v6ops Working Group L. Colitti Internet-Draft Google, LLC Intended status: Informational J. Linkova, Ed. Expires: 31 August 2023 X. Ma, Ed. Google 27 February 2023 Using DHCP-PD to Allocate /64 per Host in Broadcast Networks draft-collink-v6ops-ent64pd-02 Abstract This document discusses the IPv6 deployment scenario when individual hosts connected to broadcast networks (like WiFi hotspots or enterprise networks) are allocated /64 subnets via DHCP-PD. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 31 August 2023. Copyright Notice Copyright (c) 2023 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 (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Colitti, et al. Expires 31 August 2023 [Page 1] Internet-Draft MultAddrr February 2023 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Design Principles . . . . . . . . . . . . . . . . . . . . . . 3 4. Prefix Length Considerations . . . . . . . . . . . . . . . . 4 5. Routing Considerations . . . . . . . . . . . . . . . . . . . 5 5.1. First-Hop Routers Requirements . . . . . . . . . . . . . 5 5.2. Topologies with Multiple First-Hop Routers . . . . . . . 5 5.3. Preventing Routing Loops . . . . . . . . . . . . . . . . 5 6. DHCPv6-PD Server Considerations . . . . . . . . . . . . . . . 6 7. Antispoofing and SAVI Interaction . . . . . . . . . . . . . . 6 8. Migration Strategies and Co-existence with SLAAC Using Prefixes From PIO . . . . . . . . . . . . . . . . . . . . . . . . 7 9. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7 10. Applicability and Limitations . . . . . . . . . . . . . . . . 8 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 13. Security Considerations . . . . . . . . . . . . . . . . . . . 9 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 14.1. Normative References . . . . . . . . . . . . . . . . . . 9 14.2. Informative References . . . . . . . . . . . . . . . . . 10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction Unlike IPv4, IPv6 allows (and often requires) hosts to have multiple addresses. At the very least, a host can be expected to have one link-local address, one temporary address and, in most cases, one stable global address. On an IPv6-only network the device would need to have a dedicated 464XLAT address, which brings the total number of addresses to 4. If the network is multihomed and uses two different prefixes, or during graceful renumbering (when the old prefix is deprecated), or if an enterprise uses ULAs, the number of global addresses can easily double, bringing the total number of addresses to 7. Devices running containers/namespaces might need even more addresses per physical host. On one hand multiple addresses could be considered as a significant advantage of IPv6. On the other hand, however, they are sometimes seen as a drawback for the following reasons: Colitti, et al. Expires 31 August 2023 [Page 2] Internet-Draft MultAddrr February 2023 * Increased number of addresses introduces scalability issues on the network infrastructure. Network devices need to maintain various types of tables/hashes (Neighbor Cache on first-hop routers, Neighbor Discovery Proxy caches on L2 devices etc). On VXLAN [RFC7348] networks each address might be represented as a route so 8 addresses instead of 1 requires the devices to support 8 times more routes, etc. * An operator might need to know all addresses used by a given device in the past for forensics or troubleshooting purposes. * If an infrastructure device resources are exhausted, the device might drop some IPv6 addresses from the corresponding tables. The host, however, might be still using the address to send traffic. This leads to traffic blackholing and degraded customer experience. [RFC7934] discusses this aspect and explicitly states that IPv6 deployments SHOULD NOT limit the number of IPv6 addresses a host can have. However it seems inevitable that some limits might need to be imposed by the network in an attempt to protect the network resources and prevent DoS attacks (see [I-D.linkova-v6ops-ipmaclimi]). Therefore it would be beneficial for IPv6 deployments to address the above mentioned issues while still allowing hosts to have multiple IPv6 addresses. One of the very promising approaches is allocating an unique /64 prefix per host ([RFC8273]). The same principle has been actively used in mobile IPv6 deployments. However it's very uncommon in enterprise-style networks, where nodes are usually connected to broadcast segments/VLANs and each segment has a single /64 subnet assigned. This document expands the approach defined in [RFC8273] to allocate an unique IPv6 /64 prefix per host using DHCP- PD. 2. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Design Principles Instead of all hosts on a given segment forming addresses from the same /64 assigned to that segment: Colitti, et al. Expires 31 August 2023 [Page 3] Internet-Draft MultAddrr February 2023 * A host acts as DHCP-PD client and requests /64 via DHCPv6-PD by sending IA_PD. * The first-hop router acts as a DHCPv6-PD relay and sends the request to the DHCPv6-PD servers. In smaller networks it's entirely possible for the first-hop router to act as a DHCPv6-PD server and assign /64 from a larger pool allocated for the given segment or the whole site. * The allocated /64 is installed into the first-hop router routing table as a route pointing to the client's link-local address. For the router and all other infrastructure devices that prefix is considered off-link, so no traffic to that prefix does not trigger any ND packets. * The host uses the delegated /64 to allocate addresses to its interfaces, containers etc. For example, the device can include that /64 into Router Advertisements sent to virtual systems or to any other devices connected to its downstream interface. 