Network Working Group                                      J. Chroboczek
Internet-Draft                                 IRIF, University of Paris
Updates: 8966 (if approved)                                13 April 2021
Intended status: Standards Track
Expires: 15 October 2021

    IPv4 routes with an IPv6 next-hop in the Babel routing protocol


   This document defines an extension to the Babel routing protocol that
   allows annoncing 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

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   This Internet-Draft will expire on 15 October 2021.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   3
   2.  Protocol operation  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Announcing v4-via-v6 routes . . . . . . . . . . . . . . .   3
     2.2.  Receiving v4-via-v6 routes  . . . . . . . . . . . . . . .   4
     2.3.  Prefix and seqno requests . . . . . . . . . . . . . . . .   4
     2.4.  Other TLVs  . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  ICMPv4 and PMTU discovery . . . . . . . . . . . . . . . . . .   5
   4.  Backwards compatibility . . . . . . . . . . . . . . . . . . .   6
   5.  Protocol encoding . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Prefix encoding . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Changes for existing TLVs . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Traditionally, a routing table maps a network prefix of a given
   address family to a next-hop address in the same address family.  The
   sole purpose of this next-hop address is to serve as an input to a
   protocol that will map it to a link-layer address, Neighbour
   Discovery (ND) [RFC4861] in the case of IPv6, Address Resolution
   (ARP) [RFC0826] in the case of IPv4.  Therefore, there is no reason
   why the address family of the next hop address should match that of
   the prefix being announced: an IPv6 next-hop yields a link-layer
   address that is suitable for forwarding both IPv6 or IPv4 traffic.

   We call a route towards an IPv4 prefix that uses an IPv6 next hop a
   "v4-via-v6" route.  Since an IPv6 next-hop can use a link-local
   address that is autonomously configured, the use of v4-via-v6 routes
   enables a mode of operation where the network core has no statically
   assigned IP addresses of either family, thus significantly reducing
   the amount of manual configuration.

   This document describes an extension that allows the Babel routing
   protocol [RFC8966] to announce routes towards IPv6 prefixes with IPv4
   next hops.  The extension is inspired by a previously defined
   extension to the BGP protocol [RFC5549].

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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 double-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.  This new
   AE is only allowed in TLVs that carry network prefixes: TLVs that
   carry a neighbour address use the normal encodings for IPv6

2.1.  Announcing v4-via-v6 routes

   A Babel node that needs to announce an IPv4 route over an interface
   that has no assigned IPv4 address MAY make a v4-via-v6 announcement.
   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); it then sends
   an Update with AE equal to TBD containing the IPv4 prefix being

   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 use a v4-via-v6 announcement.  A node SHOULD
   NOT send both ordinary IPv4 and v4-via-v6 annoucements 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 doubles the amount of routing traffic while
   providing no benefit.

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2.2.  Receiving v4-via-v6 routes

   Upon reception of an Update TLV with a v4-via-v6 AE 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 silently 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

   *  the prefix, plen, router-id, seqno, 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
   (Appendix C of [RFC8966]).  If a node cannot install v4-via-v6
   routes, eg., due to hardware or software limitations, then routes to
   an IPv4 prefix with an IPv6 next-hop MUST NOT be selected, as
   described in Section 3.5.3 of [RFC8966].

2.3.  Prefix and seqno requests

   Prefix 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 TBD (v4-via-v6); instead, it SHOULD request IPv4
   updates using requests with the AE field being set to 1 (IPv4).

   When receiving requests, AEs 1 (IPv4) and TBD (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 sent with AE 1 or TBD, as described in Section 2.1 above,
   irrespective of which AE was used in the request.

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   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 or AE
   TBD, as described in Section 2.1 above.

2.4.  Other TLVs

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

3.  ICMPv4 and PMTU discovery

   The Internet Control Message Protocol (ICMPv4, or simply ICMP)
   [RFC792] 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 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.

   There are various ways to meet this requirement, and choosing between
   them is left to the implementation.  For example, if a router has an
   interface that has been assigned an IPv4 address, or if an IPv4
   address has been assigned to the router itself (to the "loopback
   interface"), then that IPv4 address may be "borrowed" to serve as the
   source of originated ICMPv4 packets.  If no IPv4 address is
   available, a router may use a dummy IPv4 address as the source of
   outgoing ICMPv4 packets, for example an address taken from a private
   address range [RFC1918] that is known to not be used in the local

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   routing domain (either dynamically chosen, for example drawn randomly
   or derived algorithmically from an IPv6 address, or statically
   configured).  Note however that using the same address on multiple
   routers may hamper debugging and fault isolation, e.g., when using
   the "traceroute" utility.

4.  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 TBD,
   which, as stated in Section 2.3, are NOT RECOMMENDED.

   Using a new AE introduces a new compression state, used to parse the
   network prefixes.  As this compression state is separate from other
   AEs' states, 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 it is updated, and therefore 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

5.  Protocol encoding

   This extension defines the v4-via-v6 AE, whose value is TBD.  This AE
   is solely used to tag network prefixes, and MUST NOT be used to tag
   peers' addresses, eg. in Next-Hop or IHU TLVs.

   This extension defines no new TLVs or sub-TLVs.

5.1.  Prefix encoding

   Network prefixes tagged with AE TBD MUST be encoded and decoded as
   prefixes tagged with AE 1 (IPv4), as described in Section 4.3.1 of

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

5.2.  Changes for existing TLVs

   The following TLVs MAY be tagged with AE TBD:

   *  Update (Type = 8)

   *  Route Request (Type = 9)

   *  Seqno Request (Type = 10)

   As AE TBD is suitable only to tag network prefixes, IHU (Type = 5)
   and Next-Hop (Type = 7) TLVs MUST NOT be tagged with AE TBD.  Such
   (incorrect) TLVs MUST be silently ignored upon reception.

5.2.1.  Update

   An Update (Type = 8) TLV with AE = TBD is constructed as described in
   Section 4.6.9 of [RFC8966] for AE 1 (IPv4), with the following

   *  Prefix.  The Prefix field is constructed according to the
      Section 5.1 above.

   *  Next hop.  The next hop is determined as described in Section 2.2

5.2.2.  Other valid TLVs tagged with AE = TBD

   Any other valid TLV tagged with AE = TBD MUST be constructed and
   decoded as described in Section 4.6 of [RFC8966].  Network prefixes
   within MUST be constructed and decoded as described in Section 5.1

6.  IANA Considerations

   IANA is requested to allocate a value (4 suggested) in the "Babel
   Address Encodings" registry as follows:

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                   | AE  | Name      | Reference       |
                   | TBD | v4-via-v6 | (this document) |

                                  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
   some network administatoris, which could conceivably lead to security

   For example, if an island of IPv4-only hosts is separated from the
   IPv4 Internet by an area of 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 (Appendix C of [RFC8966]) together with
   matching packet filters in the data plane.

8.  Acknowledgments

   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 Hoiland-
   Jorgensen, and David Schinazi.

9.  References

9.1.  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,

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

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   [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,

9.2.  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,

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <>.

   [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,

Author's Address

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


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