IPv4 routes with an IPv6 next hop
draft-ietf-intarea-v4-via-v6-02

Internet Area Working Group                                J. Chroboczek
Internet-Draft                                 IRIF, University of Paris
Intended status: Standards Track                               W. Kumari
Expires: 23 February 2026                                    Google, LLC
                                                    T. Høiland-Jørgensen
                                                                 Red Hat
                                                          22 August 2025

                   IPv4 routes with an IPv6 next hop
                    draft-ietf-intarea-v4-via-v6-02

Abstract

   This document proposes "v4-via-v6" routing, a technique that uses
   IPv6 next-hop addresses for routing IPv4 packets, thus making it
   possible to route IPv4 packets across a network where routers have
   not been assigned IPv4 addresses.  The document both describes the
   technique, as well as discussing its operational implications.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://wkumari.github.io/draft-chroboczek-intarea-v4-via-v6/draft-
   ietf-intarea-v4-via-v6.html.  Status information for this document
   may be found at https://datatracker.ietf.org/doc/draft-ietf-intarea-
   v4-via-v6/.

   Discussion of this document takes place on the Internet Area Working
   Group Working Group mailing list (mailto:int-area@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/int-area/.
   Subscribe at https://www.ietf.org/mailman/listinfo/int-area/.

   Source for this draft and an issue tracker can be found at
   https://github.com/wkumari/draft-chroboczek-intarea-v4-via-v6.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 23 February 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Structure of the routing table  . . . . . . . . . . . . .   4
     3.2.  Operation of the forwarding plane . . . . . . . . . . . .   5
     3.3.  Operation of routing protocols  . . . . . . . . . . . . .   5
   4.  ICMP Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Arista EOS  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  The Babel routing protocol  . . . . . . . . . . . . . . .   7
     5.3.  Linux . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       5.3.1.  Example:  . . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Mikrotik RouterOS . . . . . . . . . . . . . . . . . . . .   8
       5.4.1.  Example . . . . . . . . . . . . . . . . . . . . . . .   8
     5.5.  Cisco NX-OS . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     Version 00-01 . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   The dominant form of routing in the Internet is next-hop routing,
   where a routing protocol constructs a routing table which is used by
   a forwarding process to forward packets.  The routing table is a data
   structure that maps network prefixes in a given family (IPv4 or IPv6)
   to next hops, pairs of an outgoing interface and a neighbor's network
   address, for example:

       destination                      next hop
     2001:db8:0:1::/64               eth0, fe80::1234:5678
     203.0.113.0/24                  eth0, 192.0.2.1

   When a packet is routed according to a given routing table entry, the
   forwarding plane uses a neighbor discovery protocol (the Neighbor
   Discovery protocol (ND) [RFC4861] in the case of IPv6, the Address
   Resolution Protocol (ARP) [RFC0826] in the case of IPv4) to map the
   next-hop address to a link-layer address (a "MAC address"), which is
   then used to construct the link-layer frames that encapsulate
   forwarded packets.

   It is apparent from the description above that there is no
   fundamental reason why the destination prefix and the next-hop
   address should be in the same address family: there is nothing
   preventing an IPv6 packet from being routed through a next hop with
   an IPv4 address (in which case the next hop's MAC address will be
   obtained using ARP), or, conversely, an IPv4 packet from being routed
   through a next hop with an IPv6 address.  (In fact, it is even
   possible to store link-layer addresses directly in the next-hop entry
   of the routing table, thus avoiding the use of an address resolution
   protocol altogether, which is commonly done in networks using the OSI
   protocol suite.)

   This document focuses on the specific case of routing IPv4 packets
   through an IPv6 next-hop.  This case is particularly interesting,
   since it makes it possible to build networks that have no IPv4
   addresses except at the edges and still provide IPv4 connectivity to
   edge hosts.  In addition, since an IPv6 next hop can use a link-local
   address that is autonomously configured, the use of such routes
   enables a mode of operation where the network core has no statically
   assigned IP addresses of either family, which significantly reduces
   the amount of manual configuration required.  (See also [RFC7404] for
   a discussion of the issues involved with such an approach.)

