Internet-Draft Opaque May 1997
Expiration Date: November 1997 FORE Systems
File name: draft-ietf-ospf-opaque-01.txt
The OSPF Opaque LSA Option
Rob Coltun
FORE Systems
(301) 571-2521
rcoltun@fore.com
Status Of This Memo
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Table Of Contents
1.0 Abstract ................................................. 3
2.0 Overview ................................................. 3
2.1 Organization Of This Document ............................ 3
2.2 Acknowledgments .......................................... 4
3.0 The Opaque LSA ........................................... 4
3.1 Flooding Opaque LSAs ..................................... 5
3.2 Modifications To The Neighbor State Machine .............. 6
4.0 Protocol Data Structures ................................. 8
4.1 Additions To The OSPF Neighbor Structure ................. 8
5.0 References ............................................... 8
Appendix A: OSPF Data Formats ................................ 10
A.1 The Options Field ........................................ 10
A.2 Opaque LSA ............................................... 12
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1.0 Abstract
This memo documents enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs. The
Opaque LSA option defines a general mechanism to allow for future
extensibility of OSPF. The information contained in Opaque LSAs may be
used directly by OSPF or by other protocols. Opaque LSAs contain some
number of octets padded to 32-bit alignment. The standard OSPF link-
state database flooding mechanisms are use for distribution of Opaque
LSAs. Opaque LSAs are flooded throughout all or some limited portion
of the OSPF topology.
2.0 Overview
Over the last few years the OSPF routing protocol [OSPF] has been
widely deployed throughout the Internet. As a result of this deploy-
ment and the evolution of networking technology, OSPF has been
extended to support many options; this evolution will obviously con-
tinue.
This memo documents enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs which
defines an optional generalized mechanism to allow for future extensi-
bility of OSPF. The information contained in Opaque LSAs may be used
directly by OSPF or by other protocols. For example, the OSPF LSA may
be used to distribute BGP AS Path information (as documented in The
OSPF External Attributes LSA [EAL]) which is then used by BGP route-
leaking mechanisms. The option may also be used to distribute IP QoS
information which may be used directly by an OSPF path computation.
The exact use of Opaque LSAs is beyond the scope of this draft.
The data contained in an Opaque LSA is some number of 32-bit aligned
octets. Like any other LSA, the Opaque LSA uses the link-state data-
base distribution mechanism for flooding this information throughout
the topology. The link-state type of the Opaque LSA identifies the
Flooding Scope or range of the topology to which this LSA may be dis-
tributed to.
2.1 Organization Of This Document
This document first defines the three types of Opaque LSAs followed by
a description of OSPF packet processing which includes modifications
to the flooding procedure and the neighbor state machine needed to
support the Opaque LSA. Appendix A then gives the packet formats.
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2.2 Acknowledgments
The author would like to thank Dennis Ferguson, Acee Lindem, John Moy,
Sandra Murphy, Zhaohui "Jeffrey" Zhang and the rest of the OSPF Work-
ing Group for the ideas and support they have given to this project.
3.0 The Opaque LSA
Opaque LSAs are types 9, 10 and 11 link-state advertisements. Each
type has a unique flooding scope and may be originated by any router.
The data contained in the Opaque LSA consists of some number of octets
aligned to a 32-bit boundary. Like any other LSA, Opaque LSAs use the
link-state database distribution mechanism for flooding this informa-
tion throughout the topology. The Opaque LSA's link-state type iden-
tifies the range of the topology to which this LSA may be distributed
to. This section documents the flooding of Opaque LSAs.
The following are possible values of the link-state type filed with
their related flooding scope.
o Link-state type 9 denotes a link-local scope. Type 9 Opaque
LSAs are not flooded beyond the local (sub)network.
o Link-state type 10 denotes an area-local scope. Type 10 Opaque
LSAs are not flooded beyond the area that they are originated
into.
o Link-state type 11 denotes that the LSA is flooded throughout
the Autonomous System (AS). Type 11 LSAs maintain the flooding
scope of existing OSPF areas. Specifically type 11 Opaque LSAs
are 1) flooded throughout all transit areas, 2) not flooded into
stub areas from the backbone and 3) not originated within stub
areas. As with type 5 LSAs, if a type 11 Opaque LSA is received
in a stub area from a neighboring router within the stub area the
LSA is rejected.
Origination of Opaque LSAs are unique to the application using it.
The link-state ID of the Opaque LSA is divided into a type field (the
first 8 bits) a type-specific ID (the remaining 24 bits). The packet
format of the Opaque LSA is given in Appendix A.
