Network Working Group S. Yamamoto Internet-Draft C. Williams Expires: September 6, 2007 KDDI R&D Labs F. Parent consultant H. Yokota KDDI R&D Labs March 5, 2007 Softwire Security Analysis and Requirements draft-ietf-softwire-security-requirements-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 13, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). 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 Yamamoto, et al. Expires September 13, 2007 [Page 1] Internet-Draft Softwire security considerations March 2007 mechanism such as authentication, integrity and confidentiality to the softwire control and data packets. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Hubs and Spokes Security Guidelines . . . . . . . . . . . . . 4 3.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 5 3.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 6 3.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 7 3.4. Softwire Security Guidelines . . . . . . . . . . . . . . . 10 3.4.1. Authentication . . . . . . . . . . . . . . . . . . . . 11 3.4.2. Softwire Security Protocol . . . . . . . . . . . . . . 11 3.5. Guidelines for Usage of IPsec in Softwire . . . . . . . . 12 3.5.1. Authentication Issues . . . . . . . . . . . . . . . . 12 3.5.2. IPsec Pre-Shared Keys for Authentication . . . . . . . 13 3.5.3. Inter-operability guidelines . . . . . . . . . . . . . 13 3.5.4. IPsec filtering details . . . . . . . . . . . . . . . 13 3.5.5. IPsec SPD entries example . . . . . . . . . . . . . . 14 4. Mesh Security Guidelines . . . . . . . . . . . . . . . . . . . 15 4.1. Deployment Scenario . . . . . . . . . . . . . . . . . . . 15 4.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 16 4.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 17 4.3.1. Attacks on the Control Plane . . . . . . . . . . . . . 17 4.3.2. Attacks on the Data Plane . . . . . . . . . . . . . . 18 4.4. Applicability of Security Protection Mechanism . . . . . . 18 4.5. Guidelines for Usage of Security Protection Mechanism . . 19 4.5.1. Security Protection Mechanism for Control Plane . . . 19 4.5.2. Security Protection Mechanism for Data Plane . . . . . 21 5. Security Considerations . . . . . . . . . . . . . . . . . . . 21 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.1. Normative References . . . . . . . . . . . . . . . . . . . 22 6.2. Informative References . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . . . 26 Yamamoto, et al. Expires September 13, 2007 [Page 2] Internet-Draft Softwire security considerations March 2007 1. Introduction TThe 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 in such two Softwire solution spaces such as "Hubs and Spokes" and "Mesh" scenarios Section 3andSection 4described in [I-D.ietf-softwire-problem-statement]. The protocols selected for Softwire connectivity require the Security consideration on more specific deployment scenarios for each solution. Layer Two Tunneling Protocol (L2TPv2) selected for "Hubs and Spokes" solution MUST use IPsec if the secure communication is required[RFC3193]. This document provides the implementation guidance (and proper usage) of IPsec as the security protection mechanism by considering the various security vulnerabilities in "Hubs and Spokes" scenarios. IKEv2 SHOULD be used in the key management protocol for IPsec as the reason of the future proven technology as opposed to IKEv1. In the "Mesh" solution, Multi-Protocol Border Gateway Protocol (MP- BGP) is used as the signaling protocol to establish the Softwire connectivities among the access islands with same address families across the transit core to exchange the reachability information and softwire encapsulation attributes. As BGP is vulnerable to various security attakcs[RFC4272], the adequate security protection mechanism MUST be implemented in BGP. When the networks associated with Softwire connectivity include untrusted devices or have possibility of connections of those devices, the proper security protection mechanism MUST be used for BGP signaling together with filering at the Softwire end-point nodes and the secure encapsulation method MUST be used for data traffic. This document provides the implementation guidance of IPsec as the security protection mechanism for BGP by referencing to the security framework for the Provider-Provisioned Virtual Private Networks (PPVPNs) [RFC4111]. 2. Terminology The terminology is based on the softwire problem statement document [I-D.ietf-softwire-problem-statement]. AF(i) - Address Family. IPv4 or IPv6. Notation used to indicate Yamamoto, et al. Expires September 13, 2007 [Page 3] Internet-Draft Softwire security considerations March 2007 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 . 