IPv4 routes with an IPv6 next-hop in the Babel routing protocol
IRIF, University of Paris
Case 7014
75205 Paris Cedex 13
France
jch@irif.fr
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.
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)
in the case of IPv6, Address Resolution (ARP) 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 to announce routes towards IPv6
prefixes with IPv4 next hops. The extension is inspired by a previously
defined extension to the BGP protocol .
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.
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 addresses.
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 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 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.
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
. 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 ; the parameters of the route are as follows:
the prefix, plen, router-id, seqno, metric MUST be computed as for an
IPv4 route, as described in Section 4.6.9 of ;
the next-hop MUST be computed as for an IPv6 route, as described in
Section 4.6.9 of : 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 .
As usual, a node MAY ignore the update, e.g., due to filtering
(Appendix C of ). 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 .
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 . If an Update is sent, then
it MAY be sent with AE 1 or TBD, as described in
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 3.8.1.1 of
. Any IPv4 routes contained in the route dump
MAY use either AE 1 or AE TBD, as described in
above.
The only other TLVs defined by that carry an
AE field are Next-Hop and TLV. Next-Hop and IHU TLVs MUST NOT carry the
AE TBD (v4-via-v6).
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.
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 choose
a source address from a prefix known to be unused, for example from
a suitably chosen private address range . If no
more suitable address is available, then a router MAY use the IPv4 dummy
address 192.0.0.8 as the source address of the IMCPv4 packets that it
sends. Note however that using the same address on multiple routers may
hamper debugging and fault isolation, e.g., when using the "traceroute"
utility.
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.
Network prefixes tagged with AE TBD MUST be encoded and decoded just
like prefixes tagged with AE 1 (IPv4), as described in Section 4.3.1
of .
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
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 for network prefixes, IHU (Type = 5) and
Next-Hop (Type = 7) TLVs MUST NOT be tagged with AE TBD. Such (incorrect)
TLVs MUST be ignored upon reception.
An Update (Type = 8) TLV with AE = TBD is constructed as described in
Section 4.6.9 of for AE 1 (IPv4), with the
following specificities:
Prefix. The Prefix field is constructed according to
above.
Next hop. The next hop is determined as described in
above.
When tagged with the AE TBD, Route Request and Seqno Request updates
MUST be constructed and decoded as described in Section 4.6 of
, and the network prefixes contained within them
decoded as described in above.
This protocol extension adds no new TLVs or sub-TLVs.
This protocol extension uses a new AE. As discussed in Appendix D of
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 , 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 , 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 is requested to allocate a value (4 suggested) in the "Babel Address
Encodings" registry as follows:
AENameReference
TBDv4-via-v6(this document)
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 issues.
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 )
together with matching packet filters in the data plane.
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.
Key words for use in RFCs to Indicate Requirement Levels
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
The Babel Routing Protocol
Internet Control Message Protocol
An Ethernet Address Resolution Protocol: Or Converting Network
Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet
Hardware
Neighbor Discovery for IP version 6 (IPv6)
Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop
Path MTU discovery
Packetization Layer Path MTU Discovery
Address Allocation for Private Internets