Network working group D. Cheng
Internet Draft Huawei Technologies
Category: Informational M. Boucadair
Expires: August 5, 2011 France Telecom
February 5, 2011
Routing for IPv4-embedded IPv6 Packets
draft-cheng-ospf-ipv4-embedded-ipv6-routing-03
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Abstract
This document describes routing packets destined to IPv4-embedded
IPv6 addresses across IPv6 transit core using OSPFv3 with a separate
routing table.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents
1. Introduction...................................................3
1.1. The scenario..............................................3
1.2. Routing Solution per RFC5565..............................4
1.3. An Alternative Routing Solution with OSPFv3...............4
1.4. OSPFv3 Routing with a Specific Topology...................5
2. Provisioning...................................................6
2.1. Deciding the IPv4-embedded IPv6 Topology..................6
2.2. Maintaining a Dedicated IPv4-embedded IPv6 Routing Table..6
2.3. OSPFv3 Topology with a Separate Instance ID...............7
2.4. OSPFv3 Topology with the Default Instance.................7
3. IP Packets Translation.........................................7
3.1. Address Translation.......................................8
4. Advertising IPv4-embedded IPv6 Routes..........................8
4.1. Advertising IPv4-embedded IPv6 Routes into IPv6 Transit
Network........................................................8
4.1.1. Routing Metrics......................................9
4.1.2. Forwarding Address...................................9
4.2. Advertising IPv4 Addresses into Client Networks...........9
5. Aggregation on IPv4 Addresses and Prefixes.....................9
6. Forwarding....................................................10
7. MTU Issue.....................................................10
8. Backdoor Connections..........................................11
9. Security......................................................11
10. IANA Considerations..........................................11
11. Acknowledgements.............................................11
12. References...................................................11
12.1. Normative References....................................11
12.2. Informative References..................................12
13. Authors' Addresses...........................................12
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1. Introduction
This document describes a routing scenario where IPv4 packets are
transported over IPv6 network.
In this document the following terminology is used:
- An IPv4-embedded IPv6 address denotes an IPv6 address which
contains an embedded 32-bit IPv4 address constructed according
to the rules defined in [RFC6052].
- IPv4-embedded IPv6 packets are packets of which destination
addresses are IPv4-embedded IPv6 addresses.
- AFBR (Address Family Border Router, [RFC5565]) refers to an edge
router, which supports both IPv4 and IPv6 address families, of a
backbone that supports only IPv4 or IPv6 address family.
- AFXLBR (Address Family Translation Border Router) is defined in
this document. It refers to a border router that supports both
IPv4 and IPv6 address families, located on the boundary of IPv4-
only network and IPv6-only network, and is capable of performing
IP header translation between IPv4 and IPv6 according to
[I-D.draft-ietf-behave-v6v4-xlate].
1.1. The scenario
Due to exhaustion of public IPv4 addresses, there has been
continuing effort within IETF on IPv6 transitional techniques. In
the course of transition, it is certain that networks based on IPv4
and IPv6 transfer capabilities, respectively, will co-exist at least
for some time. One scenario of the co-existence is that IPv4-only
networks inter-connecting with IPv6-only networks, and in
particular, when an IPv6-only network serves as a transit network
that inter-connects several segregated IPv4-only networks. In this
scenario, IPv4 packets are transported over the IPv6 transit network
between IPv4 networks. In order to forward an IPv4 packet from a
source IPv4 network to the destination IPv4 network, IPv4
reachability information must be exchanged among involved networks
by dedicated means.
Unlike dual-stack networks, operating an IPv6-only network would
allow optimize OPEX and maintenance operations in particular. Some
solutions have been proposed to allow delivery of IPv4 services over
an IPv6-only network. This document focuses on an engineering
techniques which aims to separate the routing instance dedicated to
IPv4-embedded IPv6 destination from native IPv6 ones.
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The purpose of running separate instances or topologies for IPv4-
embedded IPv6 traffic is to distinguish from the native IPv6 routing
topology, and the topology that is used for routing IPv4-embedded
IPv6 datagram only. Separate instances/topologies are also meant to
prevent any overload of the native IPv6 routing tables by IPv4-
embedded IPv6 routes.
