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Network Working Group B. Storer
Request for Comments: 5571 C. Pignataro, Ed.
Category: Standards Track M. Dos Santos
Cisco Systems
B. Stevant, Ed.
L. Toutain
TELECOM Bretagne
J. Tremblay
Videotron Ltd.
June 2009
Softwire Hub and Spoke Deployment Framework
with Layer Two Tunneling Protocol Version 2 (L2TPv2)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Storer, et al. Standards Track [Page 1]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Abstract This document describes the framework of the Softwire "Hub and Spoke" solution with the Layer Two Tunneling Protocol version 2 (L2TPv2). The implementation details specified in this document should be followed to achieve interoperability among different vendor implementations. Table of Contents 1. Introduction ....................................................4 1.1. Abbreviations ..............................................5 1.2. Requirements Language ......................................5 1.3. Considerations .............................................6 2. Applicability of L2TPv2 for Softwire Requirements ...............6 2.1. Traditional Network Address Translation (NAT and NAPT) .....6 2.2. Scalability ................................................7 2.3. Routing ....................................................7 2.4. Multicast ..................................................7 2.5. Authentication, Authorization, and Accounting (AAA) ........7 2.6. Privacy, Integrity, and Replay Protection ..................7 2.7. Operations and Management ..................................8 2.8. Encapsulations .............................................8 3. Deployment Scenarios ............................................8 3.1. IPv6-over-IPv4 Softwires with L2TPv2 .......................9 3.1.1. Host CPE as Softwire Initiator ......................9 3.1.2. Router CPE as Softwire Initiator ...................10 3.1.3. Host behind CPE as Softwire Initiator ..............11 3.1.4. Router behind CPE as Softwire Initiator ............12 3.2. IPv4-over-IPv6 Softwires with L2TPv2 ......................14 3.2.1. Host CPE as Softwire Initiator .....................14 3.2.2. Router CPE as Softwire Initiator ...................15 3.2.3. Host behind CPE as Softwire Initiator ..............16 3.2.4. Router behind CPE as Softwire Initiator ............16 4. References to Standardization Documents ........................17 4.1. L2TPv2 ....................................................18 4.2. Securing the Softwire Transport ...........................18 4.3. Authentication, Authorization, and Accounting .............18 4.4. MIB .......................................................18 4.5. Softwire Payload Related ..................................19 4.5.1. For IPv6 Payloads ..................................19 4.5.2. For IPv4 Payloads ..................................19 5. Softwire Establishment .........................................20 5.1. L2TPv2 Tunnel Setup .......................................22 5.1.1. Tunnel Establishment ...............................22 5.1.1.1. AVPs Required for Softwires ...............25 5.1.1.2. AVPs Optional for Softwires ...............25 5.1.1.3. AVPs Not Relevant for Softwires ...........26 Storer, et al. Standards Track [Page 2]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 5.1.2. Tunnel Maintenance .................................26 5.1.3. Tunnel Teardown ....................................27 5.1.4. Additional L2TPv2 Considerations ...................27 5.2. PPP Connection ............................................27 5.2.1. MTU ................................................27 5.2.2. LCP ................................................27 5.2.3. Authentication .....................................28 5.2.4. IPCP ...............................................28 5.2.4.1. IPV6CP ....................................28 5.2.4.2. IPv4CP ....................................28 5.3. Global IPv6 Address Assignment to Endpoints ...............28 5.4. DHCP ......................................................29 5.4.1. DHCPv6 .............................................29 5.4.2. DHCPv4 .............................................29 6. Considerations about the Address Provisioning Model ............30 6.1. Softwire Endpoints' Addresses .............................30 6.1.1. IPv6 ...............................................30 6.1.2. IPv4 ...............................................31 6.2. Delegated Prefixes ........................................31 6.2.1. IPv6 Prefixes ......................................31 6.2.2. IPv4 Prefixes ......................................31 6.3. Possible Address Provisioning Scenarios ...................31 6.3.1. Scenarios for IPv6 .................................32 6.3.2. Scenarios for IPv4 .................................32 7. Considerations about Address Stability .........................32 8. Considerations about RADIUS Integration ........................33 8.1. Softwire Endpoints ........................................33 8.1.1. IPv6 Softwires .....................................33 8.1.2. IPv4 Softwires .....................................33 8.2. Delegated Prefixes ........................................34 8.2.1. IPv6 Prefixes ......................................34 8.2.2. IPv4 Prefixes ......................................34 9. Considerations for Maintenance and Statistics ..................34 9.1. RADIUS Accounting .........................................35 9.2. MIBs ......................................................35 10. Security Considerations .......................................35 11. Acknowledgements ..............................................36 12. References ....................................................37 12.1. Normative References .....................................37 12.2. Informative References ...................................38 Storer, et al. Standards Track [Page 3]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 1. Introduction The Softwires Working Group has selected Layer Two Tunneling Protocol version 2 (L2TPv2) as the phase 1 protocol to be deployed in the Softwire "Hub and Spoke" solution space. This document describes the framework for the L2TPv2 "Hub and Spoke" solution, and the implementation details specified in this document should be followed to achieve interoperability among different vendor implementations. In the "Hub and Spoke" solution space, a Softwire is established to provide the home network with IPv4 connectivity across an IPv6-only access network, or IPv6 connectivity across an IPv4-only access network. When L2TPv2 is used in the Softwire context, the voluntary tunneling model applies. The Softwire Initiator (SI) at the home network takes the role of the L2TP Access Concentrator (LAC) client (initiating both the L2TP tunnel/session and the PPP link) while the Softwire Concentrator (SC) at the ISP takes the role of the L2TP Network Server (LNS). The terms voluntary tunneling and compulsory tunneling are defined in Section 1.1 of [RFC3193]. Since the L2TPv2 compulsory tunneling model does not apply to Softwires, it SHOULD NOT be requested or honored. This document identifies all the voluntary tunneling related L2TPv2 attributes that apply to Softwires and specifies the handling mechanism for such attributes in order to avoid ambiguities in implementations. This document also identifies the set of L2TPv2 attributes specific to the compulsory tunneling model that does not apply to Softwires and specifies the mechanism to ignore or nullify their effect within the Softwire context. The SI and SC MUST follow the L2TPv2 operations described in [RFC2661] when performing Softwire establishment, teardown, and Operations, Administration, and Management (OAM). With L2TPv2, a Softwire consists of an L2TPv2 Control Connection (also referred to as Control Channel), a single L2TPv2 Session, and the PPP link negotiated over the Session. To establish the Softwire, the SI first initiates an L2TPv2 Control Channel to the SC, which accepts the request and terminates the Control Channel. L2TPv2 supports an optional mutual Control Channel authentication that allows both SI and SC to validate each other's identity at the initial phase of hand-shaking before proceeding with Control Channel establishment. After the L2TPv2 Control Channel is established between the SI and SC, the SI initiates an L2TPv2 Session to the SC. Then the PPP/IP link is negotiated over the L2TPv2 Session between the SI and SC. After the PPP/IP link is established, it acts as the Softwire between the SI and SC for tunneling IP traffic of one Address Family (AF) across the access network of another Address Family. Storer, et al. Standards Track [Page 4]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 During the life of the Softwire, both SI and SC send L2TPv2 keepalive HELLO messages to monitor the health of the Softwire and the peer L2TP Control Connection Endpoint (LCCE), and to potentially refresh the NAT/NAPT (Network Address Translation / Network Address Port Translation) entry at the CPE or at the other end of the access link. Optionally, Link Control Protocol (LCP) ECHO messages can be used as keepalives for the same purposes. In the event of keepalive timeout or administrative shutdown of the Softwire, either the SI or the SC MAY tear down the Softwire by tearing down the L2TPv2 Control Channel and Session as specified in [RFC2661]. 