes" scenario. The document also
addresses cases where the security protocol is not necessarily
mandated.
The softwire "Mesh" solution MUST support various levels of security
mechanisms to protect the data packets being transmitted on a
softwire tunnel from the access networks with one address family
across the transit core operating with a different address family
[RFC4925]. The security mechanism for the control plane is also
required to be protected from control-data modification, spoofing
attacks, etc. In the "Mesh" solution, BGP is used for distributing
softwire routing information in the transit core; meanwhile, security
issues for BGP are being discussed in other working groups. This
document provides the proper usage of security mechanisms for
softwire mesh deployment scenarios.
2. Terminology
2.1. Abbreviations
The terminology is based on the "Softwire Problem Statement"
[RFC4925].
AF(i) - Address Family. IPv4 or IPv6. Notation used to indicate
that prefixes, a node, or network only deal with a single IP AF.
AF(i,j) - Notation used to indicate that a node is dual-stack or that
a network is composed of dual-stack nodes.
Address Family Border Router (AFBR) - A dual-stack router that
interconnects two networks that use either the same or different
address families. An AFBR forms peering relationships with other
AFBRs, adjacent core routers, and attached Customer Edge (CE)
routers; performs softwire discovery and signaling; advertises client
ASF(i) reachability information; and encapsulates/decapsulates
customer packets in softwire transport headers.
Customer Edge (CE) - A router located inside an AF access island that
peers with other CE routers within the access island network and with
one or more upstream AFBRs.
Customer Premise Equipment (CPE) - An equipment, host or router,
located at a subscriber's premises and connected with a carrier's
access network.
Provider Edge (PE) - A router located at the edge of a transit core
network that interfaces with the CE in an access island.
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.
Softwire Encapsulation Set (SW-Encap) - A softwire encapsulation set
contains tunnel header parameters, order of preference of the tunnel
header types, and the expected payload types (e.g., IPv4) carried
inside the softwire.
Softwire Next_Hop (SW-NHOP) - This attribute accompanies client AF
reachability advertisements and is used to reference a softwire on
the ingress AFBR leading to the specific prefixes. It contains a
softwire identifier value and a softwire next_hop IP address denoted
as <SW ID:SW-NHOP address>. Its existence in the presence of client
AF prefixes (in advertisements or entries in a routing table) infers
the use of softwire to reach that prefix.
2.2. 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 [RFC2119].
3. Hubs and Spokes Security Guidelines
3.1. Deployment Scenarios
To provide the security guidelines, discussion of the possible
deployment scenario and the trust relationship in the network is
important.
The softwire initiator (SI) always resides in the customer network.
The node in which the SI 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 a PC, appliance, sensor, etc.
However, the host device may not always have direct access to its
home carrier network, to which the user has subscribed. For example,
the SI in the laptop PC can access various access networks such as
Wi-Fi hot-spots, visited office networks, etc. This is the nomadic
case, which the softwire SHOULD support.
As the softwire deployment model, the following three cases as shown
in Figure 1 should be considered. Cases 2 and 3 are typical for a
nomadic node, but are also applicable to a stationary node. In order
to securely connect a legitimate SI and SC to each other, the
authentication process between SI and SC is normally performed using
Authentication, Authorization, and Accounting (AAA) servers.
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: Authentication Model for Hubs and Spokes
The AAA server shown in Figure 1 interacts with the SC, which acts as
a AAA client. The AAA may consists of multiple AAA servers, and the
proxy AAA may be intermediate between the SC and the AAA servers.
This document refers to the AAA server in the home network service
provider as the home AAA server (AAAh) and to that in the visited
network service provider as the visited AAA server (AAAv).
The "Softwire Problem Statement" [RFC4925] states that the softwire
solution must be able to be integrated with commonly deployed AAA
solutions. L2TPv2 used in softwire supports PPP and L2TP
authentications that can be integrated with common AAA servers.
When the softwire is used in an unprotected network, a stronger
authentication process is required (e.g., IKEv2). The proper
selection of the authentication processes is discussed in Section 3.4
with respect to the various security threats.
Case 1: The SI connects to the SC that belongs to the home network
service provider via the home access provider network that operates a
different address family. It is assumed that the home access
provider network and the home network service provider for the SC are
under the same administrative system.
Note that the IP address of the host device, in which the SI resides,
is static or dynamic depending on the subscribed service. The
discovery of the SC may be automatic. But in this document, the
information on the SC, e.g., the DNS name or IP address, is assumed
to be configured by the user or the provider of the SI in advance.
Case 2: The SI connects to the SC that belongs to the home network
service provider via the visited access network. For the nomadic
case, the SI/user does not subscribe to the visited access provider.
For network access through the public network, such as Wi-Fi hot-
spots, the home network service provider does not have a trust
relationship with the access network.
Note that the IP address of the host device, in which the SI resides,
may be changed periodically due to the home network service
provider's policy.
Case 3: The SI connects to the SC that belongs to the visited network
service provider via the visited access network. This is typical of
the nomadic access case. When the SI is mobile, it may roam from the
home ISP providing the home access network to the visited access
network, e.g., Wi-Fi hot-spot network provided by the different ISP.
