Network Working Group S. Yamamoto
Internet-Draft NICT/KDDI R&D Labs
Intended status: Standards Track C. Williams
Expires: September 11, 2009 KDDI R&D Labs
F. Parent
Beon Solutions
H. Yokota
KDDI R&D Labs
March 10, 2009
Softwire Security Analysis and Requirements
draft-ietf-softwire-security-requirements-07
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Abstract
This document describes the security guidelines for the softwire
"Hubs and Spokes" and "Mesh" solutions. Together with the discussion
of the softwire deployment scenarios, the vulnerability to the
security attacks is analyzed to provide the security protection
mechanism such as authentication, integrity and confidentiality to
the softwire control and data packets.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . 10
3.4.1. Authentication . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . 14
3.5.3. Inter-Operability Guidelines . . . . . . . . . . . . . 15
3.5.4. IPsec Filtering Details . . . . . . . . . . . . . . . 15
4. Mesh Security Guidelines . . . . . . . . . . . . . . . . . . . 18
4.1. Deployment Scenario . . . . . . . . . . . . . . . . . . . 18
4.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 19
4.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 20
4.4. Applicability of Security Protection Mechanism . . . . . . 20
4.4.1. Security Protection Mechanism for Control Plane . . . 21
4.4.2. Security Protection Mechanism for Data Plane . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . 25
A.1. IPv6 over IPv4 Softwire with L2TPv2 example for IKE . . . 25
A.2. IPv4 over IPv6 Softwire with example for IKE . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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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 the connectivity to enable global reachability
of both address families by reusing or extending exisiting
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 such two softwire solution spaces such as
"Hubs and Spokes" and "Mesh" scenarios . "Hubs and Spokes" and
"Mesh" problems are described in [RFC4925] Section 2 and Section 3,
respectively. The protocols selected for softwire connectivity
require the security consideration on more specific deployment
scenarios for each solution.
Layer Two Tunneling Protocol (L2TPv2) is selected as 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 appropriate security
protocol such as IPsec described in [RFC3193]. Note that other
adequate security mechanisms are applicable, the security protocol
for softwire is not necessarily mandated. This document provides the
implementation guidance and proper usage of IPsec as the security
protection mechanism by considering the security vulnerabilities in
"Hubs and Spokes" scenario. The new implementation SHOULD use IKEv2
in the key management protocol for IPsec because of more reliable
protocol and integration of required protocols in a sigle platform.
The softwire "Mesh" solution should support various levels of
security mechanism to protect the data packets from an attacker being
transmitted on a softwire tunnel from the access networks with one
address family across the transit core operating with different
address family [RFC4925]. The security mechanism for the control
plane is also required to be protected from control data
modification, spoofing attack, etc. In the "Mesh" solution, BGP is
used for distributing softwire routing information in the transit
core. As the security considerations for BGP is being discussed in
other working groups, this document provides general guidelines for
the deployment scenario being envisaged.
2. Terminology
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2.1. Abbreviations
The terminology is based on the softwire problem statement document
[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 CE routers, perform
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 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 transit core
network that interfaces with CE in 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.
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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, the 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 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 network. 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 taking account of the nomadic case.
In order to securely connect legitimate SI and SC each other, the
authentication process between SI and SC is performed normally using
AAA servers.
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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: Hubs and Spokes model
The AAA server shown in Figure 1 interacts with the SC which acts as
an 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 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 common deployed AAA
solution. L2TPv2 used in softwire supports PPP and L2TP
authentications which 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.
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Case 1: The SI connects to the SC that belongs to the home network
service provider via the home access provider network operating
different address family. It is assumed that the home access network
and the home network providing SC are under the same administrative
system.
Note that the IP address of the host device, in which 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 network via the visited access network. For the
nomadic case, the SI/user does not subscribe to the visited access
provider. For the network access through the public network such as
WiFi hot-spots, the home network service provider does not have the
trust relationship with the access network.
Note that the IP address of the host device, in which 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
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. WiFi hot-spot network provided by the different ISP.
The SI does not connect to the SC in the home network, for example,
due to the geographical reason. 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 SI resides, is
provided the visited network service provider's policy.
3.2. Trust Relationship
The establishment of the trust relationship between SI and SC is
different for three cases. The security consideration 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
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for the SI authentication. Since the SI 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, the access to the home network
service provider through the visited access network provider is
allowed. The trust relationship between SI and the SC in the home
network must be established. When the visited access network is a
public network, the 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 Case3, if the SI roams into a different network service provider's
administrative domain and 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, 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 be weaken
when the hop counts of the proxy chain is longer. For example, the
accounting information exchanged among AAA proxies is attracive 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], [I-D.ietf-softwire-hs-
framework-l2tpv2]. In the softwire "Hubs and Spokes" model, L2TPv2
is used in a voluntary tunnel model only. The SI acts as a L2TP
Access Concentrator (LAC) and PPP endpoint. The L2TPv2 tunnel is
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always initiated from the SI.