1.1. Abbreviations AF Address Family, IPv4 or IPv6. CPE Customer Premises Equipment. LCCE L2TP Control Connection Endpoint, an L2TP node that exists at either end of an L2TP Control Connection. (See [RFC3931].) LNS L2TP Network Server, a node that acts as one side of an L2TP tunnel (Control Connection) endpoint. The LNS is the logical termination point of a PPP session that is being tunneled from the remote system by the peer LCCE. (See [RFC2661].) SC Softwire Concentrator, the node terminating the Softwire in the service provider network. (See [RFC4925].) SI Softwire Initiator, the node initiating the Softwire within the customer network. (See [RFC4925].) SPH Softwire Payload Header, the IP headers being carried within a Softwire. (See [RFC4925].) STH Softwire Transport Header, the outermost IP header of a Softwire. (See [RFC4925].) SW Softwire, a shared-state "tunnel" created between the SC and SI. (See [RFC4925].) 1.2. 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]. Storer, et al. Standards Track [Page 5]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 1.3. Considerations Some sections of this document contain considerations that are not required for interoperability and correct operation of Softwire implementations. These sections' titles are marked beginning with "Considerations". 2. Applicability of L2TPv2 for Softwire Requirements A list of Softwire "Hub and Spoke" requirements has been identified by the Softwire Problem Statement [RFC4925]. The following sub- sections describe how L2TPv2 fulfills each of them. 2.1. Traditional Network Address Translation (NAT and NAPT) A "Hub and Spoke" Softwire must be able to traverse Network Address Translation (NAT) and Network Address Port Translation (NAPT, also referred to as Port Address Translation or PAT) devices [RFC3022] in case the scenario in question involves a non-upgradable, preexisting IPv4 home gateway performing NAT/NAPT or some carrier equipment at the other end of the access link performing NAT/NAPT. The L2TPv2 Softwire (i.e., Control Channel and Session) is capable of NAT/NAPT traversal since L2TPv2 can run over UDP. Since L2TPv2 does not detect NAT/NAPT along the path, L2TPv2 does not offer the option of disabling UDP. The UDP encapsulation is present regardless of NAT/NAPT presence. Both NAT/NAPT "autodetect" and UDP "bypass" are optional requirements in Section 2.3 of [RFC4925]. As mentioned in Section 8.1 of [RFC2661] and Section 4 of [RFC3193], an L2TP Start-Control-Connection-Reply (SCCRP) responder (SC) can chose a different IP address and/or UDP port than those from the initiator's Start-Control-Connection-Request (SCCRQ) (SI). This may or may not traverse a NAT/NAPT depending on the NAT/NAPT's Filtering Behavior (see Section 5 of [RFC4787]). Specifically, any IP address and port combination will work with Endpoint-Independent Filtering, but changing the IP address and port will not work through Address- Dependent or Address-and-Port-Dependent Filtering. Given this, responding from a different IP address and/or UDP port is NOT RECOMMENDED. Storer, et al. Standards Track [Page 6]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 2.2. Scalability In the "Hub and Spoke" model, a carrier must be able to scale the solution to millions of Softwire Initiators by adding more hubs (i.e., Softwire Concentrators). L2TPv2 is a widely deployed protocol in broadband services, and its scalability has been proven in multiple large-scale IPv4 Virtual Private Network deployments, which scale up to millions of subscribers each. 2.3. Routing There are no dynamic routing protocols between the SC and SI. A default route from the SI to the SC is used. 2.4. Multicast Multicast protocols simply run transparently over L2TPv2 Softwires together with other regular IP traffic. 2.5. Authentication, Authorization, and Accounting (AAA) L2TPv2 supports optional mutual Control Channel authentication and leverages the optional mutual PPP per-session authentication. L2TPv2 is well integrated with AAA solutions (such as RADIUS) for both authentication and authorization. Most L2TPv2 implementations available in the market support the logging of authentication and authorization events. L2TPv2 integration with RADIUS accounting (RADIUS Accounting extension for tunnel [RFC2867]) allows the collection and reporting of L2TPv2 Softwire usage statistics. 2.6. Privacy, Integrity, and Replay Protection Since L2TPv2 runs over IP/UDP in the Softwire context, IPsec Encapsulating Security Payload (ESP) can be used in conjunction to provide per-packet authentication, integrity, replay protection, and confidentiality for both L2TPv2 control and data traffic [RFC3193] and [RFC3948]. For Softwire deployments in which full payload security is not required, the L2TPv2 built-in Control Channel authentication and the inherited PPP authentication and PPP Encryption Control Protocol can be considered. Storer, et al. Standards Track [Page 7]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 2.7. Operations and Management L2TPv2 supports an optional in-band keepalive mechanism that injects HELLO control messages after a specified period of time has elapsed since the last data or control message was received on a tunnel (see Section 5.5 of [RFC2661]). If the HELLO control message is not reliably delivered, then the Control Channel and its Session will be torn down. In the Softwire context, the L2TPv2 keepalive is used to monitor the connectivity status between the SI and SC and/or as a refresh mechanism for any NAT/NAPT translation entry along the access link. LCP ECHO offers a similar mechanism to monitor the connectivity status, as described in [RFC1661]. Softwire implementations SHOULD use L2TPv2 Hello keepalives, and in addition MAY use PPP LCP Echo messages to ensure Dead End Detection and/or to refresh NAT/NAPT translation entries. The combination of these two mechanisms can be used as an optimization. The L2TPv2 MIB [RFC3371] supports the complete suite of management operations such as configuration of Control Channel and Session, polling of Control Channel and Session status and their traffic statistics and notifications of Control Channel and Session UP/DOWN events. 2.8. Encapsulations L2TPv2 supports the following encapsulations: o IPv6/PPP/L2TPv2/UDP/IPv4 o IPv4/PPP/L2TPv2/UDP/IPv6 o IPv4/PPP/L2TPv2/UDP/IPv4 o IPv6/PPP/L2TPv2/UDP/IPv6 Note that UDP bypass is not supported by L2TPv2 since L2TPv2 does not support "autodetect" of NAT/NAPT. 3. Deployment Scenarios For the "Hub and Spoke" problem space, four scenarios have been identified. In each of these four scenarios, different home equipment plays the role of the Softwire Initiator. This section elaborates each scenario with L2TPv2 as the Softwire protocol and Storer, et al. Standards Track [Page 8]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 other possible protocols involved to complete the solution. This section examines the four scenarios for both IPv6-over-IPv4 (Section 3.1) and IPv4-over-IPv6 (Section 3.2) encapsulations. 3.1. IPv6-over-IPv4 Softwires with L2TPv2 The following sub-sections cover IPv6 connectivity (SPH) across an IPv4-only access network (STH) using a Softwire. 3.1.1. Host CPE as Softwire Initiator The Softwire Initiator (SI) is the host CPE (directly connected to a modem), which is dual-stack. There is no other gateway device. The IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 1. IPv6 or dual-stack IPv4-only dual-stack |------------------||-----------------||----------| I SC SI N +-----+ +----------+ T | | | v4/v6 | E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....