Internet Engineering Task Force (IETF) I. Chen
Request for Comments: 7949 Ericsson
Updates: 5838 A. Lindem
Category: Standards Track Cisco
ISSN: 2070-1721 R. Atkinson
August 2016 OSPFv3 over IPv4 for IPv6 Transition
This document defines a mechanism to use IPv4 to transport OSPFv3
packets. Using OSPFv3 over IPv4 with the existing OSPFv3 Address
Family extension can simplify transition from an OSPFv2 IPv4-only
routing domain to an OSPFv3 dual-stack routing domain. This document
updates RFC 5838 to support virtual links in the IPv4 unicast address
family when using OSPFv3 over IPv4.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
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Table of Contents
1. Introduction ....................................................2
1.1. IPv4-Only Use Case .........................................3
2. Requirements Language ...........................................4
3. Encapsulation in IPv4 ...........................................4
3.1. Source Address .............................................6
3.2. Destination Address ........................................6
3.3. OSPFv3 Header Checksum .....................................6
3.4. Operation over Virtual Links ...............................7
4. Management Considerations .......................................7
4.1. Coexistence with OSPFv2 ....................................7
5. Security Considerations .........................................8
6. References ......................................................8
6.1. Normative References .......................................8
6.2. Informative References .....................................9
Authors' Addresses ................................................11
Using OSPFv3 [RFC5340] over IPv4 [RFC791] with the existing OSPFv3
address family extension can simplify transition from an IPv4-only
routing domain to an IPv6 [RFC2460] or dual-stack routing domain.
Dual-stack routing protocols, such as the Border Gateway Protocol
[RFC4271], have an advantage during the transition, because both IPv4
and IPv6 address families can be advertised using either IPv4 or IPv6
transport. Some IPv4-specific and IPv6-specific routing protocols
share enough similarities in their protocol packet formats and
protocol signaling that it is trivial to deploy an initial IPv6
routing domain by transporting the routing protocol over IPv4,
thereby allowing IPv6 routing domains to be deployed and tested
before decommissioning IPv4 and moving to an IPv6-only network.
In the case of the Open Shortest Path First (OSPF) interior gateway
routing protocol (IGP), OSPFv2 [RFC2328] is the IGP deployed over
IPv4, while OSPFv3 [RFC5340] is the IGP deployed over IPv6. OSPFv3
further supports multiple address families [RFC5838], including both
the IPv6 unicast address family and the IPv4 unicast address family.
Consequently, it is possible to deploy OSPFv3 over IPv4 without any
changes to either OSPFv3 or IPv4. During the transition to IPv6,
future OSPF extensions can focus on OSPFv3, and OSPFv2 can move to
This document specifies how to use IPv4 to transport OSPFv3 packets.
The mechanism takes advantage of the fact that OSPFv2 and OSPFv3
share the same IP protocol number, 89. Additionally, the OSPF packet
header for both OSPFv2 and OSPFv3 includes the OSPF header version
(i.e., the field that distinguishes an OSPFv2 packet from an OSPFv3
packet) in the same location (i.e., the same offset from the start of
If the IPv4 topology and IPv6 topology are not identical, the most
likely cause is that some parts of the network deployment have not
yet been upgraded to support both IPv4 and IPv6. In situations where
the IPv4 deployment is a superset of the IPv6 deployment, it is
expected that OSPFv3 packets would be transported over IPv4, until
the rest of the network deployment is upgraded to support IPv6 in
addition to IPv4. In situations where the IPv6 deployment is a
superset of the IPv4 deployment, it is expected that OSPFv3 would be
transported over IPv6.
Throughout this document, "OSPF" is used when the text applies to
both OSPFv2 and OSPFv3. "OSPFv2" or "OSPFv3" is used when the text
is specific to one version of the OSPF protocol. Similarly, "IP" is
used when the text describes either version of the Internet Protocol.
"IPv4" or "IPv6" is used when the text is specific to a single
version of the Internet Protocol.
1.1. IPv4-Only Use Case
OSPFv3 only requires IPv6 link-local addresses to form adjacencies,
and does not require IPv6 global-scope addresses to establish an IPv6
routing domain. However, IPv6 over Ethernet [RFC2464] uses a
different EtherType (0x86dd) from IPv4 (0x0800) and the Address
Resolution Protocol (ARP) (0x0806) [RFC826] used with IPv4.
