Announcing IPv4 routes with an IPv6 next-hop in the Babel routing protocol
Ecole Normale Superieure, Paris
France
theophile.bastian@ens.fr
IRIF, University of Paris-Diderot
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-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 to announce routes towards IPv6
prefixes with IPv4 next hops. The extension is inspired by a previously
defined extension to the BGP protocol .
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 addresses.
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 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-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.
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
. 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.
As usual, a node MAY ignore the update, e.g., due to filtering
(Appendix C of ). 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 .
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 . 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 TLV defined by that carries
an AE field is the IHU TLV. IHU TLVs MUST NOT carry the AE TBD
(v4-over-v6).
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-over-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-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 announced.
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.
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
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 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.
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 the
above.
Next hop. The next hop is determined as described in
above.
Any other valid TLV tagged with AE = TBD MUST be constructed and decoded as
described in Section 4.6 of . Network prefixes
within MUST be constructed and decoded as described in above.
IANA is instructed to add the following entry to the "Babel Address
Encodings" registry:
AENameReference
TBDv4-over-v6(this document)
This extension does not fundamentally change the security properties of
the Babel protocol: as described in Section 6 of ,
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 the use of
suitable filtering rules (Appendix C of )
together with matching access control rules in the data plane.
The Babel Routing 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