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Network Working Group                                         T. Bastian
Internet-Draft                           Ecole Normale Superieure, Paris
Updates: 6126bis (if approved)                             J. Chroboczek
Intended status: Experimental          IRIF, University of Paris-Diderot
Expires: November 15, 2020                                  May 14, 2020

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


   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

   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
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   This Internet-Draft will expire on November 15, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   3
   2.  Protocol operation  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Announcing v4-over-v6 routes  . . . . . . . . . . . . . .   3
     2.2.  Receiving v4-over-v6 routes . . . . . . . . . . . . . . .   3
     2.3.  Prefix and seqno requests . . . . . . . . . . . . . . . .   4
     2.4.  Other TLVs  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Backwards compatibility . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol encoding . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Prefix encoding . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Changes for existing TLVs . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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-over-v6" route.  Since an IPv6 next-hop can use a link-local
   address that is autonomously configured, the use of v4-over-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 [RFC6126bis] 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-over-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-over-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-over-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-over-v6 announcement.  A node SHOULD
   NOT send both ordinary IPv4 and v4-over-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.

2.2.  Receiving v4-over-v6 routes

   Upon reception of an Update TLV with a v4-over-v6 AE, a Babel node
   computes the IPv6 next-hop, as described in Section 4.6.9 of
   [RFC6126bis].  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
   [RFC6126bis]; the parameters of the route are as follows:

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   o  the prefix, plen, router-id, seqno, metric MUST be computed as for
      an IPv4 route, as described in Section 4.6.9 of [RFC6126bis];

   o  the next-hop MUST be computed as for an IPv6 route, as described
      in Section 4.6.9 of [RFC6126bis]: 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.

   As usual, a node MAY ignore the update, e.g., due to filtering
   (Appendix C of [RFC6126bis]).  If a node cannot install v4-over-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 [RFC6126bis].

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 ordinary IPv4 and v4-over-v6 requests.

   A node SHOULD NOT send requests of either kind with the AE field
   being set to TBD (v4-over-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-over-v6) MUST be
   treated in the same manner: the receiver processes the request as
   described in Section 3.8 of [RFC6126bis].  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.

   When receiving a request with AE 0 (wildcard), the receiver SHOULD
   send a full route dump, as described in Section of
   [RFC6126bis].  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 TLV defined by [RFC6126bis] that carries an AE field
   is the IHU TLV.  IHU TLVs MUST NOT carry the AE TBD (v4-over-v6).

3.  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
   [RFC6126bis] and specified in the same document, implementations that

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   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-over-v6 routes.  They will also ignore
   requests with AE TBD, which, as stated in Section 2.3, are NOT

   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-over-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

4.  Protocol encoding

   This extension defines the v4-over-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.

4.1.  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

   A new compression state for AE TBD (v4-over-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

4.2.  Changes for existing TLVs

   The following TLVs MAY be tagged with AE TBD:

   o  Update (Type = 8)

   o  Route Request (Type = 9)

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   o  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
   TLVs MUST be silently ignored.

4.2.1.  Update

   An Update (Type = 8) TLV with AE = TBD is constructed as described in
   Section 4.6.9 of [RFC6126bis] for AE 1 (IPv4), with the following

   o  Prefix.  The Prefix field is constructed according to the
      Section 4.1 above.

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

4.2.2.  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 [RFC6126bis].  Network
   prefixes within MUST be constructed and decoded as described in
   Section 4.1 above.

5.  IANA Considerations

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

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

6.  Security Considerations

   This extension does not fundamentally change the security properties
   of the Babel protocol: as described in Section 6 of [RFC6126bis],
   Babel must be protected by a suitable cryptographic mechanism in
   order to be made secure.

   However, enabling this extension will allow IPv4 traffic to flow
   through sections of a network that have not been assigned IPv4
   addresses, which, in turn, might allow IPv4 traffic to reach areas of
   the network that were previously inaccessible to such traffic.  If
   this is undesirable, the flow of IPv4 traffic must be restricted by

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   the use of suitable filtering rules (Appendix C of [RFC6126bis])
   together with matching access control rules in the data plane.

7.  References

7.1.  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.

              Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", draft-ietf-babel-rfc6126bis-17 (work in
              progress), February 2020.

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

7.2.  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.

   [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.

Authors' Addresses

   Theophile Bastian
   Ecole Normale Superieure, Paris

   Email: theophile.bastian@ens.fr

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

   Email: jch@irif.fr

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