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   We call a route towards an IPv4 prefix that uses an IPv6 next hop a
   "v4-via-v6" route.  V4-via-v6 routing is not restricted to routers,
   and could usefully be applied to hosts, although doing so would
   require solving the issue of host configuration, for example by
   extending either DHCPv4 or DHCPv6 to publish an IPv4 default route
   with an IPv6 next hop.

   [RFC8950] discusses advertising of IPv4 NLRI with a next-hop address
   that belongs to the IPv6 protocol, but confines itself to how this is
   carried and advertised in the BGP protocol.  This document, on the
   other hand, discusses the concept of v4-via-v6 routes independently
   of any specific routing protocol, their design and operational
   considerations, and the implications of using them.

   { Editor note, to be removed before publication.  This document is
   heavily based on draft-ietf-babel-v4viav6.  When draft-ietf-babel-
   v4viav6 was going through IESG eval, Warren raised concerns that
   something this fundamental deserved to be documented in a separate,
   standalone document, so that it can be more fully discussed, and,
   more importantly, referenced cleanly in the future.}

2.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Operation

   Next-hop routing is implemented by two separate components, the
   routing protocol and the forwarding plane, that communicate through a
   shared data structure, the routing table.

3.1.  Structure of the routing table

   The routing table is a data structure that maps address prefixes to
   next-hops, pairs of the form (interface, address).  In traditional
   next-hop routing, the routing table maps IPv4 prefixes to IPv4 next
   hops, and IPv6 addresses to IPv6 next hops.  With v4-via-v6 routing,
   the routing table is extended so that an IPv4 prefix may map to an
   IPv6 next hop, or an IPv6 prefix to an IPv4 next hop.

   Resolution may be recursive: the next-hop may itself be a prefix that
   requires further resolution to map to the outgoing interface and L2
   address.  V4-via-v6 routing does not prevent recursive resolution.

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3.2.  Operation of the forwarding plane

   The forwarding plane is the part of the routing implementation that
   is executed for every forwarded packet.  As a packet arrives, the
   forwarding plane consults the routing table, selects a single route
   matching the packet, and forwards the packet out the outgoing
   interface to the associated next-hop address.

   With v4-via-v6 routing, the address family of the next-hop address is
   no longer determined by the address family of the prefix: since the
   routing table may map an IPv4 prefix to either an IPv4 or an IPv6
   next-hop, the forwarding plane must be able to determine, on a per-
   packet basis, which address resolution protocol (ARP for IPv4, ND for
   IPv6) to consult.

3.3.  Operation of routing protocols

   The routing protocol is the part of the routing implementation that
   is executed asynchronously from the forwarding plane, and whose role
   is to build the routing table.  Since v4-via-v6 routing is a
   generalization of traditional next-hop routing, v4-via-v6 can
   interoperate with existing routing protocols: a traditional routing
   protocol produces a traditional next-hop routing table, which can be
   used by an implementation supporting v4-via-v6 routing.

   However, in order to use the additional flexibility provided by
   v4-via-v6 routing, routing protocols need to be extended with the
   ability to populate the routing table with v4-via-v6 routes when an
   IPv4 address is not available or when the available IPv4 addresses
   are not suitable for use as a next-hop.

   Some protocols already support the advertisement of IPv4 routes with
   an IPv6 next-hop, including Babel [RFC8966] and BGP [RFC8950].  Other
   protocol advertise both IPv4 and IPv6 prefixes over a single
   neighbor; these include: * Multi-Topology (MT) Routing in OSPF
   ([RFC4915]) * Multi-Topology (MT) Routing in IS-IS ([RFC5120]) While
   both of these employ a common control plane, they use separate data
   planes, and therefore don't implement v4-via-v6 routing.

