]> Asteroid International B.V.

remco@asteroidhq.com

intarea Internet-Draft This document requests the allocation of the IPv4 special-purpose address 192.0.0.11/32 to enable IPv4 communication over IPv6-only networks without subnets, ARP, tunneling, or translation. Hosts configured with this address as their IPv4 default gateway resolve the next-hop link-layer address from the IPv6 neighbor cache instead of via ARP. IPv4 packets are carried natively, end-to-end. About This Document Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction Networks migrating to IPv6-only infrastructure still need to carry IPv4 traffic. Traditional mechanisms such as dual-stack, tunneling, and translation all reintroduce IPv4 at the infrastructure level. This document defines a special-purpose IPv4 address, 192.0.0.11, that signals to a host stack that link-layer resolution for the IPv4 default gateway is derived from the link-layer address entry for the IPv6 default router in the neighbor cache , rather than via ARP. The IPv4 routing table entry is unchanged; only the ARP resolution path is modified. This eliminates the need for IPv4 subnets and ARP on the local segment, removing the requirement for tunneling or translation at the first hop.

Conventions and Definitions 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.

Problem Statement IPv4 next-hop resolution on a local link depends on ARP, which requires an IPv4 subnet to be configured on the link. In an IPv6-only network, no such subnet exists. Existing solutions -- dual-stack, tunneling, and translation-based approaches such as NAT64 -- generally require changes beyond the local segment. This document closes the first-hop resolution gap for IPv4 hosts on IPv6-only segments without requiring changes to host software, packet formats, or DHCPv4 clients.

Host Behavior and Next-Hop Resolution When a host is configured to use 192.0.0.11 as its IPv4 default gateway, the host's operating system MUST implement the following logic:

1. The host MUST maintain a functional IPv6 Neighbor Discovery implementation per on the same interface, including default router discovery and neighbor cache maintenance. No additional action specific to this mechanism is required at interface configuration time.
2. When the next hop for an IPv4 packet is 192.0.0.11, the host MUST NOT perform ARP. Instead, it consults the IPv6 default router list and neighbor cache for the link-layer address, scoped to the interface on which 192.0.0.11 is configured as the IPv4 default gateway.
3. If the IPv6 default router link-layer address is in a usable NUD state (REACHABLE, STALE, DELAY, or PROBE per ), the IPv4 packet is sent in a link-layer frame addressed to that destination.
4. If no reachable IPv6 default router is known after the interface has completed initial configuration (i.e., at least one RA has been processed), the packet MAY be queued or dropped per implementation policy. If a last-known router address is available, a Neighbor Solicitation SHOULD be sent to that address. For behavior prior to first RA reception, see the startup paragraph below.

Host stacks MUST treat 192.0.0.11 as a reserved address requiring IPv6-based next-hop resolution, regardless of other address configuration on the interface. This behavior is unconditional and not dependent on any additional signaling. When a DHCPv4 lease configuring 192.0.0.11 expires and is not renewed, the host SHOULD remove 192.0.0.11 as the IPv4 default gateway and cease IPv6-based resolution on that interface. For statically configured deployments, removal is governed by local administrative policy. Cross-interface resolution MUST NOT be performed. On multi-homed hosts, each interface independently resolves 192.0.0.11 against its own IPv6 neighbor cache state. The following pseudocode defines the resolution logic: Router selection uses Default Router Preference as defined in . When multiple routers have equal preference and reachability, the tiebreaker is implementation-defined; use of most-recently-heard RA is one reasonable approach. If the selected IPv6 default router becomes unreachable during an active session, the host SHOULD re-evaluate the default router list and select an alternative. Existing transport sessions will be disrupted if the link-layer next-hop changes; this is consistent with IPv6 router failure behavior and is not specific to this mechanism. Packets queued for a router that has become unreachable SHOULD be flushed and re-evaluated against the updated router selection. Each time an interface transitions to an operational state and begins RA solicitation, a host may receive DHCPv4 configuration before any RA has been processed. Until a reachable IPv6 default router is known, IPv4 packets MUST be queued rather than silently dropped, pending RA reception. Implementations SHOULD bound this queue duration to avoid indefinite resource consumption. On queue timeout, packets SHOULD be dropped and an ICMPv4 Host Unreachable message MAY be generated toward the sending application.