The responsibility for proper handling of the Opaque LSA's flooding
scope is placed on both the sender and receiver of the LSA. The
receiver must always store a valid received Opaque LSA in its link-
state database. The receiver must not accept Opaque LSAs that violate
the flooding scope (i.e., a type 11 (domain-wide) Opaque LSA is not
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accepted in a stub area). Flooding scope effects both the building of
the Database summary list during the initial synchronization of the
link-state database and the flooding procedure.
In order to make the use of the Opaque LSAs predictable, it is recom-
mended that all routers within the scope of use have the same Opaque
LSA capabilities. For example, if the Opaque LSA is to be used for
flooding Opaque information throughout a single area, all routers
within the area should support the Opaque option.
The following describes the modifications to these procedures that are
necessary to insure proper use of the Opaque LSA's Scoping Rules.
3.1 Flooding Opaque LSAs
The flooding of Opaque LSAs must follow the rules of Flooding Scope as
specified in this section. The flooding mechanisms must suppress the
flooding of Opaque LSAs as described in the following.
o If the Opaque LSA is type 9 (the flooding scope is link-local)
and the interface that the LSA was received on is not the same
as the target interface (e.g., the interface associated with a
particular neighbor), the Opaque LSA must not be flooded out that
interface (or to that neighbor). An implementation should keep
track of the IP interface associated with each Opaque LSA having
a link-local flooding scope.
o If the Opaque LSA is type 10 (the flooding scope is area-local)
and the area associated with Opaque LSA is not the area associ-
ated with a particular interface, the Opaque LSA must not be
flooded out the interface. An implementation should keep track
of the OSPF area associated with each Opaque LSA having an area-
local flooding scope.
o If the Opaque LSA is type 11 (the flooding scope is the entire
AS) and 1) the area associated with Opaque LSA is not the area
associated with a particular interface (i.e., the target area is
not the same) and 2) the target area is a stub area, the Opaque
LSA must not be flooded out the interface.
o If an Opaque LSA is received on an interface associated with a
stub area the LSA, the LSA is to be discarded and not ack-
nowledged since neighboring router has flooded the LSA in error
(see Section 13 of [OSPF], receiving LSAs having unknown LS
types).
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When opaque-capable routers and non-opaque-capable OSPF routers are
mixed together in a routing domain, the Opaque LSAs are not flooded to
the non-opaque-capable routers. As a general design principle,
optional OSPF advertisements are only flooded to those routers that
understand them.
An opaque-capable router learns of its neighbor's opaque capability at
the beginning of the "Database Exchange Process" (see Section 10.6 of
[OSPF], receiving Database Description packets from a neighbor in
state ExStart). A neighbor is opaque-capable if and only if it sets
the O-bit in the Options field of its Database Description packets.
Then, in the next step of the Database Exchange process, Opaque LSAs
are included in the Database summary list sent to the neighbor (see
Sections 3.2 below and 10.3 of [OSPF]) if and only if the neighbor is
opaque capable.
When flooding Opaque-LSAs to adjacent neighbors, a opaque-capable
router looks at the neighbor's opaque capability. Opaque LSAs are
only flooded to opaque capable neighbors. To be more precise, in Sec-
tion 13.3 of [OSPF], Opaque LSAs are only placed on the link-state
retransmission lists of opaque-capable neighbors. Note however that
when sending Link State Update packets as multicasts, a non-opaque-
capable neighbor may (inadvertently) receive Opaque LSAs. The non-
opaque-capable router will then simply discard the LSA (see Section 13
of [OSPF], receiving LSAs having unknown LS types).
3.2 Modifications To The Neighbor State Machine
The state machine as it exists in section 10.3 of [OSPF] remains
unchanged except for the action associated with State: ExStart, Event:
NegotiationDone which is where the Database summary list is built. To
incorporate the Opaque LSA in OSPF the action is changed to the fol-
lowing.
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link-state database in the neighbor Database summary
list. The area link-state database consists of the
Router LSAs, Network LSAs, Summary LSAs and types 9
and 10 Opaque LSAs contained in the area structure,
along with AS External and type 11 Opaque LSAs
contained in the global structure. AS External and
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type 11 Opaque LSAs are omitted from a virtual
neighbor's Database summary list. AS External LSAs
and type 11 Opaque LSAs are omitted from the
Database summary list if the area has been
configured as a stub (see Section 3.6 of [OSPF]).
