| Internet-Draft | Prefer ULAs over IPv4 addresses | November 2023 |
| Buraglio, et al. | Expires 24 May 2024 | [Page] |
This document updates [RFC6724] based on operational experience gained since its publication over ten years ago. In particular it updates the precedence of Unique Local Addresses (ULAs) in the default address selection policy table, which as originally defined by [RFC6724] has lower precedence than legacy IPv4 addressing. The update places both IPv6 Global Unicast Addresses (GUAs) and ULAs ahead of all IPv4 addresses on the policy table to better suit operational deployment and management of ULAs in production. In updating the [RFC6724] default policy table, this document also demotes the preference for 6to4 addresses. These changes to default behavior improve supportability of common use cases such as, but not limited to, automatic / unmanaged scenarios. It is recognized that some less common deployment scenarios may require explicit configuration or custom changes to achieve desired operational parameters.¶
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When [RFC6724] was published in 2012 it was expected that the default policy table may need to be updated from operational experience; section 2.1 says "It is important that implementations provide a way to change the default policies as more experience is gained" and points to the examples in Section 10, including Section 10.6 which considers a ULA example.¶
This document is written on the basis of such operational experience, in particular for scenarios where ULAs are used within a site.¶
The current default policy table in [RFC6724] leads to preference for IPv6 GUAs over IPv4 globals, which is widely considered to be preferential behavior to support greater use of IPv6 in dual-stack environments, and to allow sites to phase out IPv4 as its use becomes ever lower.¶
However, the default policy table also puts IPv6 ULAs below all IPv4 addresses, including [RFC1918] addresses. For many site operators this behavior will be counter-intuitive, and may create difficulties with respect to planning, operational, and security implications for environments where ULA addressing is used in certain IPv4/IPv6 dual-stack network scenarios. The expected prioritization of IPv6 traffic over IPv4 by default, as happens with IPv6 GUA addressing, will not happen for ULAs.¶
An IPv6 deployment, whether enterprise, residential or other, may use combinations of IPv6 GUAs, IPv6 ULAs, IPv4 globals, IPv4 RFC 1918 addressing, and may or may not use some form of NAT.¶
This document makes no comment or recommendation on how ULAs are used, or on the use of NAT in an IPv6 network. As the default policy table stands, operationally where GUAs and ULAs are used alongside RFC 1918 addressing, an IPv6 GUA would be selected to reach an IPv6 GUA destination. However where there are only ULAs and RFC1918 addressing in use, RFC 1918 addresses will be preferred.¶
This document updates the default policy table to elevate the preference for ULAs such that ULAs will be preferred over all IPv4 addresses, providing more consistent and less confusing behavior for operators.¶
This change aims to improve the default handling of address selection for common cases, and unmanaged / automatic scenarios rather than those where DHCPv6 is deployed. Sites using DHCPv6 for host configuration management can make use of implementations of [RFC7078] to apply changes to the [RFC6724] policy table.¶
The changes should also assist operators in phasing out IPv4 from dual-stack environments, since IPv6 GUAs and ULAs will be preferred over any IPv4 addresses. This is an extremely important enabler towards IPv6-only networking.¶
The changes are discussed in more detail in the following sections, with a further section providing a summary of the proposed updates.¶
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 preference for use of IPv6 addressing over IPv4 addressing in [RFC6724] is inconsistent. As written, [RFC6724] section 10.3 states:¶
"The default policy table gives IPv6 addresses higher precedence than IPv4 addresses. This means that applications will use IPv6 in preference to IPv4 when the two are equally suitable. An administrator can change the policy table to prefer IPv4 addresses by giving the ::ffff:0.0.0.0/96 prefix a higher precedence".¶
The expected behavior would be that ULA address space would be preferred over legacy IPv4, however this is not the case. This presents an issue with any environment that will use ULA addressing alongside legacy IPv4, whether global or RFC 1918. This is counter to the standard expectations for legacy IPv4 / IPv6 dual-stack behavior in preferring IPv6, which is the case for GUA addressing.¶
There are demonstrated and easily repeatable operational examples of the impact of the current [RFC6724] behaviour, i.e., ULAs not being preferred in OS and network equipment over legacy IPv4 addresses. These reinforce the need to update [RFC6724] to both better reflect the original intent of the RFC and to facilitate the depreciation and eventual removal of IPv4 in network environments.¶
When the default policy table in a given operating system is referenced it dictates the behavior of getaddrinfo() or analogous process. More specifically, where getaddrinfo() or a comparable API is used, the sorting behavior should take into account both the source address of the requesting host as well as the destination addresses returned and sort according to both source and destination addresses, i.e, when a ULA address is returned, the source address selection should return and use a ULA address if available. Similarly, if a GUA address is returned the source address selection should return a GUA source address if available.¶
However, there are clearly evidenced example of three failure scenarios:¶
ULA per is less preferred (the precedence value is lower) than all legacy IPv4 (represented by ::ffff:0:0/96 in the aforementioned table).¶
Because of the lower precedence value of fc00::/7, if a host has legacy IPv4 enabled, it will use legacy IPv4 before using ULA.¶
A dual-stacked client will source the traffic from the legacy IPv4 address, meaning it will require a corresponding legacy IPv4 destination address.¶
For scenario number 3, when a host resolves through DNS a destination with A and AAAA DNS records, the host will choose the A record to get an legacy IPv4 address for the destination, meaning ULA IPv6 is rendered unused.¶