The SI does not connect to the SC in the home network, for example,
due to geographical reasons. The SI/user does not subscribe to the
visited network service provider, but the visited network service
provider has some roaming agreement with the home network service
provider.
Note that the IP address of the host, in which the SI resides, is
provided with the visited network service provider's policy.
3.2. Trust Relationship
The establishment of a trust relationship between the SI and SC is
different for three cases. The security considerations must be taken
into account for each case.
In Case 1, the SC and the home AAA server in the same network service
provider MUST have a trust relationship and communications between
them MUST be secured. When the SC authenticates the SI, the SC
transmits the authentication request message to the home AAA server
and obtains the accept message together with the Attribute Value Pair
for the SI authentication. Since the SI is in the service provider
network, the provider can take measures to protect the entities
(e.g., SC, AAA servers) against a number of security threats,
including the communication between them.
In Case 2, when the SI is mobile, access to the home network service
provider through the visited access network provider is allowed. The
trust relationship between the SI and the SC in the home network MUST
be established. When the visited access network is a public network,
various security attacks must be considered. Especially for SI to
connect to the legitimate SC, the authentication from SI to SC MUST
be performed together with that from SC to SI.
In Case 3, if the SI roams into a different network service
provider's administrative domain, the visited AAA server communicates
with the home AAA server to obtain the information for SI
authentication. The visited AAA server MUST have a trust
relationship with the home AAA server and the communication between
them MUST be secured in order to properly perform the roaming
services that have been agreed upon under specified conditions.
Note that the path for the communications between the home AAA server
and the visited AAA server may consist of several AAA proxies. In
this case, the AAA proxy threat model SHOULD be considered [RFC2607].
A malicious AAA proxy may launch passive or active security attacks.
The trustworthiness of proxies in AAA proxy chains will weaken when
the hop counts of the proxy chain is longer. For example, the
accounting information exchanged among AAA proxies is attractive for
an adversary. The communication between a home AAA server and a
visited AAA server MUST be protected.
3.3. Softwire Security Threat Scenarios
Softwire can be used to connect IPv6 networks across public IPv4
networks and IPv4 networks across public IPv6 networks. The control
and data packets used during the softwire session are vulnerable to
the security attacks.
A complete threat analysis of softwire requires examination of the
protocols used for the softwire setup, the encapsulation method used
to transport the payload, and other protocols used for configuration
(e.g., router advertisements, DHCP).
The softwire solution uses a subset of the Layer Two Tunneling
Protocol (L2TPv2) functionality ([RFC2661], [RFC5571]). In the
softwire "Hubs and Spokes" model, L2TPv2 is used in a voluntary
tunnel model only. The SI acts as an L2TP Access Concentrator (LAC)
and PPP endpoint. The L2TPv2 tunnel is always initiated from the SI.
The generic threat analysis done for L2TP using IPsec [RFC3193] is
applicable to softwire "Hubs and Spokes" deployment. The threat
analysis for other protocols such as MIPv6 (Mobile IPv6) [RFC4225],
PANA (Protocol for Carrying Authentication for Network Access)
[RFC4016], NSIS (Next Steps in Signaling) [RFC4081], and Routing
Protocols [RFC4593] are applicable here as well and should be used as
references.
First, the SI that resides in the customer network sends a Start-
Control-Connection-Request (SCCRQ) packet to the SC for the
initiation of the softwire. L2TPv2 offers an optional tunnel
authentication system (which is similar to CHAP -- the Challenge
Handshake Authentication Protocol) during control connection
establishment. This requires a shared secret between the SI and SC
and no key management is offered for this L2TPv2.
When the L2TPv2 control connection is established, the SI and SC
optionally enter the authentication phase after completing PPP Link
Control Protocol (LCP) negotiation. PPP authentication supports one-
way or two-way CHAP authentication, and can leverage existing AAA
infrastructure. PPP authentication does not provide per-packet
authentication.
PPP encryption is defined but PPP Encryption Control Protocol (ECP)
negotiation does not provide for a protected cipher suite
negotiation. PPP encryption provides a weak security solution
[RFC3193]. PPP ECP implementation cannot be expected. PPP
authentication also does not provide scalable key management.
Once the L2TPv2 tunnel and PPP configuration are successfully
established, the SI is connected and can start using the connection.
These steps are vulnerable to man-in-the-middle (MITM), denial-of-
service (DoS), and service-theft attacks, which are caused by the
following adversary actions.
Adversary attacks on softwire include:
1. An adversary may try to discover identities and other
confidential information by snooping data packets.
2. An adversary may try to modify both control and data packets.
This type of attack involves integrity violations.
3. An adversary may try to eavesdrop and collect control messages.
By replaying these messages, an adversary may successfully hijack
the L2TP tunnel or the PPP connection inside the tunnel. An
adversary might mount MITM, DoS, and theft-of-service attacks.
