RFC 2740
OSPF for IPv6
Network Working Group R. Coltun Requests for Comments: 2740 Siara Systems Category: Standards Track D. Ferguson Juniper Networks J. Moy Sycamore Networks December 1999 OSPF for IPv6 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved.Abstract
This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, DR election, area support, SPF calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. Changes between OSPF for IPv4 and this document include the following. Addressing semantics have been removed from OSPF packets and the basic LSAs. New LSAs have been created to carry IPv6 addresses and prefixes. OSPF now runs on a per-link basis, instead of on a per-IP-subnet basis. Flooding scope for LSAs has been generalized. Authentication has been removed from the OSPF protocol itself, instead relying on IPv6's Authentication Header and Encapsulating Security Payload. Most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4, even with the larger IPv6 addresses. Most field-XSand packet-size limitations present in OSPF for IPv4 have been relaxed. In addition, option handling has been made more flexible.
All of OSPF for IPv4's optional capabilities, including on-demand circuit support, NSSA areas, and the multicast extensions to OSPF (MOSPF) are also supported in OSPF for IPv6.Table of Contents
1 Introduction ........................................... 4 1.1 Terminology ............................................ 4 2 Differences from OSPF for IPv4 ......................... 4 2.1 Protocol processing per-link, not per-subnet ........... 5 2.2 Removal of addressing semantics ........................ 5 2.3 Addition of Flooding scope ............................. 5 2.4 Explicit support for multiple instances per link ....... 6 2.5 Use of link-local addresses ............................ 6 2.6 Authentication changes ................................. 7 2.7 Packet format changes .................................. 7 2.8 LSA format changes ..................................... 8 2.9 Handling unknown LSA types ............................ 10 2.10 Stub area support ..................................... 10 2.11 Identifying neighbors by Router ID .................... 11 3 Implementation details ................................ 11 3.1 Protocol data structures .............................. 12 3.1.1 The Area Data structure ............................... 13 3.1.2 The Interface Data structure .......................... 13 3.1.3 The Neighbor Data Structure ........................... 14 3.2 Protocol Packet Processing ............................ 15 3.2.1 Sending protocol packets .............................. 15 3.2.1.1 Sending Hello packets ................................. 16 3.2.1.2 Sending Database Description Packets .................. 17 3.2.2 Receiving protocol packets ............................ 17 3.2.2.1 Receiving Hello Packets ............................... 19 3.3 The Routing table Structure ........................... 19 3.3.1 Routing table lookup .................................. 20 3.4 Link State Advertisements ............................. 20 3.4.1 The LSA Header ........................................ 21 3.4.2 The link-state database ............................... 22 3.4.3 Originating LSAs ...................................... 22 3.4.3.1 Router-LSAs ........................................... 25 3.4.3.2 Network-LSAs .......................................... 27 3.4.3.3 Inter-Area-Prefix-LSAs ................................ 28 3.4.3.4 Inter-Area-Router-LSAs ................................ 29 3.4.3.5 AS-external-LSAs ...................................... 29 3.4.3.6 Link-LSAs ............................................. 31 3.4.3.7 Intra-Area-Prefix-LSAs ................................ 32 3.5 Flooding .............................................. 35 3.5.1 Receiving Link State Update packets ................... 36 3.5.2 Sending Link State Update packets ..................... 36 3.5.3 Installing LSAs in the database ....................... 38
3.6 Definition of self-originated LSAs .................... 39 3.7 Virtual links ......................................... 39 3.8 Routing table calculation ............................. 39 3.8.1 Calculating the shortest path tree for an area ........ 40 3.8.1.1 The next hop calculation .............................. 41 3.8.2 Calculating the inter-area routes ..................... 42 3.8.3 Examining transit areas' summary-LSAs ................. 42 3.8.4 Calculating AS external routes ........................ 42 3.9 Multiple interfaces to a single link .................. 43 References ............................................ 44 A. OSPF data formats ..................................... 46 A.1 Encapsulation of OSPF packets ......................... 46 A.2 The Options field ..................................... 47 A.3 OSPF Packet Formats ................................... 48 A.3.1 The OSPF packet header ................................ 49 A.3.2 The Hello packet ...................................... 50 A.3.3 The Database Description packet ....................... 52 A.3.4 The Link State Request packet ......................... 54 A.3.5 The Link State Update packet .......................... 55 A.3.6 The Link State Acknowledgment packet .................. 56 A.4 LSA formats ........................................... 57 A.4.1 IPv6 Prefix Representation ............................ 