OSPFv3 Address Families: Support of Address Families in OSPFv3
How OSPFv3 routes IPv4 and multicast prefixes by running one separate OSPFv3 instance per address family.
Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and R. Aggarwal, "Support of Address Families in OSPFv3", RFC 5838, DOI 10.17487/RFC5838, April 2010, <https://www.rfc-editor.org/info/rfc5838>.
@misc{rfc5838,
series = {Request for Comments},
number = 5838,
howpublished = {RFC 5838},
publisher = {RFC Editor},
doi = {10.17487/RFC5838},
url = {https://www.rfc-editor.org/info/rfc5838},
author = {A. Lindem and S. Mirtorabi and A. Roy and M. Barnes and R. Aggarwal},
title = {{Support of Address Families in OSPFv3}},
pagetotal = 13,
year = 2010,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Lindem, Ed.
Request for Comments: 5838 Ericsson
Category: Standards Track S. Mirtorabi
ISSN: 2070-1721 A. Roy
M. Barnes
Cisco Systems
R. Aggarwal
Juniper Networks
April 2010
Support of Address Families in OSPFv3
Abstract
This document describes a mechanism for supporting multiple address
families (AFs) in OSPFv3 using multiple instances. It maps an AF to
an OSPFv3 instance using the Instance ID field in the OSPFv3 packet
header. This approach is fairly simple and minimizes extensions to
OSPFv3 for supporting multiple AFs.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5838.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Design Considerations . . . . . . . . . . . . . . . . . . 2
1.2. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Instance ID Values for New AFs . . . . . . . . . . . . . . 3
2.2. OSPFv3 Options Changes . . . . . . . . . . . . . . . . . . 4
2.3. Advertising Prefixes in AFs Other Than IPv6 . . . . . . . 4
2.4. Changes to the Hello Packet Processing . . . . . . . . . . 4
2.5. Next-Hop Calculation for IPv4 Unicast and Multicast AFs . 5
2.6. AS-External-LSA and NSSA-LSA Forwarding Address for
IPv4 Unicast and IPv4 Multicast AFs . . . . . . . . . . . 5
2.7. Database Description Maximum Transmission Unit (MTU)
Specification for Non-IPv6 AFs . . . . . . . . . . . . . . 6
2.8. Operation over Virtual Links . . . . . . . . . . . . . . . 8
3. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 12
1. Introduction
OSPFv3 [OSPFV3] has been defined to support the base IPv6 unicast
address family (AF). There are requirements to advertise other AFs
in OSPFv3, including multicast IPv6, unicast IPv4, and multicast
IPv4. This document supports these other AFs in OSPFv3 by mapping
each AF to a separate Instance ID and OSPFv3 instance.
1.1. Design Considerations
This section describes the rationale for using the multiple Instance
ID approach to support multiple address families in OSPFv3. As
described earlier, OSPFv3 is designed to support multiple instances.
Hence, mapping an instance to an address family doesn't introduce any
new mechanisms to the protocol. It minimizes the protocol extensions
required, and it simplifies the implementation. The presence of a
separate link state database per address family is also easier to
debug and operate. Additionally, it doesn't change the existing
instance, area, and interface-based configuration model in most
OSPFv3 implementations.
1.2. Requirements Notation
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-KEYWORDS].
2. Protocol Details
Currently, the entire Instance ID number space is used for IPv6
unicast. This specification assigns different Instance ID ranges to
different AFs in order to support other AFs in OSPFv3. Each Instance
ID implies a separate OSPFv3 instance with its own neighbor
adjacencies, link state database, protocol data structures, and
shortest path first (SPF) computation.
Additionally, the current Link State Advertisements (LSAs) defined to
advertise IPv6 unicast prefixes can be used to advertise prefixes
from other AFs without modification.
It should be noted that OSPFv3 runs on top of IPv6 and uses IPv6 link
local addresses for OSPFv3 control packets. Therefore, it is
required that IPv6 be enabled on an OSPFv3 link, although the link
may not be participating in any IPv6 AFs.