4. An adversary can flood the softwire node with bogus signaling
messages to cause DoS attacks by terminating L2TP tunnels or PPP
connections.
5. An adversary may attempt to disrupt the softwire negotiation in
order to weaken or remove confidentiality protection.
6. An adversary may wish to disrupt the PPP LCP authentication
negotiation.
When AAA servers are involved in softwire tunnel establishment, the
security attacks can be mounted on the communication associated with
AAA servers. Specifically, for Case 3 stated in Section 3.2, an
adversary may eavesdrop on the packets between AAA servers in the
home and visited network and compromise the authentication data. An
adversary may also disrupt the communication between the AAA servers,
causing a service denial. Security of AAA server communications is
out of scope of this document.
In environments where the link is shared without cryptographic
protection and weak authentication or one-way authentication is used,
these security attacks can be mounted on softwire control and data
packets.
When there is no prior trust relationship between the SI and SC, any
node can pretend to be a SC. In this case, an adversary may
impersonate the SC to intercept traffic (e.g., "rogue" softwire
concentrator).
The rogue SC can introduce a denial-of-service attack by blackholing
packets from the SI. The rogue SC can also eavesdrop on all packets
sent from or to the SI. Security threats of a rogue SC are similar
to a compromised router.
The deployment of ingress filtering is able to control malicious
users' access [RFC4213]. Without specific ingress filtering checks
in the decapsulator at the SC, it would be possible for an attacker
to inject a false packet, leaving the system vulnerable to attacks
such as DoS. Using ingress filtering, invalid inner addresses can be
rejected. Without ingress filtering of inner addresses, another kind
of attack can happen. The malicious users from another ISP could
start using its tunneling infrastructure to get free inner-address
connectivity, effectively transforming the ISP into an inner-address
transit provider.
Ingress filtering does not provide complete protection in the case
that address spoofing has happened. In order to provide better
protection against address spoofing, authentication with binding
between the legitimate address and the authenticated identity MUST be
implemented. This can be implemented between the SC and the SI using
IPsec.
3.4. Softwire Security Guidelines
Based on the security threat analysis in Section 3.3 of this
document, the softwire security protocol MUST support the following
protections.
1. Softwire control messages between the SI and SC MUST be protected
against eavesdropping and spoofing attacks.
2. The softwire security protocol MUST be able to protect itself
against replay attacks.
3. The softwire security protocol MUST be able to protect the device
identifier against the impersonation when it is exchanged between
the SI and the SC.
4. The softwire security protocol MUST be able to securely bind the
authenticated session to the device identifier of the client, to
prevent service theft.
5. The softwire security protocol MUST be able to protect disconnect
and revocation messages.
The softwire security protocol requirement is comparable to
[RFC3193].
For softwire control packets, authentication, integrity, and replay
protection MUST be supported, and confidentiality SHOULD be
supported.
For softwire data packets, authentication, integrity, and replay
protection SHOULD be supported, and confidentiality MAY be supported.
The "Softwire Problem Statement" [RFC4925] provides some requirements
for the "Hubs and Spoke" solution that are taken into account in
defining the security protection mechanisms.
1. The control and/or data plane MUST be able to provide full
payload security when desired.
2. The deployed technology MUST be very strongly considered.
This additional security protection must be separable from the
softwire tunneling mechanism.
Note that the scope of this 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. But this is out of
scope of this document.
3.4.1. Authentication
The softwire security protocol MUST support user authentication in
the control plane in order to authorize access to the service and
provide adequate logging of activity. Although several
authentication protocols are available, security threats must be
considered to choose the protocol.
For example, consider the SI/user using Password Authentication
Protocol (PAP) access to the SC with a cleartext password. In many
circumstances, this represents a large security risk. The adversary
may spoof as a legitimate user by using the stolen password. The
Challenge Handshake Authentication Protocol (CHAP) [RFC1994] encrypts
a password with a "challenge" sent from the SC. The theft of
password can be mitigated. However, as CHAP only supports
unidirectional authentication, the risk of a man-in-the-middle or
rogue SC cannot be avoided. Extensible Authentication Protocol-
Transport Layer Security (EAP-TLS) [RFC5216] mandates mutual
authentication and avoids the rogue SC.
When the SI established a connection to the SC through a public
network, the SI may want proof of the SC identity. Softwire MUST
support mutual authentication to allow for such a scenario.
In some circumstances, however, the service provider may decide to
allow non-authenticated connection [RFC5571]. For example, when the
customer is already authenticated by some other means, such as closed
networks, cellular networks at Layer 2, etc., the service provider
may decide to turn authentication off. If no authentication is
conducted on any layer, the SC acts as a gateway for anonymous
connections. Running such a service MUST be configurable by the SC
administrator and the SC SHOULD take some security measures, such as
ingress filtering and adequate logging of activity. It should be
noted that anonymous connection service cannot provide the security
functionalities described in this document (e.g., integrity, replay
protection, and confidentiality).
L2TPv2 selected as the softwire phase 1 protocol supports PPP
authentication and L2TPv2 authentication. PPP authentication and
L2TPv2 have various security threats, as stated in Section 3.3. They
will be used in the limited condition as described in the next
subsections.