58 A.4.1.1 Prefix Options ........................................ 58 A.4.2 The LSA header ........................................ 59 A.4.2.1 LS type ............................................... 60 A.4.3 Router-LSAs ........................................... 61 A.4.4 Network-LSAs .......................................... 64 A.4.5 Inter-Area-Prefix-LSAs ................................ 65 A.4.6 Inter-Area-Router-LSAs ................................ 66 A.4.7 AS-external-LSAs ...................................... 67 A.4.8 Link-LSAs ............................................. 69 A.4.9 Intra-Area-Prefix-LSAs ................................ 71 B. Architectural Constants ............................... 73 C. Configurable Constants ................................ 73 C.1 Global parameters ..................................... 73 C.2 Area parameters ....................................... 74 C.3 Router interface parameters ........................... 75 C.4 Virtual link parameters ............................... 77 C.5 NBMA network parameters ............................... 77 C.6 Point-to-MultiPoint network parameters ................ 78 C.7 Host route parameters ................................. 78 Security Considerations ............................... 79 Authors' Addresses .................................... 79 Full Copyright Statement .............................. 80
1. Introduction
This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, DR election, area support, SPF calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. This document is organized as follows. Section 2 describes the differences between OSPF for IPv4 and OSPF for IPv6 in detail. Section 3 provides implementation details for the changes. Appendix A gives the OSPF for IPv6 packet and LSA formats. Appendix B lists the OSPF architectural constants. Appendix C describes configuration parameters.1.1. Terminology
This document attempts to use terms from both the OSPF for IPv4 specification ([Ref1]) and the IPv6 protocol specifications ([Ref14]). This has produced a mixed result. Most of the terms used both by OSPF and IPv6 have roughly the same meaning (e.g., interfaces). However, there are a few conflicts. IPv6 uses "link" similarly to IPv4 OSPF's "subnet" or "network". In this case, we have chosen to use IPv6's "link" terminology. "Link" replaces OSPF's "subnet" and "network" in most places in this document, although OSPF's Network-LSA remains unchanged (and possibly unfortunately, a new Link-LSA has also been created). The names of some of the OSPF LSAs have also changed. See Section 2.8 for details.2. Differences from OSPF for IPv4
Most of the algorithms from OSPF for IPv4 [Ref1] have preserved in OSPF for IPv6. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. The following subsections describe the differences between this document and [Ref1].
2.1. Protocol processing per-link, not per-subnet
IPv6 uses the term "link" to indicate "a communication facility or medium over which nodes can communicate at the link layer" ([Ref14]). "Interfaces" connect to links. Multiple IP subnets can be assigned to a single link, and two nodes can talk directly over a single link, even if they do not share a common IP subnet (IPv6 prefix). For this reason, OSPF for IPv6 runs per-link instead of the IPv4 behavior of per-IP-subnet. The terms "network" and "subnet" used in the IPv4 OSPF specification ([Ref1]) should generally be relaced by link. Likewise, an OSPF interface now connects to a link instead of an IP subnet, etc. This change affects the receiving of OSPF protocol packets, and the contents of Hello Packets and Network-LSAs.2.2. Removal of addressing semantics
In OSPF for IPv6, addressing semantics have been removed from the OSPF protocol packets and the main LSA types, leaving a network- protocol-independent core. In particular: o IPv6 Addresses are not present in OSPF packets, except in LSA payloads carried by the Link State Update Packets. See Section 2.7 for details. o Router-LSAs and Network-LSAs no longer contain network addresses, but simply express topology information. See Section 2.8 for details. o OSPF Router IDs, Area IDs and LSA Link State IDs remain at the IPv4 size of 32-bits. They can no longer be assigned as (IPv6) addresses. o Neighboring routers are now always identified by Router ID, where previously they had been identified by IP address on broadcast and NBMA "networks".2.3. Addition of Flooding scope
Flooding scope for LSAs has been generalized and is now explicitly coded in the LSA's LS type field. There are now three separate flooding scopes for LSAs:
o Link-local scope. LSA is flooded only on the local link, and
no further. Used for the new Link-LSA (see Section A.4.8).
o Area scope. LSA is flooded throughout a single OSPF area
only. Used for Router-LSAs, Network-LSAs, Inter-Area-Prefix-
LSAs, Inter-Area-Router-LSAs and Intra-Area-Prefix-LSAs.
o AS scope. LSA is flooded throughout the routing domain. Used
for AS-external-LSAs.
2.4. Explicit support for multiple instances per link
OSPF now supports the ability to run multiple OSPF protocol instances
on a single link. For example, this may be required on a NAP segment
shared between several providers -- providers may be running separate
OSPF routing domains that want to remain separate even though they
have one or more physical network segments (i.e., links) in common.