| host CPE | R [network] | | [ network ] | | N | LNS | |LAC Client| E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _() <-- IPv6 traffic PPP o L2TPv2 o UDP o IPv4 (SPH) Softwire <------------------> IPV6CP: capable of /64 Intf-Id assignment or uniqueness check |------------------>/64 prefix RA |------------------>DNS, etc. DHCPv6 Figure 1: Host CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, the IPv6 Control Protocol (IPV6CP) negotiates IPv6-over-PPP, which also provides the capability for the ISP to assign the 64-bit Interface-Identifier to the host CPE or perform uniqueness validation for the two interface identifiers at the two PPP ends [RFC5072]. After IPv6-over-PPP is up, IPv6 Storer, et al. Standards Track [Page 9]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Stateless Address Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP link, and the LNS can inform the host CPE of a prefix to use for stateless address autoconfiguration through a Router Advertisement (RA) while other non-address configuration options (such as DNS [RFC3646] or other servers' addresses that might be available) can be conveyed to the host CPE via DHCPv6. 3.1.2. Router CPE as Softwire Initiator The Softwire Initiator (SI) is the router CPE, which is a dual-stack device. The IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 2. IPv6 or dual-stack IPv4-only dual-stack |------------------||-----------------||---------------------| I SC SI N +-----+ +----------+ T | | | v4/v6 | +-----+ E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....| CPE |----|v4/v6| R [network] | | [ network ] | | | host| N | LNS | |LAC Client| +-----+ E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _() <-------- IPv6 traffic PPP o L2TPv2 o UDP o IPv4 (SPH) Softwire <------------------> IPV6CP: capable of /64 Intf-Id assignment or uniqueness check |------------------>/64 prefix RA |------------------>/48 prefix, DHCPv6 DNS, etc. |------->/64 prefix RA |-------> DNS, etc. DHCPv4/v6 Figure 2: Router CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPV6CP negotiates IPv6-over-PPP, which also provides the capability for the ISP to assign the 64-bit Storer, et al. Standards Track [Page 10]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Interface-Identifier to the router CPE or perform uniqueness validation for the two interface identifiers at the two PPP ends [RFC5072]. After IPv6-over-PPP is up, IPv6 Stateless Address Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP link, and the LNS can inform the router CPE of a prefix to use for stateless address autoconfiguration through a Router Advertisement (RA). DHCPv6 can be used to perform IPv6 Prefix Delegation (e.g., delegating a prefix to be used within the home network [RFC3633]) and convey other non-address configuration options (such as DNS [RFC3646]) to the router CPE. 3.1.3. Host behind CPE as Softwire Initiator The CPE is IPv4-only. The Softwire Initiator (SI) is a dual-stack host (behind the IPv4-only CPE), which acts as an IPv6 host CPE. The IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 3. IPv6 or dual-stack IPv4-only dual-stack |------------------||----------------------------||----------| I SC SI N +-----+ +----------+ T | | +-------+ | v4/v6 | E <==[ IPv6 ]....|v4/v6|....[IPv4-only]....|v4-only|--| host | R [network] | | [ network ] | CPE | | | N | LNS | +-------+ |LAC Client| E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv6 PPP o L2TPv2 o UDP o IPv4 traffic Softwire (SPH) <------------------------------> IPV6CP: capable of /64 Intf-Id assignment or uniqueness check |------------------------------>/64 prefix RA |------------------------------>DNS, etc. DHCPv6 Figure 3: Host behind CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPV6CP negotiates IPv6-over-PPP, which also provides the capability for the ISP to assign the 64-bit Storer, et al. Standards Track [Page 11]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Interface-Identifier to the host or perform uniqueness validation for the two interface identifiers at the two PPP ends [RFC5072]. After IPv6-over-PPP is up, IPv6 Stateless Address Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP link, and the LNS can inform the host of a prefix to use for stateless address autoconfiguration through a Router Advertisement (RA) while other non-address configuration options (such as DNS [RFC3646]) can be conveyed to the host via DHCPv6. 3.1.4. Router behind CPE as Softwire Initiator The CPE is IPv4-only. The Softwire Initiator (SI) is a dual-stack device (behind the IPv4-only CPE) acting as an IPv6 CPE router inside the home network. The IPv4 traffic SHOULD NOT traverse the Softwire. See Figure 4. Storer, et al. Standards Track [Page 12]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 IPv6 or dual-stack IPv4-only dual-stack |------------------||-------------------------||-------------| I SC SI N +-----+ +----------+ T | | +-------+ | v4/v6 | E <==[ IPv6 ]....|v4/v6|..[IPv4-only]..|v4-only|---| router | R [network] | | [ network ] | CPE | | | | N | LNS | +-------+ | |LAC Client| E +-----+ | +----------+ T | ---------+-----+ |v4/v6| | host| _ _ _ _ _ _ _ _ _ _ _ _ _ +-----+ ()_ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv6 PPP o L2TPv2 o UDP o IPv4 traffic Softwire (SPH) <---------------------------> IPV6CP: capable of /64 Intf-Id assignment or uniqueness check |--------------------------->/64 prefix RA |--------------------------->/48 prefix, DHCPv6 DNS, etc. |----> /64 RA prefix |----> DNS, DHCPv6 etc. Figure 4: Router behind CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPV6CP negotiates IPv6-over-PPP, which also provides the capability for the ISP to assign the 64-bit Interface-Identifier to the v4/v6 router or perform uniqueness validation for the two interface identifiers at the two PPP ends [RFC5072]. After IPv6-over-PPP is up, IPv6 Stateless Address Autoconfiguration / Neighbor Discovery runs over the IPv6-over-PPP link, and the LNS can inform the v4/v6 router of a prefix to use for stateless address autoconfiguration through a Router Advertisement Storer, et al. Standards Track [Page 13]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 (RA). DHCPv6 can be used to perform IPv6 Prefix Delegation (e.g., delegating a prefix to be used within the home network [RFC3633]) and convey other non-address configuration options (such as DNS [RFC3646]) to the v4/v6 router. 3.2. IPv4-over-IPv6 Softwires with L2TPv2 The following sub-sections cover IPv4 connectivity (SPH) across an IPv6-only access network (STH) using a Softwire. 3.2.1. Host CPE as Softwire Initiator The Softwire Initiator (SI) is the host CPE (directly connected to a modem), which is dual-stack. There is no other gateway device. The IPv6 traffic SHOULD NOT traverse the Softwire. See Figure 5. IPv4 or dual-stack IPv6-only dual-stack |------------------||-----------------||----------| I SC SI N +-----+ +----------+ T | | | v4/v6 | E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....| host CPE | R [network] | | [ network ] | | N | LNS | |LAC Client| E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _() <-- IPv4 traffic PPP o L2TPv2 o UDP o IPv6 (SPH) Softwire <------------------> IPCP: capable of global IP assignment and DNS, etc. Figure 5: Host CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, the IP Control Protocol (IPCP) negotiates IPv4-over-PPP, which also provides the capability for the ISP to assign a global IPv4 address to the host CPE. A global IPv4 address can also be assigned via DHCP. Other configuration options (such as DNS) can be conveyed to the host CPE via IPCP [RFC1877] or DHCP [RFC2132]. Storer, et al. Standards Track [Page 14]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 3.2.2. Router CPE as Softwire Initiator The Softwire Initiator (SI) is the router CPE, which is a dual-stack device. The IPv6 traffic SHOULD NOT traverse the Softwire. See Figure 6. IPv4 or dual-stack IPv6-only dual-stack Home |------------------||-----------------||-------------------| I SC SI N +-----+ +----------+ T | | | v4/v6 | +-----+ E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....