3.4.1.1. PPP Authentication
PPP can provide mutual authentication between the SI and SC using
CHAP [RFC1994] during the connection-establishment phase (via the
Link Control Protocol, 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
unprotected on a public IP network. If other appropriate protected
mechanisms have been already applied, PPP CHAP authentication MAY be
used.
Optionally, other authentication methods such as PAP, MS-CHAP, and
EAP MAY be supported.
3.4.1.2. L2TPv2 Authentication
L2TPv2 provides an optional CHAP-like tunnel authentication during
the control connection establishment [RFC2661], Section 5.1.1.
L2TPv2 authentication MUST NOT be used unprotected on a public IP
network, similar to the same restriction applied to PPP CHAP
authentication.
3.4.2. Softwire Security Protocol
To meet the above requirements, all softwire-security-compliant
implementations MUST implement the following security protocols.
IPsec ESP [RFC4303] in transport mode is used for securing softwire
control and data packets. The Internet Key Exchange (IKE) protocol
[RFC4306] MUST be supported for authentication, security association
negotiation, and key management for IPsec. The applicability of
different versions of IKE is discussed in Section 3.5.
The softwire security protocol MUST support NAT traversal. UDP
encapsulation of IPsec ESP packets[RFC3948] and negotiation of NAT-
traversal in IKE [RFC3947] MUST be supported when IPsec is used.
3.5. Guidelines for Usage of IPsec in Softwire
When the softwire "Hubs and Spokes" solution implemented by L2TPv2 is
used in an untrustworthy network, softwire MUST be protected by
appropriate security protocols, such as IPsec. This section provides
guidelines for the usage of IPsec in L2TPv2-based softwire.
[RFC3193] discusses how L2TP can use IKE [RFC2409] and IPsec
[RFC2401] to provide tunnel authentication, privacy protection,
integrity checking, and replay protection. Since the publication of
[RFC3193], the revisions to IPsec protocols have been published
(IKEv2 [RFC4306], ESP [RFC4303], NAT-traversal for IKE [RFC3947], and
ESP [RFC3948]).
Given that deployed technology must be very strongly considered
[RFC4925] for the 'time-to-market' solution, [RFC3193] MUST be
supported. However, the new implementation SHOULD use IKEv2
[RFC4306] for IPsec because of the numerous advantages it has over
IKE [RFC2409]. In new deployments, IKEv2 SHOULD be used as well.
Although [RFC3193] can be applied in the softwire "Hubs and Spokes"
solution, softwire requirements such as NAT-traversal, NAT-traversal
for IKE [RFC3947], and ESP [RFC3948] MUST be supported.
Meanwhile, IKEv2 [RFC4306] integrates NAT-traversal. IKEv2 also
supports EAP authentication, with the authentication using shared
secrets (pre-shared key) or a public key signature (certificate).
The selection of pre-shared key or certificate depends on the scale
of the network for which softwire is to be deployed, as described in
Section 3.5.2. However, pre-shared keys and certificates only
support the machine authentication. When both machine and user
authentications are required as, for example, in the nomadic case,
EAP SHOULD be used.
Together with EAP, IKEv2 [RFC4306] supports legacy authentication
methods that may be useful in environments where username- and
password-based authentication is already deployed.
IKEv2 is a more reliable protocol than IKE [RFC2409] in terms of
replay-protection capability, DoS-protection-enabled mechanism, etc.
Therefore, new implementations SHOULD use IKEv2 over IKE.
The following sections will discuss using IPsec to protect L2TPv2 as
applied in the softwire "Hubs and Spokes" model. Unless otherwise
stated, IKEv2 and the new IPsec architecture [RFC4301] is assumed.
3.5.1. Authentication Issues
IPsec implementation using IKE only supports machine authentication.
There is no way to verify a user identity and to segregate the tunnel
traffic among users in the multi-user machine environment. IKEv2 can
support user authentication with EAP payload by leveraging the
existing authentication infrastructure and credential database. This
enables traffic segregation among users when user authentication is
used by combining the legacy authentication. The user identity
asserted within IKEv2 will be verified on a per-packet basis.
If the AAA server is involved in security association establishment
between the SI and SC, a session key can be derived from the
authentication between the SI and the AAA server. Successful EAP
exchanges within IKEv2 run between the SI and the AAA server to
create a session key, which is securely transferred to the SC from
the AAA server. The trust relationship between the involved entities
follows Section 3.2 of this document.
3.5.2. IPsec Pre-Shared Keys for Authentication
With IPsec, when the identity asserted in IKE is authenticated, the
resulting derived keys are used to provide per-packet authentication,
integrity, and replay protection. As a result, the identity verified
in the IKE is subsequently verified on reception of each packet.
Authentication using pre-shared keys can be used when the number of
SI and SC is small. As the number of SI and SC grows, pre-shared
keys become increasingly difficult to manage. A softwire security
protocol MUST provide a scalable approach to key management.
