IPv4 routes with an IPv6 next hop

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Table of Contents

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).¶

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.¶

[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.¶

4. Operational Considerations

The routing "logic" is not fundamentally different between IPv4 and IPv6, and the primary thing preventing many implementations from supporting v4-via-v6 operations is the command line / configuration syntax. This means that the required changes to support v4-via-v6 routing in many implementations are relatively small - basically just changing the command line parsing to allow specifying an IPv6 address as a next-hop for an IPv4 route.¶

5. 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).¶

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 experimental 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. Note that this mirrors the behavior in Section 3 of [RFC9229].¶

{Editor note: It might be surprising for operators to see something like:¶

> $ traceroute -n 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
 1  192.168.0.1  1.894 ms  1.953 ms  1.463 ms
 2  192.0.0.8  9.012 ms  8.852 ms  12.211 ms
 3  192.0.0.8  8.445 ms  9.426 ms  9.781 ms
 4  192.0.0.8  9.984 ms  10.282 ms  10.763 ms
 5  192.0.0.8  13.994 ms  13.031 ms  12.948 ms
 6  192.0.0.8  27.502 ms  26.895 ms
 7  8.8.8.8  26.509 ms

Is this a problem though? If this becomes common practice, operators will come to understand that the repeated 192.0.0.8 is not actually a looping packet, but rather that the packet is (probably!) making forward progress.¶

In addition, [I-D.fenner-intarea-extended-icmp-hostid] 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 packet:¶

This document introduces a similar ICMP extension for Node
Identification.  It allows 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.

This mechanism may be used to provide a more meaningful source address in the future. This is not a requirement for this document, but it is worth noting. }¶

6. Implementation Status

( This section to be removed before publication. )¶

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 which 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.¶

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

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 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

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

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

[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

7. 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.¶

8. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

We would like to thank Joe Abley, Bill Fenner, John Gilmore, Bob Hinden, Gyan Mishra, tom petch, Herbie Robinson, Behcet Sarikaya, David Schinazi, and Ole Troan for their helpful comments and suggestions on this document.¶

The authors would like to thank the members of the Babel community for the insightful discussions that led to the creation of this document.¶

Authors' Addresses