| CPE |--|v4/v6| R [network] | | [ network ] | | | host| N | LNS | |LAC Client| +-----+ E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _() <--------- IPv4 traffic PPP o L2TPv2 o UDP o IPv6 (SPH) Softwire <------------------> IPCP: capable of global IP assignment and DNS, etc. |------------------> DHCPv4: prefix, mask, PD private/ |------> global DHCP IP, DNS, etc. Figure 6: Router CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPCP negotiates IPv4-over-PPP, which also provides the capability for the ISP to assign a global IPv4 address to the router CPE. A global IPv4 address can also be assigned via DHCP. Other configuration options (such as DNS) can be conveyed to the router CPE via IPCP [RFC1877] or DHCP [RFC2132]. For IPv4 Prefix Delegation for the home network, DHCP [SUBNET-ALL] can be used. Storer, et al. Standards Track [Page 15]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 3.2.3. Host behind CPE as Softwire Initiator The CPE is IPv6-only. The Softwire Initiator (SI) is a dual-stack host (behind the IPv6 CPE), which acts as an IPv4 host CPE. The IPv6 traffic SHOULD NOT traverse the Softwire. See Figure 7. IPv4 or dual-stack IPv6-only dual-stack |------------------||----------------------------||----------| I SC SI N +-----+ +----------+ T | | +-------+ | v4/v6 | E <==[ IPv4 ]....|v4/v6|....[IPv6-only]....|v6-only|--| host | R [network] | | [ network ] | CPE | | | N | LNS | +-------+ |LAC Client| E +-----+ +----------+ T _ _ _ _ _ _ _ _ _ _ _ _ _ _ ()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <-- IPv4 PPP o L2TPv2 o UDP o IPv6 traffic Softwire (SPH) <------------------------------> IPCP: capable of global IP assignment and DNS, etc. Figure 7: Host behind CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPCP negotiates IPv4-over-PPP, which also provides the capability for the ISP to assign a global IPv4 address to the host. A global IPv4 address can also be assigned via DHCP. Other configuration options (such as DNS) can be conveyed to the host CPE via IPCP [RFC1877] or DHCP [RFC2132]. 3.2.4. Router behind CPE as Softwire Initiator The CPE is IPv6-only. The Softwire Initiator (SI) is a dual-stack device (behind the IPv6-only CPE) acting as an IPv4 CPE router inside the home network. The IPv6 traffic SHOULD NOT traverse the Softwire. See Figure 8. Storer, et al. Standards Track [Page 16]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 IPv4 or dual-stack IPv6-only dual-stack |------------------||-------------------------||------------| I SC SI N +-----+ +----------+ T | | +-------+ | v4/v6 | E <==[ IPv4 ]....|v4/v6|..[IPv6-only]..|v6-only|---| router | R [network] | | [ network ] | CPE | | | | N | LNS | +-------+ | |LAC Client| E +-----+ | +----------+ T | --------+-----+ |v4/v6| | host| _ _ _ _ _ _ _ _ _ _ _ _ _ +-----+ ()_ _ _ _ _ _ _ _ _ _ _ _ _() <--- IPv4 PPP o L2TPv2 o UDP o IPv4 traffic Softwire (SPH) <---------------------------> IPCP: assigns global IP address and DNS, etc. |---------------------------> DHCPv4: prefix, mask, PD private/ |----> global DHCP IP, DNS, etc. Figure 8: Router behind CPE as Softwire Initiator In this scenario, after the L2TPv2 Control Channel and Session establishment and PPP LCP negotiation (and optionally PPP Authentication) are successful, IPCP negotiates IPv4-over-PPP, which also provides the capability for the ISP to assign a global IPv4 address to the v4/v6 router. A global IPv4 address can also be assigned via DHCP. Other configuration options (such as DNS) can be conveyed to the v4/v6 router via IPCP [RFC1877] or DHCP [RFC2132]. For IPv4 Prefix Delegation for the home network, DHCP [SUBNET-ALL] can be used. 4. References to Standardization Documents This section lists and groups documents from the Internet standardization describing technologies used to design the framework of the Softwire "Hub and Spoke" solution. This emphasizes the motivation of Softwire to reuse as many existing standards as Storer, et al. Standards Track [Page 17]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 possible. This list contains both Standards Track (Proposed Standard, Draft Standard, and Standard) and Informational documents. The list of documents and their status should only be only used for description purposes. 4.1. L2TPv2 RFC 2661 "Layer Two Tunneling Protocol 'L2TP'" [RFC2661]. * For both IPv4 and IPv6 payloads (SPH), support is complete. * For both IPv4 and IPv6 transports (STH), support is complete. 4.2. Securing the Softwire Transport RFC 3193 "Securing L2TP using IPsec" [RFC3193]. RFC 3948 "UDP Encapsulation of IPsec ESP Packets" [RFC3948]. * IPsec supports both IPv4 and IPv6 transports. 4.3. Authentication, Authorization, and Accounting RFC 2865 "Remote Authentication Dial In User Service (RADIUS)" [RFC2865]. * Updated by [RFC2868], [RFC3575], and [RFC5080]. RFC 2867 "RADIUS Accounting Modifications for Tunnel Protocol Support" [RFC2867]. RFC 2868 "RADIUS Attributes for Tunnel Protocol Support" [RFC2868]. RFC 3162 "RADIUS and IPv6" [RFC3162]. 4.4. MIB RFC 1471 "The Definitions of Managed Objects for the Link Control Protocol of the Point-to-Point Protocol" [RFC1471]. RFC 1473 "The Definitions of Managed Objects for the IP Network Control Protocol of the Point-to-Point Protocol" [RFC1473]. Storer, et al. Standards Track [Page 18]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 RFC 3371 "Layer Two Tunneling Protocol "L2TP" Management Information Base" [RFC3371]. RFC 4087 "IP Tunnel MIB" [RFC4087]. * Both IPv4 and IPv6 transports are supported. 4.5. Softwire Payload Related 4.5.1. For IPv6 Payloads RFC 4861 "Neighbor Discovery for IP version 6 (IPv6)" [RFC4861]. RFC 4862 "IPv6 Stateless Address Autoconfiguration" [RFC4862]. RFC 5072 "IP Version 6 over PPP" [RFC5072]. RFC 3315 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" [RFC3315]. RFC 3633 "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6" [RFC3633]. RFC 3646 "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" [RFC3646]. RFC 3736 "Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6" [RFC3736]. 4.5.2. For IPv4 Payloads RFC 1332 "The PPP Internet Protocol Control Protocol (IPCP)" [RFC1332]. RFC 1661 "The Point-to-Point Protocol (PPP)" [RFC1661]. RFC 1877 "PPP Internet Protocol Control Protocol Extensions for Name Server Addresses" [RFC1877]. RFC 2131 "Dynamic Host Configuration Protocol" [RFC2131]. RFC 2132 "DHCP Options and BOOTP Vendor Extensions" [RFC2132]. DHCP Subnet Allocation "Subnet Allocation Option". * Work in progress, see [SUBNET-ALL]. Storer, et al. Standards Track [Page 19]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 5. Softwire Establishment A Softwire is established in three distinct steps, potentially preceded by an optional IPsec-related step 0 (see Figure 9). First, an L2TPv2 tunnel with a single session is established from the SI to the SC. Second, a PPP session is established over the L2TPv2 session and the SI obtains an address. Third, the SI optionally gets other information through DHCP such as a delegated prefix and DNS servers. SC SI | | |<-------------IKEv1------------->| Step 0 | | IPsec SA establishment | | (optional) | | |<-------------L2TPv2------------>| Step 1 | | L2TPv2 Tunnel establishment | | |<--------------PPP-------------->| Step 2 |<-----Endpoint Configuration---->| PPP and Endpoint | | configuration | | |<------Router Configuration----->| Step 3 | | Additional configuration | | (optional) Figure 9: Steps for the Establishment of a Softwire Figure 10 depicts details of each of these steps required to establish a Softwire. Storer, et al. Standards Track [Page 20]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 SC SI | | | | Step 0 |<------------IKEv1-------------->| = IKEv1 (Optional) | | | | Step 1 |<------------SCCRQ---------------| - |-------------SCCRP-------------->| | |<------------SCCCN---------------| | |<------------ICRQ----------------| | L2TPv2 |-------------ICRP--------------->| | |<------------ICCN----------------| - | | | | Step 2 |<-----Configuration-Request------| - |------Configuration-Request----->| | PPP |--------Configuration-Ack------->| | LCP |<-------Configuration-Ack--------| - | | |-----------Challenge------------>| - PPP Authentication |<----------Response--------------| | (Optional - CHAP) |------------Success------------->| - | | |<-----Configuration-Request------| - |------Configuration-Request----->| | PPP NCP |--------Configuration-Ack------->| | (IPV6CP or IPCP) |<-------Configuration-Ack--------| - | | |<------Router-Solicitation-------| - Neighbor Discovery |-------Router-Advertisement----->| | (IPv6 only) | | - | | | | Step3 | | DHCP (Optional) |<-----------SOLICIT--------------| - |-----------ADVERTISE------------>| | DHCPv6 |<---------- REQUEST--------------| | (IPv6 SW, Optional) |-------------REPLY-------------->| - | | or |<---------DHCPDISCOVER-----------| - |-----------DHCPOFFER------------>| | DHCPv4 |<---------DHCPREQUEST------------| | (IPv4 SW, Optional) |------------DHCPACK------------->| - Figure 10: Detailed Steps in the Establishment of a Softwire Storer, et al. Standards Track [Page 21]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 The IPsec-related negotiations in step 0 are optional. The L2TPv2 negotiations in step 1 are described in Section 5.1. The PPP Network Control Protocol (NCP) negotiations in step 2 use IPV6CP for IPv6- over-IPv4 Softwires, and IPCP for IPv4-over-IPv6 Softwires (see Section 5.2.4). The optional DHCP negotiations in step 3 use DHCPv6 for IPv6-over-IPv4 Softwires, and DHCPv4 for IPv4-over-IPv6 Softwires (see Section 5.4). Additionally, for IPv6-over-IPv4 Softwires, the DHCPv6 exchange for non-address configuration (such as DNS) can use Stateless DHCPv6, the two-message exchange with Information-Request and Reply messages (see Section 1.2 of [RFC3315] and [RFC3736]). 