3.1. Automatic DHCPv6 RELEASEs

Some CE routers are known to automatically send DHCPv6-PD RELEASE messages upon restart events. However, this may inadvertently trigger a flash-renumbering scenario, along with the associated problems discussed in [RFC8978], that this document attempts to mitigate.¶

As a result, requirement WPD-9 from Section 3 specifies that CE routers SHOULD NOT automatically send DHCPv6-PD RELEASE messages upon restart events.¶

3.2. Stability of IAIDs

[RFC8415] requires that the IAID for an IA MUST be consistent across restarts of the DHCP client. However, some popular CE routers are known to select new random IAIDs, e.g., every time the underlying PPP session is established or when the device is rebooted. This could be the result of extrapolating the behavior described in [RFC7844] or simply a consequence of not storing IAIDs on stable storage along with failure to employ an algorithm that consistently generates the same IAID upon reboots. Thus, requirement WPD-10 from Section 3 prevents CE routers from inadvertently triggering flash-renumbering events on the local network.¶

3.3. Interface between the WAN Side and LAN Side

The "Preferred Lifetime" and "Valid Lifetime" of Prefix Information Options (PIOs) [RFC4861] corresponding to prefixes learned via DHCPv6-PD on the WAN side MUST NOT span past the remaining preferred and valid lifetimes of the corresponding DHCPv6-PD prefixes. This means that the "Preferred Lifetime" and the "Valid Lifetime" advertised in PIOs by the CE router MUST be dynamically adjusted such that they never span past the remaining preferred and valid lifetimes of the corresponding prefixes delegated via DHCPv6-PD on the WAN side.¶

Similarly, the "preferred-lifetime" and "valid-lifetime" of DHCPv6 IA Address options and DHCPv6 IA Prefix options employed with DHCPv6 on the LAN side MUST NOT span past the remaining preferred and valid lifetimes of the corresponding prefixes learned via DHCPv6-PD on the WAN side. This means that the "preferred-lifetime" and "valid-lifetime" of DHCPv6 IA Address options and DHCPv6 IA Prefix options employed with DHCPv6 on the LAN side MUST be dynamically adjusted such that they never span past the remaining preferred and valid lifetimes of the corresponding prefixes delegated to the CE router on the WAN side via DHCPv6-PD.¶

RATIONALE:¶

• The lifetime values employed for the "Preferred Lifetime" (AdvPreferredLifetime) and "Valid Lifetime" (AdvValidLifetime) of SLAAC Prefix Information Options must never be larger than the remaining lifetimes of the corresponding prefixes (as learned via DHCPv6-PD on the WAN side). This is in line with the requirement from Section 6.3 of [RFC8415], which states:¶

In particular, if the delegated prefix or a prefix derived from it is advertised for stateless address autoconfiguration [RFC4862], the advertised preferred and valid lifetimes MUST NOT exceed the corresponding remaining lifetimes of the delegated prefix.¶

• The lifetime values of prefixes advertised on the LAN side via SLAAC must be dynamically updated (rather than static values); otherwise, the advertised lifetimes would eventually span past the DHCPv6-PD lifetimes.¶
• The same considerations apply for the "valid-lifetime" and "preferred-lifetime" of IA Address options and IA Prefix options employed with DHCPv6 on the LAN side.¶

3.4. LAN-Side Option Lifetimes

CE routers SHOULD override the default lifetime values of Neighbor Discovery options that depend in any way on changes in the prefix employed for address configuration on the LAN side, and employ shorter lifetime values to improve the robustness to renumbering events, while complying with the requirements from Section 3.3 of this document and the recommendations in [RFC7772].¶

CE routers SHOULD set the "Router Lifetime" of Router Advertisement (RA) messages to ND_PREFERRED_LIMIT.¶