Some existing deployed link-layer equipment only supports IPv4 and
ARP. Such equipment contains hardware filters keyed on the EtherType
field of the Ethernet frame to filter which frames will be accepted
by that link-layer equipment. Because IPv6 uses a different
EtherType, IPv6 framing for OSPFv3 will not work with that equipment.
In other cases, Point-to-Point Protocol (PPP) might be used over a
serial interface, but again only IPv4 over PPP might be supported
over such an interface. It is hoped that equipment with such
limitations will be eventually upgraded or replaced.
In some locations, especially locations with less communications
infrastructure, satellite communications (SATCOM) are used to reduce
deployment costs for data networking. SATCOM often has lower cost to
deploy than running new copper or optical cables over long distances
to connect remote areas. Also, in a wide range of locations
including places with good communications infrastructure, Very Small
Aperture Terminals (VSATs) often are used by banks and retailers to
connect their branches and stores to a central location.
Some widely deployed VSAT equipment has either (A) Ethernet
interfaces that only support the Ethernet Address Resolution Protocol
(ARP) and IPv4, or (B) serial interfaces that only support IPv4 and
PPP packets. Such deployments and equipment still can deploy and use
OSPFv3 over IPv4 today, and then later migrate to OSPFv3 over IPv6
after equipment is upgraded or replaced. This can have lower
operational costs than running OSPFv2 and then trying to make a flag-
day switch to OSPFv3. By running OSPFv3 over IPv4 now, the eventual
transition to dual-stack, and then to IPv6-only, can be orchestrated.
2. Requirements Language
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 [RFC2119].
3. Encapsulation in IPv4
An OSPFv3 packet can be directly encapsulated within an IPv4 packet
as the payload, without the IPv6 packet header, as illustrated in
Figure 1. For OSPFv3 transported over IPv4, the IPv4 packet has an
IPv4 protocol type of 89, denoting that the payload is an OSPF
packet. The payload of the IPv4 packet consists of an OSPFv3 packet,
beginning with the OSPF packet header having its OSPF version field
set to 3.
An OSPFv3 packet followed by an OSPF link-local signaling (LLS)
extension data block [RFC5613] encapsulated in an IPv4 packet is
illustrated in Figure 2.
Since an IPv4 header without options is only 20 octets long and is
shorter than an IPv6 header, an OSPFv3 packet encapsulated in a
20-octet IPv4 header is shorter than an OSPFv3 packet encapsulated in
an IPv6 header. Consequently, the link MTU for IPv6 is sufficient to
transport an OSPFv3 packet encapsulated in a 20-octet IPv4 header.
If the link MTU is not sufficient to transport an OSPFv3 packet in
IPv4, then OSPFv3 can rely on IP fragmentation and reassembly
3.1. Source Address
For OSPFv3 over IPv4, the source address is the primary IPv4 address
for the interface over which the packet is transmitted. All OSPFv3
routers on the link should share the same IPv4 subnet for IPv4
transport to function correctly.
While OSPFv2 operates on a subnet, OSPFv3 operates on a link
[RFC5340]. Accordingly, an OSPFv3 router implementation MAY support
adjacencies with OSPFv3 neighbors on different IPv4 subnets. If this
is supported, the IPv4 data plane MUST resolve IPv4 addresses to
Layer 2 addresses using ARP on multi-access networks and point-to-
point over LAN [RFC5309] for direct next hops on different IPv4
subnets. When OSPFv3 adjacencies on different IPv4 subnets are
supported, Bidirectional Forwarding Detection (BFD) [RFC5881] cannot
be used for adjacency loss detection since BFD is restricted to a
3.2. Destination Address
As defined in OSPFv2, the IPv4 destination address of an OSPF
protocol packet is either an IPv4 multicast address or the IPv4
unicast address of an OSPFv2 neighbor. Two well-known link-local
multicast addresses are assigned to OSPFv2, the AllSPFRouters address
(18.104.22.168) and the AllDRouters address (22.214.171.124). The multicast
address used depends on the OSPF packet type, the OSPF interface
type, and the OSPF router's role on multi-access networks.
Thus, for an OSPFv3-over-IPv4 packet to be sent to AllSPFRouters, the
destination address field in the IPv4 packet MUST be 126.96.36.199. For
an OSPFv3-over-IPv4 packet to be sent to AllDRouters, the destination
address field in the IPv4 packet MUST be 188.8.131.52.