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. 3. Hubs and Spokes Security Guidelines Yamamoto, et al. Expires September 13, 2007 [Page 4] Internet-Draft Softwire security considerations March 2007 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 softwire initiator 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 models, the following three cases as shown in Figure 1 should be considered. In these cases, the automated discovery of the softwire concentrator (SC) may be used. But in this document, the information on the SC such as the DNS name or IP address is assumed to be configured by the user, or by the provider of the softwire initiator in advance. Case 1: The SI connects to the SC that belongs to the home network service provider via the home access provider network. The IP address of the host may be changed periodically due to the home network service provider's policy. Case 2: The SI connects to the SC that belongs to the home network service provider via the visited access network. This is typical of nomadic access use case. The host does not subscribe to the visited access provider, but this provider has some roaming agreement with the home network service provider of the host. The IP address of the host may be changed periodically due to the home network service provider's policy. Case 3: The SI connects to the SC that belongs to the visited network service provider via the visited access network. This is also typical of nomadic access use case. The host does not subscribe to the visited network service provider, but this provider has some roaming agreement with the home network service provider of the host. If this is the case, the IP address of the host is determined by the visited network service provider's policy. The trust relationship for these three cases will also be different. The security consideration must take them into account. In particular, to allow cases 2 and 3, the authentication infrastructure Yamamoto, et al. Expires September 13, 2007 [Page 5] Internet-Draft Softwire security considerations March 2007 between the SI and the SC is needed to establish the trust relationship. The softwire problem statement states that the softwire solution must be able to be integrated with commonly deployed AAA solution. In these cases, AAA interactions between the home network service provider and visited access/service provider should be considered. The details of this scenario are given in Section Section 3.2. 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 3.2. Trust Relationship To perform authentication between the SC and the SI, the AAA server needs to be involved. One or more AAA servers should reside in the same administrative domain as the SC to authenticate the SI. When the SI is mobile, it may roam from the home ISP network to another, e.g. a WiFi hot-spot network. In such a situation, the SI may not always connect to the same SC. From the SI's viewpoint, the AAA server that is in the same administrative domain is called the home Yamamoto, et al. Expires September 13, 2007 [Page 6] Internet-Draft Softwire security considerations March 2007 AAA server and those not in the same administrative domain are called visited AAA servers. The trust relationships between those nodes are as follows: It can be assumed that the SC and the AAA in the same administrative domain share a trust relationship. When the SC needs to authenticate the SI, the SC communicates with the AAA server to request authentication and/or to obtain security information. If the SI roams into a network that is not in the same administrative domain, the visited AAA server communicates with the home AAA server that has the SI's security information. Therefore, the communication between the SC and the AAA server must be protected. It can be usually assumed that the home and visited AAA servers share a trust relationship and the connection between them is protected. It can be assumed that the SI and the home AAA server share a trust relationship. The home AAA server provides security information on the SI when it is requested by the visited AAA server. The SI and the visited AAA server do not usually have a trust relationship; however, if the SI can confirm that the home AAA server is involved with the authentication of the SI and the visited AAA server does not alter security information from the home AAA server, the visited AAA server can be trusted by the SI. The communication between the SI, the home and visited AAA servers must be protected. The SI and the SC do not necessarily share a trust relationship especially when the SI roams into a different administrative domain. When they are mutually authenticated by means of e.g. AAA servers, they can start trusting each other. Unless authentication is successfully performed, the softwire protocol should not be initiated. 