1.2. Routing Solution per RFC5565
The aforementioned scenario is described in [RFC5565], i.e.- IPv4-
over-IPv6 scenario, where the network core is IPv6-only, and the
inter-connected IPv4 networks are called IPv4 client networks. The P
routers in the core only support IPv6 but the AFBRs (Address Family
Border Routers) support IPv4 on interface facing IPv4 client
networks, and IPv6 on interface facing the core. The routing
solution defined in [RFC5565] for this scenario is to run i-BGP
among AFBRs to exchange IPv4 routing information with each other,
and the IPv4 packets are forwarded from one IPv4 client network to
the other through a softwire using tunneling technology such as MPLS
LSP, GRE, L2TPv3, etc.
1.3. An Alternative Routing Solution with OSPFv3
In this document, we propose an alternative routing solution for the
scenario described in Section 1.1, where several segregated IPv4
networks, called IPv4 client networks, are interconnected by an IPv6
transit network, and in particular, we name the border node on the
boundary of an IPv4 client network and the IPv6 transit network as
Address Family Translation Border Router, or AFXLBR, which supports
both IPv4 and IPv6 address families, and is capable of translating
an IPv4 packet to an IPv6 packet, and vice versa, according to
[I-D.draft-ietf-behave-v6v4-xlate].
Since the scenario occurs most in a single ISP operating
environment, an IPv6 prefix can be locally allocated and used to
construct IPv4-embedded IPv6 addresses according to [RFC6052] by
each AFXLBR, where the embedded IPv4 addresses are associated with
an IPv4 client network that is connected to the AFXLBR, and each
IPv4 address is an individual IPv4 address or prefix. An AFXLBR
injects IPv4-embedded IPv6 addresses/prefixes into the IPv6 transit
network using OSPFv3 and also installs those advertised by other
AFXLBRs. When an IPv4 packet is sent from one IPv4 client network to
the other, it is first encapsulated with an IPv6 header, where the
source and destination IPv6 address are constructed, in a stateless
manner, as IPv4-embedded IPv6 address, respectively, and then
forwarded to the destination AFXLBR that is the advertising router
of the destination IPv4-embedded IPv6 address. The destination
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AFXLBR replaces the IPv6 header by the corresponding IPv4 header,
where the source and destination IPv4 addresses are derived from the
IPv4-embedded IPv6 source and destination addresses, respectively,
and then forwards the IPv4 packet using its IPv4 routing table in
the attached IPv4 client network.
There are use cases where the proposed routing solution is useful.
One case is that some border nodes do not participate in i-BGP for
routes exchange (one example is documented in [I-D.boucadair-
softwire-dslite-v6only]), or i-BGP is not used at all. Another case
is that tunnel mechanism is not used in the IPv6 transit network, or
native IPv6 forwarding is preferred. Note also that with this
routing solution, the IPv4-IPv6 inter-connection and associated
header translation that occurs at an AFXLBR is stateless.
1.4. OSPFv3 Routing with a Specific Topology
Routing IPv4-embedded IPv6 packets in the IPv6 transit network using
OSPFv3, in general, may be performed by the OSPFv3 operation that is
already running in the IPv6 network. One concern however, is that
IPv4-embedded IPv6 routes would flood throughout the entire transit
network and stored on every router, which may not be desirable.
Also, since all IPv6 routes are stored in the same routing table, it
might be more difficult to manage the resource required for routing
and forwarding based on traffic category, if so desired. To solve
this problem and to ease the separation between native IPv6 and
IPv4-inferred routing policies, a separate OSPFv3 routing table can
be constructed that is dedicated to IPv4-embedded IPv6 topology, and
that table is solely used for routing IPv4-embedded IPv6 packets
(i.e., IPv4 part of the Internet) in the transit network. Further,
only a set of routers in the transit network are required to be
involved in such routing scheme, including AFXLBRs that connect to
IPv4 client networks along with a set of P routers in the core for
routing path.
There are two methods to build a separate OSPFv3 routing table for
IPv4-embedded IPv6 routing.
- The first one is to run a separate OSPFv3 instance for IPv4-
embedded IPv6 topology in the IPv6 transit network according to
[RFC5838],
- The second one is to stay with the existing OSPFv3 instance that
already operates in the transit network, but maintain a separate
IPv4-embedded topology, according to [I-D.ietf-ospf-mt-ospfv3].