Whenever possible, authentication with certificates is preferred.
When pre-shared keys are used, group pre-shared keys MUST NOT be used
because of its vulnerability to man-in-the-middle attacks ([RFC3193],
Section 5.1.4).
3.5.3. Inter-Operability Guidelines
The L2TPv2/IPsec inter-operability concerning tunnel teardown,
fragmentation, and per-packet security checks given in [RFC3193],
Section 3 must be taken into account.
Although the L2TP specification allows the responder (SC in softwire)
to use a new IP address or to change the port number when sending the
Start-Control-Connection-Request-Reply (SCCRP), a softwire
concentrator implementation SHOULD NOT do this ([RFC3193], Section
4).
However, for some reasons, for example, "load-balancing" between SCs,
the IP address change is required. To signal an IP address change,
the SC sends a StopCCN message to the SI using the Result and Error
Code AVP in an L2TPv2 message. A new IKE_SA and CHILD_SA MUST be
established to the new IP address.
Since ESP transport mode is used, the UDP header carrying the L2TP
packet will have an incorrect checksum due to the change of parts of
the IP header during transit. Section 3.1.2 of [RFC3948] defines 3
procedures that can be used to fix the checksum. A softwire
implementation MUST NOT use the "incremental update of checksum"
(option 1 described in [RFC3948]) because IKEv2 does not have the
information required (NAT-OA payload) to compute that checksum.
Since ESP is already providing validation on the L2TP packet, a
simple approach is to use the "do not check" approach (option 3 in
[RFC3948]).
3.5.4. IPsec Filtering Details
If the old IPsec architecture [RFC2401] and IKE [RFC2409] are used,
the security policy database (SPD) examples in [RFC3193], Appendix A
can be applied to softwire model. In that case, the initiator is
always the client (SI), and the responder is the SC. IPsec SPD
examples for IKE [RFC2409] are also given in Appendix A of this
document.
The revised IPsec architecture [RFC4301] redefined the SPD entries to
provide more flexibility (multiple selectors per entry, list of
address range, peer authentication database (PAD), "populate from
packet" (PFP) flag, etc.). The Internet Key Exchange (IKE) has also
been revised and simplified in IKEv2 [RFC4306]. The following
sections provide the SPD examples for softwire to use the revised
IPsec architecture and IKEv2.
3.5.4.1. IPv6-over-IPv4 Softwire L2TPv2 Example for IKEv2
If IKEv2 is used as the key management protocol, [RFC4301] provides
the guidance of the SPD entries. In IKEv2, we can use the PFP flag
to specify the SA, and the port number can be selected with the TSr
(Traffic Selector - Responder) payload during CREATE_CHILD_SA. The
following describes PAD entries on the SI and SC, respectively. The
PAD entries are only example configurations. The PAD entry on the SC
matches user identities to the L2TP SPD entry. This is done using a
symbolic name type specified in [RFC4301].
SI PAD:
- IF remote_identity = SI_identity
Then authenticate (shared secret/certificate/)
and authorize CHILD_SA for remote address SC_address
SC PAD:
- IF remote_identity = user_1
Then authenticate (shared secret/certificate/EAP)
and authorize CHILD_SAs for symbolic name "l2tp_spd_entry"
The following describes the SPD entries for the SI and SC,
respectively. Note that IKEv2 and ESP traffic MUST be allowed
(bypass). These include IP protocol 50 and UDP port 500 and 4500.
The IPv4 packet format when ESP protects and L2TPv2 carries an IPv6
packet is shown in Table 1, which is similar to Table 1 in [RFC4891].
+----------------------------+------------------------------------+
| Components (first to last) | Contains |
+----------------------------+------------------------------------+
| IPv4 header | (src = IPv4-SI, dst = IPv4-SC) |
| ESP header | |
| UDP header | (src port=1701, dst port=1701) |
| L2TPv2 header | |
| PPP header | |
| IPv6 header | |
| (payload) | |
| ESP ICV | |
+----------------------------+------------------------------------+
Table 1: Packet Format for L2TPv2 with ESP Carrying IPv6 Packet
SPD for Softwire Initiator:
Softwire Initiator SPD-S
- IF local_address=IPv4-SI
remote_address=IPv4-SC
Next Layer Protocol=UDP
local_port=1701
remote_port=ANY (PFP=1)
Then use SA ESP transport mode
Initiate using IDi = user_1 to address IPv4-SC
SPD for Softwire Concentrator:
Softwire Concentrator SPD-S
- IF name="l2tp_spd_entry"
local_address=IPv4-SC
remote_address=ANY (PFP=1)
Next Layer Protocol=UDP
local_port=1701
remote_port=ANY (PFP=1)
Then use SA ESP transport mode
3.5.4.2. IPv4-over-IPv6 Softwire L2TPv2 Example for IKEv2
The PAD entries for SI and SC are shown as examples. These example
configurations are similar to those in Section 3.5.4.1 of this
document.