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 attack. 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 Layer2Tunneling 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 Softwire Initiator (SI) acts as a Yamamoto, et al. Expires September 13, 2007 [Page 7] Internet-Draft Softwire security considerations March 2007 L2TP Access Concentrator (LAC) and PPP endpoint. The L2TPv2 tunnel is 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, SI resided in the customer network sends Start-Control- Connection-Request(SCCRQ) packet to SC for the initiation of Softwire. Optionally, L2TP exchanges Challenge and Response AVPs for tunnel mutual authentication in L2TPv2 tunnel creation. For the CHAP authentication key, L2TPv2 protocol does not provide the key management facilities. Once L2TPv2 process has been completed, 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, which can be interworked with AAA. Other authentication PAP authentication, MS-CHAP, and EAP MAY be supported. But 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 access is granted to the SI, other protocols start for network configuration and the node in the SI side will exchange data with other nodes in the network connected through SC. 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 Yamamoto, et al. Expires September 13, 2007 [Page 8] Internet-Draft Softwire security considerations March 2007 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. In environments where the link is shared without cryptographic protections and the weak authentication or one-way authentication is used, these security attacks can be mounted on softwire control and data packets. To access the SC through the public networks, any node can pretend to be a SC, if there is no prior trust relationship between SI and SC. In this case, an adversary may impersonate the SC to intercept traffic ("rogue" softwire concentrator). The rogue SC captures all of necessary information (including keys if security is present) of a legitimate Softwire node and remove the message of the subgroup of the network. The rogue SC can introduce a black hole attack in which the attacker sends out forged routing packets and setup a route to some destination via itself and when the actual data packets get in they are simply dropped, forming a black hole at the SC - where data enters but never leaves. Another possibility is for the attacker to forge routes pointing into an area where the destination node is not located. Everything will be routed into this area but nothing will leave. The deployment of ingress filtering is able to control the malicious users' access. Without specific ingress filtering checks in the decapsulator at 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 this 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. Yamamoto, et al. Expires September 13, 2007 [Page 9] Internet-Draft Softwire security considerations March 2007 3.4. Softwire Security Guidelines Based on the security threat analysis in Section 3.3 in this document, Softwire security protocol must support the following protections. 1. Softwire control messages between the SI and the 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 [I-D.ietf-softwire-problem-statement] 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. Yamamoto, et al. Expires September 13, 2007 [Page 10] Internet-Draft Softwire security considerations March 2007 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. The protocol SHOULD offer mutual authentication in scenarios where the SI requires identity proof from the SC, for example, SI accesses to SC across the public network. In some circumstances, the service provider may decide to allow non- authenticated connection [I-D.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 it 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). 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 unprotected on a public IP network. Optionally, other authentication methods such as PAP, MS-CHAP EAP MAY be supported. 3.4.1.1.1. L2TPv2 Authentication L2TPv2 provides an optional CHAP-like[RFC1994] tunnel authentication during the control connection establishment [RFC2661, 5.1.1]. The same restrictions apply to L2TPv2 authentication and PPP CHAP. 