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With both methods, there would be a dedicated IPv4-embedded IPv6
topology that is maintained by OSPFv3 speakers and thus a dedicated
IPv4-embedded IPv6 routing table, which is then used for routing
IPv4-embedded IPv6 packets (i.e., packets destined to an IPv4
destination). It would be operators' preference as which method is
going to be used. This document elaborates on how configuration is
done for each method and related routing issues that is common to
both.
This document only focuses on unicast routing for IPv4-embedded IPv6
packets using OSPFv3.
2. Provisioning
2.1. Deciding the IPv4-embedded IPv6 Topology
Before making appropriate configuration in order to generate a
separate OSPFv3 routing table for IPv4-embedded IPv6
addresses/prefixes, decision must be made on the set of routers and
their interfaces in the IPv6 transit network that should be on the
IPv4-embedded IPv6 topology.
For the purpose of this topology, all AFXLBRs that connect to IPv4
client networks should be members of this topology, and also at
least some of their network core facing interfaces, which depends on
which P routers in the IPv6 transit network would be on this
topology.
The IPv4-embedded IPv6 topology is a sub-topology of the entire IPv6
transit network, and if all routers (including AFXLBRs and P-
routers) and their interfaces are included, the two topologies
converge. In general, as more P routers and their interfaces are
configured on this sub-topology, it would increase the inter-
connectivity and potentially, there would be more routing paths
cross the transit network from one IPv4 client network to the other,
at the cost that more routers need to participate the IPv4-embedded
IPv6 routing. In any case, the IPv4-embedded IPv6 topology must be
continuous with no partitions.
2.2. Maintaining a Dedicated IPv4-embedded IPv6 Routing Table
In an IPv6 transit network, in order to maintain a separate IPv6
routing table that contains routes for IPv4-embedded IPv6
destinations only, OSPFv3 needs to use the mechanism defined either
in [RFC5838] or [I-D.ietf-ospf-mt-ospfv3] with required
configuration tasks, as described in the following sub-sections.
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2.3. OSPFv3 Topology with a Separate Instance ID
It is assumed that the scenario as described in this document is
under a single ISP and as such, an OSPFv3 instance ID (IID) is
allocated locally and used for an OSPFv3 operation dedicated to
unicast IPv4-embedded IPv6 routing in an IPv6 transit network. This
IID is configured on each OSPFv3 interface of routers that
participates in this routing instance.
The range for a locally configured OSPFv3 IID is from 128 to 255,
inclusively, and this number must be used to encode the "Instance
ID" field in the OSPFv3 packet header on every router that executes
this instance in the IPv6 transit network.
In addition, the "AF" bit in the OSPFv3 Option field must be set.
During the Hello packets processing, adjacency may only be
established when received Hello packets contain the same Instance
ID as configured on the receiving interface for OSPFv3 instance
dedicated to the IPv4-embedded IPv6 routing.
For more details, the reader is referred to [RFC5838].
2.4. OSPFv3 Topology with the Default Instance
Similar to that as described in the previous section, an OSPFv3
multi-topology ID (MT-ID) is locally allocated and used for an
OSPFv3 operation including unicast IPv4-embedded IPv6 routing in an
IPv6 transit network. This MTID is configured on each OSPFv3
interface of routers that participates in this routing topology.
The range for a locally configured OSPFv3 MT-ID is from 32 to 255,
inclusively, and this number must be used to encode the "MT-ID"
field that is included in some of the extended LSAs as documented in
[I-D.ietf-ospf-mt-ospfv3].
In addition, the MT bit in the OSPFv3 Option field must be set.
For more details, the reader is referred to [I-D.ietf-ospf-mt-
ospfv3].
3. IP Packets Translation
When transporting IPv4 packets across an IPv6 transit network with
the mechanism described above, an IPv4 packet is translated to an
IPv6 packet at ingress AFXLBR, and the IPv6 packet is translated
back to the original IPv4 packet at egress AFXLBR. The IP packet
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translation is accomplished in stateless manner according to rules
specified in [I-D.draft-ietf-behave-v6v4-xlate], with the address
translation detail explained in the next sub-section.
3.1. Address Translation
Prior to the operation, an IPv6 prefix is allocated by the ISP and
it is used to form an IPv4-embedded IPv6 address.