As a result, the use of ULAs is not a viable option for dual-stack networking transition planning, large scale network modeling, network lab environments or other modes of large scale networking that run both IPv4 and IPv6 concurrently with the expectation that IPv6 will be preferred by default.¶
The anycast prefix for 6to4 relays was deprecated by [RFC7526] in 2015, and since that time the use of 6to4 addressing has further declined to the point where it is generally not seen and can be considered to all intents and purposes deprecated in use.¶
This document therefore demotes the precedence of the 6to4 prefix in the policy table to the same minimum preference as carried by the deprecated site local and 6bone address prefixes.¶
This update will make two specific changes: first, to update the default policy table, and second, change the next hop advertised prefix by the next hop to a MUST.¶
This update alters the default policy table listed in Rule 2.1 of [RFC6724].¶
The table below reflects the current [RFC6724] state on the left, and the updated state defined by this RFC on the right:¶
RFC 6724 Updated
Prefix Precedence Label Prefix Precedence Label
::1/128 50 0 ::1/128 50 0
::/0 40 1 ::/0 40 1
::ffff:0:0/96 35 4 ::ffff:0:0/96 20 4 (*)
2002::/16 30 2 2002::/16 5 2 (*)
2001::/32 5 5 2001::/32 5 5
fc00::/7 3 13 fc00::/7 30 13 (*)
::/96 1 3 ::/96 1 3
fec0::/10 1 11 fec0::/10 1 11
3ffe::/16 1 12 3ffe::/16 1 12
(*) value(s) changed in update
¶
This preference table update moves 2002::/16 to de-preference its status in line with RFC 7526 and changes the default address selection to move fc00::/7 above legacy IPv4, with ::ffff:0:0/96 now set to precedence 20.¶
As designed, ULAs are defined to have a /48 site prefix. An implementation SHOULD automatically add rows for all covering ULA site prefixes received in Router Advertisements (RAs) [RFC4861] within Prefix Information Options (PIOs) or Route Information Options (RIOs) [RFC4191]. These known-local ULA /48s SHOULD have a precedence of 45. All Nodes SHOULD provide a mechanism to configure the policy table. Any Node that does not provide a mechanism for policy table configuration MUST implement the automated increased precedence for known-local /48s of ULA. Nodes implementing the automated increased precedence for known-local /48s of ULA MAY set the default precedence for the ULA label (fc00::/7) to 10. Otherwise, the default precedence for the ULA label (fc00::/7) MUST be 30.¶
The heuristic for address selection defined in Section 5.5 of [RFC6724] to prefer addresses in a prefix advertised by a next-hop router has proven to be very useful. [RFC6724] does not state any requirement for SHOULD or MUST for this heuristic to be used; this update therefore amends section 5.5 to reflect that a system MUST apply the next-hop tracking heuristic.¶
As with most adjustments to standards, and using [RFC6724] itself as a measuring stick, the updates defined in this document will likely take between 8-20 years to become common enough for consistent behavior within most operating systems. At the time of writing, it has been over 10 years since [RFC6724] has been published but we continue to see existing commercial and open source operating systems exhibiting [RFC3484] behavior.¶
While it should be noted that [RFC6724] defines a solution to adjust the address preference selection table that is functional theoretically, operationally the solution is operating system dependent and in practice policy table changes cannot be signaled by any currently deployed network mechanism. While [RFC7078] defines such a DHCPv6 option, it is not by any means widely implemented. This lack of an intra-protocol or network-based ability to adjust address selection preference, along with the inability to adjust a notable number of operating systems either programmatically or manually, renders operational scalability of such a mechanism challenging.¶
It is especially important to note this behavior in the long lifecycle equipment that exists in industrial control and operational technology environments due to their very long mean time to replacement/lifecycle.¶
In practice this means that network operators and those who design networks need to keep these considerations in mind. Should the current ULA and IPv4 preference issue be of concern then 'workarounds' do exist. One is to use IPv6-only networking, i.e., not deploy dual-stack, and another is to only use GUA IPv6 addresses which are preferred by default over all IPv4 addresses.¶
The authors would like to acknowledge the valuable input and contributions of the 6man WG including Brian Carpenter, XiPeng Xiao, Eduard Vasilenko, David Farmer, Bob Hinden, Ed Horley, Tom Coffeen, Scott Hogg, Chris Cummings, Paul Wefel, Dale Carder, Erik Auerswald, Ole Troan, Eric Vyncke, Timothy Winters, Kyle Rose, Lorenzo Colitti, Jen Linkova, and Mark Smith.¶
There are no direct security considerations in this document.¶
The mixed preference for IPv6 over IPv4 from the default policy table in [RFC6724] represents a potential security issue, given an operator may expect ULAs to be used when in practice [RFC1918] addresses are used instead.¶
When using the updated ULA source address selection defined in this document, network operators MUST follow Section 4.3 of [RFC4193] for firewall/packet filtering as "routers be configured by default to keep any packets with Local IPv6 addresses from leaking outside of the site and to keep any site prefixes from being advertised outside of their site." Following this security practice is critical when ULAs have more broad reachability.¶
Operators should be mindful of cases where one node is compliant with [RFC6724] as originally published and another node is compliant with the update presented in this document, as there may be inconsistent behaviour for communications initiated in each direction. Similarly, differences between current RFC 6724-compliant and RFC 3484-compliant nodes may also be observed. Ultimately all nodes should be made compliant to the updated specification described in this document.¶
None.¶
Update to default preference table moving 6to4 address block 2002::/16 to de-preference status in line with [RFC7526],¶
Change the default address selection to move fc00::/7 to preference 30, above legacy IPv4,¶
Change ::ffff:0:0/96 to preference 20,¶
Add section relating to Happy Eyeballs,¶
Change section 5.5 to require interface prefix tracking.¶