When an OSPF router sends a unicast OSPF packet over a connected
interface, the destination of such an IP packet is the address
assigned to the receiving interface. Thus, a unicast OSPFv3 packet
transported in an IPv4 packet would specify the OSPFv3 neighbor's
IPv4 address as the destination address.
3.3. OSPFv3 Header Checksum
For IPv4 transport, the pseudo-header used in the checksum
calculation will contain the IPv4 source and destination addresses,
the OSPFv3 protocol ID, and the OSPFv3 length from the OSPFv3 header
(Appendix A.3.1 of [RFC5340]). The format is similar to the UDP
pseudo-header as described in [RFC768] and is illustrated in
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Source Address |
| Destination Address |
| 0 | Protocol (89) | OSPFv3 Packet Length |
Figure 3: Pseudo-header for OSPFv3 over IPv43.4. Operation over Virtual Links
When an OSPF router sends an OSPF packet over a virtual link, the
receiving router might not be directly connected to the sending
router. Thus, the destination IP address of the IP packet must be a
reachable unicast IP address for the virtual link endpoint. Because
IPv6 is the presumed Internet protocol and an IPv4 destination is not
routable, the OSPFv3 address family extension [RFC5838] specifies
that only virtual links in the IPv6 address family are supported.
As illustrated in Figure 1, this document specifies OSPFv3 transport
over IPv4. As a result, OSPFv3 virtual links can be supported with
IPv4 address families by simply setting the IPv4 destination address
to a reachable IPv4 unicast address for the virtual link endpoint.
Hence, the restriction in Section 2.8 of RFC 5838 [RFC5838] is
relaxed since virtual links can now be supported for IPv4 address
families as long as the transport is also IPv4. If IPv4 transport,
as specified herein, is used for IPv6 address families, virtual links
cannot be supported. Hence, in OSPF routing domains that require
virtual links, the IP transport MUST match the address family (IPv4
4. Management Considerations
4.1. Coexistence with OSPFv2
Since OSPFv2 [RFC2328] and OSPFv3 over IPv4 as described herein use
exactly the same protocol and IPv4 addresses, OSPFv2 packets may be
delivered to the OSPFv3 process and vice versa. When this occurs,
the mismatched protocol packets will be dropped due to validation of
the version in the first octet of the OSPFv2/OSPFv3 protocol header.
Note that this will not prevent the packets from being delivered to
the correct protocol process as standard socket implementations will
deliver a copy to each socket matching the selectors.
Implementations of OSPFv3 over IPv4 transport SHOULD implement
separate counters for a protocol mismatch and SHOULD provide means to
suppress the ospfIfRxBadPacket and ospfVirtIfRxBadPacket SNMP
notifications as described in [RFC4750] and the ospfv3IfRxBadPacket
and ospv3VirtIfRxBadPacket SNMP notifications as described in
[RFC5643] when an OSPFv2 packet is received by the OSPFv3 process or
5. Security Considerations
OSPFv3 [RFC5340] relies on IPsec [RFC4301] for authentication and
confidentiality. "Authentication/Confidentiality in OSPFv3"
[RFC4552] specifies how IPsec is used with OSPFv3 over IPv6
transport. In order to use OSPFv3 with IPv4 transport as specified
herein, further work such as "Authentication/Confidentiality in
OSPFv2" [IPsec-OSPF] would be required.
An optional OSPFv3 Authentication Trailer [RFC7166] also has been
defined as an alternative to using IPsec. The calculation of the
authentication data in the Authentication Trailer includes the source
IPv6 address to protect an OSPFv3 router from man-in-the-middle
attacks. For IPv4 encapsulation as described herein, the IPv4 source
address should be placed in the first 4 octets of Apad followed by
the hexadecimal value 0x878FE1F3 repeated (L-4)/4 times, where L is
the length of the hash measured in octets.
The processing of the optional Authentication Trailer is contained
entirely within the OSPFv3 protocol. In other words, each OSPFv3
router instance is responsible for the authentication, without
involvement from IPsec or any other IP-layer function. Consequently,
except for calculation of the Apad value, transporting OSPFv3 packets
using IPv4 does not change the generation or validation of the
optional OSPFv3 Authentication Trailer.
6.1. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,