Internet Engineering Task Force (IETF) O. Troan, Ed. Request for Comments: 7157 Cisco Category: Informational D. Miles ISSN: 2070-1721 Google Fiber S. Matsushima Softbank Telecom T. Okimoto NTT West D. Wing Cisco March 2014 IPv6 Multihoming without Network Address Translation
AbstractNetwork Address and Port Translation (NAPT) works well for conserving global addresses and addressing multihoming requirements because an IPv4 NAPT router implements three functions: source address selection, next-hop resolution, and (optionally) DNS resolution. For IPv6 hosts, one approach could be the use of IPv6-to-IPv6 Network Prefix Translation (NPTv6). However, NAT and NPTv6 should be avoided, if at all possible, to permit transparent end-to-end connectivity. In this document, we analyze the use cases of multihoming. We also describe functional requirements and possible solutions for multihoming without the use of NAT in IPv6 for hosts and small IPv6 networks that would otherwise be unable to meet minimum IPv6-allocation criteria. We conclude that DHCPv6-based solutions are suitable to solve the multihoming issues described in this document, but NPTv6 may be required as an intermediate solution. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7157.
Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. IPv6 Multihomed Network Scenarios . . . . . . . . . . . . . . 6 3.1. Classification of Network Scenarios for Multihomed Host . 6 3.2. Multihomed Network Environment . . . . . . . . . . . . . 8 3.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 9 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. End-to-End Transparency . . . . . . . . . . . . . . . . . 11 4.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 11 5. Problem Analysis . . . . . . . . . . . . . . . . . . . . . . 11 5.1. Source Address Selection . . . . . . . . . . . . . . . . 11 5.2. Next Hop Selection . . . . . . . . . . . . . . . . . . . 12 5.3. DNS Recursive Name Server Selection . . . . . . . . . . . 13 6. Implementation Approach . . . . . . . . . . . . . . . . . . . 13 6.1. Source Address Selection . . . . . . . . . . . . . . . . 14 6.2. Next Hop Selection . . . . . . . . . . . . . . . . . . . 14 6.3. DNS Recursive Name Server Selection . . . . . . . . . . . 15 6.4. Other Algorithms Available in RFCs . . . . . . . . . . . 16 7. Considerations for MHMP Deployment . . . . . . . . . . . . . 16 7.1. Non-MHMP Host Consideration . . . . . . . . . . . . . . . 16 7.2. Coexistence Considerations . . . . . . . . . . . . . . . 17 7.3. Policy Collision Consideration . . . . . . . . . . . . . 17 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 19 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 10.1. Normative References . . . . . . . . . . . . . . . . . . 20 10.2. Informative References . . . . . . . . . . . . . . . . . 20
RFC6296], or; 2. refining IPv6 specifications to resolve the problems with IPv6 multihoming. This document concerns itself with the latter and explores the solution space. We hope this will encourage the development of solutions to the problem so that, in the long run, NPTv6 can be avoided. IPv6 provides enough globally unique addresses to permit every conceivable host on the Internet to be uniquely addressed without the requirement for Network Address Port Translation (NAPT) [RFC3022], offering a renaissance in end-to-end transparent connectivity. Unfortunately, this may not be possible in every case, due to the possible necessity of NAT even in IPv6, because of multihoming. Though there are mechanisms to implement multihoming, such as BGP multihoming [RFC4116] at the network level and multihoming based on the Stream Control Transmission Protocol (SCTP) [RFC4960] in the transport layer, there is no mechanism in IPv6 that serves as a replacement for NAT-based multihoming in IPv4. In IPv4, for a host or a small network, NAT-based multihoming is easily deployable and is an already-deployed technique. Whenever a host or small network (that does not meet minimum IPv6 allocation criteria) is connected to multiple upstream networks, an IPv6 address is assigned by each respective service provider resulting in hosts with multiple global scope IPv6 addresses with different prefixes. As each service provider is allocated a different address space from its Internet Registry, it, in turn, assigns a different address space to the end-user network or host. For example, a remote access user's host or router may use a VPN to simultaneously connect to a remote network and retain a default route to the Internet for other purposes.
