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IPv4 Routes with an IPv6 Next Hop in the Babel Routing Protocol
RFC 9229

Document Type RFC - Experimental (May 2022)
Author Juliusz Chroboczek
Last updated 2022-05-06
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Martin Vigoureux
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RFC 9229

Internet Engineering Task Force (IETF)                     J. Chroboczek
Request for Comments: 9229                     IRIF, University of Paris
Category: Experimental                                          May 2022
ISSN: 2070-1721

    IPv4 Routes with an IPv6 Next Hop in the Babel Routing Protocol


   This document defines an extension to the Babel routing protocol that
   allows announcing routes to an IPv4 prefix with an IPv6 next hop,
   which makes it possible for IPv4 traffic to flow through interfaces
   that have not been assigned an IPv4 address.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2022 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
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Specification of Requirements
   2.  Protocol Operation
     2.1.  Announcing v4-via-v6 Routes
     2.2.  Receiving v4-via-v6 Routes
     2.3.  Route and Seqno Requests
     2.4.  Other TLVs
   3.  ICMPv4 and PMTU Discovery
   4.  Protocol Encoding
     4.1.  Prefix Encoding
     4.2.  Changes to Existing TLVs
   5.  Backwards Compatibility
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Author's Address

1.  Introduction

   The role of a routing protocol is to build a routing table, a data
   structure that maps network prefixes in a given family (IPv4 or IPv6)
   to next hops, which are (at least conceptually) pairs of an outgoing
   interface and a neighbour's network address.  For example:

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

   When a packet is routed according to a given routing table entry, the
   forwarding plane typically uses a neighbour discovery protocol (the
   Neighbour Discovery (ND) protocol [RFC4861] in the case of IPv6 and
   the Address Resolution Protocol (ARP) [RFC0826] in the case of IPv4)
   to map the next-hop address to a link-layer address (a "Media Access
   Control (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, which is commonly done in networks using the
   OSI protocol suite).

   The case of routing IPv4 packets through an IPv6 next hop 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.)

   We call a route towards an IPv4 prefix that uses an IPv6 next hop a
   "v4-via-v6" route.  This document describes an extension that allows
   the Babel routing protocol [RFC8966] to announce v4-via-v6 routes
   across interfaces that have no IPv4 addresses assigned but are
   capable of forwarding IPv4 traffic.  Section 3 describes procedures
   that ensure that all routers can originate ICMPv4 packets, even if
   they have not been assigned any IPv4 addresses.

   The extension described in this document is inspired by a previously
   defined extension to BGP [RFC5549].

1.1.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  Protocol Operation

   The Babel protocol fully supports dual-stack operation: all data that
   represent a neighbour address or a network prefix are tagged by an
   Address Encoding (AE), a small integer that identifies the address
   family (IPv4 or IPv6) of the address of prefix and describes how it
   is encoded.  This extension defines a new AE, called "v4-via-v6",
   which has the same format as the existing AE for IPv4 addresses (AE
   1).  This new AE is only allowed in TLVs that carry network prefixes:
   TLVs that carry an IPv6 neighbour address use one of the normal
   encodings for IPv6 addresses.

2.1.  Announcing v4-via-v6 Routes

   A Babel node can use a v4-via-v6 announcement to announce an IPv4
   route over an interface that has no assigned IPv4 address.  In order
   to do so, it first establishes an IPv6 next-hop address in the usual
   manner (either by sending the Babel packet over IPv6, or by including
   a Next Hop TLV containing an IPv6 address and using AE 2 or 3); it
   then sends an Update, with AE equal to 4 (v4-via-v6) containing the
   IPv4 prefix being announced.

   If the outgoing interface has been assigned an IPv4 address, then, in
   the interest of maximising compatibility with existing routers, the
   sender SHOULD prefer an ordinary IPv4 announcement; even in that
   case, however, it MAY send a v4-via-v6 announcement.  A node SHOULD
   NOT send both ordinary IPv4 and v4-via-v6 announcements for the same
   prefix over a single interface (if the update is sent to a multicast
   address) or to a single neighbour (if sent to a unicast address),
   since doing that provides no benefit while doubling the amount of
   routing traffic.