4.  ICMP Considerations

   The Internet Control Message Protocol (ICMPv4, or simply ICMP)
   [RFC0792] is a protocol related to IPv4 that is primarily used to
   carry diagnostic and debugging information.  ICMPv4 packets may be
   originated by end hosts (e.g., the "destination unreachable, port
   unreachable" ICMPv4 packet), but they may also be originated by
   intermediate routers (e.g., most other kinds of "destination
   unreachable" packets).

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   Some protocols deployed in the Internet rely on ICMPv4 packets sent
   by intermediate routers.  Most notably, path MTU Discovery (PMTUd)
   [RFC1191] is an algorithm executed by end hosts to discover the
   maximum packet size that a route is able to carry.  While there exist
   variants of PMTUd that are purely end-to-end [RFC4821], the variant
   most commonly deployed in the Internet has a hard dependency on
   ICMPv4 packets originated by intermediate routers: if intermediate
   routers are unable to send ICMPv4 packets, PMTUd may lead to
   persistent black-holing of IPv4 traffic.

   Due to this kind of dependency, every router that is able to forward
   IPv4 traffic SHOULD be able originate ICMPv4 traffic.  Since the
   extension described in this document enables routers to forward IPv4
   traffic received over an interface that has not been assigned an IPv4
   address, a router implementing this extension MUST be able to
   originate ICMPv4 packets even when the outgoing interface has not
   been assigned an IPv4 address.

   In such a situation, if the router has an interface that has been
   assigned a publicly routable IPv4 address (other than the loopback
   address), or if an IPv4 address has been assigned to the router
   itself (to the "loopback interface"), then that IPv4 address may be
   used as the source of originated ICMPv4 packets.  If no IPv4 address
   is available, the router should use the mechanism described in
   Requirement R-22 of Section 4.8 [RFC7600], which consists of using
   the dummy address 192.0.0.8 as the source address of originated
   ICMPv4 packets.  Note however that using the same address on multiple
   routers may hamper debugging and fault isolation, e.g., when using
   the "traceroute" utility.  This mirrors the behavior in Section 3 of
   [RFC9229].

   [I-D.draft-ietf-intarea-extended-icmp-nodeid] provides a possible
   solution to this issue, by allowing the ICMP packet to carry a "host
   identifier" that can be used to identify the router that originated
   the ICMP by providing a unique IP address and/or a textual name for
   the node, in the case where each node may not have a unique IP
   address.

5.  Implementation Status

   ( This section to be removed before publication. )

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   As this document does not really define a protocol, this
   implementation status section is much less formal.  Instead, it is
   being used as a place to list implementations that are known to
   support this functionality, examples, notes, etc.  This information
   is provided as a guide to the reader, and is not intended to be a
   complete list, nor endorsement, etc.  If you know of an
   implementation which is not listed, please let the authors know.

5.1.  Arista EOS

   Arista has supported static IPv4 routes with IPv6 nexthops since EOS-
   4.30.1.

5.2.  The Babel routing protocol

   As noted above, this document is heavily based on RFC9229 (nee draft-
   ietf-babel-v4viav6), and this functionality is supported by babeld.

   Pasted below is email sent to the babel mailing list (archived at
   https://mailarchive.ietf.org/arch/msg/babel/
   QtFi3F4TFfF7fXXlkHSpEnuT44Y/)

   A route across three IPv6-only nodes:

  $ ip route show 10.0.0.2
  10.0.0.2 via inet6 fe80::216:3eff:fe00:1 dev lxcbr0 proto babel onlink

   Here's how it's logged by babeld:

  10.0.0.2/32 from 0.0.0.0/0 metric 384 (384) refmetric 288 id
  02:16:3e:ff:fe:9a:5e:22 seqno 36425 chan (255) age 15 via lxcbr0 neigh
  fe80::216:3eff:fe00:1 (installed)