In IPv4, a common solution to the multihoming problem is to employ NAPT on a border router and use private address space for individual host addressing. The use of NAPT allows hosts to have exactly one IP address visible on the public network, and the combination of NAPT with provider-specific outside addresses (one for each uplink) and destination-based routing insulates a host from the impacts of multiple upstream networks. The border router may also implement a DNS cache or DNS policy to resolve address queries from hosts. It is our goal to avoid the IPv6 equivalent of NAT. So, the goals for IPv6 multihoming defined in [RFC3582] do not match the goals of this document. Also, regardless of what the NPTv6 specification is, we are trying to avoid any form of network address translation technique that may not be visible to either of the end hosts. To reach this goal, several mechanisms are needed for end-user hosts to have multiple address assignments and resolve issues such as which address to use for sourcing traffic to which destination: o If multiple routers exist on a single link, the host must select the appropriate next hop for each connected network. Each router is in turn connected to a different service provider network, which provides independent address assignment. Routing protocols that would normally be employed for router-to-router network advertisement seem inappropriate for use by individual hosts. o Source address selection becomes difficult whenever a host has more than one address of the same address scope. Current address selection criteria may result in hosts using an arbitrary or random address when sourcing upstream traffic. Unfortunately, for the host, the appropriate source address is a function of the upstream network for which the packet is bound. If an upstream service provider uses IP anti-spoofing or ingress filtering, it is conceivable that the packets that have an inappropriate source address for the upstream network would never reach their destination. o In a multihomed environment, different DNS scopes or partitions may exist in each independent upstream network. A DNS query sent to an arbitrary upstream DNS recursive name server may result in incorrect or poisoned responses. In short, while IPv6 facilitates hosts having more than one address in the same address scope, the application of this causes significant issues for a host from routing, source address selection, and DNS resolution perspectives. A possible consequence of assigning a host multiple identically scoped addresses is severely impaired IP connectivity.
If a host connects to a network behind an IPv4 NAPT, the host has one private address in the local network. There is no confusion. The NAT becomes the gateway of the host and forwards the packet to an appropriate network when it is multihomed. It also operates a DNS cache server or DNS proxy, which receives all DNS inquires, and gives a correct answer to the host. RFC6296]. NAPT Network Address Port Translation as described in [RFC3022]. In other contexts, NAPT is often pronounced "NAT" or written as "NAT". MHMP Multihomed with multi-prefix. A host implementation that supports the mechanisms described in this document; namely, source address selection policy, next hop selection, and DNS selection policy.
+------+ ___________ | | / \ +---| rtr1 |=====/ network \ | | | \ 1 / +------+ | +------+ \___________/ | | | | hosts|-----+ | | | +------+ | +------+ ___________ | | | / \ +---| rtr2 |=====/ network \ | | \ 2 / +------+ \___________/ Figure 1: Single Uplink, Multiple Next Hop, Multiple Prefix (Scenario 1) Figure 1 illustrates the host connecting to rtr1 and rtr2 via a shared link. Networks 1 and 2 are reachable via rtr1 and rtr2, respectively. When the host sends packets to network 1, the next hop to network 1 is rtr1. Similarly, rtr2 is the next hop to network 2. Example: multiple broadband service providers (Internet, VoIP, IPTV, etc.)