   Updates with infinite metric are retractions: they indicate that a
   previously announced route is no longer available.  Retractions do
   not require a next hop; therefore, there is no difference between
   v4-via-v6 retractions and ordinary retractions.  A node MAY send IPv4
   retractions only, or it MAY send v4-via-v6 retractions on interfaces
   that have not been assigned an IPv4 address.

2.2.  Receiving v4-via-v6 Routes

   Upon reception of an Update TLV with AE equal to 4 (v4-via-v6) and
   finite metric, a Babel node computes the IPv6 next hop, as described
   in Section 4.6.9 of [RFC8966].  If no IPv6 next hop exists, then the
   Update MUST be ignored.  If an IPv6 next hop exists, then the node
   MAY acquire the route being announced, as described in Section 3.5.3
   of [RFC8966]; the parameters of the route are as follows:

   *  The prefix, plen, router-id, seqno, and metric MUST be computed as
      for an IPv4 route, as described in Section 4.6.9 of [RFC8966].

   *  The next hop MUST be computed as for an IPv6 route, as described
      in Section 4.6.9 of [RFC8966].  It is taken from the last
      preceding Next Hop TLV with an AE field equal to 2 or 3; if no
      such entry exists and if the Update TLV has been sent in a Babel
      packet carried over IPv6, then the next hop is the network-layer
      source address of the packet.

   An Update TLV with a v4-via-v6 AE and metric equal to infinity is a
   retraction: it announces that a previously available route is being
   retracted.  In that case, no next hop is necessary, and the
   retraction is treated as described in Section 4.6.9 of [RFC8966].

   As usual, a node MAY ignore the update, e.g., due to filtering (see
   Appendix C of [RFC8966]).  If a node cannot install v4-via-v6 routes,
   e.g., due to hardware or software limitations, then routes to an IPv4
   prefix with an IPv6 next hop MUST NOT be selected.

2.3.  Route and Seqno Requests

   Route and seqno requests are used to request an update for a given
   prefix.  Since they are not related to a specific next hop, there is
   no semantic difference between IPv4 and v4-via-v6 requests.
   Therefore, a node SHOULD NOT send requests of either kind with the AE
   field being set to 4 (v4-via-v6); instead, it SHOULD request IPv4
   updates by sending requests with the AE field being set to 1 (IPv4).

   When receiving requests, AEs 1 (IPv4) and 4 (v4-via-v6) MUST be
   treated in the same manner: the receiver processes the request as
   described in Section 3.8 of [RFC8966].  If an Update is sent, then it
   MAY be an ordinary IPv4 announcement (AE = 1) or a v4-via-v6
   announcement (AE = 4), as described in Section 2.1, irrespective of
   which AE was used in the request.

   When receiving a request with AE 0 (wildcard), the receiver SHOULD
   send a full route dump, as described in Section of [RFC8966].
   Any IPv4 routes contained in the route dump may use either AE 1
   (IPv4) or AE 4 (v4-via-v6), as described Section 2.1.

2.4.  Other TLVs

   The only other TLVs defined by [RFC8966] that carry an AE field are
   Next Hop and IHU.  Next Hop and IHU TLVs MUST NOT carry the AE 4 (v4-

3.  ICMPv4 and PMTU Discovery

   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).

   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 blackholing of IPv4 traffic.

   Due to this kind of dependency, every Babel router that is able to
   forward IPv4 traffic MUST 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 a Babel router has an interface that has been
   assigned an IPv4 address (other than a 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, a Babel
   router could use the experimental mechanism described in Requirement
   R-22 of Section 4.8 of [RFC7600], which consists of using the dummy
   address 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.

4.  Protocol Encoding

   This extension defines the v4-via-v6 AE, whose value is 4.  This AE
   is solely used to tag network prefixes and MUST NOT be used to tag
   neighbour addresses, e.g., in Next Hop or IHU TLVs.

   This extension defines no new TLVs or sub-TLVs.

4.1.  Prefix Encoding

   Network prefixes tagged with AE 4 (v4-via-v6) MUST be encoded and
   decoded just like prefixes tagged with AE 1 (IPv4), as described in
   Section 4.1.5 of [RFC8966].

