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Internet IPv6 Maintenance ipv6 slaac pio router advertisement renumbering IPv6 routers employ Router Advertisements (RAs) to disseminate essential network configuration data to hosts. RAs play a vital role in Stateless address autoconfiguration (SLAAC) and providing IPv6 connectivity. Timely updates via RAs become paramount as network configurations change to prevent service outages. This document modifies RFC4861, recommending immediate propagation of configuration information changes by routers.

Introduction Router Advertisements (RAs) contain information that IPv6 hosts use to configure their networks and establish connectivity. Examples of such information include but are not limited to:

• Prefix Information Options (): Advertise network prefixes, allowing hosts to automatically configure IPv6 addresses (SLAAC).
• Link MTU (): Informs hosts about the maximum packet size supported on the link, preventing fragmentation.
• DNS Recursive Server Addresses (): Provides DNS servers for domain name resolution.
• NAT64 Prefix (): Indicates the presence of a NAT64 translator, enabling communication between IPv6 and IPv4 networks.

Network configurations can change over time, and these changes must be communicated to hosts. Routers propagate updates either by sending unsolicited RAs or by responding with an RA to a host's Router Solicitation (RS) message. While some changes (like adding a new prefix or DNS server) can be safely included in the next scheduled RA, others require immediate action to prevent service disruptions. In particular, the removal of a prefix, DNS server, or NAT64 service necessitates a timely update to ensure hosts discontinue using outdated and non-functional resources. A frequent issue arises during network renumbering, when the prefix used for SLAAC and/or on-link communication is modified. If the router fails to send an immediate unsolicited RA upon a prefix change, hosts might continue selecting source addresses from the outdated prefix for up to MaxRtrAdvInterval (65535 seconds, ). Communications using those addresses will fail since the old prefix is no longer valid. Residential networks, where prefixes are often delegated to Customer Edge (CE) routers via DHCPv6-PD, are particularly susceptible to this issue. To ensure timely prefix change notifications, Requirement L-13 of states: If the delegated prefix changes, i.e., the current prefix is replaced with a new prefix without any overlapping time period, then the IPv6 CE router MUST immediately advertise the old prefix with a Preferred Lifetime of zero and a Valid Lifetime of either a) zero or b) the lower of the current Valid Lifetime and two hours (which must be decremented in real time) in a Router Advertisement message as described in Section 5.5.3, (e) of [RFC4862]. It's important to note that this requirement applies specifically to CE routers and addresses prefix changes only. Section 6.2.4 of says: The information contained in Router Advertisements may change through actions of system management. For instance, the lifetime of advertised prefixes may change, new prefixes could be added, a router could cease to be a router (i.e., switch from being a router to being a host), etc. In such cases, the router MAY transmit up to MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the same rules as when an interface becomes an advertising interface. Consequently, routers not classified as CE equipment and therefore exempt from are not required to immediately notify hosts about network configuration changes. This can lead to degraded user experience and contribute to a negative perception of IPv6 reliability.

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.

Changes to RFC4861 This document makes the following changes to Section 6.2.4 of , OLD TEXT: === The information contained in Router Advertisements may change through actions of system management. For instance, the lifetime of advertised prefixes may change, new prefixes could be added, a router could cease to be a router (i.e., switch from being a router to being a host), etc. In such cases, the router MAY transmit up to MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the same rules as when an interface becomes an advertising interface. === NEW TEXT === The information contained in Router Advertisements may change through actions of system management. For instance, the lifetime of advertised prefixes may change, new prefixes could be added, a router could cease to be a router (i.e., switch from being a router to being a host), etc. In such cases, the router SHOULD transmit up to MAX_INITIAL_RTR_ADVERTISEMENTS unsolicited advertisements, using the same rules as when an interface becomes an advertising interface. ===

Security Considerations This document does not introduce any security considerations.

Privacy Considerations

IANA Considerations This memo does not introduce any IANA considerations.

References Normative References Informative References

Acknowledgements Thanks to Erik Kline for the discussions, the input and all contribution.