The IPv6 prefix can either be a well-known IPv6 prefix (WKP)
64:FF9B::/96, or a network-specific prefix that is unique to the
ISP, and for the later case, the IPv6 prefix length may be 32, 40,
48, 56 or 64. In either case, this IPv6 prefix is used during the
address translation between an IPv4 address and an IPv4-embedded
IPv6 address, which is performed according to [RFC6052].
During translation from an IPv4 header to an IPv6 header at an
ingress AFXLBR, the source IPv4 address and destination IPv4 address
are translated into the corresponding IPv6 source address and
destination IPv6 address, respectively, and during translation from
an IPv6 header to an IPv4 header at an egress AFXLBR, the source
IPv6 address and destination IPv6 address are translated into the
corresponding IPv4 source address and destination IPv4 address,
respectively. Note that the address translation is accomplished in a
stateless manner.
4. Advertising IPv4-embedded IPv6 Routes
In order to forward IPv4 packets to the proper destination across
IPv6 transit network, IPv4 reachability needs to be disseminated
throughout the IPv6 transit network and this work is performed by
AFXLBRs that connect to IPv4 client networks using OSPFv3.
With the scenario described in this document, i.e. - a set of
AFXLBRs that inter-connect a bunch of IPv4 client networks with an
IPv6 transit network, we view that IPv4 networks and IPv6 networks
belong to separate Autonomous Systems, and as such, these AFXLBRs
are OSPFv3 ASBRs.
4.1. Advertising IPv4-embedded IPv6 Routes into IPv6 Transit Network
IPv4 addresses and prefixes in an IPv4 client network are translated
into IPv4-embedded IPv6 addresses and prefixes, respectively, using
the same IPv6 prefix allocated by the ISP and the method specified
in [RFC6052], and then advertised by one or more attached ASBRs into
the IPv6 transit network using AS External LSA ([RFC5340]), i.e. -
with the advertising scope throughout the entire Autonomous System.
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4.1.1. Routing Metrics
By default, the metric in an AS External LSA that carries an IPv4-
embedded IPv6 address or prefixes is a Type 1 external metric, which
is then to be added to the metric of an intra-AS path during OSPFv3
routes calculation. By configuration on an ASBR, the metric can be
set to a Type 2 external metric, which is considered much larger
than that on any intra-AS path. The detail is referred to OSPFv3
specification [RFC5340]. In either case, an external metric may be
exact the same unit as in an IPv4 network (running OSPFv2 or
others), but may also be specified by a routing policy, the detail
is outside of the scope of this document.
4.1.2. Forwarding Address
If the "Forwarding Address" field of an OSPFv3 AS External LSA is
used to carry an IPv6 address, that must also be an IPv4-embedded
IPv6 address where the embedded IPv4 address is the actual address
in an IPv4 client network to which, data traffic is forwarded to.
However, since an AFXLBR sits on the border of an IPv4 network and
an IPv6 network, it is recommended that the "Forwarding Address"
field not to be used by setting the F bit in the associated
OSPFv3 AS-external-LSA to zero, so that the AFXLBR can make the
forwarding decision based on its own IPv4 routing table.
4.2. Advertising IPv4 Addresses into Client Networks
IPv4-embedded IPv6 routes injected into the IPv6 transit network
from one IPv4 client network may be advertised into another IPv4
client network, after the associated destination addresses/prefixes
are translated back to IPv4 addresses/prefixes format. This
operation is similar to the regular OSPFv3 operation, wherein an AS
External LSA can be advertised in a non-backbone area by default.
An IPv4 client network that does not want to receive such
advertisement can be configured as a stub area or with other routing
policy.
For the purpose of this document, IPv4-embedded IPv6 routes must not
advertised into any IPv6 client networks that also connected to the
IPv6 transit network.
5. Aggregation on IPv4 Addresses and Prefixes
In order to reduce the amount of AS External LSAs that are injected
to the IPv6 transit network, effort must be made to aggregate IPv4
addresses and prefixes at each AFXLBR before advertising.
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6. Forwarding
There are three cases in forwarding IP packets in the scenario as
described in this document, as follows:
1) On an AFXLBR, if an IPv4 packet that is received on an interface
connecting to an IPv4 client network with the destination IPv4
address belong to another IPv4 client network, the header of the
packet is translated to a corresponding IPv6 header as described
in Section 3, and the packet is then forwarded to the destination
AFXLBR that advertises the IPv4-embedded IPv6 address through the
IPv6 transit network.