5.1. L2TPv2 Tunnel Setup L2TPv2 [RFC2661] was originally designed to provide private network access to end users connected to a public network. In the L2TPv2 incoming call model, the end user makes a connection to an L2TP Access Concentrator (LAC). The LAC then initiates an L2TPv2 tunnel to an L2TP Network Server (LNS). The LNS then transfers end-user traffic between the L2TPv2 tunnel and the private network. In the Softwire "Hub and Spoke" model, the Softwire Initiator (SI) assumes the role of the LAC Client and the Softwire Concentrator (SC) assumes the role of the LNS. In the Softwire model, an L2TPv2 packet MUST be carried over UDP. The underlying version of the IP protocol may be IPv4 or IPv6, depending on the Softwire scenario. In the following sections, the term "Tunnel" follows the definition from Section 1.2 of [RFC2661], namely: "The Tunnel consists of a Control Connection and zero or more L2TP Sessions". 5.1.1. Tunnel Establishment Figure 11 describes the messages exchanged and Attribute Value Pairs (AVPs) used to establish a tunnel between an SI (LAC) and an SC (LNS). The messages and AVPs described here are only a subset of those defined in [RFC2661]. This is because Softwires use only a subset of the L2TPv2 functionality. The subset of L2TP Control Connection Management AVPs that is applicable to Softwires is grouped into Required AVPs and Optional AVPs on a per-control-message basis (see Figure 11). For each control message, Required AVPs include all the "MUST be present" AVPs from [RFC2661] for that control message, and Optional AVPs include the "MAY be present" AVPs from [RFC2661] that are used in the Softwire context on that control message. Note that in the Softwire environment, the SI always initiates the tunnel. L2TPv2 AVPs SHOULD NOT be hidden. Storer, et al. Standards Track [Page 22]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 SC SI |<--------SCCRQ---------| Required AVPs: Message Type Protocol Version Host Name Framing Capabilities Assigned Tunnel ID Optional AVPs: Receive Window Size Challenge Firmware Revision Vendor Name |---------SCCRP-------->| Required AVPs: Message Type Protocol Version Framing Capabilities Host Name Assigned Tunnel ID Optional AVPs: Firmware Revision Vendor Name Receive Window Size Challenge Challenge Response |<--------SCCCN---------| Required AVPs: Message Type Optional AVPs: Challenge Response Figure 11: Control Connection Establishment In L2TPv2, generally, the tunnel between an LAC and LNS may carry the data of multiple users. Each of these users is represented by an L2TPv2 session within the tunnel. In the Softwire environment, the tunnel carries the information of a single user. Consequently, there is only one L2TPv2 session per tunnel. Figure 12 describes the messages exchanged and the AVPs used to establish a session between an SI (LAC) and an SC (LNS). The messages and AVPs described here are only a subset of those defined in [RFC2661]. This is because Softwires use only a subset of the L2TPv2 functionality. The subset of L2TP Call Management (i.e., Session Management) AVPs that is applicable to Softwires is grouped into Required AVPs and Optional AVPs on a per-control-message basis (see Figure 12). For each Storer, et al. Standards Track [Page 23]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 control message, Required AVPs include all the "MUST be present" AVPs from [RFC2661] for that control message, and Optional AVPs include the "MAY be present" AVPs from [RFC2661] that are used in the Softwire context on that control message. Note that in the Softwire environment, the SI always initiates the session. An L2TPv2 session setup for a Softwire uses only the incoming call model. No outgoing or analog calls (sessions) are permitted. L2TPv2 AVPs SHOULD NOT be hidden. SC SI |<--------ICRQ---------| Required AVPs: Message Type Assigned Session ID Call Serial Number |---------ICRP-------->| Required AVPs: Message Type Assigned Session ID |<--------ICCN---------| Required AVPs: Message Type (Tx) Connect Speed Framing Type Figure 12: Session Establishment The following sub-sections (5.1.1.1 through 5.1.1.3) describe in more detail the Control Connection and Session establishment AVPs (see message flows in Figures 11 and 12, respectively) that are required, optional and not relevant for the L2TPv2 Tunnel establishment of a Softwire. Specific L2TPv2 protocol messages and flows that are not explicitly described in these sections are handled as defined in [RFC2661]. The mechanism for hiding AVP Attribute values is used, as described in Section 4.3 of [RFC2661], to hide sensitive control message data such as usernames, user passwords, or IDs, instead of sending the AVP contents in the clear. Since AVPs used in L2TP messages for the Softwire establishment do not transport such sensitive data, L2TPv2 AVPs SHOULD NOT be hidden. Storer, et al. Standards Track [Page 24]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 5.1.1.1. AVPs Required for Softwires This section prescribes specific values for AVPs that are required (by [RFC2661]) to be present in one or more of the messages used for the Softwire establishment, as they are used in the Softwire context. It combines all the Required AVPs from all the control messages in Section 5.1.1, and provides Softwire-specific use guidance. Host Name AVP This AVP is required in SCCRQ and SCCRP messages. This AVP MAY be used to authenticate users, in which case it would contain a user identification. If this AVP is not used to authenticate users, it may be used for logging purposes. Framing Capabilities AVP Both the synchronous (S) and asynchronous (A) bits SHOULD be set to 1. This AVP SHOULD be ignored by the receiver. Framing Type AVP The synchronous bit SHOULD be set to 1 and the asynchronous bit to 0. This AVP SHOULD be ignored by the receiver. (Tx) Connect Speed AVP (Tx) Connect Speed is a required AVP but is not meaningful in the Softwire context. Its value SHOULD be set to 0 and ignored by the receiver. Message Type AVP, Protocol Version AVP, Assigned Tunnel ID AVP, Call Serial Number AVP, and Assigned Session ID AVP As defined in [RFC2661]. 5.1.1.2. AVPs Optional for Softwires This section prescribes specific values for AVPs that are Optional (not required by [RFC2661]) but used in the Softwire context. It combines all the Optional AVPs from all the control messages in Section 5.1.1, and provides Softwire-specific use guidance. Storer, et al. Standards Track [Page 25]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Challenge AVP and Challenge Response AVP These AVPs are not required, but are necessary to implement tunnel authentication. Since tunnel authentication happens at the beginning of L2TPv2 tunnel creation, it can be helpful in preventing denial-of-service (DoS) attacks. See Section 5.1.1 of [RFC2661]. The usage of these AVPs in L2TP messages is OPTIONAL, but SHOULD be implemented in the SC. Receive Window Size AVP, Firmware Revision AVP, and Vendor Name AVP As defined in [RFC2661]. 