2.1. Instance ID Values for New AFs
Instance ID zero is already defined by default for the IPv6 unicast
AF. When this specification is used to support multiple AFs, we
define the following ranges for different AFs. The first value of
each range is the default value for the corresponding AF.
Instance ID # 0 - # 31 IPv6 unicast AF
Instance ID # 32 - # 63 IPv6 multicast AF
Instance ID # 64 - # 95 IPv4 unicast AF
Instance ID # 96 - # 127 IPv4 multicast AF
Instance ID # 128 - # 255 Unassigned
OSPFv3 Instance IDs
2.2. OSPFv3 Options Changes
A new AF-bit is added to the OSPFv3 Options field. The V6-bit is
only applicable to the IPv6 unicast AF.
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+-+-+-+-+--+
| | | | | | | | | | | | | | | |AF|*|*|DC|R|N|x|E|V6|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+--+-+-+-+-+--+
The Options field
OSPFv3 Options
V6-bit
The V6-bit is used in OSPFv3 to exclude a node from IPv6 unicast
route calculation but allow it in the SPF calculation for other
address families. Since the Instance ID now denotes the AF
explicitly, this bit is ignored in AFs other than IPv6 unicast.
AF-bit
When an OSPFv3 router is supporting AFs as described in this
specification, it MUST set the AF-bit in the OSPFv3 Options field
of Hello packets, Database Description packets, and LSAs.
2.3. Advertising Prefixes in AFs Other Than IPv6
Each prefix advertised in OSPFv3 has a prefix Length field [OSPFV3].
This facilitates advertising prefixes of different lengths in
different AFs. The existing LSAs defined in OSPFv3 are used for this
and there is no need to define new LSAs.
Prefixes that don't conform to the AF of an OSPFv3 instance MUST NOT
be used in the route computation for that instance.
2.4. Changes to the Hello Packet Processing
When an OSPFv3 router does not support this specification and it is
configured with the corresponding Instance ID, packets could be black
holed. This could happen due to misconfiguration or a router
software downgrade. Black holing is possible because a router that
doesn't support this specification can still be included in the SPF
calculated path as long as it establishes adjacencies using the
Instance ID corresponding to the AF. Note that Router-LSAs and
Network-LSAs are AF independent.
In order to avoid the above situation, Hello packet processing is
changed in order to only establish adjacencies with routers that have
the AF-bit set in their Options field.
Receiving Hello packets is specified in section 4.2.2.1 of [OSPFV3].
The following check is added to Hello packet reception:
o When an OSPFv3 router participates in an AF (sets the AF-bit in
the Options field), it MUST discard Hello packets having the AF-
bit clear in the Options field. The only exception is the Base
IPv6 unicast AF, where this check MUST NOT be done (for backward
compatibility).
2.5. Next-Hop Calculation for IPv4 Unicast and Multicast AFs
OSPFv3 runs on top of IPv6 and uses IPv6 link local addresses for
OSPFv3 control packets and next-hop calculations. Although IPv6 link
local addresses could be used as next hops for IPv4 address families,
it is desirable to have IPv4 next-hop addresses. For example, in the
IPv4 multicast AF, the Protocol Independent Multicast (PIM) [PIM]
neighbor address and the next-hop address should both be IPv4
addresses in order for the Reverse Path Forwarding (RPF) lookup to
work correctly. Troubleshooting is also easier when the prefix
address and next-hop address are in the same AF.
In order to achieve this, the link's IPv4 address will be advertised
in the "link local address" field of the IPv4 instance's Link-LSA.
This address is placed in the first 32 bits of the "link local
address" field and is used for IPv4 next-hop calculations. The
remaining bits MUST be set to zero.
We denote a Direct Interface Address (DIA) as an IPv4 or IPv6 address
that is both directly reachable via an attached link and has an
available layer 3 to layer 2 mapping. Note that there is no explicit
need for the IPv4 link addresses to be on the same subnet. An
implementation SHOULD resolve layer 3 to layer 2 mappings via the
Address Resolution Protocol (ARP) [ARP] or Neighbor Discovery (ND)
[ND] for a DIA even if the IPv4 address is not on the same subnet as
the router's interface IP address.