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 PANA [RFC4016], NSIS [RFC4081],
and Routing Protocols [RFC4593] are applicable here as well and
should be used as reference.
First, the SI resided in the customer network sends Start-Control-
Connection-Request(SCCRQ) packet to the SC for the initiation of the
softwire. L2TPv2 offers an optional CHAP-like tunnel authentication
system during control connection establishment. This requires a
shared secret between the SI and SC and no key management is offered
for this L2TPv2.
When L2TPv2 control connection is established, the SI and SC
optionally enter 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 the 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 as the
consequence of the following adversary actions.
Adversary attacks on softwire include:
1. An adversary may try to discover identities 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.
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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.
In environments where the link is shared without the cryptographic
protection and the 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 dinial 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 compromisd router.
The deployment of ingress filtering is able to control the 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. This causes DoS attack. The inner address
ingress filtering can reject invalid inner source address. Without
inner address ingress filtering, another kind of attack can happen.
The malicious users from another ISP could start using its tunneling
infrastructure to get free inner address connectivity, transforming
effectively the ISP into an inner address transit provider.
While the ingress filtering does not provide the complete protection
in the case an address spoofing has been happened. To protect
address spoofing, authentication MUST be implemented in the tunnel
encapsulation.
3.4. Softwire Security Guidelines
Based on the security threat analysis in Section 3.3 in this
document, the softwire security protocol must support the following
protections.
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1. Softwire control messages between the SI and SC MUST BE protected
against eavesdropping and spoofing attacks.
2. Softwire security protocol MUST be able to protect itself against
replay attacks.
3. 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. Softwire security protocol MUST be able to securely bind the
authenticated session to the device identifier of the client, to
prevent service theft.
5. 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 MUST be supported and confidentiality MAY be supported.
The softwire problem statement [RFC4925] provides some requirements
for "Hubs and Spoke" solution that are taken into account in defining
the security protection mechanisms.
1. Control and/or data plane must be able to provide full payload
security when desired.
2. Deployed technology must be very strongly considered.
This additional security protection must be separable from the
softwire tunneling mechanism.
Note that 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. 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
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authentication protocols are available, the security threats must be
considered to choose the protocol.
For example, the SI/user using Password Authentication Protocol (PAP)
access to the SC with the cleartext password. In many circumstances,
this represents a large security risk. The adversary may spoof as a
legitimate user by using the stolen password. Challenge Handshake
Authentication Protocol (CHAP) [RFC1994] encrypts a password with
"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 avoid the
rogue SC.
When the SI established a connection to the SC through a public
network, the SI may want to a proof of the SC identity. Softwire
MUST support mutual authentication to allow for such scenario.
In some circumstances, however, the service provider may decide to
allow non-authenticated connection
[I-D.ietf-softwire-hs-framework-l2tpv2]. 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 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 (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 for
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unprotected on a public IP network. If other appropriate protected
mechanism has been already applied, PPP CHAP authentication may be
used.
Optionally, other authentication methods such as PAP, MS-CHAP EAP MAY
be supported.
3.4.1.2. L2TPv2 Authentication
L2TPv2 provides an optional CHAP-like tunnel authentication during
the control connection establishment [RFC2661, 5.1.1]. L2TPv2
authentication MUST NOT be used for unprotected on a public IP
network as the same restriction applied to PPP CHAP.
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. Internet Key Exchange (IKE)
protocol[RFC4306] MUST be supported for authentication, security
association negotiation and key management for IPsec. The
applicability of different version 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 softwire "Hubs and Spokes" solution implemented by L2TPv2 is
used in untrustworthy network, softwire MUST be protected by
appropriate security protocol such as IPsec. This section provides
guidelines for the usage of IPsec in L2TPv2 based softwire.
[RFC3193] discusses how L2TP can use IPsec to provide tunnel
authentication, privacy protection, integrity checking and replay
protection. Since its publication, the revision to IPsec protocols
have been published (IKEv2 [RFC4306], ESP [RFC4303], NAT-traversal
for IKE [RFC3947] and ESP[RFC3948]).
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.
