Network Working Group S. Yamamoto Internet-Draft NICT/KDDI R&D Labs Intended status: Standards Track C. Williams Expires: August 28, 2008 KDDI R&D Labs F. Parent Beon Solutions H. Yokota KDDI R&D Labs February 25, 2008 Softwire Security Analysis and Requirements draft-ietf-softwire-security-requirements-06 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 August 28, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). 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 Yamamoto, et al. Expires August 28, 2008 [Page 1] Internet-Draft Softwire security considerations February 2008 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 . . . . . . . . . . . . 7 3.4. Softwire Security Guidelines . . . . . . . . . . . . . . . 10 3.4.1. Authentication . . . . . . . . . . . . . . . . . . . . 11 3.4.2. Softwire Security Protocol . . . . . . . . . . . . . . 12 3.5. Guidelines for Usage of IPsec in Softwire . . . . . . . . 12 3.5.1. Authentication Issues . . . . . . . . . . . . . . . . 13 3.5.2. IPsec Pre-Shared Keys for Authentication . . . . . . . 13 3.5.3. Inter-Operability Guidelines . . . . . . . . . . . . . 13 3.5.4. IPsec Filtering Details . . . . . . . . . . . . . . . 14 4. Mesh Security Guidelines . . . . . . . . . . . . . . . . . . . 17 4.1. Deployment Scenario . . . . . . . . . . . . . . . . . . . 17 4.2. Trust Relationship . . . . . . . . . . . . . . . . . . . . 18 4.3. Softwire Security Threat Scenarios . . . . . . . . . . . . 19 4.4. Applicability of Security Protection Mechanism . . . . . . 19 4.4.1. Security Protection Mechanism for Control Plane . . . 20 4.4.2. Security Protection Mechanism for Data Plane . . . . . 20 5. Security Considerations . . . . . . . . . . . . . . . . . . . 21 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 8.1. Normative References . . . . . . . . . . . . . . . . . . . 22 8.2. Informative References . . . . . . . . . . . . . . . . . . 23 Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . 24 A.1. IPv6 over IPv4 Softwire with L2TPv2 example for IKE . . . 24 A.2. IPv4 over IPv6 Softwire with example for IKE . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 Intellectual Property and Copyright Statements . . . . . . . . . . 28 Yamamoto, et al. Expires August 28, 2008 [Page 2] Internet-Draft Softwire security considerations February 2008 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 Yamamoto, et al. Expires August 28, 2008 [Page 3] Internet-Draft Softwire security considerations February 2008 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 . 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. Yamamoto, et al. Expires August 28, 2008 [Page 4] Internet-Draft Softwire security considerations February 2008 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 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 SI 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 Yamamoto, et al. Expires August 28, 2008 [Page 5] Internet-Draft Softwire security considerations February 2008 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. In this 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 between the SI and SC is needed to establish the trust relationship. The softwire problem statement[RFC4925] 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 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 Yamamoto, et al. Expires August 28, 2008 [Page 6] Internet-Draft Softwire security considerations February 2008 3.2. Trust Relationship To perform authentication between the SC and 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 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. Yamamoto, et al. Expires August 28, 2008 [Page 7] Internet-Draft Softwire security considerations February 2008 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 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. 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 the AAA server. Other authentication of 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 the 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: Yamamoto, et al. Expires August 28, 2008 [Page 8] Internet-Draft Softwire security considerations February 2008 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. 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. To access the SC through the public networks, any node can pretend to be a SC, if there is no prior trust relationship between the SI and SC. In this case, an adversary may impersonate the SC to intercept traffic (e.g. "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 an 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 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. Yamamoto, et al. Expires August 28, 2008 [Page 9] Internet-Draft Softwire security considerations February 2008 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. 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 Yamamoto, et al. Expires August 28, 2008 [Page 10] Internet-Draft Softwire security considerations February 2008 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. The protocol SHOULD offer mutual authentication in scenarios where the SI requires identity proof from the SC, for example, the SI accesses to the SC across the public network. In some circumstances, 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 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 for unprotected on a public IP network. This means that there is no reason to prohibit PPP CHAP authentication if appropriate protected mechanism has been applied. 