Scenario 2: In this scenario, a single gateway router connects the host to two or more upstream service provider networks. This gateway router would receive prefix delegations and a different set of DNS recursive name servers from each independent service provider network. The gateway, in turn, advertises the provider prefixes to the host, and for DNS, may either act as a lightweight DNS cache server or advertise the complete set of service provider DNS recursive name servers to the hosts. +------+ ___________ +-----+ | | / \ | |=======| rtr1 |=====/ network \ | |port1 | | \ 1 / +------+ | | +------+ \___________/ | | | | | hosts|-----| GW | | | | rtr | +------+ | | +------+ ___________ | |port2 | | / \ | |-------| rtr2 |=====/ network \ +-----+ | | \ 2 / +------+ \___________/ Figure 2: Single Uplink, Single Next Hop, Multiple Prefix (Scenario 2) Figure 2 illustrates the host connected to GW rtr. GW rtr connects to networks 1 and 2 via port1 and 2, respectively. As the figure shows a logical topology of the scenario, port1 could be a pseudo- interface for tunneling, which connects to network 1 through network 2 and vice versa. When the host sends packets to either network 1 or 2, the next hop is GW rtr. When the packets are sent to network 1 (network 2), GW rtr forwards the packets to port1 (port2). Example: Internet + VPN / Application Service Provider (ASP)
Scenario 3: In this scenario, a host has more than one active interface that connects to different routers and service provider networks. Each router provides the host with a different address prefix and set of DNS recursive name servers, resulting in a host with a unique address per link/interface. +------+ +------+ ___________ | | | | / \ | |-----| rtr1 |=====/ network \ | | | | \ 1 / | | +------+ \___________/ | | | host | | | | | +------+ ___________ | | | | / \ | |=====| rtr2 |=====/ network \ | | | | \ 2 / +------+ +------+ \___________/ Figure 3: Multiple Uplink, Multiple Next Hop, Multiple Prefix (Scenario 3) Figure 3 illustrates the host connecting to rtr1 and rtr2 via a direct connection or a virtual link. When the host sends packets to network 1, the next hop to network 1 is rtr1. Similarly, rtr2 is the next hop to network 2. Example: Mobile Wifi + 3G, ISP A + ISP B
requirements for IPv6 multihomed environments. A destination prefix/ route is often used on the gateway router to separate traffic between the networks. +------+ ___________ | | / \ +---| rtr1 |=====/ network \ | | | \ 1 / +------+ +-----+ | +------+ \___________/ | IPv4 | | | | | hosts|-----| GW |---+ | | | rtr | | +------+ +-----+ | +------+ ___________ (NAPT&DNS) | | | / \ (private +---| rtr2 |=====/ network \ address | | \ 2 / space) +------+ \___________/ Figure 4: IPv4 Multihomed Environment with Gateway Router Performing NAPT Section 3.1, a number of connectivity issues are identified: Scenario 1: The host has been assigned an address from each router and recognizes both rtr1 and rtr2 as valid default routers (in the default routers list).
o The source address selection policy on the host does not deterministically resolve a source address. Ingress filtering or filter policies will discard traffic with source addresses that the operator did not assign. o The host will select one of the two routers as the active default router. No traffic is sent to the other router. Scenario 2: The host has been assigned two different addresses from the single gateway router. The gateway router is the only default router on the link. o The source address selection policy on the host does not deterministically resolve a source address. Ingress filtering or filter policies will discard traffic with source addresses that the operator did not assign. o The gateway router does not have an autonomous mechanism for determining which traffic should be sent to which network. If the gateway router is implementing host functions (i.e., processing Router Advertisement (RA)), then two valid default routers may be recognized. Scenario 3: A host has two separate interfaces, and each interface has a different address assigned. Each link has its own router. o The host does not have enough information to determine which traffic should be sent to which upstream routers. The host will select one of the two routers as the active default router, and no traffic is sent to the other router. The default address selection rules select the address assigned to the outgoing interface as the source address. So, if a host has an appropriate routing table, an appropriate source address will be selected. All scenarios: o In network deployments utilizing local namespaces, the host may choose to communicate with a "wrong" DNS recursive server unable to serve a local namespace.
RFC5245]). Therefore, the IPv6 multihoming solution should strive to avoid NPTv6 to achieve end-to-end transparency. Section 3 can be classified into these three types: o Wrong source address selection o Wrong next hop selection o Wrong DNS server selection This section reviews the problem statements presented above and the proposed functional requirements to resolve the issues. RFC6724] may not deterministically select the correct source address. [RFC7078] describes the use of the policy table (as discussed in [RFC6724]) to resolve this problem, using a DHCPv6 mechanism for host policy table management.