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 for securing softwire control and Yamamoto, et al. Expires September 13, 2007 [Page 11] Internet-Draft Softwire security considerations March 2007 data packets. Internet Key Exchange (IKE) protocol[RFC3947] 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 IKEv1 is used. 3.5. Guidelines for Usage of IPsec in Softwire [RFC3193] discusses how L2TP can use IPsec to provide tunnel authentication, privacy protection, integrity checking and replay protection [RFC4306]. Since it's publication, 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. To meet softwire requirements such as NAT-traversal, NAT- traversal for IKE [RFC3947] and ESP[RFC3948] MUST be supported. IKEv2 [RFC4306] offers NAT-traversal. IKEv2 also supports EAP authentication with the authentication using shared secrets and public key signatures. IKE is more reliable protocol than IKEv1 and the future proof technology. New implementations SHOULD use IKEv2 over IKEv1. There are cases where IKEv1 may be needed, e.g. an existing deployment of clients using L2TPv2 with IKEv1. IKEv2 [RFC4306] supports legacy authentication methods that may be useful in environments where username and password based authentication is already deployed. The following sections will discuss using IPsec to protect L2TPv2 as applied in the softwire "Hubs and Spokes" model. Both IKEv1 (based on [RFC3193]) and IKEv2 examples will be given. 3.5.1. Authentication Issues IPsec implementation using IKEv1 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. When the user identity is required, the extension of IKE is required, for example, Xauth is commonly used but not standardized. Whereas, the 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 Yamamoto, et al. Expires September 13, 2007 [Page 12] Internet-Draft Softwire security considerations March 2007 verified on a per-packet basis. If the AAA server is involved to establish a security association between the SI and SC, a session key can be derived from the authentication between the SI and the AAA server. Such a scenario can be found in [I-D.draft-eronen-ipsec-ikev2-eap-auth-05]. 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 packets. [RFC3193, 5.1] Authentication using pre-shared keys can be used when the number of SI and SC is small. AS the number of SI and SC grow, pre- shared 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. ([RFC3193], 4.1). If 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 must be followed by guidelines given in ([RFC3193] section 3). 3.5.4. IPsec filtering details The IPsec filtering details from [RFC3193] section 4 are applicable to softwire "Hubs and Spokes" model. Although the L2TP specification allows the responder (SC in softwire) to use a new IP address when sending the Start-Control-Connection- Request-Reply (SCCRP), a softwire concentrator implementation SHOULD NOT do this ([RFC3193] section 4). Note that this feature may be needed for "load-balancing" between SCs. Yamamoto, et al. Expires September 13, 2007 [Page 13] Internet-Draft Softwire security considerations March 2007 3.5.5. IPsec SPD entries example The SPD examples in [RFC3193] appendix A can be applied to softwire model. In that case, the initiator is always the client (SI), and responder is the SC. Note that the examples are IKEv1 specific. 3.5.5.1. IPv6 over IPv4 Softwire with L2TPv2 example In this example, the softwire initiator and concentrator are denoted with IPv4 addresses IPv4-SI and IPv4-SC respectively. src dst Protocol Action ----- ------ -------- ------ IPV4-SI IPV4-SC ESP (ports 500,4500) 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 src dst Protocol Action ----- ------ -------- ------ * IPV4-SC ESP (ports 500,4500) BYPASS * IPV4-SC IKE BYPASS * IPV4-SC UDP, src * , dst 1701 PROTECT(ESP,transport) Softwire concentrator SPD 3.5.5.2. IPv4 over IPv6 Softwire with example In this example, the softwire initiator and concentrator are denoted with IPv6 addresses IPv6-SI and IPv6-SC respectively. src dst Protocol Action ----- ------ -------- ------ IPV6-SI IPV6-SC ESP (ports 500,4500) 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) Yamamoto, et al. Expires September 13, 2007 [Page 14] Internet-Draft Softwire security considerations March 2007 Softwire initiator SPD src dst Protocol Action ----- ------ -------- ------ * IPV6-SC ESP (ports 500,4500) BYPASS * IPV6-SC IKE BYPASS * IPV6-SC UDP, src * , dst 1701 PROTECT(ESP,transport) Softwire concentrator SPD 4. Mesh Security Guidelines 4.1. Deployment Scenario In the softwire "Mesh" solution, 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 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, point to multi-point connectivity among AFBRs is dynamically established by announcing the reachability and the encapsulation method using Multiprotocol Extensions for BGP-4 (MP-BGP) [RFC2858]. AFBR nodes 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. For the connectivity between CE and PE routers, the following two cases should be considered. Note that the CE-PE connection includes dedicated physical circuits, logical circuits (such as Frame Relay and ATM), and shared medium access (such as Ethernet-based access). Yamamoto, et al. Expires September 13, 2007 [Page 15] Internet-Draft Softwire security considerations March 2007 Case 1: When AFBRs are PE routers located at the edge of the provider core networks, this is similar architecture of Provider Provisioned PE-based VPN. 