Using DHCP-PD to Allocate Unique IPv6 Prefix per Host in Broadcast Networks

Using DHCP-PD to Allocate Unique IPv6 Prefix per Host in Broadcast Networks Google, LLC

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OPS Area v6ops Working Group IPv6 SLAAC DHCP-PD This document discusses the IPv6 deployment scenario when individual hosts connected to broadcast networks (like WiFi hotspots or enterprise networks) are allocated unique prefixes via DHCP-PD.

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:

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 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.

discusses this aspect and explicitly states that IPv6 deployments SHOULD NOT limit the number of IPv6 addresses a host can have. However it's been observed that networks do impose such limits, likely in an attempt to protect the network resources and prevent DoS attacks. The most common scenario of network-imposed limitations is Neighbor Discovery (ND) proxy. Many enterprise-scale wireless solutions implement ND proxy to reduce amount of broadcast and multicast downstream (AP to clients) traffic. To perform ND proxy a device usually maintains a table, containing IPv6 and MAC addresses of connected clients. At least some implementations have hardcoded limits on how many IPv6 addresses per a single MAC such a table can contain. When the limit is exceeded the behaviour is implementation-dependent. Some vendors just fail to install N+1 address to the table. Other delete the oldest entry for this MAC and replace it with the new address. In any case the affected addresses lose network connectivity without receiving any implict signal, with traffic being silently dropped. It would be beneficial for IPv6 deployments to address the above mentioned scalability issues while still allowing hosts to have multiple IPv6 addresses. One of the very promising approaches is allocating an unique IPv6 prefix per host (). The same principle has been actively used in cellular 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 to allocate an unique IPv6 prefix per host using DHCP-PD.

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 when, and only when, they appear in all capitals, as shown here.

Design Principles Instead of all hosts on a given segment forming addresses from the same /64 assigned to that segment:

A host acts as DHCP-PD client and requests a prefix via DHCPv6-PD by sending an IA_PD request.
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 the prefix from a larger pool allocated for the given segment or the whole site.
The allocated prefix 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 traffic to that prefix does not trigger any ND packets.
The host uses the delegated prefix to allocate addresses to its interfaces, containers etc. For example, the device can include the prefix into Router Advertisements sent to virtual systems or to any other devices connected to its downstream interface.

Prefix Length Considerations DHCPv6 prefix delegation supports delegating prefixes of any size. However at the time of writing, the only prefix size that will allow the host to use SLAAC is 64 bits (see also ). As a result delegating a /64 prefix 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 a /64 allows hosts to extend the network arbitrarily, similarily to using NAT in IPv4 but with full support for end-to-end communication. Chosing longer prefixes would require the host and any connected system to use some other form of address assignment and therefore would drastically limit the applicability of the proposed solution. The extensive analysis provided in is fully applicable to selecting the delegated prefix in the proposed deployment model. 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.

Routing Considerations

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 . 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 .

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 ). As per 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 () 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 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. To ensure that routes to the delegated prefixes are preserved even if a relay is rebooted or replaced, the operator MUST ensure that all relays in the network infrastructure support DHCPv6 Bulk Leasequery as defined in . While Section 4.3 of lists keeping active prefix delegations in persistent storage as an alternative to DHCPv6 Bulk Leasequery, relying on persistent storage has the following drawbacks:

In a network with multiple relays, network state can change significantly while the relay was rebooting (new prefixes delegated, some prefixes expiring etc).
Persistent storage might not be preserved if the router is physically replaced.

Preventing Routing Loops To prevent routing loops caused by traffic to unused addresses from the delegated prefix the client MUST drop all packets to such addresses (see the requirement WPD-5 in ).

DHCPv6-PD Server Considerations The following requirements are applicable to the DHCPv6-PD server delegating prefixes to endhosts:

The server MUST follow recommendations on processing prefix-length hints.
The server MUST provide a prefix short enough for the client to assign addresses to its interfaces and connected systems via SLAAC.
If the server receives the same SOLICIT message from the same client multiple times through multiple relays, it MUST reply to all of them with the same prefix(es). This ensures that all the relays will correctly configure routes to the delegated prefixes.
The server MUST 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 ).

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, ) 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 ). Using FCFS SAVI () 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.

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 prefixes 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 a unique prefix 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 ), 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, defines a new PIO flag, indicating that DHCPv6-PD is preferred method of obtaining prefixes.

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 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 prefixes, there is no reason to advertise a common /64 assigned to that segment in PIO with 'A' flag set. Therefore it is possible to 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 .
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.
A dedicated prefix per host allows the network administrator to create per-host security policies (ACLs) even if the host is using temporary addresses. This mitigates one of the issues described in .
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 a prefix allows the host to provide connectivity to other hosts, like it is possible in IPv4 with NAT.

Applicability and Limitations Delegating a unique prefix per host provides all the benefits of both SLAAC and DHCPv6 address allocation, but at the cost of greater address space usage. This design would substantially benefit some networks (see ), in which the addional cost of an additional service (DHCPv6 prefix delegation) and allocating a larger amount of address space can easily be justified. Examples of such networks include but are not limited to:

Large-scale networks where even 3-5 addresses per client might introduce scalability issues.
Networks with high number of virtual hosts, so physical endpoints require multiple addresses.
Managed networks where extensive troubleshooting, host traffic logging or forensics might be required.

In smaller networks, such as home networks, with smaller address space and lower number of clients, SLAAC is a better and simpler option.

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 () but for delegated prefixes:

The host requests another 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.

IANA Considerations This memo includes no request to IANA.

Security Considerations A malicious or just misbehaving host might exhaust the DHCP-PD pool by sending a large number of requests with various DUIDs. This is not a new issue as the same attack might be implemented in DHCPv4 or DHCPv6 for IA_NA requests. To prevent a misbehaving client from denying service to other clients, the DHCPv6 server or relay MUST support limiting the number of prefixes delegated to a given host at any given time. 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 . Spoofing scenarios and prevention mechanisms are discussed in .

References Normative References Informative References

Acknowledgements Thanks to Nick Buraglio, Brian Carpenter, Gert Doering, David Farmer, Fernando Gont, Bob Hinden, Martin Hunek, Erik Kline, David Lamparter, Andrew McGregor, Tomek Mrugalski, Pascal Thubert, Ole Troan, Eduard Vasilenko, Timothy Winters for the discussions, their input and all contribution.

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