Again, by employing DHCPv6, the server could restrict address assignment (of additional prefixes) only to hosts that support policy table management. Scenario 1: Host needs to support the solution for this problem. Scenario 2: Host needs to support the solution for this problem. Scenario 3: If Host supports the next hop selection solution, there is no need to support the address selection functionality on the host. It is noted that the network's DHCP server and DHCP-forwarding routers must also support the Address Selection option [RFC7078]. RFC4861] to distribute default route/next-hop information to the host or gateway router. In this case, the host or gateway router may select any valid default router from the default routers list, resulting in traffic being sent to the wrong router and discarded by the upstream service provider. Using the above scenarios as an example, whenever the host wishes to reach a destination in network 2 and there is no connectivity between networks 1 and 2 (as is the case for a walled-garden or closed service), the host or gateway router does not know whether to forward traffic to rtr1 or rtr2 to reach a destination in network 2. The host or gateway router may choose rtr1 as the default router, but traffic will fail to reach the destination server. The host or gateway router requires route information for each upstream service provider, but the use of a routing protocol between the gateway and the two routers causes both configuration and scaling issues. In IPv4, gateway routers are often pre-configured with static routes or use the Classless Static Route Options [RFC3442] for DHCPv4. An extension to Router Advertisements through Default Router Preference and More-Specific Routes [RFC4191] provides for link-specific preferences but does not address per-host configuration in a multi- access topology because of its reliance on Router Advertisements. Scenario 1: Host needs to support the solution for this problem. Scenario 2: GW rtr needs to support the solution for this problem. Scenario 3: Host needs to support the solution for this problem.
RFC3646] or RA [RFC6106]. When the host or gateway router sends a DNS query, it would normally choose one of the available DNS recursive name servers for the query. In the IPv6 gateway router scenario, the Broadband Forum (BBF) [TR-124] requires that the query be sent to all DNS recursive name servers and that the gateway wait for the first reply. In IPv6, given our use of specific destination-based policy for both routing and source address selection, it is desirable to extend a policy- based concept to DNS recursive name server selection. Doing so can minimize DNS recursive name server load and avoid issues where DNS recursive name servers in different networks have connectivity issues, or the DNS recursive name servers are not publicly accessible. In the worst case, a DNS query for a name from a local namespace may not be resolved correctly if sent towards a DNS server not aware of said local namespace, resulting in a lack of connectivity. It is not an issue of the Domain Name System model itself, but an IPv6 multihomed host or gateway router should have the ability to select appropriate DNS recursive name servers for each service based on the domain space for the destination, and each service should provide rules specific to that network. [RFC6731] proposes a solution for distributing DNS server selection policy using a DHCPv6 option. Scenario 1: Host needs to support the solution for this problem. Scenario 2: GW rtr needs to support the solution for this problem. Scenario 3: Host needs to support the solution for this problem. It is noted that the network's DHCP server and DHCP-forwarding routers must also support the Address Selection option [RFC6731]. Section 5, in the multi-prefix environment, we have three problems: source address selection, next hop selection, and DNS recursive name server selection. In this section, possible solutions for each problem are introduced and evaluated against the requirements in Section 4.
RFC5220]. When solutions are examined against the requirements in Section 4, the proactive approaches, such as the policy table distribution mechanism and the routing hints mechanism, are more appropriate in that they can propagate the network administrator's policy directly. The policy distribution mechanism has an advantage with regard to the host's protocol stack impact and the static nature of the assumed target network environment. RFC4191], and the CPE WAN Management Protocol (CWMP) [TR069] standardized at BBF. The RA-based mechanism doesn't handle distribution of per-host routing information easily. Dynamic routing protocols are not typically used between residential users and ISPs, because of their scalability and security implications. The DHCPv6 mechanism does not have these problems and has the advantage of relay functionality. It is commonly used and is thus easy to deploy. [TR069], mentioned above, defines a possible solution mechanism for routing information distribution to customer premises equipment (CPE). It assumes, however, that IP reachability to the Auto Configuration Server (ACS) has been established. Therefore, if the CPE requires routing information to reach the ACS, CWMP [TR069] cannot be used to distribute this information.
RFC6731], where several pairs of DNS recursive name server addresses and DNS domain suffixes are defined as part of a policy and conveyed to hosts in a new DHCP option. In an environment where there is a home gateway router, that router can act as a DNS recursive name server, interpret this option, and distribute DNS queries to the appropriate DNS servers according to the policy.