SI PAD:
- IF remote_identity = SI_identity
Then authenticate (shared secret/certificate/)
and authorize CHILD_SA for remote address SC_address
SC PAD:
- IF remote_identity = user_2
Then authenticate (shared secret/certificate/EAP)
and authorize CHILD_SAs for symbolic name "l2tp_spd_entry"
The following describes the SPD entries for the SI and SC,
respectively. In this example, the SI and SC are denoted with IPv6
addresses IPv6-SI and IPv6-SC, respectively. Note that IKEv2 and ESP
traffic MUST be allowed (bypass). These include IP protocol 50 and
UDP port 500 and 4500.
The IPv6 packet format when ESP protects and L2TPv2 carries an IPv4
packet is shown in Table 2, which is similar to Table 1 in [RFC4891].
+----------------------------+------------------------------------+
| Components (first to last) | Contains |
+----------------------------+------------------------------------+
| IPv6 header | (src = IPv6-SI, dst = IPv6-SC) |
| ESP header | |
| UDP header | (src port=1701, dst port=1701) |
| L2TPv2 header | |
| PPP header | |
| IPv4 header | |
| (payload) | |
| ESP ICV | |
+----------------------------+------------------------------------+
Table 2: Packet Format for L2TPv2 with ESP Carrying IPv4 Packet
SPD for Softwire Initiator:
Softwire Initiator SPD-S
- IF local_address=IPv6-SI
remote_address=IPv6-SC
Next Layer Protocol=UDP
local_port=1701
remote_port=ANY (PFP=1)
Then use SA ESP transport mode
Initiate using IDi = user_2 to address IPv6-SC
SPD for Softwire Concentrator:
Softwire Concentrator SPD-S
- IF name="l2tp_spd_entry"
local_address=IPv6-SC
remote_address=ANY (PFP=1)
Next Layer Protocol=UDP
local_port=1701
remote_port=ANY (PFP=1)
Then use SA ESP transport mode
4. Mesh Security Guidelines
4.1. Deployment Scenario
In the softwire "Mesh" solution ([RFC4925], [RFC5565]), it is
required to establish connectivity to access network islands of one
address family type across a transit core of a differing address
family type. To provide reachability across the transit core, AFBRs
are installed between the access network island and transit core
network. These AFBRs can perform as Provider Edge routers (PE)
within an autonomous system or perform peering across autonomous
systems. The AFBRs establish and encapsulate softwires in a mesh to
the other islands across the transit core network. The transit core
network consists of one or more service providers.
In the softwire "Mesh" solution, a pair of PE routers (AFBRs) use BGP
to exchange routing information. AFBR nodes in the transit network
are Internal BGP speakers and will peer with each other directly or
via a route reflector to exchange SW-encap sets, perform softwire
signaling, and advertise AF access island reachability information
and SW-NHOP information. If such information is advertised within an
autonomous system, the AFBR node receiving them from other AFBRs does
not forward them to other AFBR nodes. To exchange the information
among AFBRs, the full mesh connectivity will be established.
The connectivity between CE and PE routers includes dedicated
physical circuits, logical circuits (such as Frame Relay and ATM),
and shared medium access (such as Ethernet-based access).
When AFBRs are PE routers located at the edge of the provider core
networks, this architecture is similar to the L3VPN described in
[RFC4364]. The connectivity between a CE router in an access island
network and a PE router in a transit network is established
statically. The access islands are enterprise networks accommodated
through PE routers in the provider's transit network. In this case,
the access island networks are administrated by the provider's
autonomous system.
The AFBRs may have multiple connections to the core network, and also
may have connections to multiple client access networks. The client
access networks may connect to each other through private networks or
through the Internet. When the client access networks have their own
AS number, a CE router located inside access islands forms a private
BGP peering with an AFBR. Further, an AFBR may need to exchange full
Internet routing information with each network to which it connects.
4.2. Trust Relationship
All AFBR nodes in the transit core MUST have a trust relationship or
an agreement with each other to establish softwires. When the
transit core consists of a single administrative domain, it is
assumed that all nodes (e.g., AFBR, PE, or Route Reflector, if
applicable) are trusted by each other.
If the transit core consists of multiple administrative domains,
intermediate routers between AFBRs may not be trusted.
There MUST be a trust relationship between the PE in the transit core
and the CE in the corresponding island, although the link(s) between
the PE and the CE may not be protected.
4.3. Softwire Security Threat Scenarios
As the architecture of the softwire mesh solution is very similar to
that of the provider-provisioned VPN (PPVPN). The security threat
considerations on the PPVPN operation are applicable to those in the
softwire mesh solution [RFC4111].
Examples of attacks to data packets being transmitted on a softwire
tunnel include:
1. An adversary may try to discover confidential information by
sniffing softwire packets.
2. An adversary may try to modify the contents of softwire packets.
3. An adversary may try to spoof the softwire packets that do not
belong to the authorized domains and to insert copies of once-
legitimate packets that have been recorded and replayed.
4. An adversary can launch denial-of-service (DoS) attacks by
deleting softwire data traffic. DoS attacks of the resource
exhaustion type can be mounted against the data plane by spoofing
a large amount of non-authenticated data into the softwire from
the outside of the softwire tunnel.