   Traceroute is a little confusing:

   $ traceroute 10.0.0.2
   traceroute to 10.0.0.2 (10.0.0.2), 30 hops max, 60 byte packets
    1  192.0.0.8 (192.0.0.8)  0.079 ms  0.019 ms  0.014 ms
    2  192.0.0.8 (192.0.0.8)  0.040 ms  0.023 ms  0.042 ms
    3  192.0.0.8 (192.0.0.8)  0.061 ms  0.030 ms  0.030 ms
    4  10.0.0.2 (10.0.0.2)  0.060 ms  0.040 ms  0.039 ms

   PMTUD works fine (thanks to Toke):

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   19:58:47.402871 IP 192.168.0.27.60046 > 10.0.0.2.22: Flags [.],\
   seq 33:1481, ack 33, win 502, options [nop,nop,TS val 917354570\
   ecr 1849974691], length 1448
   19:58:47.402874 IP 192.168.0.27.60046 > 10.0.0.2.22: Flags [P.],\
   seq 1481:1537, ack 33, win 502, options [nop,nop,TS val 917354570\
   ecr 1849974691], length 56
   19:58:47.402906 IP 192.0.0.8 > 192.168.0.27: ICMP 10.0.0.2 \
   unreachable- need to frag (mtu 1420), length 556
   19:58:47.402919 IP 10.0.0.2.22 > 192.168.0.27.60046: Flags [.],\
   ack 33, win 509, options [nop,nop,TS val 1849974692 \
   ecr 917354569,nop,nop,sac 1 {1481:1537}], length 0
   19:58:47.402934 IP 192.168.0.27.60046 > 10.0.0.2.22: Flags [.], \
   seq 33:1401, ack 33, win 502, options [nop,nop,TS val 917354570 \
   ecr 1849974692], length 1368

   -- Juliusz

5.3.  Linux

   Linux has supported v4-via-v6 routes since kernel version 5.2,
   released on 2019-07-07.

5.3.1.  Example:

   rincewind ~ #
   ip -4 r a 192.0.2.23/32 via inet6 2001:db8::2342

   rincewind ~ # ip r s 192.0.2.23/32
   192.0.2.23 via inet6 2001:db8::2342 dev wlp36s0.25

5.4.  Mikrotik RouterOS

   Mikrotik RouterOS has supported v4-via-v6 routes since (at least)
   version 7.11beta2

   {Editor note: I'm not sure when support was added.  I tested this in
   Version 7.11beta2, and it worked there, but I believe that this
   functionality has existed for a while.  I'll try to find out when it
   was added.}

5.4.1.  Example

  [wkumari@Dulles-CCR] /ip/route> print
  Flags: D - DYNAMIC; I - INACTIVE, A - ACTIVE; c - CONNECT, s - STATIC,
  d -DHCP, v - VPN; H - HW-OFFLOADED
  Columns: DST-ADDRESS, GATEWAY, DISTANCE
  #      DST-ADDRESS       GATEWAY                             DISTANCE
  0  As  192.0.2.0/24      fe80::201:5cff:feb2:1646%1_Comcast         1

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5.5.  Cisco NX-OS

   Cisco NX-OS has supported v4-via-v6 routes "for more than 8 years" --
   Krishnaswamy Ananthamurthy

6.  Security Considerations

   The techniques described in this document make routing more flexible
   by allowing IPv4 routes to propagate across a section of a network
   that has only been assigned IPv6 addresses.  This additional
   flexibility might invalidate otherwise reasonable assumptions made by
   network administrators, which could potentially cause security
   issues.