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 operated within the provider's autonomous system. When the access island networks have their own AS number, inter-AS model can be applied for the connections among the access island networks. CE routers located inside access islands form a peering relationship with AFBRs in the transit network autonomous system to exchange AF access island reachability information using eBGP. Case 2: As alternative model, a single-stack AF(j) PE node is applicable, the AFBR function of the dual-stack AF(i,j) processing is moved to CE routers located at the edge of a customer site. This is the dual-stack CE model. The CE device has the IP connectivity with service provider's PE device over the access connection. The customer's access network belongs to provider's autonomous system. This model might evolve inter-CE BGP peering to exchange users' AF prefixes/next-hops. For this managed CE-based model, users in access networks have to have a fairly high level of trust that the service provider will properly provision and manage the CE devices. 4.2. Trust Relationship In case 1, all AFBR nodes in the transit core MUST have a trust relationship or an agreement with each other to establish softwires. Within an autonomous system, 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 autonomous systems, intermediate routers between AFBRs may not be trusted when back-to- back AFBRs are not available. 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. For the dual-stack CE model in Case 2, CE nodes of iBGP speakers in the access island network MUST have the trust relationship with each other. In addition, users in the access island networks and the transit core provider MUST have the trust relationship. The security protection mechanism can be applied for CE-to-CE in either a provider-provisioned or a user provisioned model. Note that the user-provisioned CE-CE security protection mechanism is outside the Yamamoto, et al. Expires September 13, 2007 [Page 16] Internet-Draft Softwire security considerations March 2007 scope of this document. 4.3. Softwire Security Threat Scenarios The architecture of softwire mesh solution is very similar to that of the provider provisioned VPN (PPVPN) [RFC 4111]. The security threats considerations on the PPVPN operation are applicable to those in the softwire mesh solution. The security attacks can be mounted on both the control plane and the data plane. In softwire mesh solution, softwires encapsulation will be setup by using MP-BGP. MP-BGP does not change the security issues inherent in BGP. In terms of the control plane security, the general BGP security vulnerabilities are applicable [RFC4272]. 4.3.1. Attacks on the Control Plane BGP is subject to the following attacks [RFC4272]. 1. The routing data carried in BGP is carried in cleartext, so eavesdropping is a possible attack against routing data confidentiality. (confidentiality violations) 2. BGP does not provide for replay protection of its message. (replay) 3. BGP does not provide protection against insertion of messages. However, because BGP uses TCP, when the connection is fully established, message insertion by an outsider would require accurate sequence number prediction or session-stealing attacks.(message insertion) 4. BGP does not provide protection against deletion messages. This attack is more difficult against a mature TCP implementation, but is not entirely out of question. (message deletion) 5. BGP does not provide protection against modification of messages. A modification that was syntactically correct and did not change the length of the TCP payload would in general not be detectable. (message modification) 6. BGP does not provide protection against man-in-the-middle attacks. As BGP does not perform peer entity authentication, it is vulnerable to a man-in-the-middle attack. (man-in-the-middle) 7. While bogus routing data can present a DoS attack on the end systems that are trying to transmit data through network and on the network infrastructure itself, certain bogus information can Yamamoto, et al. Expires September 13, 2007 [Page 17] Internet-Draft Softwire security considerations March 2007 present a DoS on the BGP routing protocol. (denial-of-service) 4.3.2. Attacks on the Data Plane Examples of attacks 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 there and to insert of copies of once-legitimate packets that have been recorded and replayed. 