Router Behavior

End-to-End Packet Flow In this model, end hosts are assigned IPv4 addresses with a /32 prefix length. No IPv4 prefix is configured on the link; a sending host directs all IPv4 traffic to the first-hop router using the link-layer address derived from the IPv6 neighbor cache. A host cannot resolve another host's IPv4 address on the local link without router assistance; direct host-to-host IPv4 communication on the segment may occur via ICMPv4 redirect (, Section 3.2.2.2) from the gateway, but cannot be initiated by the host alone. For return traffic to reach end hosts, operators MUST ensure that host /32 routes with an IPv6 next-hop per are present in the routing infrastructure, allowing routers to forward IPv4 traffic toward the correct first-hop without requiring IPv4 addresses on any router interface. The mechanism by which the routing infrastructure learns these host routes is outside the scope of this document. The following diagram illustrates the end-to-end packet flow. Router-to-router forwarding uses ; no IPv4 address is configured on any router interface. IPv4 addresses used in the diagram are from the documentation ranges defined in and are not globally routable. Router R1 Router R2 (no IPv4 address) (no IPv4 address configured) IPv6 link-local: fe80::R1 IPv6 link-local: fe80::R2 [2] FIB lookup: 203.0.113.5/32 via fe80::R2 (RFC 8950) L2 dst: MAC(R2) -- resolved from ND cache, no ARP ----------------------------------------------------> Router R2 Host B (no IPv4 address) IPv4: 203.0.113.5/32 IPv6 link-local: fe80::R2 IPv6: 2001:db8:2::2 [3] FIB lookup: 203.0.113.5/32 via fe80::HostB (ND) L2 dst: MAC(HostB) -- resolved from ND cache, no ARP ----------------------------------------------------> [4] IPv4 packet delivered to Host B ]]> No ARP is exchanged at any point.

Router Ingress Behavior Routers MUST treat 192.0.0.11 as an interface-scoped address -- valid only on the interface on which it is configured, and only for locally-terminated traffic. Specifically:

• It MUST NOT be injected into any routing protocol.
• It MUST NOT trigger overlapping-subnet checks.
• It MUST NOT appear as source or destination in any forwarded packet.

A router MAY respond to ICMPv4 echo requests addressed to 192.0.0.11 and MAY generate ICMPv4 Time Exceeded messages using 192.0.0.11 as the source address. All such messages are interface-local. ICMPv4 error generation on IPv6-only transit routers is out of scope; see .

Backward Compatibility: Router ARP Response The use of 192.0.0.11 as the DHCPv4 Router Option (Option 3) value is fully conformant with , which imposes no requirement that the router address be reachable via ARP on the same subnet. Unmodified hosts receiving 192.0.0.11 as their IPv4 default gateway will issue an ARP request for it. A router SHOULD respond to such ARP requests with its own MAC address. This is not proxy ARP: no subnet exists, no remote host is being proxied. This enables a two-tier deployment model on the same L2 segment:

• Unmodified hosts: router answers ARP; IPv4 forwarding works with zero host-side changes.
• Updated hosts: link-layer address resolved from IPv6 neighbor cache; ARP eliminated entirely.

Both tiers interoperate, allowing incremental deployment. The router requires no per-host state to support both tiers simultaneously: updated hosts will not send ARP requests for 192.0.0.11, so the router's ARP response behavior is triggered only by unmodified hosts.

Deployment Considerations This mechanism complements (IPv6-Only Preferred Option). RFC 8925 allows hosts to signal a preference for IPv6-only operation, but operators must still provide IPv4 for hosts or applications that require it. This document provides exactly that fallback: IPv4 connectivity on an IPv6-only segment, without requiring a dual-stack infrastructure or an IPv4 subnet on the local link. A host that receives Option 108 and transitions to IPv6-only operation retains functional IPv4 connectivity via 192.0.0.11 without any additional configuration. Hosts without an IPv6 stack are outside the scope of this document. On segments with multiple routers advertising equal Default Router Preference -- common in datacenter ECMP fabrics -- hosts may make inconsistent router selections based on RA timing. Operators SHOULD configure explicit Default Router Preference values per to ensure deterministic behavior. Implementations in which IPv4 and IPv6 stacks are managed by separate processes -- as is common on mobile operating systems -- will require inter-process communication to expose the IPv6 neighbor cache to the IPv4 forwarding path. This is an implementation consideration and does not affect the on-wire behavior defined in this document.

Security Considerations In the updated-host deployment model, ARP is eliminated from the network entirely. In the unmodified-host model, ARP is constrained to a single well-known address, reducing the ARP attack surface relative to conventional dual-stack networks. The mechanism relies on the integrity of IPv6 Neighbor Discovery. Rogue RA risks apply as in any IPv6 deployment and can be mitigated with RA Guard . Subnet scanning is mitigated since hosts carry /32 addresses only. A host receiving 192.0.0.11 as its IPv4 default gateway on a network that does not implement this mechanism will issue an ARP request that receives no response, causing IPv4 connectivity to fail silently. Operators SHOULD ensure 192.0.0.11 is only offered via DHCPv4 on segments where the mechanism is deployed. DHCPv4 snooping and dynamic ARP inspection, where used, MUST be configured to permit ARP responses for 192.0.0.11 from the first-hop router. This mechanism does not interact with IPv4 link-local address configuration per . A host configured with 192.0.0.11 as its gateway and a link-local IPv4 source address will follow the same resolution logic defined in Section 4 (Host Behavior and Next-Hop Resolution).