4. Prefix Length Considerations As per [RFC7421] /64 is the only prefix size that will allow the host to use SLAAC. As a result delegating a /64 prefix to the client allows the client to provide limitless addresses to IPv6 nodes connected to it (e.g., virtual machines, tethered devices), because all IPv6 nodes are required to support SLAAC. In other words, delegating /64 allows the hosts to extend the network arbitrarily, similarily to using NAT in IPv4. Chosing longer prefixes would require the host and any connected system to use some other form of address assignment and therefore would drastically limit applicability of the proposed solution. Section 9.2 of [RFC7934] demonstrates that if a network uses 10.0.0.0/8 to address hosts, /40 would be sufficient to provide each host with /64. In multi-site networks the calculations might get more complex as each site IPv6 prefix needs to be larger enough to be globally routable and accepted by eBGP peers, for example /48. Let's consider an enterprise network which has 8000 sites (~2^13). Imagine that site has up to 64 (2^6) different network types and each network requires its own /48. So each network can provide /64 to 65K clients (an equivalent of using /16 IPv4 subnet to address hosts). In that case such an enterprise would need /29 (48 - 6 - 13) to provide /64 to its devices. Networks of such size usually have multiple allocations from RIRs so /29 sounds reasonable. In real life there are very few networks of that scale and a single /32 would be sufficient for most deployments. Colitti, et al. Expires 31 August 2023 [Page 4] Internet-Draft MultAddrr February 2023 5. Routing Considerations 5.1. First-Hop Routers Requirements The design described in this document is targeted to large networks were the number of clients combined with multiple IPv6 addresses per client creates scalability issues. In such networks DHCPv6 servers are usually deployed as dedicated systems, so the first-hop routers act as DHCP relays. To delegate IPv6 prefixes to hosts the first hop router needs to implement DHCPv6-PD relay functions and meet the requirements defined in [RFC8987]. In particular, if the same DHCPv6-PD pool is used for clients connected to multiple routers, dynamic routing protocols are required to propagate the routes to the allocated prefixes. Each relay needs to advertise the learned delegated leases as per requirement R-4 specified in Section 4.2 of [RFC8987]. 5.2. Topologies with Multiple First-Hop Routers Traditionally DHCPv6-PD is used in environments where a DHCPv6-PD client (a home CPE, for example) is connected to a single router which performs DHCPv6-PD relay functions. In the topology with redundant first-hop routers, all those routers need to snoop DHCPv6 traffic, install the delegated prefixes into its routing table and, if needed, advertise those prefixes to the network. That means that all relays the host is connected to must be able to snoop DHCPv6-PD traffic, in particular Reply messages sent by the server (as those messages contain the delegated prefix). Normally the client uses multicast to reach all servers or an individual server (see Section 14 of [RFC8415]). As per Section 18.4 of [RFC8415] the server is not supposed to accept unicast traffic when it is not explicitly configured to do, and unicast transmission is only allowed for some messages and only if the Server Unicast option ([RFC8415], Section 21.12) is used. Therefore, in the topologies with multiple first-hop routers the DHCPv6 servers MUST be configured not to use the Server Unicast option (it should be noted that [I-D.dhcwg-dhc-rfc8415bis] deprecates the Server Unicast option exactly because it is not compatible with multiple relays topology). Therefore as long as the Server Unicast option is not used, all first-hop routers shall be able to install the route for the delegates prefix. 5.3. Preventing Routing Loops To prevent routing loops caused by traffic to unused addresses from the delegated /64, the client MUST drop all packets to such addresses (see the requirement WPD-5 in Section 4.2 of [RFC7084]. Colitti, et al. Expires 31 August 2023 [Page 5] Internet-Draft MultAddrr February 2023 6. DHCPv6-PD Server Considerations The following requirements are applicable to the DHCPv6-PD server delegating prefixes to endhosts: * The server MUST follow [RFC8168] recommendations on processing prefix-length hints. Specifically, the server MUST NOT delegate prefixes longer than /64 if the client sets the prefix-length field of the IA_PD option to 64. This is required so the host can use SLAAC to assign addresses from the delegated /64 to its interfaces or virtual instances. The client MAY ignore any delegated prefixes longer that /64. * Do not send the Server Unicast option to the client unless the network topology guarantees that no client is connected to a segment with multiple relays (see Section 5.2). 