5. An adversary may try to sniff softwire packets and to examine
aspects or meta-aspects of them that may be visible even when the
packets themselves are encrypted. An attacker might gain useful
information based on the amount and timing of traffic, packet
sizes, source and destination addresses, etc.
The security attacks can be mounted on the control plane as well. In
the softwire mesh solution, softwire encapsulation will be set up by
using BGP. As described in [RFC4272], BGP is vulnerable to various
security threats such as confidentiality violation; replay attacks;
insertion, deletion, and modification of BGP messages; man-in-the-
middle attacks; and denial-of-service attacks.
4.4. Applicability of Security Protection Mechanism
Given that security is generally a compromise between expense and
risk, it is also useful to consider the likelihood of different
attacks. There is at least a perceived difference in the likelihood
of most types of attacks being successfully mounted in different
deployment.
The trust relationship among users in access networks, transit core
providers, and other parts of networks described in Section 4.2 is a
key element in determining the applicability of the security
protection mechanism for the specific softwire mesh deployment.
4.4.1. Security Protection Mechanism for Control Plane
The "Softwire Problem Statement" [RFC4925] states that the softwire
mesh setup mechanism to advertise the softwire encapsulation MUST
support authentication, but the transit core provider may decide to
turn it off in some circumstances.
The BGP authentication mechanism is specified in [RFC2385]. The
mechanism defined in [RFC2385] is based on a one-way hash function
(MD5) and use of a secret key. The key is shared between a pair of
peer routers and is used to generate 16-byte message authentication
code values that are not readily computed by an attacker who does not
have access to the key.
However, the security mechanism for BGP transport (e.g., TCP-MD5) is
inadequate in some circumstances and also requires operator
interaction to maintain a respectable level of security. The current
deployments of TCP-MD5 exhibit some shortcomings with respect to key
management as described in [RFC3562].
Key management can be especially cumbersome for operators. The
number of keys required and the maintenance of keys (issue/revoke/
renew) has had an additive effect as a barrier to deployment. Thus,
automated means of managing keys, to reduce operational burdens, is
available in the BGP security system ([BGP-SEC], [RFC4107]).
Use of IPsec counters the message insertion, deletion, and
modification attacks, as well as man-in-the-middle attacks by
outsiders. If routing data confidentiality is desired, the use of
IPsec ESP could provide that service. If eavesdropping attacks are
identified as a threat, ESP can be used to provide confidentiality
(encryption), integrity, and authentication for the BGP session.
4.4.2. Security Protection Mechanism for Data Plane
To transport data packets across the transit core, the mesh solution
defines multiple encapsulations: L2TPv3, IP-in-IP, MPLS (LDP-based
and RSVP-TE based), and GRE. To securely transport such data
packets, the softwire MUST support IPsec tunnel.
IPsec can provide authentication and integrity. The implementation
MUST support ESP with null encryption [RFC4303] or else AH (IP
Authentication Header) [RFC4302]. If some part of the transit core
network is not trusted, ESP with encryption MAY be applied.
Since the softwires are created dynamically by BGP, the automated key
distribution MUST be performed by IKEv2 [RFC4306] with either pre-
shared key or public key management. For dynamic softwire IPsec
tunnel creation, the pre-shared key will be the same in all routers.
Namely, pre-shared key indicates here "group key" instead of
"pairwise-shared" key.
If security policy requires a stronger key management, the public key
SHOULD be used. If a public key infrastructure is not available, the
IPsec Tunnel Authentication sub-TLV specified in [RFC5566] MUST be
used before SA is established.
If the link(s) between the user's site and the provider's PE is not
trusted, then encryption MAY be used on the PE-CE link(s).
Together with the cryptographic security protection, the access-
control technique reduces exposure to attacks from outside the
service provider networks (transit networks). The access-control
technique includes packet-by-packet or packet-flow-by-packet-flow
access control by means of filters as well as by means of admitting a
session for a control/signaling/management protocol that is being
used to implement softwire mesh.
The access-control technique is an important protection against
security attacks of DoS, etc., and a necessary adjunct to
cryptographic strength in encapsulation. Packets that match the
criteria associated with a particular filter may be either discarded
or given special treatment to prevent an attack or to mitigate the
effect of a possible future attack.
5. Security Considerations
This document discusses various security threats for the softwire
control and data packets in the "Hubs and Spokes" and "Mesh" time-to-
market solutions. With these discussions, the softwire security
protocol implementations are provided by referencing "Softwire
Problem Statement" [RFC4925], "Securing L2TP using IPsec" [RFC3193],
"Security Framework for PPVPNs" [RFC4111], and "Guidelines for
Specifying the Use of IPsec" [RFC5406]. The guidelines for the
security protocol employment are also given considering the specific
deployment context.
Note that this document discusses softwire tunnel security protection
and does not address end-to-end protection.
6. Acknowledgments
The authors would like to thank Tero Kivinen for reviewing the
document and Francis Dupont for substantive suggestions.