In OSPF for IPv4 this was supported in a haphazard fashion using the
authentication fields in the OSPF for IPv4 header.
Another use for running multiple OSPF instances is if you want, for
one reason or another, to have a single link belong to two or more
OSPF areas.
Support for multiple protocol instances on a link is accomplished via
an "Instance ID" contained in the OSPF packet header and OSPF
interface structures. Instance ID solely affects the reception of
OSPF packets.
2.5. Use of link-local addresses
IPv6 link-local addresses are for use on a single link, for purposes
of neighbor discovery, auto-configuration, etc. IPv6 routers do not
forward IPv6 datagrams having link-local source addresses [Ref15].
Link-local unicast addresses are assigned from the IPv6 address range
FF80/10.
OSPF for IPv6 assumes that each router has been assigned link-local
unicast addresses on each of the router's attached physical segments.
On all OSPF interfaces except virtual links, OSPF packets are sent
using the interface's associated link-local unicast address as
source. A router learns the link-local addresses of all other
routers attached to its links, and uses these addresses as next hop
information during packet forwarding.
On virtual links, global scope or site-local IP addresses must be
used as the source for OSPF protocol packets.
Link-local addresses appear in OSPF Link-LSAs (see Section 3.4.3.6). However, link-local addresses are not allowed in other OSPF LSA types. In particular, link-local addresses must not be advertised in inter-area-prefix-LSAs (Section 3.4.3.3), AS-external-LSAs (Section 3.4.3.5) or intra-area-prefix-LSAs (Section 3.4.3.7).2.6. Authentication changes
In OSPF for IPv6, authentication has been removed from OSPF itself. The "AuType" and "Authentication" fields have been removed from the OSPF packet header, and all authentication related fields have been removed from the OSPF area and interface structures. When running over IPv6, OSPF relies on the IP Authentication Header (see [Ref19]) and the IP Encapsulating Security Payload (see [Ref20]) to ensure integrity and authentication/confidentiality of routing exchanges. Protection of OSPF packet exchanges against accidental data corruption is provided by the standard IPv6 16-bit one's complement checksum, covering the entire OSPF packet and prepended IPv6 pseudo- header (see Section A.3.1).2.7. Packet format changes
OSPF for IPv6 runs directly over IPv6. Aside from this, all addressing semantics have been removed from the OSPF packet headers, making it essentially "network-protocol-independent". All addressing information is now contained in the various LSA types only. In detail, changes in OSPF packet format consist of the following: o The OSPF version number has been increased from 2 to 3. o The Options field in Hello Packets and Database description Packet has been expanded to 24-bits. o The Authentication and AuType fields have been removed from the OSPF packet header (see Section 2.6). o The Hello packet now contains no address information at all, and includes an Interface ID which the originating router has assigned to uniquely identify (among its own interfaces) its interface to the link. This Interface ID becomes the Netowrk-LSA's Link State ID, should the router become Designated-Router on the link.
o Two option bits, the "R-bit" and the "V6-bit", have been added to
the Options field for processing Router-LSAs during the SPF
calculation (see Section A.2). If the "R-bit" is clear an OSPF
speaker can participate in OSPF topology distribution without
being used to forward transit traffic; this can be used in multi-
homed hosts that want to participate in the routing protocol. The
V6-bit specializes the R-bit; if the V6-bit is clear an OSPF
speaker can participate in OSPF topology distribution without
being used to forward IPv6 datagrams. If the R-bit is set and the
V6-bit is clear, IPv6 datagrams are not forwarded but diagrams
belonging to another protocol family may be forwarded.
o TheOSPF packet header now includes an "Instance ID" which allows
multiple OSPF protocol instances to be run on a single link (see
Section 2.4).
2.8. LSA format changes
All addressing semantics have been removed from the LSA header, and
from Router-LSAs and Network-LSAs. These two LSAs now describe the
routing domain's topology in a network-protocol-independent manner.
New LSAs have been added to distribute IPv6 address information, and
data required for next hop resolution. The names of some of IPv4's
LSAs have been changed to be more consistent with each other.