IKEv2 [RFC4306] integrates NAT-traversal. IKEv2 also supports EAP
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authentication with the authentication using shared secrets (pre-
shared key) or public key signature (certificate). The selection of
pre-shared key and certificate depends on the scale of the network
for softwire to be deployed as described in Section 3.5.2. However,
pre-shared key and certificates only support the machine
authentication. When both machine and user authentications are
required, 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 more reliable protocol than IKE [RFC2409]in terms of the
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 existing
authentication infrastructure and credential database. This enables
the 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 runs between the SI and the AAA server create
a session key and it 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
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keys becomes 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],
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, with 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 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. [RFC3948] section 3.1.2 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 the 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 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
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been revised and simplified in IKEv2 [RFC4306]. The following
sections provides 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 PFP flag to
specify SA and the port number can be selected with Traffic Selector
with TSr 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.
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 of ESP protecting L2TPv2 carrying IPv6 packet
is shown in Table 1 by using the similar Table 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.
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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 are only example configurations. The PAD entries
specify that the IP address in the traffic selector payload
(SC_address and SI_address) is used for matching against the SPD.
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 remote address SI_address
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 of ESP protecting L2TPv2 carrying IPv4 packet
is shown in Table 2 by using similar one in [RFC4891].
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+----------------------------+------------------------------------+
| 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 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],
[I-D.ietf-softwire-mesh-framework], 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
access network island and transit core network. These AFBRs can
perform as Provider Edge routers (PE) within an autonomous system or
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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 is similar architecture of the L3VPN described in
[RFC4364]. The connectivity between a CE router in access island
network and a PE router in transit network is established by static
way. 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 the multiple connections to the core network, and
also may have the connections to the multiple client access networks.
The client access networks may connect 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 a 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 with each other.
If the transit core consists of multiple administrative domains,
intermediate routers between AFBRs may not be trusted.
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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 softwire mesh solution is very similar to that
of the provider provisioned VPN (PPVPN). The security threats
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 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) attack 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, sources and destination addresses, etc.
The security attacks can be mounted on the control plane as well. In
softwire mesh solution, softwires encapsulation will be setup by
using BGP. As described in [RFC4272], BGP is vulnerable to various
security threats such as confidential violation, replay attacks,
insertion, deletion and modification of BGP messages, main-in-the-
middle, and denial-of-service.
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
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deployment.
The trust relationship among users in access networks, transit core
provider, and other parts of networks described in section 4.2 is a
key element in determining the applicability of 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 of 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 BGP security system [I-D.ietf-rpsec-bgpsecrec],
[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 attack is
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 packet,
the softwire must support IPsec tunnel.
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IPsec can provide authentication and integrity. The implementation
MUST support ESP with null encryption RFC4303. If some part of the
transit core network is not trusted, ESP with encryption may be
applied.
The automated key distribution can be performed by IKE with the pre-
shared key management. But the implementation of IPsec with
automatic key management depends on the operational requirements, for
example, the scalability requirement, etc.
To provide replay protection, automated key management system using
IKEv2 can be used. IKEv2 can be applied using shared secrets for
authentication when the number of BGP peers is small. When the
number of BGP peers is large, managing the shared secrets on all
peers does not scale. In this scenario, public-key digital signature
or key encryption authentication in IKE SHOULD be used.
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 the 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 "Hubs and Spokes" and "Mesh" time-to-
market solutions. With these discussions, the softwire security
protocol implementations are provided referencing to Softwire Problem
Statement [RFC4925], Securing L2TP using IPsec [RFC3193], Security
Framework for PPVPNs [RFC4111], and Guidelines for Mandating the Use
of IPsec [RFC5406]. The guidelines for the security protocol
employment are also given considering the specific deployment
context.
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Note that this document discusses the softwire tunnel security
protection and does not address the end-to-end protection.
6. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC5226].
7. Acknowledgments
The authors would like to thank Tero Kivinen for reviewing the
document and Francis Dupont for substative 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 Softwire Hub & Spoke
Deployment Framework document for providing the text concerning
security.
8. References
8.1. 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",
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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.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
8.2. Informative References
[I-D.ietf-rpsec-bgpsecrec]
Christian, B. and T. Tauber, "BGP Security Requirements",
draft-ietf-rpsec-bgpsecrec-10 (work in progress),
November 2008.
[I-D.ietf-softwire-hs-framework-l2tpv2]
Storer, B., Pignataro, C., Santos, M., Stevant, B., and J.
Tremblay, "Softwire Hub & Spoke Deployment Framework with
L2TPv2", draft-ietf-softwire-hs-framework-l2tpv2-12 (work
in progress), March 2009.
[I-D.ietf-softwire-mesh-framework]
Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", draft-ietf-softwire-mesh-framework-06 (work in
progress), February 2009.
[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.
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[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.
[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.
Appendix A.
If the old IPsec architecture [RFC2401] and IKE [RFC2409] are used,
the SPD examples in [RFC3193] is applicable to "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,
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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)
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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
Florent Parent
Beon Solutions
Quebec, QC
Canada
Phone: +1-418-353-0857
Email: Florent.Parent@beon.ca
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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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