Internet Engineering Task Force (IETF) J. Chroboczek

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

Abstract

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
evaluation.

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
https://www.rfc-editor.org/info/rfc9229.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

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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
Acknowledgments
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
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 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",
"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.

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 3.8.1.1 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-
via-v6).

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 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.

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
[RFC8966]

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
announced.

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
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 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,
<https://www.rfc-editor.org/info/rfc792>.

[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/info/rfc2119>.

[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/info/rfc8174>.

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

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,
<https://www.rfc-editor.org/info/rfc826>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/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/info/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/info/rfc4861>.

[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,
<https://www.rfc-editor.org/info/rfc5549>.

[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/info/rfc7404>.

[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/info/rfc7600>.

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 Høiland-
Jørgensen, David Schinazi, and Donald Sharp.

Author's Address

Juliusz Chroboczek
IRIF, University of Paris
Case 7014
75205 Paris Cedex 13
France
Email: [email protected]