7. Antispoofing and SAVI Interaction Enabling the unicast Reverse Path Forwarding (uRPF) on the first-hop router interfaces towards clients provides the first layer of defence agains spoofing. If the malicious client sends a spoofed packet it would be dropped by the router unless the spoofed address belongs to a prefix delegated to another client on the same interface. Therefore the malicious client can only spoof addresses already delegated to another client on the same segment or another host link- local address. Source Address Validation Improvement (SAVI, [RFC7039]) provides more reliable protection against address spoofing. Administrators deploying the proposed solution on the SAVI-enabled infastructure should ensure that SAVI perimeter devices support DHCPv6-PD snooping to create the correct binding for the delegated prefixes (see [RFC7513]). Using FCFS SAVI ([RFC6620]) for protecting link-local addresses and creating SAVI bindings for DHCPv6-PD assigned prefixes would prevent spoofing. It should be noted that using DHCPv6-PD makes it harder for an attacker to select the spoofed source address. When all hosts are using the same /64 to form addresses, the attacker might learn addresses used by other hosts by listening to multicast Neighbor Solicitations and Neighbour Advertisements. In DHCPv6-PD environments, however, the attacker can only learn about other hosts global addresses by listening to multicast DHCPv6 messages, which are not transmitted so often. Colitti, et al. Expires 31 August 2023 [Page 6] Internet-Draft MultAddrr February 2023 8. Migration Strategies and Co-existence with SLAAC Using Prefixes From PIO It would be beneficial for the network to explicitly indicate its support of DHCPv6-PD for hosts. * In small networks (e.g. home ones), where the number of devices is not too high, the number of available /64 becomes a limiting factor. If every phone or laptop in a home network would request an unique /64, the home network might run out of /64s, if the prefix allocated to the CPE by its ISP is too small (e.g. if an ISP allocates /60, it would only allow 16 devices to request /64). So while the enterprise network administrator might want all phones in the network to request /64, it would be highly undesirable for the same phone to request /64 when connecting to a home network. * When the network supports both /64 per host and /64 advertised in PIO as address assignment methods, it's highly desirable for the host NOT to use the PIO prefix tto form global addresses and only use the prefix delegated via DHCPv6-PD. Starting both SLAAC using the PIO prefix and DHCPv6-PD and deprecating that SLAAC addresses after receiving a delegated prefix would be very disruptive for applications. If the host continues to use addresses formed from the PIO prefix it would not only undermine the benefits of the proposed solution (see Section 9), but would also introduce complexity and unpredictability in the source address selection. Therefore the host needs to know what address assignment method to use and whether to use the prefix in PIO or not, if the network provides the PIO with A flag set. To allow the network to signal DHCPv6-PD support a new PIO flag is proposed (link to the 6man draft will be added here after this draft is adopted). 9. Benefits The proposed solution provides the following benefits: * The network devices resources (e.g. memory) need to scale to the number of devices, not the number of IPv6 addresses. The first- hop routers have a single /64 route per host pointing to the host's link-local address. * If all devices connected to the given network segment support this mode of operation and can generate addresses from the delegated /64, there is no reason to advertise a common /64 assigned to that segment in PIO with 'A' flag set. Therefore it is possible to Colitti, et al. Expires 31 August 2023 [Page 7] Internet-Draft MultAddrr February 2023 remove the global /64 prefix from that network segment and the router interface completely, so no global addresses are on-link for the segment. This would lead to reducing the attack surface for Neighbor Discovery attacks described in [RFC6583]. * DHCP-PD logs and first-hop routers routing tables provide complete information on IPv6 to MAC mapping, which can be used for forensics and troubleshooting. Such information is much less dynamic than ND cache and therefore it's much easier for an operator to collect and process it. * The cost of having multiple addresses is offloaded to the endpoint. Devices are free to create and use as many addresses as they need. * Fate sharing: all global addresses used by a given device are routed as a single /64. Either all of them work or not, which makes the failures easier to diagnoze and mitigate. * Ability to extend the network transparently. If the hosts use SLAAC, delegating /64 allows the host to provide connectivity to other hosts, like it is possible in IPv4 with NAT. 10. Applicability and Limitations The solution described in this document shouldn't be seen as an attempt to deprecate SLAAC. Delegating /64 provides an alternative for SLAAC rather than replacing it, so network administrators and OS developers have a choice. This design would substantially benefit some networks (see Section 9), so running an additional service and allocation larger amount