2.6. AS-External-LSA and NSSA-LSA Forwarding Address for IPv4 Unicast
and IPv4 Multicast AFs
For OSPFv3, this address is an IPv6 host address (128 bits). If
included, data traffic for the advertised destination will be
forwarded to this address. For IPv4 unicast and IPv4 multicast AFs,
the Forwarding Address in AS-external-LSAs and NSSA-LSAs MUST encode
an IPv4 address. To achieve this, the IPv4 Forwarding Address is
advertised by placing it in the first 32 bits of the Forwarding
Address field in AS-external-LSAs and NSSA-LSAs. The remaining bits
MUST be set to zero.
2.7. Database Description Maximum Transmission Unit (MTU) Specification
for Non-IPv6 AFs
For address families other than IPv6, both the MTU for the instance
address family and the IPv6 MTU used for OSPFv3 maximum packet
determination MUST be considered. The MTU in the Database
Description packet MUST always contain the MTU corresponding to the
advertised address family. For example, if the instance corresponds
to an IPv4 address family, the IPv4 MTU for the interface MUST be
specified in the interface MTU field. As specified in Section 10.6
of [OSPFV2], the Database Description packet will be rejected if the
MTU is greater than the receiving interface's MTU for the address
family corresponding to the instance. This behavior will assure that
an adjacency is not formed and address family specific routes are not
installed over a path with conflicting MTUs.
The value used for OSPFv3 maximum packet size determination MUST also
be compatible for an adjacency to be established. Since only a
single MTU field is specified, the M6-bit is defined by this
specification. If the M6-bit is clear, the specified MTU SHOULD also
be checked against the IPv6 MTU, and the Database Description packet
SHOULD be rejected if the MTU is larger than the receiving
interface's IPv6 MTU. An OSPFv3 router SHOULD NOT set the M6-bit if
its IPv6 MTU and address family specific MTU are the same.
If the IPv6 and IPv4 MTUs differ, the M6-bit MUST be set for non-IPv6
address families. If the M6-bit is set, the IPv6 MTU is dictated by
the presence or absence of an IPv6 MTU TLV in the link-local
signaling (LLS) [LLS] block. If this TLV is present, it carries the
IPv6 MTU that SHOULD be compared with the local IPv6 MTU. If this
TLV is absent, the minimum IPv6 MTU of 1280 octets SHOULD be used for
the comparison (refer to [IPV6]).
If the M6-bit is set in a received Database Description packet for a
non-IPv6 address family, the receiving router MUST NOT check the
Interface MTU in the Database Description packet against the
receiving interface's IPv6 MTU.
The figure below graphically depicts the changed fields in octets
20-23 of the OSPFv3 Database Description packet:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+--+
| Interface MTU | 0 |0|0|0|M6|0|I|M|MS|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+-+-+-+--+
OSPFv3 Database Description Packet Changes
The changed fields in the Database Description packet are described
below. The remaining fields are unchanged from [OSPFV3].
Interface MTU
The size in octets of the largest address family specific datagram
that can be sent on the associated interface without
fragmentation. The MTUs of common Internet link types can be
found in Table 7-1 of [MTUDISC]. The Interface MTU SHOULD be set
to 0 in Database Description packets sent over virtual links.
M6-bit
The IPv6 MTU bit - this bit indicates that the sender is using a
different IPv6 MTU than the MTU for the AF.
An IPv6 MTU TLV can be optionally carried in an LLS block as
described above. This TLV carries the IPv6 MTU for the interface.
The length field of the TLV is set to 4 bytes.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 17 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Format of IPv6 MTU TLV
Only one instance of the IPv6 MTU TLV MAY appear in the LLS block.
Instances subsequent to the first are not processed, and the LLS
inconsistency SHOULD be logged.