In detail, changes in LSA format consist of the following:
o The Options field has been removed from the LSA header, expanded
to 24 bits, and moved into the body of Router-LSAs, Network-LSAs,
Inter-Area-Router-LSAs and Link-LSAs. See Section A.2 for details.
o The LSA Type field has been expanded (into the former Options
space) to 16 bits, with the upper three bits encoding flooding
scope and the handling of unknown LSA types (see Section 2.9).
o Addresses in LSAs are now expressed as [prefix, prefix length]
instead of [address, mask] (see Section A.4.1). The default route
is expressed as a prefix with length 0.
o The Router and Network LSAs now have no address information, and
are network-protocol-independent.
o Router interface information may be spread across multiple Router
LSAs. Receivers must concatenate all the Router-LSAs originated by
a given router when running the SPF calculation.
o A new LSA called the Link-LSA has been introduced. The LSAs have
local-link flooding scope; they are never flooded beyond the link
that they are associated with. Link-LSAs have three purposes: 1)
they provide the router's link-local address to all other routers
attached to the link, 2) they inform other routers attached to the
link of a list of IPv6 prefixes to associate with the link and 3)
they allow the router to assert a collection of Options bits to
associate with the Network-LSA that will be originated for the
link. See Section A.4.8 for details.
In IPv4, the router-LSA carries a router's IPv4 interface
addresses, the IPv4 equivalent of link-local addresses. These are
only used when calculating next hops during the OSPF routing
calculation (see Section 16.1.1 of [Ref1]), so they do not need to
be flooded past the local link; hence using link-LSAs to
distribute these addresses is more efficient. Note that link-local
addresses cannot be learned through the reception of Hellos in all
cases: on NBMA links next hop routers do not necessarily exchange
hellos, but rather learn of each other's existence by way of the
Designated Router.
o The Options field in the Network LSA is set to the logical OR of
the Options that each router on the link advertises in its Link-
LSA.
o Type-3 summary-LSAs have been renamed "Inter-Area-Prefix-LSAs".
Type-4 summary LSAs have been renamed "Inter-Area-Router-LSAs".
o The Link State ID in Inter-Area-Prefix-LSAs, Inter-Area-Router-
LSAs and AS-external-LSAs has lost its addressing semantics, and
now serves solely to identify individual pieces of the Link State
Database. All addresses or Router IDs that were formerly expressed
by the Link State ID are now carried in the LSA bodies.
o Network-LSAs and Link-LSAs are the only LSAs whose Link State ID
carries additional meaning. For these LSAs, the Link State ID is
always the Interface ID of the originating router on the link
being described. For this reason, Network-LSAs and Link-LSAs are
now the only LSAs whose size cannot be limited: a Network-LSA must
list all routers connected to the link, and a Link-LSA must list
all of a router's addresses on the link.
o A new LSA called the Intra-Area-Prefix-LSA has been introduced.
This LSA carries all IPv6 prefix information that in IPv4 is
included in Router-LSAs and Network-LSAs. See Section A.4.9 for
details.
o Inclusion of a forwarding address in AS-external-LSAs is now
optional, as is the inclusion of an external route tag (see
[Ref5]). In addition, AS-external-LSAs can now reference another
LSA, for inclusion of additional route attributes that are outside
the scope of the OSPF protocol itself. For example, this can be
used to attach BGP path attributes to external routes as proposed
in [Ref10].
2.9. Handling unknown LSA types
Handling of unknown LSA types has been made more flexible so that,
based on LS type, unknown LSA types are either treated as having
link-local flooding scope, or are stored and flooded as if they were
understood (desirable for things like the proposed External-
Attributes-LSA in [Ref10]). This behavior is explicitly coded in the
LSA Handling bit of the link state header's LS type field (see
Section A.4.2.1).
The IPv4 OSPF behavior of simply discarding unknown types is
unsupported due to the desire to mix router capabilities on a single
link. Discarding unknown types causes problems when the Designated
Router supports fewer options than the other routers on the link.
2.10. Stub area support
In OSPF for IPv4, stub areas were designed to minimize link-state
database and routing table sizes for the areas' internal routers.
This allows routers with minimal resources to participate in even
very large OSPF routing domains.
In OSPF for IPv6, the concept of stub areas is retained. In IPv6, of
the mandatory LSA types, stub areas carry only router-LSAs, network-
LSAs, Inter-Area-Prefix-LSAs, Link-LSAs, and Intra-Area-Prefix-LSAs.
This is the IPv6 equivalent of the LSA types carried in IPv4 stub
areas: router-LSAs, network-LSAs and type 3 summary-LSAs.
However, unlike in IPv4, IPv6 allows LSAs with unrecognized LS types
to be labeled "Store and flood the LSA, as if type understood" (see
the U-bit in Section A.4.2.1). Uncontrolled introduction of such LSAs
could cause a stub area's link-state database to grow larger than its
component routers' capacities.
To guard against this, the following rule regarding stub areas has
been established: an LSA whose LS type is unrecognized may only be
flooded into/throughout a stub area if both a) the LSA has area or
link-local flooding scope and b) the LSA has U-bit set to 0. See
Section 3.5 for details.