4. An adversary can launch Denial-of-Service 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. 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 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. The Softwire Problem Statement [I-D.ietf-softwire-problem-statement] states that the softwire mesh setup mechanism MUST support authentication, but the transit core provider may decide to turn it off in some circumstances. If a routing protocol is used to advertise the softwire encapsulation, it must also support authentication. In the data plane, the softwire must support IPsec and a IPsec Yamamoto, et al. Expires September 13, 2007 [Page 18] Internet-Draft Softwire security considerations March 2007 profile must be defined. In particular, it determines where encryption should be applied, as follows [RFC4111] - 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). - If some part of the transit core network is not trusted, PE-PE path may be encrypted. The access control technique reduces the exposure to attacks from outside the service provider 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. 4.5. Guidelines for Usage of Security Protection Mechanism 4.5.1. Security Protection Mechanism for Control Plane A BGP has the three fundamental vulnerabilities to the security threats [RFC4272]. 1. BGP has no internal mechanism that provides strong protection of the integrity, freshness, and peer authenticity of the message in peer-peer BGP communications. 2. No mechanism has been specified within BGP to validate the authority of a BGP peer to announce NLRI information. 3. No mechanism has been specified within BGP to ensure the authenticity of the path attributes announced by a BGP peer. The BGP specification requires that a BGP must support the authentication mechanism specified in [RFC2385]. However the security mechanism for BGP transport (e.g. TCP-MD5) is inadequate and requires significant operator interaction to maintain a respectable level of security. The current deployments of TCP-MD5 exhibit serious shortcomings with respect of key management as Yamamoto, et al. Expires September 13, 2007 [Page 19] Internet-Draft Softwire security considerations March 2007 described in [RFC3562] The mechanism defined in RFC 2385 is based on a one-way hash function (MD5) and use of a secret key. The key is shared between 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. 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, MUST be available in BGP security systems[I-D.rpsec-bgpsecrec], [RFC4107] 4.5.1.1. IKE/IPsec 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. To provide replay protection, automated key management system using IKEv2 must 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, assuming that the peers have the necessary computation available. 4.5.1.2. Secure BGP The deeper security issues raised by BGP are not addressed by IPsec or any other transmission security mechanism. As cryptographic-based mechanism, both TCP MD5 and IPsec assume that the cryptographic algorithms are secure, that secrets used are protected from exposure and are chosen well so as not to be guessable, that the platforms are securely managed and operated to prevent break-ins, etc. These mechanisms do not prevent attacks that arise from a router's legitimate BGP peers [RFC4272]. The S-BGP countermeasures use IPsec, Public Key Infrastructure (PKI) technology, and a new BGP path attribute ("attestations") to ensure the authenticity and integrity of BGP communication on a point-to- Yamamoto, et al. Expires September 13, 2007 [Page 20] Internet-Draft Softwire security considerations March 2007 point basis, and to validate BGP routing UPDATE's on a source to destination basis[I-D.clynn-s-bgp-protocol]. To implement the secure BGP, Secure Origin BGP (soBGP) and Pretty Secure BGP (psBGP) are also proposed. The detail comparison was made in[Wan05]. 4.5.2. Security Protection Mechanism for Data Plane The protection mechanisms discussed are intended to describe methods by which some security threats can be mitigated. They are not intended as requirements for all softwire mesh implementations. The several of the attacks are outlined in 4.3.2. In order to protect against such threats, the softwire SHOULD provide for replay and integrity protection for softwire data packets and MAY protect confidentiality of data packets. Automated key management in the softwire mesh solution may be necessary per [RFC4107]. IPsec can provide replay protection, integrity and confidentiality of IP data packets, which would protect against most threats identified in 4.3.2. In softwire mesh solution, IPsec