5.1.1.3. AVPs Not Relevant for Softwires L2TPv2 specifies numerous AVPs that, while allowed for a given message, are irrelevant to Softwires. They can be irrelevant to Softwires because they do not apply to the Softwire establishment flow (e.g., they are only used in the Outgoing Call establishment message exchange, while Softwires only use the Incoming Call message flow), or because they are Optional AVPs that are not used. L2TPv2 AVPs that are relevant to Softwires were covered in Sections 5.1.1, 5.1.1.1, and 5.1.1.2. Softwire implementations SHOULD NOT send AVPs that are not relevant to Softwires. However, they SHOULD ignore them when they are received. This will simplify the creation of Softwire applications that build upon existing L2TPv2 implementations. 5.1.2. Tunnel Maintenance Periodically, the SI/SC MUST transmit a message to the peer to detect tunnel or peer failure and maintain NAT/NAPT contexts. The L2TPv2 HELLO message provides a simple, low-overhead method of doing this. The default values specified in [RFC2661] for L2TPv2 HELLO messages could result in a dead-end detection time of 83 seconds. Although these retransmission timers and counters SHOULD be configurable (see Section 5.8 of [RFC2661]), these values may not be adapted for all situations, where a quicker dead-end detection is required, or where NAT/NAPT context needs to be refreshed more frequently. In such cases, the SI/SC MAY use, in combination with L2TPv2 HELLO, LCP ECHO messages (Echo-Request and Echo-Reply codes) described in [RFC1661]. When used, LCP ECHO messages SHOULD have a re-emission timer lower than the value for L2TPv2 HELLO messages. The default value recommended in Section 6.5 of [RFC2661] for the HELLO message retransmission interval is 60 seconds. When used, a set of suggested values (included here only for guidance) for the LCP ECHO message Storer, et al. Standards Track [Page 26]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 request interval is a default of 30 seconds, a minimum of 10 seconds, and a maximum of the lesser of the configured L2TPv2 HELLO retransmission interval and 60 seconds. 5.1.3. Tunnel Teardown Either the SI or SC can tear down the session and tunnel. This is done as specified in Section 5.7 of [RFC2661], by sending a StopCCN control message. There is no action specific to Softwires in this case. 5.1.4. Additional L2TPv2 Considerations In the Softwire "Hub and Spoke" framework, L2TPv2 is layered on top of UDP, as part of an IP-in-IP tunnel; Section 8.1 of [RFC2661] describes L2TP over UDP/IP. Therefore, the UDP guidelines specified in [RFC5405] apply, as they pertain to the UDP tunneling scenarios carrying IP-based traffic. Section 3.1.3 of [RFC5405] specifies that for this case, specific congestion control mechanisms for the tunnel are not necessary. Additionally, Section 3.2 of [RFC5405] provides message size guidelines for the encapsulating (outer) datagrams, including the recommendation to implement Path MTU Discovery (PMTUD). 5.2. PPP Connection This section describes the PPP negotiations between the SI and SC in the Softwire context. 5.2.1. MTU The MTU of the PPP link presented to the SPH SHOULD be the link MTU minus the size of the IP, UDP, L2TPv2, and PPP headers together. On an IPv4 link with an MTU equal to 1500 bytes, this could typically mean a PPP MTU of 1460 bytes. When the link is managed by IPsec, this MTU SHOULD be lowered to take into account the ESP encapsulation (see [SW-SEC]). The value for the MTU may also vary according to the size of the L2TP header, as defined by the leading bits of the L2TP message header (see [RFC2661]). Additionally, see [RFC4623] for a detailed discussion of fragmentation issues. 5.2.2. LCP Once the L2TPv2 session is established, the SI and SC initiate the PPP connection by negotiating LCP as described in [RFC1661]. The Address-and-Control-Field-Compression configuration option (ACFC) [RFC1661] MAY be rejected. Storer, et al. Standards Track [Page 27]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 5.2.3. Authentication After completing LCP negotiation, the SI and SC MAY optionally perform authentication. If authentication is chosen, Challenge Handshake Authentication Protocol (CHAP) [RFC1994] authentication MUST be supported by both the Softwire Initiator and Softwire Concentrator. Other authentication methods such as Microsoft CHAP version 1 (MS-CHAPv1) [RFC2433] and Extensible Authentication Protocol (EAP) [RFC3748] MAY be supported. A detailed discussion of Softwire security is contained in [SW-SEC]. 5.2.4. IPCP The only Network Control Protocol (NCP) negotiated in the Softwire context is IPV6CP (see Section 5.2.4.1) for IPv6 as SPH, and IPCP (see Section 5.2.4.2) for IPv4 as SPH. 5.2.4.1. IPV6CP In the IPv6-over-IPv4 scenarios (see Section 3.1), after the optional authentication phase, the Softwire Initiator MUST negotiate IPV6CP as defined in [RFC5072]. IPV6CP provides a way to negotiate a unique 64-bit Interface-Identifier to be used for the address autoconfiguration at the local end of the link. 5.2.4.2. IPv4CP In the IPv4-over-IPv6 scenarios (see Section 3.2), a Softwire Initiator MUST negotiate IPCP [RFC1332]. The SI uses IPCP to obtain an IPv4 address from the SC. IPCP MAY also be used to obtain DNS information as described in [RFC1877]. 5.3. Global IPv6 Address Assignment to Endpoints In several scenarios defined in Section 3.1, global IPv6 addresses are expected to be allocated to Softwire endpoints (in addition to the Link-Local addresses autoconfigured using the IPV6CP negotiated interface identifier). The Softwire Initiator assigns global IPv6 addresses using the IPV6CP negotiated interface identifier and using Stateless Address Autoconfiguration [RFC4862], and/or using Privacy Extensions for Stateless Address Autoconfiguration [RFC4941], (as described in Section 5 of [RFC5072]), and/or using DHCPv6 [RFC3315]. Storer, et al. Standards Track [Page 28]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 The Softwire Initiator of an IPv6 Softwire MUST send a Router Solicitation message to the Softwire Concentrator after IPV6CP is completed. The Softwire Concentrator MUST answer with a Router Advertisement. This message MUST contain the global IPv6 prefix of the PPP link if Neighbor Discovery is used to configure addresses of Softwire endpoints. If DHCPv6 is available for address delegation, the M bits of the Router Advertisement SHOULD be set. The Softwire Initiator MUST then send a DHCPv6 Request to configure the address of the Softwire endpoint. Duplicate Address Detection ([RFC4861]) MUST be performed on the Softwire in both cases. 5.4. DHCP The Softwire Initiator MAY use DHCP to get additional information such as delegated prefix and DNS servers. 5.4.1. DHCPv6 In the scenarios in Section 3.1, if the SI supports DHCPv6, it SHOULD send a Solicit message to verify if more information is available. If an SI establishing an IPv6 Softwire acts as a router (i.e., in the scenarios in Sections 3.1.2 and 3.1.4) it MUST include the Identity Association for Prefix Delegation (IA_PD) option [RFC3633] in the DHCPv6 Solicit message [RFC3315] in order to request an IPv6 prefix. When delegating an IPv6 prefix to the SI by returning a DHCPv6 Advertise message with the IA_PD and IP_PD Prefix options [RFC3633], the SC SHOULD inject a route for this prefix in the IPv6 routing table in order to forward the traffic to the relevant Softwire. Configuration of DNS MUST be done as specified in [RFC3646] and transmitted according to [RFC3315] and [RFC3736]. In general, all DHCPv6 options MUST be transmitted according to [RFC3315] and [RFC3736]. 5.4.2. DHCPv4 An SI establishing an IPv4 Softwire MAY send a DHCP request containing the Subnet Allocation option [SUBNET-ALL]. This practice is not common, but it may be used to connect IPv4 subnets using Softwires, as defined in Sections 3.2.2 and 3.2.4. Storer, et al. Standards Track [Page 29]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 One Subnet-Request suboption MUST be configured with the 'h' bit set to '1', as the SI is expected to perform the DHCP server function. The 'i' bit of the Subnet-Request suboption SHOULD be set to '0' the first time a prefix is requested and to '1' on subsequent requests, if a prefix has been allocated. The Prefix length suboption SHOULD be 0 by default. If the SI is configured to support only specific prefix lengths, it SHOULD specify the longest (smallest) prefix length it supports. If the SI was previously assigned a prefix from that same SC, it SHOULD include the Subnet-Information suboption with the prefix it was previously assigned. The 'c' and 's' bits of the suboption SHOULD be set to '0'. In the scenarios in Section 3.2, when delegating an IPv4 prefix to the SI, the SC SHOULD inject a route for this prefix in the IPv4 routing table in order to forward the traffic to the relevant Softwire. 