RFC5533] and HIP [RFC5206], that may be useful in this environment. At the time of this writing, there is not enough operational experience on which to base a recommendation. Should such operational experience become available, this document may be updated in the future. Section 6). __________ / \ +---/ Internet \ gateway router | \ / +------+ +---------------------+ | \__________/ | | | | | WAN1 +--+ | host |-----|LAN| Router |--------| | | | | |NAT|WAN2+--+ +------+ +---------------------+ | __________ | / \ +---/ ASP \ \ / \__________/ Figure 5: Legacy Host The gateway router also has to support the two features, next hop selection and DNS server selection, shown in Section 6. The implementation and issues of NPTv6 are out of the scope of this document, but are discussed in Section 5 of [RFC6296].
A policy receiver is exposed to the threats of unauthorized policy, which can lead to session hijack, falsification, DoS, wiretapping, and phishing. Unauthorized policy here means a policy distributed from an entity that does not have rights to do so. Usually, only a site administrator and a network service provider have rights to distribute these policies in addition to IP address assignment and DNS server address notification. Regarding source address selection, unauthorized policy can expose an IP address that will not usually be exposed to an external server, which can be a privacy problem. To solve or mitigate the problem of unauthorized policy, one approach is to limit the use of these policy distribution mechanisms, as described in the Section 4.4 of [RFC6731]. For example, a policy should be preferred or accepted if delivered over a secure, trusted channel such as a cellular data connection. The proposed solutions are based on DHCP, so the limitation of local site communication, which is often used in WiFi access services, should be another solution or mitigation for this problem. For the DNS server selection issue, DNS Security (DNSSEC) can be another solution. For source address selection, the ingress filter at the network service provider router can be a solution. Another threat is the leakage of the policy and privacy issues resulting from that. Especially when clients receive different policies from the network service provider, that difference provides hints about the host itself and can be useful to uniquely identify the host. Encryption of the communication channel and separation of the communication channel per host can be solutions for this problem. The security threats related to IPv6 multihoming are described in [RFC4218].
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. [RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix Translation", RFC 6296, June 2011. [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, September 2012. [RFC6731] Savolainen, T., Kato, J., and T. Lemon, "Improved Recursive DNS Server Selection for Multi-Interfaced Nodes", RFC 6731, December 2012. [RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing Address Selection Policy Using DHCPv6", RFC 7078, January 2014. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4", RFC 3442, December 2002. [RFC3582] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site- Multihoming Architectures", RFC 3582, August 2003. [RFC3646] Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003. [RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V. Gill, "IPv4 Multihoming Practices and Limitations", RFC 4116, July 2005.
[RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming Solutions", RFC 4218, October 2005. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [RFC5206] Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End- Host Mobility and Multihoming with the Host Identity Protocol", RFC 5206, April 2008. [RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama, "Problem Statement for Default Address Selection in Multi- Prefix Environments: Operational Issues of RFC 3484 Default Rules", RFC 5220, July 2008. [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010. [RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim Protocol for IPv6", RFC 5533, June 2009. [RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 6106, November 2010. [TR-124] The Broadband Forum, "TR-124, Functional Requirements for Broadband Residential Gateway Devices", Issue: 2, May 2010, <http://www.broadband-forum.org/technical/download/ TR-124_Issue-2.pdf>. [TR069] The Broadband Forum, "TR-069, CPE WAN Management Protocol v1.1", Version: Issue 1 Amendment 2, December 2007, <http://www.broadband-forum.org/technical/download/ TR-069_Amendment-2.pdf>.
Authors' Addresses Ole Troan (editor) Cisco Oslo Norway EMail: firstname.lastname@example.org David Miles Google Fiber Mountain View, CA USA EMail: email@example.com Satoru Matsushima Softbank Telecom Tokyo Japan EMail: firstname.lastname@example.org Tadahisa Okimoto NTT West Osaka Japan EMail: email@example.com Dan Wing Cisco 170 West Tasman Drive San Jose USA EMail: firstname.lastname@example.org