2.11. Identifying neighbors by Router ID
In OSPF for IPv6, neighboring routers on a given link are always identified by their OSPF Router ID. This contrasts with the IPv4 behavior where neighbors on point-to-point networks and virtual links are identified by their Router IDs, and neighbors on broadcast, NBMA and Point-to-MultiPoint links are identified by their IPv4 interface addresses. This change affects the reception of OSPF packets (see Section 8.2 of [Ref1]), the lookup of neighbors (Section 10 of [Ref1]) and the reception of Hello Packets (Section 10.5 of [Ref1]). The Router ID of 0.0.0.0 is reserved, and should not be used.3. Implementation details
When going from IPv4 to IPv6, the basic OSPF mechanisms remain unchanged from those documented in [Ref1]. These mechanisms are briefly outlined in Section 4 of [Ref1]. Both IPv6 and IPv4 have a link-state database composed of LSAs and synchronized between adjacent routers. Initial synchronization is performed through the Database Exchange process, through the exchange of Database Description, Link State Request and Link State Update packets. Thereafter database synchronization is maintained via flooding, utilizing Link State Update and Link State Acknowledgment packets. Both IPv6 and IPv4 use OSPF Hello Packets to discover and maintain neighbor relationships, and to elect Designated Routers and Backup Designated Routers on broadcast and NBMA links. The decision as to which neighbor relationships become adjacencies, along with the basic ideas behind inter-area routing, importing external information in AS-external-LSAs and the various routing calculations are also the same. In particular, the following IPv4 OSPF functionality described in [Ref1] remains completely unchanged for IPv6: o Both IPv4 and IPv6 use OSPF packet types described in Section 4.3 of [Ref1], namely: Hello, Database Description, Link State Request, Link State Update and Link State Acknowledgment packets. While in some cases (e.g., Hello packets) their format has changed somewhat, the functions of the various packet types remains the same. o The system requirements for an OSPF implementation remain unchanged, although OSPF for IPv6 requires an IPv6 protocol stack (from the network layer on down) since it runs directly over the IPv6 network layer.
o The discovery and maintenance of neighbor relationships, and the
selection and establishment of adjacencies remain the same. This
includes election of the Designated Router and Backup Designated
Router on broadcast and NBMA links. These mechanisms are described
in Sections 7, 7.1, 7.2, 7.3, 7.4 and 7.5 of [Ref1].
o The link types (or equivalently, interface types) supported by
OSPF remain unchanged, namely: point-to-point, broadcast, NBMA,
Point-to-MultiPoint and virtual links.
o The interface state machine, including the list of OSPF interface
states and events, and the Designated Router and Backup Designated
Router election algorithm, remain unchanged. These are described
in Sections 9.1, 9.2, 9.3 and 9.4 of [Ref1].
o The neighbor state machine, including the list of OSPF neighbor
states and events, remain unchanged. These are described in
Sections 10.1, 10.2, 10.3 and 10.4 of [Ref1].
o Aging of the link-state database, as well as flushing LSAs from
the routing domain through the premature aging process, remains
unchanged from the description in Sections 14 and 14.1 of [Ref1].
However, some OSPF protocol mechanisms have changed, as outlined in
Section 2 above. These changes are explained in detail in the
following subsections, making references to the appropriate sections
of [Ref1].
The following subsections provide a recipe for turning an IPv4 OSPF
implementation into an IPv6 OSPF implementation.
3.1. Protocol data structures
The major OSPF data structures are the same for both IPv4 and IPv6:
areas, interfaces, neighbors, the link-state database and the routing
table. The top-level data structures for IPv6 remain those listed in
Section 5 of [Ref1], with the following modifications:
o All LSAs with known LS type and AS flooding scope appear in the
top-level data structure, instead of belonging to a specific area
or link. AS-external-LSAs are the only LSAs defined by this
specification which have AS flooding scope. LSAs with unknown LS
type, U-bit set to 1 (flood even when unrecognized) and AS
flooding scope also appear in the top-level data structure.