2) On an AFXLBR, if an IPv4-embedded IPv6 packet is received and the
embedded destination IPv4 address is in its IPv4 routing table,
the header of the packet is translated to a corresponding IPv4
header as described in Section 3, and the packet is then forwarded
accordingly.
3) On any router that is within the IPv4-embedded IPv6 topology
located in the IPv6 transit network, if an IPv4-embedded IPv6
packet is received and a route is found in the IPv4-embedded IPv6
routing table, the packet is forwarded accordingly.
The classification of IPv4-embedded IPv6 packet is according to the
IPv6 prefix of the destination address, which is either the Well
Known Prefix (i.e., 64:ff9b::/96) or locally allocated as defined in
[RFC6052].
7. MTU Issue
In the IPv6 transit network, there is no new MTU issue introduced by
this document. If a separate OSPFv3 instance (per [RFC5838]) is used
for IPv4-embedded IPv6 routing, the MTU handling in the transit
network is the same as that of the default OSPFv3 instance. If a
separate OSPFv3 topology (per [I-D.ietf-ospf-mt-ospfv3]) is used for
IPv4-embedded IPv6 routing, the MTU handling in the transit network
is the same as that of the default OSPFv3 topology.
However, the MTU in the IPv6 transit network may be different than
that of IPv4 client networks. Since an IPv6 router will never
fragment a packet, the packet size of any IPv4-embedded IPv6 packet
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entering the IPv6 transit network must be equal to or smaller than
the MTU of the IPv6 transit network. In order to achieve this
requirement, it is recommended that AFXLBRs to perform IPv6 path
discovery among themselves and the resulting MTU, after taking into
account of the difference between IPv4 header length and IPv6 header
length, must be "propagated" into IPv4 client networks, e.g.-
included in the OSPFv3 Database Description packet.
The detail of passing the proper MTU into IPv4 client networks is
beyond the scope of this document.
8. Backdoor Connections
In some deployments, there may exist direct connections among IPv4
client networks themselves in addition to the IPv6 transit network,
as "backdoor" connections referring to, where IPv4 packets can
either be transported between those IPv4 client networks via
backdoor connections, or through the IPv6 transit network. In
general, routing policies should be as such that the "backdoor" path
is preferred since the packet forwarding is within a single address
family without the need for IP header translation, among other
things.
9. Security
This document does not introduce any security issue than what has
been identified in [RFC5838], [I-D.ietf-ospf-mt-ospfv3] and
[RFC6052].
10. IANA Considerations
No new IANA assignments are required for this document.
11. Acknowledgements
Many thanks to Acee Lindem, Dan Wing and Joel Halpern for their
comments.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
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[RFC5838] Lindem, A., Mirtorabi, S., Roy, A., Barnes, M., and
Aggarwal, R., "Support of address families in OSPFv3", RFC5838,
April 2010.
[I-D.ietf-ospf-mt-ospfv3] Mirtorabi, S. and A. Roy, "Multi-
topology routing in OSPFv3 (MT-OSPFv3)",
draft-ietf-ospf-mt-ospfv3-03 (work in progress), July 2007
[RFC6052] Huitema, C., Bao, C., Bagnulo, M.
Boucadair, M., and Li, X.,
"IPv6 Addressing of IPv4/IPv6 Translators". October 2010
[I-D.draft-ietf-behave-v6v4-xlate] Li, X., Bao, C.,
Baker, F., "IP/ICMP Translation Algorithm", September 2010.
12.2. Informative References
[RFC5565] Wu, J., Cui, Y., Metz, C., and Rosen, E.,
"Softwire Mesh Network", RFC 5565, June 2009
[I-D.boucadair-softwire-dslite-v6only] Boucadair, M.,
Jacquenet, C., Grimault, JL., Kassi-Lahlou, M. Levis, P.,
Cheng, D., Lee, Y., "Deplying Dual-Stack Lite (DS-Lite)
in IPv6 Network", October 2010.
13. Authors' Addresses
Dean Cheng
Huawei Technologies,
2330 Central Expressway, CA 95050, USA
Email: dean.cheng@huawei.com
Mohamed Boucadair
France Telecom
3, Av Francois Chateaux
Rennes 35000
France
Email: mohamed.boucadair@orange-ftgroup.com
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