6. Considerations about the Address Provisioning Model This section describes how a Softwire Concentrator may manage delegated addresses for Softwire endpoints and for subnets behind the Softwire Initiator. One common practice is to aggregate endpoints' addresses and delegated prefixes into one prefix routed to the SC. The main benefit is to ease the routing scheme by isolating on the SC succeeding route injections (when delegating new prefixes for SI). 6.1. Softwire Endpoints' Addresses 6.1.1. IPv6 A Softwire Concentrator should provide globally routable addresses to Softwire endpoints. Other types of addresses such as Unique Local Addresses (ULAs) [RFC4193] may be used to address Softwire endpoints in a private network with no global connectivity. A single /64 should be assigned to the Softwire to address both Softwire endpoints. Global addresses or ULAs must be assigned to endpoints when the scenario "Host CPE as Softwire Initiator" (described in Section 3.1.1) is considered to be deployed. For other scenarios, link-local addresses may also be used. Storer, et al. Standards Track [Page 30]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 6.1.2. IPv4 A Softwire Concentrator may provide either globally routable or private IPv4 addresses. When using IPv4 private addresses [RFC1918] on the endpoints, it is not recommended to delegate an IPv4 private prefix to the SI, as it can lead to a nested-NAT situation. The endpoints of the PPP link use host addresses (i.e., /32), negotiated using IPCP. 6.2. Delegated Prefixes 6.2.1. IPv6 Prefixes Delegated IPv6 prefixes should be of global scope if the IPv6 addresses assigned to endpoints are global. Using ULAs is not recommended when the subnet is connected to the global IPv6 Internet. When using IPv6 ULAs on the endpoints, the delegated IPv6 prefix may be either of global or ULA scope. Delegated IPv6 prefixes are between /48 and /64 in length. When an SI receives a prefix shorter than 64, it can assign different /64 prefixes to each of its interfaces. An SI receiving a single /64 is expected to perform bridging if more than one interface is available (e.g., wired and wireless). 6.2.2. IPv4 Prefixes Delegated IPv4 prefixes should be routable within the address space used by assigned IPv4 addresses. Delegate non-routable IPv4 prefixes (i.e., private IPv4 prefix over public IPv4 addresses or another class of private IPv4 addresses) is not recommended as a practice for provisioning and address translation should be considered in these cases. The prefix length is between /8 and /30. 6.3. Possible Address Provisioning Scenarios This section summarizes the different scenarios for address provisioning with the considerations given in the previous sections. Storer, et al. Standards Track [Page 31]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 6.3.1. Scenarios for IPv6 This table describes the possible combination of IPv6 address scope for endpoints and delegated prefixes. +------------------+-----------------------+------------------------+ | Endpoint IPv6 | Delegated Global IPv6 | Delegated ULA IPv6 | | Address | Prefix | Prefix | +------------------+-----------------------+------------------------+ | Link Local | Possible | Possible | | | | | | ULA | Possible | Possible | | | | | | Global | Possible | Possible, but Not | | | | Recommended | +------------------+-----------------------+------------------------+ Table 1: Scenarios for IPv6 6.3.2. Scenarios for IPv4 This table describes the possible combination of IPv4 address scope for endpoints and delegated prefixes. +-------------+-----------------+-----------------------------------+ | Endpoint | Delegated | Delegated Private IPv4 Prefix | | IPv4 | Public IPv4 | | | Address | Prefix | | +-------------+-----------------+-----------------------------------+ | Private | Possible | Possible, but Not Recommended | | IPv4 | | when using NAT (cf. | | | | Section 6.1.2) | | | | | | Public IPv4 | Possible | Possible, but NAT usage is | | | | recommended (cf. Section 6.2.2) | +-------------+-----------------+-----------------------------------+ Table 2: Scenarios for IPv4 7. Considerations about Address Stability A Softwire can provide stable addresses even if the underlying addressing scheme changes, by opposition to automatic tunneling. A Softwire Concentrator should always provide the same address and prefix to a reconnecting user. However, if the goal of the Softwire service is to provide a temporary address for a roaming user, it may be provisioned to provide only a temporary address. Storer, et al. Standards Track [Page 32]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 The address and prefix are expected to change when reconnecting to a different Softwire Concentrator. However, an organization providing a Softwire service may provide the same address and prefix across different Softwire Concentrators at the cost of a more fragmented routing table. The routing fragmentation issue may be limited if the prefixes are aggregated in a location topologically close to the SC. This would be the case, for example, if several SCs are put in parallel for load-balancing purpose. 8. Considerations about RADIUS Integration The Softwire Concentrator is expected to act as a client to a AAA server, for example, a RADIUS server. During the PPP authentication phase, the RADIUS server may return additional information in the form of attributes in the Access-Accept message. The Softwire Concentrator may include the Tunnel-Type and Tunnel- Medium-Type attributes [RFC2868] in the Access-Request messages to provide a hint of the type of Softwire being configured. 8.1. Softwire Endpoints 8.1.1. IPv6 Softwires If the RADIUS server includes a Framed-Interface-Id attribute [RFC3162], the Softwire Concentrator must send it to the Softwire Initiator in the Interface-Identifier field of its IPV6CP Configuration Request message. If the Framed-IPv6-Prefix attribute [RFC3162] is included, that prefix must be used in the router advertisements sent to the SI. If Framed-IPv6-Prefix is not present but Framed-IPv6-Pool is, the SC must choose a prefix from that pool to send RAs. 8.1.2. IPv4 Softwires If the Framed-IP-Address attribute [RFC2865] is present, the Softwire Concentrator must provide that address to the Softwire Initiator during IPCP address negotiation. That is, when the Softwire Initiator requests an IP address from the Softwire Concentrator, the address provided should be the Framed-IP-Address. Storer, et al. Standards Track [Page 33]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 8.2. Delegated Prefixes 8.2.1. IPv6 Prefixes If the attribute Delegated-IPv6-Prefix [RFC4818] is present in the RADIUS Access-Accept message, it must be used by the Softwire Concentrator for the delegation of the IPv6 prefix. Since the prefix delegation is performed by DHCPv6 and the attribute is linked to a username, the SC must associate the DHCP Unique Identifier (DUID) of a DHCPv6 request to the tunnel it came from and its user. Interaction between RADIUS, PPP, and DHCPv6 server may follow the mechanism proposed in [RELAY-RAD]. In this case, during the Softwire authentication phase, PPP collects the RADIUS attributes for the user such as Delegated-IPv6-Prefix. A specific DHCPv6 relay is assigned to the Softwire. The DHCPv6 relay fills in these attributes in the Relay agent RADIUS Attribute Option (RRAO) DHCPv6 option, before forwarding the DHCPv6 requests to the DHCPv6 server. 8.2.2. IPv4 Prefixes RADIUS does not define an attribute for the delegated IPv4 Prefix. Attributes indicating an IPv4 prefix and its length (for instance the combination of the Framed-IP-Address and Framed-IP-Netmask attributes [RFC2865]) may be used by the Softwire Concentrator to delegate an IPv4 prefix to the Softwire Initiator. The Softwire Concentrator must add a corresponding route with the Softwire Initiator as next- hop. As this practice had been used, the inclusion of the Framed-IP- Netmask attribute along with the Framed-IP-Address attribute tells the Softwire Concentrator to delegate an IPv4 prefix to the Softwire Initiator (e.g., in the IPv4-over-IPv6 scenarios where the Softwire Initiator is a router, see Sections 3.2.2 and 3.2.4), as the SC should forward packets destined to any IPv4 address in the prefix to the SI. 9. Considerations for Maintenance and Statistics Existing protocol mechanics for conveying adjunct or accessory information for logging purposes, including L2TPv2 and RADIUS methods, can include informational text that the behavior is according to the Softwire "Hub and Spoke" framework (following the implementation details specified in this document). Storer, et al. Standards Track [Page 34]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 9.1. RADIUS Accounting RADIUS Accounting for L2TP and PPP are documented (see Section 4.3). When deploying Softwire solutions, operators may experience difficulties to differentiate the address family of the traffic reported in accounting information from RADIUS. This problem and some potential solutions are described in [SW-ACCT]. 