CE routers SHOULD also set the PIO "Preferred Lifetime" to the lesser of the remaining preferred lifetime of the corresponding prefix (see Section 3.3) and ND_PREFERRED_LIMIT, and set the PIO "Valid Lifetime" to the lesser of the remaining valid lifetime of the corresponding prefix and ND_VALID_LIMIT. Additionally, the "Route Lifetime" of Route Information Options (RIOs) [RFC4191], the "Lifetime" of Recursive DNS Server (RDNSS) options [RFC8106], and the "Lifetime" of DNS Search List (DNSSL) options [RFC8106] SHOULD be set to the lesser of the longest remaining valid lifetime of a prefix (leased via DHCPv6 on the WAN side) and ND_VALID_LIMIT, if any of these options are included in Router Advertisement messages.¶

NOTE: In scenarios where the valid lifetime and the preferred lifetime of prefixes learned via DHCPv6 on the WAN side are always larger than ND_VALID_LIMIT and ND_PREFERRED_LIMIT, respectively, the lifetime values advertised on the LAN side will not experience actual changes.¶

The above text refers to the Neighbor Discovery options that are typically employed by CE routers. A CE router may need to apply the same policy for setting the lifetime of other Neighbor Discovery options it employs, if and where applicable.¶

CE routers providing stateful address configuration via DHCPv6 SHOULD set the "preferred-lifetime" of a DHCPv6 IA Address option to the lesser of the remaining preferred lifetime of the corresponding prefix (see Section 3.3) and ND_PREFERRED_LIMIT, and set the "valid-lifetime" of the same option to the lesser of the remaining valid lifetime of the corresponding prefix and ND_VALID_LIMIT.¶

CE routers providing DHCPv6-PD on the LAN side SHOULD set the "preferred-lifetime" of a DHCPv6 IA Prefix option to the lesser of the remaining preferred lifetime of the corresponding prefix (see Section 3.3) and ND_PREFERRED_LIMIT, and set the "valid-lifetime" of the same option to the lesser of the remaining valid lifetime of the corresponding prefix and ND_VALID_LIMIT.¶

RATIONALE:¶

• The "Valid Lifetime" and "Preferred Lifetime" of PIOs have a direct impact on three different aspects:¶

  • The amount of time hosts may end up employing stale network configuration information (see [RFC8978]).¶
  • The amount of time CE routers need to persist trying to deprecate stale network configuration information (e.g., to handle cases where hosts miss Router Advertisement messages and thus still consider the stale information as valid).¶
  • The amount of information that CE routers need to maintain when, e.g., multiple crash-and-reboot events occur in the time span represented by the option lifetimes employed on the LAN side.¶
• CE routers need not employ the (possibly long) WAN-side DHCPv6-PD lifetimes for the "Valid Lifetime" and "Preferred Lifetime" of PIOs sent in Router Advertisement messages to advertise sub-prefixes of the leased prefix. Instead, CE routers SHOULD use shorter values for the "Valid Lifetime" and "Preferred Lifetime" of PIOs, since subsequent Router Advertisement messages will nevertheless refresh the associated lifetimes, leading to the same effective lifetimes as specified by the WAN-side DHCPv6-PD lifetimes.¶
• Similarly, CE routers need not employ the (possibly long) WAN-side DHCPv6-PD lifetimes for the "valid-lifetime" and "preferred-lifetime" of IA Address options and IA Prefix options employed by DHCPv6 on the LAN side, since the renewal of bindings by DHCPv6 clients will lead to the same effective lifetimes as specified by the WAN-side DHCPv6-PD lifetimes.¶

3.5. Signaling Stale Configuration Information

When a CE router provides LAN-side address-configuration information via SLAAC:¶

• A CE router sending RAs that advertise prefixes belonging to a dynamically learned prefix (e.g., via DHCPv6-PD) SHOULD record, on stable storage, the list of prefixes being advertised via PIOs on each network segment and the state of the "A" and "L" flags of the corresponding PIOs.¶

• Upon changes to the advertised prefixes, and after bootstrapping, the CE router advertising prefix information via SLAAC proceeds as follows:¶

  • Any prefixes that were previously advertised by the CE router via PIOs in RA messages, but that have now become stale, MUST be advertised with PIOs that have the "Valid Lifetime" and the "Preferred Lifetime" set to 0 and the "A" and "L" bits unchanged.¶