2.8. Operation over Virtual Links
OSPFv3 control packets sent over a virtual link are IPv6 packets and
may traverse multiple hops. Therefore, there MUST be a global IPv6
address associated with the virtual link so that OSPFv3 control
packets are forwarded correctly by the intermediate hops between
virtual link endpoints. Although this requirement can be satisfied
in IPv6 unicast AFs, it will not function in other AFs as there will
not be a routable global IPv6 address or forwarding path. Therefore,
virtual links are not supported in AFs other than IPv6 unicast.
3. Backward Compatibility
All modifications to OSPFv3 apply exclusively to the support of
address families other than the IPv6 unicast AF using multiple OSPFv3
instances as described in this specification. These modifications
are not applicable to IPv6 unicast topologies and do not preclude
future single instance mechanisms for supporting multiple address
families.
In this section, we will define a non-capable OSPFv3 router as one
not supporting this specification. When multiple AFs are supported
as defined herein, each new AF will have a corresponding Instance ID
and can interoperate with the existing non-capable OSPFv3 routers in
an IPv6 unicast topology. Furthermore, when a non-capable OSPFv3
router uses an Instance ID that is reserved for a given AF, no
adjacency will be formed with this router since the AF-bit in the
Options field will be clear in its OSPFv3 Hello packets. Therefore,
there are no backward compatibility issues. AFs can be gradually
deployed without disturbing OSPFv3 routing domains with non-capable
OSPFv3 routers.
4. Security Considerations
IPsec [IPsec] can be used for OSPFv3 authentication and
confidentiality as described in [OSPFV3-AUTH]. When multiple OSPFv3
instances use the same interface, they all MUST use the same Security
Association (SA), since the SA selectors do not provide selection
based on data in OSPFv3 Header fields (e.g., the Instance ID). This
restriction is documented in Section 8 of [OSPFV3-AUTH].
Security considerations for OSPFv3 are covered in [OSPFV3].
5. IANA Considerations
The following IANA assignments were made from existing registries.
o The AF-bit was assigned from the OSPFv3 Options registry as
defined in Section 2.2.
o The M6-bit was assigned from the DD Packet Flags registry as
defined in Section 2.7
o The TLV type (17) for the IPv6 MTU TLV was assigned from the OSPF
LLS TLVs registry.
IANA created a new registry, "OSPFv3 Instance ID Address Family
Values", for assignment of the mapping of OSPFv3 Instance IDs to
address families when this specification is used to support multiple
address families. Note that the Instance ID field MAY be used for
applications other than the support of multiple address families.
However, if it is being used for address families as described in
this specification, the assignments herein SHOULD be honored.
+-------------+----------------------+--------------------+
| Value/Range | Designation | Assignment Policy |
+-------------+----------------------+--------------------+
| 0 | Base IPv6 Unicast AF | Already assigned |
| | | |
| 1-31 | IPv6 Unicast AFs | Already assigned |
| | dependent on local | |
| | policy | |
| | | |
| 32 | Base IPv6 Multicast | Already assigned |
| | | |
| 33-63 | IPv6 Multicast AFs | Already assigned |
| | dependent on local | |
| | policy | |
| | | |
| 64 | Base IPv4 Unicast AF | Already assigned |
| | | |
| 65-95 | IPv4 Unicast AFs | Already assigned |
| | dependent on local | |
| | policy | |
| | | |
| 96 | Base IPv4 Multicast | Already assigned |
| | | |
| 97-127 | IPv4 Multicast AFs | Already assigned |
| | dependent on local | |
| | policy | |
| | | |
| 128-255 | Unassigned | Standards Action |
+-------------+----------------------+--------------------+
OSPFv3 Address Family Use of Instance IDs
o Instance IDs 0-127 are assigned by this specification.
o Instance IDs in the range 128-255 are not assigned at this time.
Before any assignments can be made in this range, there MUST be a
Standards Track RFC including an IANA Considerations section
explicitly specifying the AF Instance IDs being assigned.
