Internet-Draft | pio-p-flag | March 2023 |
Colitti & Linkova | Expires 28 September 2023 | [Page] |
This document defines a ‘P’ flag in the Prefix Information Option of IPv6 Router Advertisements (RAs). The flag is used to indicate that the network prefers that hosts acquire global addresses using DHCPv6 PD instead of using SLAAC for this prefix.¶
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IPv6 hosts, especially mobile hosts, usually have multiple global IPv6 addresses (e.g. stable addresses, privacy addresses, 464XLAT addresses, addresses for virtual systems etc).¶
On large networks, individually tracking these addresses can create scalability issues for the infrastructure, because routers must maintain multiple entries (neighbor cache, SAVI mappings, VXLAN routes, etc.) for each host. [I-D.collink-v6ops-ent64pd] discusses these challenges and proposes a solution that uses DHCPv6 PD [RFC8415].¶
On small networks, scaling to support multiple individual IPv6 addresses is less of a concern, because many home routers support hundreds of neighbor cache entries. On the other hand, address space is more limited compared to the number of hosts connected - the smallest home network might only have /60 prefixes, or even just a single /64.¶
A host cannot know in advance which address assignment method is most appropriate for the network, so there must be a mechanism for the network to communicate with this to the host.¶
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.¶
The information is passed to the host via a P flag in the Prefix Information Option (PIO). The reason is as follows:¶
The host SHOULD NOT use SLAAC to obtain IPv6 addresses from prefix(es) with the P bit set.¶
For each network it is currently connected to, the host MUST keep a list of every PIO it has received with the P flag. Each time the client receives a Router Advertisement containing a PIO with the P bit set that is not in the list, and every time a previously-received PIO with the P bit set becomes deprecated:¶
Whenever a Prefix Information Option’s Valid lifetime reaches zero, or its P flag changes to 0, the prefix is removed from the list. When there are no such prefixes, the host SHOULD stop the DHCPv6 client if it has no other reason to run it. The lifetimes of any DHCPv6 prefixes already obtained are unaffected.¶
When a host requests a prefix via DHCPv6 PD, it MUST use the prefix length hint Section 18.2.4 of [RFC8415] to request a prefix that is short enough to form addresses via SLAAC. To ensure that all DHCP relays on link can act on the delegated prefix, the host SHOULD NOT use the Rapid Commit option.¶
The P flag is meaningless for link-local prefixes and any Prefix Information Option containing the link-local prefix MUST be ignored as specified in Section 5.5.3 of [RFC4862].¶
For every delegated prefix:¶
In multi-prefix multihoming, the host generally needs to associate the prefix with the router that advertised it (see for example, [RFC6724] Rule 5.5). If the host supports Rule 5.5, then it SHOULD associate each prefix with the link-local address of the DHCPv6 relay from which it received the packet.¶
This document makes the following changes to Section 4.6.2 of [RFC4861]¶
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A 1-bit autonomous address-configuration flag. When set indicates that this prefix can be used for stateless address configuration as specified in [ADDRCONF].¶
Reserved1 6-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.¶
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A 1-bit autonomous address-configuration flag. When set indicates that this prefix can be used for stateless address configuration as specified in [ADDRCONF].¶
P 1-bit DHCPv6-PD flag. When set, indicates that this prefix SHOULD NOT be used for stateless address configuration. Instead the host SHOULD request a dedicated prefix via DHCPv6-PD and use that prefix for stateless address configuration.¶
Reserved1 5-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.¶
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This document makes the following changes to Section 5.5.3 of [RFC4862]:¶
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For each Prefix-Information option in the Router Advertisement:¶
a) If the Autonomous flag is not set, silently ignore the Prefix Information option.¶
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For each Prefix-Information option in the Router Advertisement:¶
a) If the P flag is set, start the DHCPv6 PD process and use the delegated prefix to assign addresses to the interfaces as described in draft-collink-6man-pio-pflag. The Prefix Information option SHOULD be processed as if A flag is set to zero.¶
b)If the Autonomous flag is not set, silently ignore the Prefix¶
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To be added¶
This memo includes no request to IANA.¶
to be added¶