   For example, if an island of IPv4-only hosts is separated from the
   IPv4 Internet by routers that have not been assigned IPv4 addresses,
   a network administrator might reasonably assume that the IPv4-only
   hosts are unreachable from the IPv4 Internet.  This assumption is
   broken if the intermediary routers implement v4-via-v6 routing, which
   might make the IPv4-only hosts reachable from the IPv4 Internet.  If
   this is not desirable, then the network administrator must filter out
   the undesirable traffic in the forwarding plane by implementing
   suitable packet filtering rules.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7600]  Despres, R., Jiang, S., Ed., Penno, R., Lee, Y., Chen, G.,
              and M. Chen, "IPv4 Residual Deployment via IPv6 - A
              Stateless Solution (4rd)", RFC 7600, DOI 10.17487/RFC7600,
              July 2015, <https://www.rfc-editor.org/rfc/rfc7600>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

8.2.  Informative References

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   [I-D.draft-ietf-intarea-extended-icmp-nodeid]
              Fenner, B. and R. Thomas, "Adding Extensions to ICMP
              Errors for Originating Node Identification", Work in
              Progress, Internet-Draft, draft-ietf-intarea-extended-
              icmp-nodeid-04, 19 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
              extended-icmp-nodeid-04>.

   [IANA-IPV4-REGISTRY]
              "IANA IPv4 Address Registry", Web 
              https://www.iana.org/assignments/iana-ipv4-special-
              registry/.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/rfc/rfc792>.

   [RFC0826]  Plummer, D., "An Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <https://www.rfc-editor.org/rfc/rfc826>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/rfc/rfc1191>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/rfc/rfc4821>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/rfc/rfc4861>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/rfc/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/rfc/rfc5120>.

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   [RFC7404]  Behringer, M. and E. Vyncke, "Using Only Link-Local
              Addressing inside an IPv6 Network", RFC 7404,
              DOI 10.17487/RFC7404, November 2014,
              <https://www.rfc-editor.org/rfc/rfc7404>.

   [RFC8950]  Litkowski, S., Agrawal, S., Ananthamurthy, K., and K.
              Patel, "Advertising IPv4 Network Layer Reachability
              Information (NLRI) with an IPv6 Next Hop", RFC 8950,
              DOI 10.17487/RFC8950, November 2020,
              <https://www.rfc-editor.org/rfc/rfc8950>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
              <https://www.rfc-editor.org/rfc/rfc8966>.

   [RFC9229]  Chroboczek, J., "IPv4 Routes with an IPv6 Next Hop in the
              Babel Routing Protocol", RFC 9229, DOI 10.17487/RFC9229,
              May 2022, <https://www.rfc-editor.org/rfc/rfc9229>.

Acknowledgments

   We are grateful to Joe Abley, Krishnaswamy Ananthamurthy, Bill
   Fenner, Tobias Fiebig, John Gilmore, Bob Hinden, David Lamparter, Jen
   Linkova, Gyan Mishra, tom petch, Herbie Robinson, Behcet Sarikaya,
   David Schinazi, Ole Troan, and Éric Vyncke, for their helpful
   comments and suggestions on this document.

   We are also indebted to the members of the Babel community for the
   discussions that led to the creation of this document.

Changes

   This section is to be removed before publication, and the primary
   change log is the git repository.  This is just a place to note some
   of the more substantive changes.

Version 00-01

   *  Added note that this works just as well for IPv6 routes with an
      IPv4 next hop. (Éric Vyncke)

   *  Cisco NX-OS has supported v4-via-v6 routes "for more than 8 years"
      (Krishnaswamy Ananthamurthy)

   *  Mention recursive next hops, and that the next hop may be a
      prefix.  (Krishnaswamy Ananthamurthy)

   *  Hosts are routers too!  (David Lamparter)

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Internet-Draft                  v4-via-v6                    August 2025

   *  Removed the claim that it's mainly a UI issue.

Authors' Addresses

   Juliusz Chroboczek
   IRIF, University of Paris
   Case 7014
   75205 Paris Cedex 13
   France
   Email: jch@irif.fr

   Warren Kumari
   Google, LLC
   Email: warren@kumari.net

   Toke Høiland-Jørgensen
   Red Hat
   Email: toke@toke.dk

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