9.2. MIBs MIB support for L2TPv2 and PPP are documented (see Section 4.4). Also, see [RFC4293]. 10. Security Considerations One design goal of the "Hub and Spoke" problem is to very strongly consider the reuse of already deployed protocols (see [RFC4925]). Another design goal is a solution with very high scaling properties. L2TPv2 [RFC2661] is the phase 1 protocol used in the Softwire "Hub and Spoke" solution space, and the L2TPv2 security considerations apply to this document (see Section 9 of [RFC2661]). The L2TPv2 Softwire solution adds the following considerations: o L2TP Tunnel Authentication (see Sections 5.1.1 and 9.1 of [RFC2661]) provides authentication at tunnel setup. It may be used to limit DoS attacks by authenticating the tunnel before L2TP and PPP resources are allocated. o In a Softwire environment, L2TPv2 AVPs do not transport sensitive data, and thus the L2TPv2 AVP hiding mechanism is not used (see Section 5.1.1). o PPP CHAP [RFC1994] provides basic user authentication. Other authentication protocols may additionally be supported (see Section 5.2.3). L2TPv2 can also be secured with IPsec to provide privacy, integrity, and replay protection. Currently, there are two different solutions for security L2TPv2 with IPsec: o Securing L2TPv2 using IPsec "version 2" (IKEv1) is specified in [RFC3193], [RFC3947], and [RFC3948]. When L2TPv2 is used in the Softwire context, the voluntary tunneling model applies. [RFC3193] describes the interaction between IPsec and L2TPv2, and Storer, et al. Standards Track [Page 35]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 is deployed. [RFC3193] MUST be supported, given that deployed technology must be very strongly considered [RFC4925] for this 'time-to-market' solution. o [SW-SEC] also specifies a new (incompatible) solution for securing L2TPv2 with IPsec "version 3" (IKEv2). Section 3.5 of [SW-SEC] describes the advantages of using IKEv2, and this solution needs to be considered for future phases. Additional discussion of Softwire security is contained in [SW-SEC]. 11. Acknowledgements The authors would like to acknowledge the following contributors who provided helpful input on this document: Florent Parent, Jordi Palet Martinez, Ole Troan, Shin Miyakawa, Carl Williams, Mark Townsley, Francis Dupont, Ralph Droms, Hemant Singh, and Alain Durand. The authors would also like to acknowledge the participants in the Softwires interim meetings held in Hong Kong, China, and Barcelona, Spain. The minutes for the interim meeting at the China University - Hong Kong (February 23-24, 2006) are at <http://www.ietf.org/proceedings/06mar/isoftwire.html>. The minutes for the interim meeting at Polytechnic University of Catalonia - Barcelona (September 14-15, 2006) are reachable at <http://www.ietf.org/proceedings/06nov/isoftwire.html>. The Softwires auxiliary page at <http://bgp.nu/~dward/softwires/> contains additional meeting information. During and after the IETF Last Call, useful comments and discussion were provided by Jari Arkko, David Black, Lars Eggert, Pasi Eronen, and Dan Romascanu. Storer, et al. Standards Track [Page 36]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 12. References 12.1. Normative References [RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol (IPCP)", RFC 1332, May 1992. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [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. [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August 1999. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC 3162, August 2001. [RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth, "Securing L2TP using IPsec", RFC 3193, November 2001. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [RFC3371] Caves, E., Calhoun, P., and R. Wheeler, "Layer Two Tunneling Protocol "L2TP" Management Information Base", RFC 3371, August 2002. [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6", RFC 3633, December 2003. Storer, et al. Standards Track [Page 37]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 [RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6", RFC 3736, April 2004. [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. [RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix Attribute", RFC 4818, April 2007. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, September 2007. [RFC5072] S.Varada, Haskins, D., and E. Allen, "IP Version 6 over PPP", RFC 5072, September 2007. 12.2. Informative References [RELAY-RAD] Lau, W., "DHCPv6 Relay agent RADIUS Attribute Option", Work in Progress, February 2006. [RFC1471] Kastenholz, F., "The Definitions of Managed Objects for the Link Control Protocol of the Point-to-Point Protocol", RFC 1471, June 1993. [RFC1473] Kastenholz, F., "The Definitions of Managed Objects for the IP Network Control Protocol of the Point-to-Point Protocol", RFC 1473, June 1993. [RFC1877] Cobb, S. and F. Baker, "PPP Internet Protocol Control Protocol Extensions for Name Server Addresses", RFC 1877, December 1995. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [RFC2433] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433, October 1998. Storer, et al. Standards Track [Page 38]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 [RFC2867] Zorn, G., Aboba, B., and D. Mitton, "RADIUS Accounting Modifications for Tunnel Protocol Support", RFC 2867, June 2000. [RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, June 2000. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote Authentication Dial In User Service)", RFC 3575, July 2003. [RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. [RFC4087] Thaler, D., "IP Tunnel MIB", RFC 4087, June 2005. [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, October 2005. [RFC4293] Routhier, S., "Management Information Base for the Internet Protocol (IP)", RFC 4293, April 2006. [RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge- to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623, August 2006. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. Storer, et al. Standards Track [Page 39]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 [RFC4925] Li, X., Dawkins, S., Ward, D., and A. Durand, "Softwire Problem Statement", RFC 4925, July 2007. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007. [RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication Dial In User Service (RADIUS) Implementation Issues and Suggested Fixes", RFC 5080, December 2007. [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for Application Designers", BCP 145, RFC 5405, November 2008. [SUBNET-ALL] Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp, "Subnet Allocation Option", Work in Progress, March 2009. [SW-ACCT] Stevant, B., Toutain, L., Dupont, F., and D. Binet, "Accounting on Softwires", Work in Progress, April 2009. [SW-SEC] Yamamoto, S., Williams, C., Parent, F., and H. Yokota, "Softwire Security Analysis and Requirements", Work in Progress, May 2009. Authors' Addresses Bill Storer Cisco Systems 170 W Tasman Dr San Jose, CA 95134 USA EMail: bstorer@cisco.com Carlos Pignataro (editor) Cisco Systems 7200 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 USA EMail: cpignata@cisco.com Storer, et al. Standards Track [Page 40]
RFC 5571 Softwire H & S Framework with L2TPv2 June 2009 Maria Alice Dos Santos Cisco Systems 170 W Tasman Dr San Jose, CA 95134 USA EMail: mariados@cisco.com Bruno Stevant (editor) TELECOM Bretagne 2 rue de la Chataigneraie CS17607 Cesson Sevigne, 35576 France EMail: bruno.stevant@telecom-bretagne.eu Laurent Toutain TELECOM Bretagne 2 rue de la Chataigneraie CS17607 Cesson Sevigne, 35576 France EMail: laurent.toutain@telecom-bretagne.eu Jean-Francois Tremblay Videotron Ltd. 612 Saint-Jacques Montreal, QC H3C 4M8 Canada EMail: jf@jftremblay.com Storer, et al. Standards Track [Page 41]