Acknowledgments to Jordi Palet Martinez, Shin Miyakawa, Yasuhiro
Shirasaki, and Bruno Stevant for their feedback.
We would like also to thank the authors of the Softwire Hub & Spoke
Deployment Framework document [RFC5571] for providing the text
concerning security.
7. References
7.1. Normative References
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,
"Securing L2TP using IPsec", RFC 3193, November 2001.
[RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
"Negotiation of NAT-Traversal in the IKE", RFC 3947,
January 2005.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
7.2. Informative References
[BGP-SEC] Christian, B. and T. Tauber, "BGP Security Requirements",
Work in Progress, November 2008.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC3562] Leech, M., "Key Management Considerations for the TCP MD5
Signature Option", RFC 3562, July 2003.
[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.
[RFC4111] Fang, L., "Security Framework for Provider-Provisioned
Virtual Private Networks (PPVPNs)", RFC 4111, July 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4225] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, December 2005.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, January 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, October 2006.
[RFC4891] Graveman, R., Parthasarathy, M., Savola, P., and H.
Tschofenig, "Using IPsec to Secure IPv6-in-IPv4 Tunnels",
RFC 4891, May 2007.
[RFC4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire
Problem Statement", RFC 4925, July 2007.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, March 2008.
[RFC5406] Bellovin, S., "Guidelines for Specifying the Use of IPsec
Version 2", BCP 146, RFC 5406, February 2009.
[RFC5565] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[RFC5566] Berger, L., White, R., and E. Rosen, "BGP IPsec Tunnel
Encapsulation Attribute", RFC 5566, June 2009.
[RFC5571] Storer, B., Pignataro, C., Dos Santos, M., Stevant, B.,
Toutain, L., and J. Tremblay, "Softwire Hub and Spoke
Deployment Framework with Layer Two Tunneling Protocol
Version 2 (L2TPv2)", RFC 5571, June 2009.
Appendix A. Examples
If the old IPsec architecture [RFC2401] and IKE [RFC2409] are used,
the SPD examples in [RFC3193] are applicable to the "Hub & Spokes"
model. In this model, the initiator is always the client (SI), and
the responder is the SC.
A.1. IPv6-over-IPv4 Softwire with L2TPv2 Example for IKE
IPv4 addresses of the softwire initiator and concentrator are denoted
by IPv4-SI and IPv4-SC, respectively. If NAT traversal is used in
IKE, UDP source and destination ports are 4500. In this SPD entry,
IKE refers to UDP port 500. * denotes wildcard and indicates ANY port
or address.
Local Remote Protocol Action
----- ------ -------- ------
IPV4-SI IPV4-SC ESP BYPASS
IPV4-SI IPV4-SC IKE BYPASS
IPv4-SI IPV4-SC UDP, src 1701, dst 1701 PROTECT(ESP,
transport)
IPv4-SC IPv4-SI UDP, src * , dst 1701 PROTECT(ESP,
transport)
Softwire Initiator SPD
Remote Local Protocol Action
------ ------ -------- ------
* IPV4-SC ESP BYPASS
* IPV4-SC IKE BYPASS
* IPV4-SC UDP, src * , dst 1701 PROTECT(ESP,
transport)
Softwire Concentrator SPD
A.2. IPv4-over-IPv6 Softwire with Example for IKE
IPv6 addresses of the softwire initiator and concentrator are denoted
by IPv6-SI and IPv6-SC, respectively. If NAT traversal is used in
IKE, UDP source and destination ports are 4500. In this SPD entry,
IKE refers to UDP port 500. * denotes wildcard and indicates ANY port
or address.
Local Remote Protocol Action
----- ------ -------- ------
IPV6-SI IPV6-SC ESP BYPASS
IPV6-SI IPV6-SC IKE BYPASS
IPv6-SI IPV6-SC UDP, src 1701, dst 1701 PROTECT(ESP,
transport)
IPv6-SC IPv6-SI UDP, src * , dst 1701 PROTECT(ESP,
transport)
Softwire Initiator SPD
Remote Local Protocol Action
------ ------ -------- ------
* IPV6-SC ESP BYPASS
* IPV6-SC IKE BYPASS
* IPV6-SC UDP, src * , dst 1701 PROTECT(ESP,
transport)
Softwire Concentrator SPD
Authors' Addresses
Shu Yamamoto
NICT/KDDI R&D Labs
1-13-16 Hakusan, Bunkyo-ku
Tokyo 113-0001
Japan
Phone: +81-3-3868-6913
EMail: shu@nict.go.jp
Carl Williams
KDDI R&D Labs
Palo Alto, CA 94301
USA
Phone: +1-650-279-5903
EMail: carlw@mcsr-labs.org
Hidetoshi Yokota
KDDI R&D Labs
2-1-15 Ohara
Fujimino, Saitama 356-8502
Japan
Phone: +81-49-278-7894
EMail: yokota@kddilabs.jp
Florent Parent
Beon Solutions
Quebec, QC
Canada
EMail: Florent.Parent@beon.ca