tunnels can be established on selective basis using Tunnel SAFI announced by AFBRs. The softwire mesh framework [wu-softwire-mesh-framework] currently supports many tunnel encapsulation type using a "Softwire Mesh Encapsulation Attribute" announced as a BGP Tunnel SAFI [nalawade-softwire-mesh- encap-attribute], [nalawade-kapoor-tunnel-safi]. To protect the data packets using IPsec, AFBRs must be configured with the proper IPsec parameters: ESP ( encryption or NULL encryption), transport or tunnel mode, key management (IKE SA parameters), selectors and SPD. [nalawade-kapoor-tunnel-safi] describes an IPsec Tunnel Information TLV that contains an IKE Identifier. The SPD and selectors must also be defined. These are directly related to the encapsulation type used between the AFBRs (e.g., selectors for L2TP will be different from IPv6-over-IPv4 tunnels). 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 Yamamoto, et al. Expires September 13, 2007 [Page 21] Internet-Draft Softwire security considerations March 2007 Statement [I-D.ietf-softwire-problem-statement], Securing L2TP using IPsec[RFC3193], Security Framework for PPVPNs [RFC4111], and Guidelines for Mandating the Use of IPsec [I-D.bellovin-useipsec].The guidelines for the security protocol employment are also given considering the specific deployment context. Note that this document discusses the softwire tunnel security protection and does not address the end-to-end protection. 6. References 6.1. Normative References [I-D.ietf-softwire-problem-statement] Li, X., "Softwire Problem Statement", draft-ietf-softwire-problem-statement-02 (work in progress), May 2006. [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [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. [RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz, "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000. [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. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. Yamamoto, et al. Expires September 13, 2007 [Page 22] Internet-Draft Softwire security considerations March 2007 [RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to Routing Protocols", RFC 4593, October 2006. 6.2. Informative References [I-D.bellovin-useipsec] Bellovin, S., "Guidelines for Mandating the Use of IPsec", draft-bellovin-useipsec-04 (work in progress), September 2005. [I-D.clynn-s-bgp-protocol] Lynn, C. and K. Seo, "Secure BGP (S-BGP)", draft-clynn-s-bgp-protocol-01 (work in progress), June 2003. [I-D.ietf-softwire-hs-framework-l2tpv2] Storer, B., "Softwires Hub & Spoke Deployment Framework with L2TPv2", draft-ietf-softwire-hs-framework-l2tpv2-03 (work in progress), February 2007. [I-D.rpsec-bgpsecrec] Christian, B. and T. Tauber, "BGP Security Requirements", draft-ietf-rpsec-bgpsecrec-06 (work in progress), April 2006. [I-D.v6ops-tunneling-requirements] Durand, A. and F. Parent, "Requirements for assisted tunneling", draft-durand-v6ops-assisted-tunneling-requirements-00 (work in progress), September 2004. [I-D.white-sobgp-architecture] White, R., "Architecture and Deployment Considerations for Secure Origin BGP (soBGP)", draft-white-sobgp-architecture-02 (work in progress), June 2006. [I-D.wu-softwire-mesh-framework] Wu, J., "Softwire Mesh Framework", draft-wu-softwire-mesh-framework-02 (work in progress), March 2007. [RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth, "Securing L2TP using IPsec", RFC 3193, November 2001. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. Yamamoto, et al. Expires September 13, 2007 [Page 23] Internet-Draft Softwire security considerations March 2007 [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. [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272, January 2006. [Wan05] Wan, T., Wan, P., and S. Kranakis, "A Selective Introduction to Border Gateway Protocol (BGP) Security Issues", URL http://www.scs.carleton.ca/research/ tech_reports/2005/TR-05-08.pdf, August 2005. Authors' Addresses Shu Yamamoto KDDI R&D Labs 2-1-15 Fujimino-shi Saitama, 356-8502 Japan Phone: 81 (49) 278-7311 Email: shu@kddilabs.jp Carl Williams KDDI R&D Labs Palo Alto, CA 94301 USA Phone: +1.650.279.5903 Email: carlw@mcsr-labs.org Florent Parent consultant Quebec, QC Canada Phone: +1 418 265 7357 Email: Florent.Parent@mac.com Yamamoto, et al. Expires September 13, 2007 [Page 24] Internet-Draft Softwire security considerations March 2007 Hidetoshi Yokota KDDI R&D Labs 2-1-15 Ohara Fujimino, Saitama 356-8502 Japan Phone: 81 (49) 278-7894 Email: yokota@kddilabs.jp Yamamoto, et al. Expires September 13, 2007 [Page 25] Internet-Draft Softwire security considerations March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Yamamoto, et al. Expires September 13, 2007 [Page 26]