  • The aforementioned advertisements MUST be performed for at least the "Valid Lifetime" previously employed for such prefixes. The CE router MUST advertise this information with unsolicited Router Advertisement messages, as described in Section 6.2.4 of [RFC4861], and MAY advertise this information via unicast Router Advertisement messages when possible and applicable.¶

    • NOTE: If requirement L-16 (Section 3) is followed, the "Valid Lifetime" need not be saved, and the stale prefix can simply be advertised for a period of ND_VALID_LIMIT.¶
• CE routers receiving DHCPv6 IA Prefix options with a 0 "valid-lifetime" MUST advertise the corresponding sub-prefixes (as they would be generated for the same leased prefix with a non-zero lifetime) with PIOs with both the "Preferred Lifetime" and the "Valid Lifetime" set to 0, for at least the WAN-side DHCPv6-PD "valid-lifetime", or for a period of ND_VALID_LIMIT if the recommended lifetimes from Section 3.4 are employed.¶

When a CE router provides LAN-side DHCPv6 (address assignment or prefix delegation), then:¶

• The CE router SHOULD record, on stable storage, the DHCPv6 address and delegated-prefix bindings corresponding to the LAN side.¶

• If the CE router finds that the prefix to be employed for address assignment and/or prefix delegation has changed (e.g., upon a crash-and-reboot event) or the CE router receives DHCPv6 IA Prefix options with 0 lifetimes, the CE router MUST:¶

  • In Replies to DHCPv6 Request, Renew, and Rebind messages, send IA Address options or IA Prefix options (as appropriate) for any address assignments or prefix delegations for the stale prefixes. The aforementioned options MUST be sent with both the "valid-lifetime" and the "preferred-lifetime" set to 0, for at least the "valid-lifetime" originally employed for them, or for a period of ND_VALID_LIMIT if the recommended lifetimes from Section 3.4 are employed.¶

  • Initiate sending Reconfigure messages, if possible (i.e., client requests Reconfigure support and the CE router offers it), to those clients with address assignments or prefix delegations for the stale prefixes.¶

RATIONALE:¶

• IPv6 network renumbering is expected to take place in a planned manner with old/stale prefixes being phased out via reduced prefix lifetimes while new prefixes (with normal lifetimes) are introduced. However, a number of scenarios may lead to the so-called "flash-renumbering" events, where a prefix being employed on a network suddenly becomes invalid and replaced by a new prefix [RFC8978]. One such scenario is when an Internet Service Provider (ISP) employs dynamic prefixes and the CE router crashes and reboots. The requirements in this section are meant to allow CE routers to deprecate stale information in such scenarios.¶
• The recommendations in this section expand from requirement L-13 in Section 4.3 of [RFC7084] and Section 6.3 of [RFC8415].¶
• Hosts configuring addresses via SLAAC on the local network may employ addresses configured for the previously advertised prefixes for at most the "Valid Lifetime" of the corresponding PIOs of the last received Router Advertisement messages. Since Router Advertisement messages may be lost or fail to be received for various reasons, CE routers need to try to deprecate stale prefixes for a period of time equal to the "Valid Lifetime" of the PIO employed when originally advertising the prefix.¶
• The requirements in this section to store information on stable storage are conveyed as "SHOULD" (as opposed to "MUST"), since they may represent a challenge for some implementations.¶
• Advertising DHCPv6-leased prefixes with zero lifetimes on the LAN side would handle the case where a CE router has no stable storage but receives the prefixes via DHCPv6 with 0 lifetimes.¶
• The above text does not include DHCPv6 Advertise messages sent in response to DHCPv6 Solicit messages, since Section 18.3.9 of [RFC8415] requires that a DHCPv6 server that is not going to assign an address or delegated prefix received as a hint in the Solicit message MUST NOT include that address or delegated prefix in the Advertise message. Additionally, any subsequent Request messages will trigger the response specified in this section and therefore cause the address or prefix to be deprecated.¶