   A new compression state for AE 4 (v4-via-v6) distinct from that of AE
   1 (IPv4) is introduced and MUST be used for address compression of
   prefixes tagged with AE 4, as described in Sections 4.5 and 4.6.9 of

4.2.  Changes to Existing TLVs

   The following TLVs MAY be tagged with AE 4 (v4-via-v6):

   *  Update (Type = 8)

   *  Route Request (Type = 9)

   *  Seqno Request (Type = 10)

   As AE 4 (v4-via-v6) is suitable only for network prefixes, IHU (Type
   = 5) and Next Hop (Type = 7) TLVs are never sent with AE 4.  Such
   (incorrect) TLVs MUST be ignored upon reception.

4.2.1.  Update

   An Update (Type = 8) TLV with AE 4 (v4-via-v6) is constructed as
   described in Section 4.6.9 of [RFC8966] for AE 1 (IPv4), with the
   following specificities:

   *  The Prefix field is constructed according to Section 4.1.

   *  The Next Hop field is built and parsed as described in Sections
      2.1 and 2.2.

4.2.2.  Requests

   When tagged with the AE 4 (v4-via-v6), Route Request and Seqno
   Request TLVs MUST be constructed and decoded as described in
   Section 4.6 of [RFC8966], and the network prefixes contained within
   them MUST be decoded as described in Section 4.1 (see also
   Section 2.3).

5.  Backwards Compatibility

   This protocol extension adds no new TLVs or sub-TLVs.

   This protocol extension uses a new AE.  As discussed in Appendix D of
   [RFC8966] and specified in the same document, implementations that do
   not understand the present extension will silently ignore the various
   TLVs that use this new AE.  As a result, incompatible versions will
   ignore v4-via-v6 routes.  They will also ignore requests with AE 4
   (v4-via-v6), which, as stated in Section 2.3, are not recommended.

   Using a new AE introduces a new compression state, which is used to
   parse the network prefixes.  As this compression state is separate
   from the states of other AEs, it will not interfere with the
   compression state of unextended nodes.

   This extension reuses the next-hop state from AEs 2 and 3 (IPv6) but
   makes no changes to the way in which it is updated.  Therefore, it
   causes no compatibility issues.

   As mentioned in Section 2.1, ordinary IPv4 announcements are
   preferred to v4-via-v6 announcements when the outgoing interface has
   an assigned IPv4 address; doing otherwise would prevent routers that
   do not implement this extension from learning the route being

6.  IANA Considerations

   IANA has allocated value 4 in the "Babel Address Encodings" registry
   as follows:

                      | AE | Name      | Reference |
                      | 4  | v4-via-v6 | RFC 9229  |

                                 Table 1

7.  Security Considerations

   The extension defined in this document does not fundamentally change
   the security properties of the Babel protocol.  However, by allowing
   IPv4 routes to be propagated across routers that have not been
   assigned IPv4 addresses, it might invalidate the assumptions made by
   network administrators, which could conceivably lead to security

   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 the extension described
   in this document, which might expose the IPv4-only hosts to traffic
   from the IPv4 Internet.  If this is undesirable, the flow of IPv4
   traffic must be restricted by the use of suitable filtering rules
   (see Appendix C of [RFC8966]) together with matching packet filters
   in the data plane.

8.  References

8.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,

8.2.  Informative References

   [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,

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

   [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,

   [RFC5549]  Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
              Layer Reachability Information with an IPv6 Next Hop",
              RFC 5549, DOI 10.17487/RFC5549, May 2009,

   [RFC7404]  Behringer, M. and E. Vyncke, "Using Only Link-Local
              Addressing inside an IPv6 Network", RFC 7404,
              DOI 10.17487/RFC7404, November 2014,

   [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, <>.


   This protocol extension was originally designed, described, and
   implemented in collaboration with Theophile Bastian.  Margaret Cullen
   pointed out the issues with ICMP and helped coin the phrase "v4-via-
   v6".  The author is also indebted to Donald Eastlake, Toke Høiland-
   Jørgensen, David Schinazi, and Donald Sharp.

Author's Address

   Juliusz Chroboczek
   IRIF, University of Paris
   Case 7014
   75205 Paris Cedex 13