6. References
6.1. Normative References
[IPV6] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.
[IPsec] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[OSPFV2] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
April 1998.
[OSPFV3] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
[OSPFV3-AUTH] Gupta, M. and S. Melam, "Authentication/
Confidentiality for OSPFv3", RFC 4552, June 2006.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFC's to Indicate
Requirement Levels", RFC 2119, March 1997.
6.2. Informative References
[ARP] Plummer, D., "Ethernet Address Resolution Protocol:
Or Converting Network Protocol Addresses to 48.bit
Ethernet Address for Transmission on Ethernet
Hardware", STD 37, RFC 826, November 1982.
[LLS] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Young, "OSPF Link-Local Signaling", RFC 5613,
August 2009.
[MTUDISC] Mogul, J. and S. Deering, "Path MTU Discovery",
RFC 1191, November 1990.
[ND] Narten, T., Nordmark, E., Simpson, W., and H.
Soliman, "Neighbor Discovery for IP version 6
(IPv6)", RFC 4861, September 2007.
[PIM] Fenner, B., Handley, M., Holbrook, H., and I.
Kouvelas, "Protocol Independent Multicast - Sparse
Mode (PIM-SM): Protocol Specification (Revised)",
RFC 4601, August 2006.
Appendix A. Acknowledgments
The RFC text was produced using Marshall Rose's xml2rfc tool.
Thanks to Tom Henderson and the folks at Boeing for implementing this
document in the Quagga routing suite, http:www.quagga.net.
Thanks to Nischal Sheth for review and comments.
Thanks to Christian Vogt for comments during the Gen-ART review.
Thanks to Adrian Farrel for comments during the IESG review.
Thanks to Alfred Hoenes for comments during the editing process.
Authors' Addresses
Acee Lindem (editor)
Ericsson
102 Carric Bend Court
Cary, NC 27519
USA
EMail: acee.lindem@ericsson.com
Sina Mirtorabi
Cisco Systems
3 West Plumeria Drive
San Jose, CA 95134
USA
EMail: smirtora@cisco.com
Abhay Roy
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Michael Barnes
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: mjbarnes@cisco.com
Rahul Aggarwal
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
EMail: rahul@juniper.net
AI ELI5
On this page
§1Why Use Multiple Instances
OSPFv3, defined in RFC 5340, supports the base IPv6 unicast address family (AF). There are requirements to advertise other AFs as well: multicast IPv6, unicast IPv4 and multicast IPv4. This document supports them by mapping each AF to a separate Instance ID and OSPFv3 instance.
The RFC gives its reasons. OSPFv3 already supports multiple instances, so mapping an instance to an AF adds no new mechanism, minimizes protocol extensions and simplifies implementations. A separate link state database per AF is also easier to debug and operate, and the existing instance, area and interface-based configuration model does not change in most implementations.
A router already knows how to keep several separate maps, each with its own number. This idea simply says: use one map for each kind of address.
§2Instance IDs per Address Family
Before this RFC the whole Instance ID space was used for IPv6 unicast. The RFC splits it into ranges, and the first value of each range is the default for that AF.
- Instance IDs 0 to 31: IPv6 unicast
- 32 to 63: IPv6 multicast
- 64 to 95: IPv4 unicast
- 96 to 127: IPv4 multicast
- 128 to 255: unassigned
Each Instance ID implies a separate OSPFv3 instance with its own neighbor adjacencies, link state database, protocol data structures and shortest path first (SPF) computation. The existing LSAs that advertise IPv6 unicast prefixes can advertise prefixes of other AFs without modification, because each prefix carries its own length field. Prefixes that do not match the AF of an instance MUST NOT be used in that instance's route computation.
OSPFv3 still runs on top of IPv6 and uses IPv6 link-local addresses for its control packets, so IPv6 must be enabled on an OSPFv3 link even if the link takes part in no IPv6 AF.
The numbers are split into blocks, like apartment floors: one floor for each kind of address. A router picks the floor, and everyone on that floor shares the same map.