3.1.1. The Area Data structure
The IPv6 area data structure contains all elements defined for IPv4 areas in Section 6 of [Ref1]. In addition, all LSAs of known type which have area flooding scope are contained in the IPv6 area data structure. This always includes the following LSA types: router-LSAs, network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs and intra-area-prefix-LSAs. LSAs with unknown LS type, U-bit set to 1 (flood even when unrecognized) and area scope also appear in the area data structure. IPv6 routers implementing MOSPF add group- membership-LSAs to the area data structure. Type-7-LSAs belong to an NSSA area's data structure.3.1.2. The Interface Data structure
In OSPF for IPv6, an interface connects a router to a link. The IPv6 interface structure modifies the IPv4 interface structure (as defined in Section 9 of [Ref1]) as follows: Interface ID Every interface is assigned an Interface ID, which uniquely identifies the interface with the router. For example, some implementations may be able to use the MIB-II IfIndex ([Ref3]) as Interface ID. The Interface ID appears in Hello packets sent out the interface, the link-local-LSA originated by router for the attached link, and the router-LSA originated by the router-LSA for the associated area. It will also serve as the Link State ID for the network-LSA that the router will originate for the link if the router is elected Designated Router. Instance ID Every interface is assigned an Instance ID. This should default to 0, and is only necessary to assign differently on those links that will contain multiple separate communities of OSPF Routers. For example, suppose that there are two communities of routers on a given ethernet segment that you wish to keep separate. The first community is given an Instance ID of 0, by assigning 0 as the Instance ID of all its routers' interfaces to the ethernet. An Instance ID of 1 is assigned to the other routers' interfaces to the ethernet. The OSPF transmit and receive processing (see Section 3.2) will then keep the two communities separate. List of LSAs with link-local scope All LSAs with link-local scope and which were originated/flooded on the link belong to the interface structure which connects to the link. This includes the collection of the link's link-LSAs.
List of LSAs with unknown LS type
All LSAs with unknown LS type and U-bit set to 0 (if unrecognized,
treat the LSA as if it had link-local flooding scope) are kept in
the data structure for the interface that received the LSA.
IP interface address
For IPv6, the IPv6 address appearing in the source of OSPF packets
sent out the interface is almost always a link-local address. The
one exception is for virtual links, which must use one of the
router's own site-local or global IPv6 addresses as IP interface
address.
List of link prefixes
A list of IPv6 prefixes can be configured for the attached link.
These will be advertised by the router in link-LSAs, so that they
can be advertised by the link's Designated Router in intra-area-
prefix-LSAs.
In OSPF for IPv6, each router interface has a single metric,
representing the cost of sending packets out the interface. In
addition, OSPF for IPv6 relies on the IP Authentication Header (see
[Ref19]) and the IP Encapsulating Security Payload (see [Ref20]) to
ensure integrity and authentication/confidentiality of routing
exchanges. For that reason, AuType and Authentication key are not
associated with IPv6 OSPF interfaces.
Interface states, events, and the interface state machine remain
unchanged from IPv4, and are documented in Sections 9.1, 9.2 and 9.3
of [Ref1] respectively. The Designated Router and Backup Designated
Router election algorithm also remains unchanged from the IPv4
election in Section 9.4 of [Ref1].
3.1.3. The Neighbor Data Structure
The neighbor structure performs the same function in both IPv6 and
IPv4. Namely, it collects all information required to form an
adjacency between two routers, if an adjacency becomes necessary.
Each neighbor structure is bound to a single OSPF interface. The
differences between the IPv6 neighbor structure and the neighbor
structure defined for IPv4 in Section 10 of [Ref1] are:
Neighbor's Interface ID
The Interface ID that the neighbor advertises in its Hello Packets
must be recorded in the neighbor structure. The router will
include the neighbor's Interface ID in the router's router-LSA
when either a) advertising a point-to-point link to the neighbor
or b) advertising a link to a network where the neighbor has
become Designated Router.
Neighbor IP address
Except on virtual links, the neighbor's IP address will be an IPv6
link-local address.
Neighbor's Designated Router
The neighbor's choice of Designated Router is now encoded as a
Router ID, instead of as an IP address.
Neighbor's Backup Designated Router
The neighbor's choice of Designated Router is now encoded as a
Router ID, instead of as an IP address.
Neighbor states, events, and the neighbor state machine remain
unchanged from IPv4, and are documented in Sections 10.1, 10.2 and
10.3 of [Ref1] respectively. The decision as to which adjacencies to
form also remains unchanged from the IPv4 logic documented in Section
10.4 of [Ref1].
3.2. Protocol Packet Processing
OSPF for IPv6 runs directly over IPv6's network layer. As such, it is
encapsulated in one or more IPv6 headers, with the Next Header field
of the immediately encapsulating IPv6 header set to the value 89.
As for IPv4, in IPv6 OSPF routing protocol packets are sent along
adjacencies only (with the exception of Hello packets, which are used
to discover the adjacencies). OSPF packet types and functions are the
same in both IPv4 and IPv4, encoded by the
Type field of the standard OSPF packet header.