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 Yamamoto, et al. Expires August 28, 2008 [Page 11] Internet-Draft Softwire security considerations February 2008 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 [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 authentication with the authentication using shared secrets and public key signatures. 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. 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. Unless otherwise stated, IKEv2 and the new IPsec architecture [RFC4301] is assumed. Yamamoto, et al. Expires August 28, 2008 [Page 12] Internet-Draft Softwire security considerations February 2008 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. Such a scenario can be found in[I-D.eronen-ipsec-ikev2-eap-auth]. 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 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). Yamamoto, et al. Expires August 28, 2008 [Page 13] Internet-Draft Softwire security considerations February 2008 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 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 etnries. 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. Yamamoto, et al. Expires August 28, 2008 [Page 14] Internet-Draft Softwire security considerations February 2008 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. 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 Yamamoto, et al. Expires August 28, 2008 [Page 15] Internet-Draft Softwire security considerations February 2008 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]. Yamamoto, et al. Expires August 28, 2008 [Page 16] Internet-Draft Softwire security considerations February 2008 +----------------------------+------------------------------------+ | 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 Yamamoto, et al. Expires August 28, 2008 [Page 17] Internet-Draft Softwire security considerations February 2008 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. Yamamoto, et al. Expires August 28, 2008 [Page 18] Internet-Draft Softwire security considerations February 2008 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 Yamamoto, et al. Expires August 28, 2008 [Page 19] Internet-Draft Softwire security considerations February 2008 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. Yamamoto, et al. Expires August 28, 2008 [Page 20] Internet-Draft Softwire security considerations February 2008 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 [I-D.bellovin-useipsec]. The guidelines for the security protocol employment are also given considering the specific deployment context. Yamamoto, et al. Expires August 28, 2008 [Page 21] Internet-Draft Softwire security considerations February 2008 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 [RFC2434]. 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. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 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, Yamamoto, et al. Expires August 28, 2008 [Page 22] Internet-Draft Softwire security considerations February 2008 January 2005. [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M. Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC 3948, January 2005. [RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, June 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. 8.2. Informative References [I-D.bellovin-useipsec] Bellovin, S., "Guidelines for Mandating the Use of IPsec Version 2", draft-bellovin-useipsec-07 (work in progress), October 2007. [I-D.eronen-ipsec-ikev2-eap-auth] Tschofenig, H. and P. Eronen, "Extension for EAP Authentication in IKEv2", draft-eronen-ipsec-ikev2-eap-auth-05 (work in progress), June 2006. [I-D.ietf-rpsec-bgpsecrec] Christian, B. and T. Tauber, "BGP Security Requirements", draft-ietf-rpsec-bgpsecrec-09 (work in progress), November 2007. [I-D.ietf-softwire-hs-framework-l2tpv2] Storer, B., Pignataro, C., Santos, M., Stevant, B., and J. Tremblay, "Softwires Hub & Spoke Deployment Framework with L2TPv2", draft-ietf-softwire-hs-framework-l2tpv2-07 (work in progress), September 2007. [I-D.ietf-softwire-mesh-framework] Wu, J., "Softwire Mesh Framework", draft-ietf-softwire-mesh-framework-02 (work in progress), July 2007. [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Yamamoto, et al. Expires August 28, 2008 [Page 23] Internet-Draft Softwire security considerations February 2008 Internet Protocol", RFC 2401, November 1998. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [RFC4016] Parthasarathy, M., "Protocol for Carrying Authentication and Network Access (PANA) Threat Analysis and Security Requirements", RFC 4016, March 2005. [RFC4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for Next Steps in Signaling (NSIS)", RFC 4081, June 2005. [RFC4111] Fang, L., "Security Framework for Provider-Provisioned Virtual Private Networks (PPVPNs)", RFC 4111, July 2005. [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. 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, IKE refers to UDP port 500. * denotes wildcard and indicates ANY port Yamamoto, et al. Expires August 28, 2008 [Page 24] Internet-Draft Softwire security considerations February 2008 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) Yamamoto, et al. Expires August 28, 2008 [Page 25] Internet-Draft Softwire security considerations February 2008 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 Yamamoto, et al. Expires August 28, 2008 [Page 26] Internet-Draft Softwire security considerations February 2008 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 August 28, 2008 [Page 27] Internet-Draft Softwire security considerations February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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 August 28, 2008 [Page 28]