]> IRIF, University of Paris

Case 7014 Paris Cedex 13 75205 France jch@irif.fr

rtg babel routing transition IPv6 transition double-stack dual-stack glorious IPv6-only future 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.

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: 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 in the case of IPv6 and the Address Resolution Protocol (ARP) 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 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 to announce v4-via-v6 routes across interfaces that have no IPv4 addresses assigned but are capable of forwarding IPv4 traffic. 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 .

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 when, and only when, they appear in all capitals, as shown here.

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.

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.

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 . 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 ; 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 .
• The next hop MUST be computed as for an IPv6 route, as described in . 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 . As usual, a node MAY ignore the update, e.g., due to filtering (see ). 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.

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 . 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 , 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 . Any IPv4 routes contained in the route dump may use either AE 1 (IPv4) or AE 4 (v4-via-v6), as described .

Other TLVs The only other TLVs defined by that carry an AE field are Next Hop and IHU. Next Hop and IHU TLVs MUST NOT carry the AE 4 (v4-via-v6).

ICMPv4 and PMTU Discovery The Internet Control Message Protocol (ICMPv4, or simply ICMP) 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) 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 , 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 , 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.

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.

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 . 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 and of

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.

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

• The Prefix field is constructed according to .
• The Next Hop field is built and parsed as described in Sections and .

Requests When tagged with the AE 4 (v4-via-v6), Route Request and Seqno Request TLVs MUST be constructed and decoded as described in , and the network prefixes contained within them MUST be decoded as described in (see also ).

Backwards Compatibility This protocol extension adds no new TLVs or sub-TLVs. This protocol extension uses a new AE. As discussed in 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 , 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 , 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.

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

AE	Name	Reference
4	v4-via-v6	RFC 9229

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 ) together with matching packet filters in the data plane.

References Normative References Informative References

Acknowledgments This protocol extension was originally designed, described, and implemented in collaboration with . pointed out the issues with ICMP and helped coin the phrase "v4-via-v6". The author is also indebted to , , , and .