3.2.1. Sending protocol packets
When an IPv6 router sends an OSPF routing protocol packet, it fills
in the fields of the standard OSPF for IPv6 packet header (see
Section A.3.1) as follows:
Version #
Set to 3, the version number of the protocol as documented in this
specification.
Type
The type of OSPF packet, such as Link state Update or Hello
Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area that the packet is being sent into.
Instance ID
The OSPF Instance ID associated with the interface that the packet
is being sent out of.
Checksum
The standard IPv6 16-bit one's complement checksum, covering the
entire OSPF packet and prepended IPv6 pseudo-header (see Section
A.3.1).
Selection of OSPF routing protocol packets' IPv6 source and
destination addresses is performed identically to the IPv4 logic in
Section 8.1 of [Ref1]. The IPv6 destination address is chosen from
among the addresses AllSPFRouters, AllDRouters and the Neighbor IP
address associated with the other end of the adjacency (which in
IPv6, for all links except virtual links, is an IPv6 link-local
address).
The sending of Link State Request Packets and Link State
Acknowledgment Packets remains unchanged from the IPv4 procedures
documented in Sections 10.9 and 13.5 of [Ref1] respectively. Sending
Hello Packets is documented in Section 3.2.1.1, and the sending of
Database Description Packets in Section 3.2.1.2. The sending of Link
State Update Packets is documented in Section 3.5.2.
3.2.1.1. Sending Hello packets
IPv6 changes the way OSPF Hello packets are sent in the following
ways (compare to Section 9.5 of [Ref1]):
o Before the Hello Packet is sent out an interface, the interface's
Interface ID must be copied into the Hello Packet.
o The Hello Packet no longer contains an IP network mask, as OSPF
for IPv6 runs per-link instead of per-subnet.
o The choice of Designated Router and Backup Designated Router are
now indicated within Hellos by their Router IDs, instead of by
their IP interface addresses. Advertising the Designated
Router (or Backup Designated Router) as 0.0.0.0 indicates that the
Designated Router (or Backup Designated Router) has not yet been
chosen.
o The Options field within Hello packets has moved around, getting
larger in the process. More options bits are now possible. Those
that must be set correctly in Hello packets are: The E-bit is set
if and only if the interface attaches to a non-stub area, the N-
bit is set if and only if the interface attaches to an NSSA area
(see [Ref9]), and the DC- bit is set if and only if the router
wishes to suppress the sending of future Hellos over the interface
(see [Ref11]). Unrecognized bits in the Hello Packet's Options
field should be cleared.
Sending Hello packets on NBMA networks proceeds for IPv6 in exactly
the same way as for IPv4, as documented in Section 9.5.1 of [Ref1].
3.2.1.2. Sending Database Description Packets
The sending of Database Description packets differs from Section 10.8
of [Ref1] in the following ways:
o The Options field within Database Description packets has moved
around, getting larger in the process. More options bits are now
possible. Those that must be set correctly in Database Description
packets are: The MC-bit is set if and only if the router is
forwarding multicast datagrams according to the MOSPF
specification in [Ref7], and the DC-bit is set if and only if the
router wishes to suppress the sending of Hellos over the interface
(see [Ref11]). Unrecognized bits in the Database Description
Packet's Options field should be cleared.
3.2.2. Receiving protocol packets
Whenever an OSPF protocol packet is received by the router it is
marked with the interface it was received on. For routers that have
virtual links configured, it may not be immediately obvious which
interface to associate the packet with. For example, consider the
Router RT11 depicted in Figure 6 of [Ref1]. If RT11 receives an OSPF
protocol packet on its interface to Network N8, it may want to
associate the packet with the interface to Area 2, or with the
virtual link to Router RT10 (which is part of the backbone). In
the following, we assume that the packet is initially associated with
the non-virtual link.
In order for the packet to be passed to OSPF for processing, the
following tests must be performed on the encapsulating IPv6 headers:
o The packet's IP destination address must be one of the IPv6
unicast addresses associated with the receiving interface (this
includes link-local addresses), or one of the IP multicast
addresses AllSPFRouters or AllDRouters.
o The Next Header field of the immediately encapsulating IPv6 header
must specify the OSPF protocol (89).
o Any encapsulating IP Authentication Headers (see [Ref19]) and the
IP Encapsulating Security Payloads (see [Ref20]) must be processed
and/or verified to ensure integrity and
authentication/confidentiality of OSPF routing exchanges.
o Locally originated packets should not be passed on to OSPF. That
is, the source IPv6 address should be examined to make sure this
is not a multicast packet that the router itself generated.