Implementation Requirements [PLACEHOLDER: This section will document conformance requirements and reference known implementations prior to IETF Last Call. An implementation is conformant if it satisfies all MUST and MUST NOT requirements in Sections 4 and 5. A reference implementation is available at https://github.com/remcovanmook/v4-with-v6-nh.]

IANA Considerations This document requests that IANA assign 192.0.0.11/32 in the "IANA IPv4 Special-Purpose Address Registry" as follows:

Field	Value
Address Block	192.0.0.11/32
Name	IPv4 Gateway via IPv6 Resolution
RFC	This document
Allocation Date	(date of publication)
Termination Date	N/A
Source	True
Destination	True
Forwardable	False
Globally Reachable	False
Reserved-by-Protocol	False

The Destination=True designation reflects that 192.0.0.11 may appear as a destination in ICMPv4 messages received by the router on a local interface (see Section 5.2). It does not imply global reachability; Forwardable=False and Globally Reachable=False together preclude any use of this address beyond the local link.

References Normative References This document describes an optional extension to Router Advertisement messages for communicating default router preferences and more-specific routes from routers to hosts. This improves the ability of hosts to pick an appropriate router, especially when the host is multi-homed and the routers are on different links. The preference values and specific routes advertised to hosts require administrative configuration; they are not automatically derived from routing tables. [STANDARDS-TRACK] This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK] This memo reiterates the assignment of an IPv4 address block (192.0.0.0/24) to IANA. It also instructs IANA to restructure its IPv4 and IPv6 Special-Purpose Address Registries. Upon restructuring, the aforementioned registries will record all special-purpose address blocks, maintaining a common set of information regarding each address block. Multiprotocol BGP (MP-BGP) specifies that the set of usable next-hop address families is determined by the Address Family Identifier (AFI) and the Subsequent Address Family Identifier (SAFI). The AFI/SAFI definitions for the IPv4 address family only have provisions for advertising a next-hop address that belongs to the IPv4 protocol when advertising IPv4 Network Layer Reachability Information (NLRI) or VPN-IPv4 NLRI. This document specifies the extensions necessary to allow the advertising of IPv4 NLRI or VPN-IPv4 NLRI with a next-hop address that belongs to the IPv6 protocol. This comprises an extension of the AFI/SAFI definitions to allow the address of the next hop for IPv4 NLRI or VPN-IPv4 NLRI to also belong to the IPv6 protocol, the encoding of the next hop to determine which of the protocols the address actually belongs to, and a BGP Capability allowing MP-BGP peers to dynamically discover whether they can exchange IPv4 NLRI and VPN-IPv4 NLRI with an IPv6 next hop. This document obsoletes RFC 5549. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. [STANDARDS-TRACK] Three IPv4 unicast address blocks are reserved for use in examples in specifications and other documents. This document describes the use of these blocks. This document is not an Internet Standards Track specification; it is published for informational purposes. This document specifies the current set of DHCP options. Future options will be specified in separate RFCs. The current list of valid options is also available in ftp://ftp.isi.edu/in-notes/iana/assignments. [STANDARDS-TRACK] To participate in wide-area IP networking, a host needs to be configured with IP addresses for its interfaces, either manually by the user or automatically from a source on the network such as a Dynamic Host Configuration Protocol (DHCP) server. Unfortunately, such address configuration information may not always be available. It is therefore beneficial for a host to be able to depend on a useful subset of IP networking functions even when no address configuration is available. This document describes how a host may automatically configure an interface with an IPv4 address within the 169.254/16 prefix that is valid for communication with other devices connected to the same physical (or logical) link. IPv4 Link-Local addresses are not suitable for communication with devices not directly connected to the same physical (or logical) link, and are only used where stable, routable addresses are not available (such as on ad hoc or isolated networks). This document does not recommend that IPv4 Link-Local addresses and routable addresses be configured simultaneously on the same interface. [STANDARDS-TRACK] Routed protocols are often susceptible to spoof attacks. The canonical solution for IPv6 is Secure Neighbor Discovery (SEND), a solution that is non-trivial to deploy. This document proposes a light-weight alternative and complement to SEND based on filtering in the layer-2 network fabric, using a variety of filtering criteria, including, for example, SEND status. This document is not an Internet Standards Track specification; it is published for informational purposes. This document specifies a stateless solution for service providers to progressively deploy IPv6-only network domains while still offering IPv4 service to customers. The solution's distinctive properties are that TCP/UDP IPv4 packets are valid TCP/UDP IPv6 packets during domain traversal and that IPv4 fragmentation rules are fully preserved end to end. Each customer can be assigned one public IPv4 address, several public IPv4 addresses, or a shared address with a restricted port set. This document specifies a DHCPv4 option to indicate that a host supports an IPv6-only mode and is willing to forgo obtaining an IPv4 address if the network provides IPv6 connectivity. It also updates RFC 2563 to specify DHCPv4 server behavior when the server receives a DHCPDISCOVER not containing the Auto-Configure option but containing the new option defined in this document.

Acknowledgements The author thanks Tobias Fiebig, Warren Kumari, and Jen Linkova.