Delegating a Prefix to a Host for Multi-addressing Purposes
draft-templin-v6ops-pdhost-01.txt

Abstract

IPv6 prefixes are typically delegated to requesting routers which then use them to number their downstream-attached links and networks. The requesting router then acts as a router between the downstream-attached hosts and the upstream provider network. The router could also act as a host under the weak end system model, and otherwise behaves as a standard router. This document considers the case when the "requesting router" is actually a host, and receives a prefix that it can use for multi-addressing purposes. The host does not connect any downstream-attached networks, and uses the prefix solely for its own multi-addressing purposes.

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Table of Contents

• 1. Introduction
• 2. Multi-Addressing
• 3. Multi-Addressing Alternatives
• 4. DAD Implications
• 5. Route Optimization
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Author's Address

1. Introduction

IPv6 provides a prefix delegation service using the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315][RFC3633]. Using DHCPv6 Prefix Delegation (PD) [RFC3633], a requesting router asks for a prefix from a delegating router. When the prefix is delegated, the requesting router assigns the prefix to its downstream-attached link (i.e., the "LAN" interface). The requesting router then acts as a router between hosts on the LAN interface and the upstream provider network (i.e., the "WAN" interface). The router could also act as a host under the weak end system model [RFC1122], and otherwise behaves as a standard router. This document considers the case when the "requesting router" is actually a simple host, and receives a prefix delegation as if it were a router. The host need not have a LAN interface, and can use the prefix solely for its own multi-addressing purpose.

2. Multi-Addressing

IPv6 allows for assignment of multiple addresses to a single interface. [I-D.ietf-v6ops-host-addr-availability] discusses options for multi-addressing as well as use cases where multi-addressing may be desirable. Multi-addressing options include Stateless Address Autoconfiguration (SLAAC) [RFC4862] or stateful DHCPv6 address delegation [RFC3315], as well as assignment of multiple addresses from a delegated prefix.

SLAAC and DHCPv6 address delegation typically obtain addresses from an on-link prefix configured on the link over which the addresses are obtained. When this happens, the address recipient is obliged to use Multicast Listener Discovery (MLD) to join the appropriate solicited-node multicast group(s) and the Duplicate Address Detection (DAD) algorithm [RFC4862] to ensure that no other node on the link configures a duplicate address. Alternatively, address delegation from a delegated prefix can be used by a node under either the weak or strong end system models [RFC1122]. In that case, the MLD/DAD procedure is not necessary, since the prefix has been delegated to the node for its own exclusive use and the prefix is NOT assigned to the link over which the prefix was obtained.

3. Multi-Addressing Alternatives

When a node receives a prefix delegation, it has many alternatives for the way in which it can provision the prefix. [RFC7278] discusses alternatives for provisioning a prefix obtained by a User Equipment (UE) device under the 3rd Generation Partnership Program (3GPP) service model. This document considers the general case when the node receives a prefix delegation in which the prefix is delegated for the exclusive use of the prefix recipient.

When the node receives the prefix (e.g., a /64), it can assign the prefix to a LAN interface and configure multiple addresses for itself on the LAN interface. The node uses link-local-only addressing on the WAN interface, and configures a default route that points to a router on the WAN link. The node can then act as both a host for its own applications and a router for any downstream-attached hosts. This approach is often known as the "tethered" configuration.

When the node does not have any LAN interfaces, it may still wish to obtain a prefix solely for multi-addressing purposes. In a first alternative, the node can receive the prefix acting as a requesting router over the WAN interface but then assign the prefix to an internal virtual interface (e.g., a loopback interface) and assign one or more addresses taken from the prefix to the virtual interface. In that case, applications on the node can use the assigned addresses according to the weak end system model.

In a second alternative, the node can receive the prefix as a requesting router over the WAN interface but then assign the prefix to a loopback interface and assign one or more addresses taken from the prefix to the WAN interface. In that case, applications on the node can use the assigned addresses according to the strong end system model.

In both of these latter two cases, the node acts as a host internally even though it behaves as a router from the standpoint of prefix delegation and neighbor discovery over the WAN interface. The host can configure as many addresses for itself as it wants.

4. DAD Implications

When a node configures addresses for itself using either SLAAC or DHCPv6 address delegation and assigns the addresses to the WAN interface, the node MUST perform MLD/DAD by sending multicast packets the solicited-node multicast address to test whether another node that configures a duplicate address is on the link. When there are many such addresses and/or many such nodes, this could result in substantial multicast traffic that affects all nodes on the link.

When a node configures addresses for itself using a delegated prefix, the node can configure as many addresses as it wants but does not perform MLD/DAD for any of the addresses over the WAN interface. This means that millions of addresses can be assigned without having any multicast messaging over the WAN link that could disturb other nodes. This becomes important when there are many such nodes and/or many addresses configured per node. Note however that nodes that assign the addresses directly to the WAN interface must be capable of disabling DAD on the WAN interface.

5. Route Optimization

When a node does not assign a non-link-local prefix on the WAN interface, it initially has only a default route pointing to a router on the WAN link. This means that all packets the node sends over the WAN interface will initially need to go through the default router even if there is a better first-hop node on the link. In that case, the default router can send a Redirect to inform the node of a better first hop. The Redirect populates an entry in the node's neighbor cache for the WAN interface, and future packets can take the more direct route without disturbing the default router. The Redirect can apply either to a singleton destination address, or to an entire destination prefix as described in AERO [I-D.templin-aerolink].

6. IANA Considerations

This document introduces no IANA considerations.

7. Security Considerations

TBD.

8. Acknowledgements

This work was motivated by recent discussions on the v6ops list. Mark Smith pointed out the need to consider MLD as well as DAD for the assignment of addresses to interfaces.

9. References

9.1. Normative References

9.2. Informative References

Author's Address