After processing the encapsulating IPv6 headers, the OSPF packet
header is processed. The fields specified in the header must match
those configured for the receiving interface. If they do not, the
packet should be discarded:
o The version number field must specify protocol version 3.
o The standard IPv6 16-bit one's complement checksum, covering the
entire OSPF packet and prepended IPv6 pseudo-header, must be
verified (see Section A.3.1).
o The Area ID found in the OSPF header must be verified. If both of
the following cases fail, the packet should be discarded. The
Area ID specified in the header must either:
(1) Match the Area ID of the receiving interface. In
this case, unlike for IPv4, the IPv6 source
address is not restricted to lie on the same IP
subnet as the receiving interface. IPv6 OSPF runs
per-link, instead of per-IP-subnet.
(2) Indicate the backbone. In this case, the packet
has been sent over a virtual link. The receiving
router must be an area border router, and the
Router ID specified in the packet (the source
router) must be the other end of a configured
virtual link. The receiving interface must also
attach to the virtual link's configured Transit
area. If all of these checks succeed, the packet
is accepted and is from now on associated with
the virtual link (and the backbone area).
o The Instance ID specified in the OSPF header must match the
receiving interface's Instance ID.
o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or Backup
(see Section 9.1).
After header processing, the packet is further processed according to
its OSPF packet type. OSPF packet types and functions are the same
for both IPv4 and IPv6.
If the packet type is Hello, it should then be further processed by
the Hello Protocol. All other packet types are sent/received only on
adjacencies. This means that the packet must have been sent by one
of the router's active neighbors. The neighbor is identified by the
Router ID appearing the the received packet's OSPF header. Packets
not matching any active neighbor are discarded.
The receive processing of Database Description Packets, Link State
Request Packets and Link State Acknowledgment Packets remains
unchanged from the IPv4 procedures documented in Sections 10.6, 10.7
and 13.7 of [Ref1] respectively. The receiving of Hello Packets is
documented in Section 3.2.2.1, and the receiving of Link State Update
Packets is documented in Section 3.5.1.
3.2.2.1. Receiving Hello Packets
The receive processing of Hello Packets differs from Section 10.5 of
[Ref1] in the following ways:
o On all link types (e.g., broadcast, NBMA, point-to- point, etc),
neighbors are identified solely by their OSPF Router ID. For all
link types except virtual links, the Neighbor IP address is set to
the IPv6 source address in the IPv6 header of the received OSPF
Hello packet.
o There is no longer a Network Mask field in the Hello Packet.
o The neighbor's choice of Designated Router and Backup Designated
Router is now encoded as an OSPF Router ID instead of an IP
interface address.
3.3. The Routing table Structure
The routing table used by OSPF for IPv4 is defined in Section 11 of
[Ref1]. For IPv6 there are analogous routing table entries: there are
routing table entries for IPv6 address prefixes, and also for AS
boundary routers. The latter routing table entries are only used to
hold intermediate results during the routing table build process (see
Section 3.8).
Also, to hold the intermediate results during the shortest-path
calculation for each area, there is a separate routing table for each
area holding the following entries:
o An entry for each router in the area. Routers are identified by
their OSPF router ID. These routing table entries hold the set of
shortest paths through a given area to a given router, which in
turn allows calculation of paths to the IPv6 prefixes advertised
by that router in Intra-area-prefix-LSAs. If the router is also an
area-border router, these entries are also used to calculate paths
for inter-area address prefixes. If in addition the router is the
other endpoint of a virtual link, the routing table entry
describes the cost and viability of the virtual link.
o An entry for each transit link in the area. Transit links have
associated network-LSAs. Both the transit link and the network-LSA
are identified by a combination of the Designated Router's
Interface ID on the link and the Designated Router's OSPF Router
ID. These routing table entries allow later calculation of paths
to IP prefixes advertised for the transit link in intra-area-
prefix-LSAs.
The fields in the IPv4 OSPF routing table (see Section 11 of [Ref1])
remain valid for IPv6: Optional capabilities (routers only), path
type, cost, type 2 cost, link state origin, and for each of the equal
cost paths to the destination, the next hop and advertising router.
For IPv6, the link-state origin field in the routing table entry is
the router-LSA or network-LSA that has directly or indirectly
produced the routing table entry. For example, if the routing table
entry describes a route to an IPv6 prefix, the link state origin is
the router-LSA or network-LSA that is listed in the body of the
intra-area-prefix-LSA that has produced the route (see Section
A.4.9).
3.3.1. Routing table lookup
Routing table lookup (i.e., determining the best matching routing
table entry during IP forwarding) is the same for IPv6 as for IPv4.
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