- 0-31IPv6 unicast — Base IPv6 unicast AF; 0 is the default.
- 32-63IPv6 multicast — 32 is the default.
- 64-95IPv4 unicast — 64 is the default.
- 96-127IPv4 multicast — 96 is the default.
- 128-255Unassigned — Needs a Standards Track RFC to assign.
§3The AF-bit and Hello Processing
A router that does not know this specification but is configured with an Instance ID reserved for an AF could still join the SPF path and then black hole packets, through misconfiguration or a software downgrade. To prevent this, a new AF-bit is added to the OSPFv3 Options field. A router supporting AFs as described MUST set it in Hello packets, Database Description packets and LSAs.
Hello reception gains one check: a router that participates in an AF MUST discard Hello packets that have the AF-bit clear. The exception is the base IPv6 unicast AF, where the check MUST NOT be done, for backward compatibility. The existing V6-bit remains applicable only to the IPv6 unicast AF and is ignored in other AFs.
Because of this, a non-capable router using a reserved Instance ID forms no adjacency, and AFs can be deployed gradually without disturbing routing domains that contain non-capable routers. All the modifications apply only to AFs other than IPv6 unicast.
Routers wear a badge to show they understand the new rules. A router that sees someone without the badge on the new floor politely refuses to team up with them.
§4Next Hops, Forwarding Addresses and Virtual Links
For IPv4 unicast and multicast AFs it is desirable to have IPv4 next hops, for example so Reverse Path Forwarding lookups work for PIM. The link's IPv4 address is therefore placed in the first 32 bits of the "link local address" field of the IPv4 instance's Link-LSA, with the remaining bits set to zero. In the same way, the Forwarding Address of AS-external-LSAs and NSSA-LSAs MUST encode an IPv4 address in its first 32 bits for the IPv4 AFs, with the remaining bits zero.
Virtual links are not supported in AFs other than IPv6 unicast. Control packets on a virtual link are IPv6 packets that may cross several hops and need a global IPv6 address, which will not exist for the other AFs.
The router writes the street address in the old IPv6 address box, using only the first part of it. Long-distance shortcuts only work for the original kind of address.
§5MTU Handling in Database Description Packets
For non-IPv6 AFs, both the AF's MTU and the IPv6 MTU used for maximum OSPFv3 packet size must be considered. The Database Description packet MUST always carry the MTU of the advertised AF, so an IPv4 instance puts the IPv4 MTU in the Interface MTU field. A Database Description packet is rejected if its MTU is larger than the receiving interface's MTU for that AF.
Since the packet has only one MTU field, the RFC defines the M6-bit to say that the sender's IPv6 MTU differs from the AF's MTU. If the IPv6 and IPv4 MTUs differ, the M6-bit MUST be set for non-IPv6 AFs. If it is set, the IPv6 MTU comes from an optional IPv6 MTU TLV (type 17) in the link-local signaling block, or, when the TLV is absent, the IPv6 minimum of 1280 octets SHOULD be used. A router SHOULD NOT set the M6-bit if the two MTUs are the same.
Two routers must agree how big a parcel can be before they trade. If the two kinds of roads allow different sizes, a flag tells the other router to check both.
§6Security and IANA Assignments
IPsec can provide OSPFv3 authentication and confidentiality as described in RFC 4552. When several OSPFv3 instances use the same interface they MUST all use the same Security Association, because SA selectors cannot select on header fields such as the Instance ID. Other security considerations are those of OSPFv3 itself.
IANA assigned the AF-bit from the OSPFv3 Options registry, the M6-bit from the DD Packet Flags registry and TLV type 17 for the IPv6 MTU TLV, and created the registry "OSPFv3 Instance ID Address Family Values". Instance IDs 0 to 127 are assigned by this RFC; assigning anything in 128 to 255 requires a Standards Track RFC with an IANA Considerations section.
All the maps on one wire have to use the same lock, because the lock cannot tell which map a message belongs to.