of address space is well justified. Examples of such networks include but are not limited to: * Large-scale networks where even 3-5 addresses per client might intorduce scalability issues. * Networks with high number of virtual hosts, so physical enpoints require multiple addresses. * Managed networks where extensive troubleshooting, host traffic logging or forensics might be required. Smaller networks (like home ones) with smaller address space and lower number of clients, SLAAC might be a better and simpler option. Colitti, et al. Expires 31 August 2023 [Page 8] Internet-Draft MultAddrr February 2023 11. Privacy Considerations Eventually, if/when the vast majority of endpoints support the proposed mechanism, an eavesdropper/information collector might be able to correlate the prefix to the device. To mitigate the threat the host might implement a mechanism similar to SLAAC temporary extensions ([RFC8981]) but for delegated prefixes: * The host requests another /64 prefix. * Upon receiving the new prefix the host deprecates all addresses from the old one. * After some time (shall it be T2 from IA_PD for the original prefix?) the host sends RELEASE for the old prefix. 12. IANA Considerations This memo includes no request to IANA. 13. Security Considerations A malicious or just misbehaving host might exhaust the DHCP-PD pool by sending a large number of requests with various DUIDs. However this is not a new issue as the same attack might be implemented in DHCPv4 or DHCPv6 for IA_NA requests. A malicious host might request a prefix and then release it very quickly, causing routing convergence events on the relays. The probability of such attack can be reduced if the network rate limits the amount of broadcast and multicast messages from the client. Delegating the same prefix for the same host introduces privacy concerns. The proposed mitigation is discussed in Section 11. Spoofing scenarios and prevention mechanisms are discussed in Section 7. 14. References 14.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Colitti, et al. Expires 31 August 2023 [Page 9] Internet-Draft MultAddrr February 2023 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, . [RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, DOI 10.17487/RFC7084, November 2013, . [RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS SAVI: First-Come, First-Served Source Address Validation Improvement for Locally Assigned IPv6 Addresses", RFC 6620, DOI 10.17487/RFC6620, May 2012, . [RFC8168] Li, T., Liu, C., and Y. Cui, "DHCPv6 Prefix-Length Hint Issues", RFC 8168, DOI 10.17487/RFC8168, May 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8273] Brzozowski, J. and G. Van de Velde, "Unique IPv6 Prefix per Host", RFC 8273, DOI 10.17487/RFC8273, December 2017, . [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, . [RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves, "Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6", RFC 8981, DOI 10.17487/RFC8981, February 2021, . [RFC8987] Farrer, I., Kottapalli, N., Hunek, M., and R. Patterson, "DHCPv6 Prefix Delegating Relay Requirements", RFC 8987, DOI 10.17487/RFC8987, February 2021, . 14.2. Informative References Colitti, et al. Expires 31 August 2023 [Page 10] Internet-Draft MultAddrr February 2023 [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational Neighbor Discovery Problems", RFC 6583, DOI 10.17487/RFC6583, March 2012, . [RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed., "Source Address Validation Improvement (SAVI) Framework", RFC 7039, DOI 10.17487/RFC7039, October 2013, . [RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S., Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit Boundary in IPv6 Addressing", RFC 7421, DOI 10.17487/RFC7421, January 2015, . [RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address Validation Improvement (SAVI) Solution for DHCP", RFC 7513, DOI 10.17487/RFC7513, May 2015, . [RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, L., Sridhar, T., Bursell, M., and C. Wright, "Virtual eXtensible Local Area Network (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014, . [RFC7934] Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi, "Host Address Availability Recommendations", BCP 204, RFC 7934, DOI 10.17487/RFC7934, July 2016, . [I-D.linkova-v6ops-ipmaclimi] Linkova, J., "Minimizing Damage of Limiting Number of IPv6 Addresses per Host", Work in Progress, Internet-Draft, draft-linkova-v6ops-ipmaclimi-00, 7 November 2022, . [I-D.dhcwg-dhc-rfc8415bis] Mrugalski, T., Volz, B., Richardson, M., Jiang, S., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", Work in Progress, Internet-Draft, draft-dhcwg- dhc-rfc8415bis-00, 7 October 2022, . Colitti, et al. Expires 31 August 2023 [Page 11] Internet-Draft MultAddrr February 2023 Acknowledgements Thanks to Brian Carpenter, Gert Doering, Fernando Gont, Martin Hunek, Erik Kline, Pascal Thubert, Eduard Vasilenko, Timothy Winters for the discussions, the input and all contribution. Contributors Authors' Addresses Lorenzo Colitti Google, LLC Shibuya 3-21-3, Japan Email: lorenzo@google.com Jen Linkova (editor) Google 1 Darling Island Rd Pyrmont NSW 2009 Australia Email: furry13@gmail.com, furry@google.com Xiao Ma (editor) Google Shibuya 3-21-3, Japan Email: xiaom@google.com Colitti, et al. Expires 31 August 2023 [Page 12]