Opaque LSAs are omitted from the Database
summary list if the following conditions are met:
1) the LSA type is type 9 (the flooding scope
is link-local) and the interface associated with
the Opaque LSA (upon reception) does not equal the
interface associated with the neighbor;
2) the LSA type is 10 (the flooding scope is
area-local) and the area associated with Opaque
LSA is not the area associated with the neighbor's
interface.
Any advertisement whose age is equal to MaxAge is
omitted from the Database summary list. It is
instead added to the neighbor's link-state
retransmission list. A summary of the Database
summary list will be sent to the neighbor in
Database Description packets. Each Database
Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database
Description Packet is allowed to be outstanding at
any one time. For more detail on the sending and
receiving of Database Description packets, see
Sections 10.8 and 10.6 of [OSPF].
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4.0 Protocol data structures
The Opaque option is described herein in terms of its operation on
various protocol data structures. These data structures are included
for explanatory uses only, and are not intended to constrain an OSPF
implementation. Besides the data structures listed below, this specif-
ication will also reference the various data structures (e.g., OSPF
neighbors) defined in [OSPF].
In an OSPF router, the following item is added to the list of global
OSPF data structures described in Section 5 of [OSPF]:
o Opaque capability. Indicates whether the router is running the
Opaque option (i.e., capable of storing Opaque LSAs). Such a
router will continue to inter-operate with non-opaque-capable
OSPF routers.
4.1 Additions To The OSPF Neighbor Structure
The OSPF neighbor structure is defined in Section 10 of [OSPF]. In an
opaque-capable router, the following items are added to the OSPF
neighbor structure:
o Neighbor Options. This field was already defined in the OSPF
specification. However, in opaque-capable routers there is a new
option which indicates the neighbor's Opaque capability. This new
option is learned in the Database Exchange process through recep-
tion of the neighbor's Database Description packets, and deter-
mines whether Opaque LSAs are flooded to the neighbor. For a more
detailed explanation of the flooding of the Opaque LSA see sec-
tion 3 of this document.
5.0 References
[OSPF] Moy, J., "OSPF Version 2", IETF Internet Draft,
Cascade, April 1997.
[MOSPF] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon,
Inc., March 1994.
[NSSA] Coltun, R. and V. Fuller, "The OSPF NSSA Option", RFC 1587,
RainbowBridge Communications, Stanford University, March 1994.
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[DEMD] Moy, J., "Extending OSPF to Support Demand Circuits", RFC 1793,
Cascade, April 1995.
[EAL] Ferguson, D., "The OSPF External Attributes LSA", work in
progress.
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Appendix A: OSPF Data formats
This appendix describes the format of the Options Field followed by
the packet format of the Opaque LSA.
A.1 The Options Field
The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all link-state advertisements. The Options
field enables OSPF routers to support (or not support) optional capa-
bilities, and to communicate their capability level to other OSPF
routers. Through this mechanism routers of differing capabilities can
be mixed within an OSPF routing domain.
When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain link-state advertisements to
a neighbor because of its reduced functionality. Lastly, listing
capabilities in link-state advertisements allows routers to forward
traffic around reduced functionality routers, by excluding them from
parts of the routing table calculation.
Seven bits of the OSPF Options field have been assigned, although only
the O-bit is described completely by this memo. Each bit is described
briefly below. Routers should reset (i.e., clear) unrecognized bits in
the Options field when sending Hello packets or Database Description
packets and when originating link-state advertisements. Conversely,
routers encountering unrecognized Option bits in received Hello Pack-
ets, Database Description packets or link-state advertisements should
ignore the capability and process the packet/advertisement normally.
+------------------------------------+
| * | O | DC | EA | N/P | MC | E | T |
+------------------------------------+
The Options Field
T-bit
This bit describes the router's TOS-based routing capability, as
specified in Sections 9.5, 10.8, 12.1.2 and 16.9 of [OSPF].
E-bit
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This bit describes the way AS-external-LSAs are flooded, as
described in Sections 3.6, 9.5, 10.8 and 12.1.2 of [OSPF].
MC-bit
This bit describes whether IP multicast datagrams are forwarded
according to the specifications in [MOSPF].
N/P-bit
This bit describes the handling of Type-7 LSAs, as specified in
[NSSA].
DC-bit
This bit describes the router's handling of demand circuits, as
specified in [DEMD].
EA-bit
This bit describes the router's willingness to receive and for-
ward External-Attributes-LSAs, as specified in [EAL].
O-bit
This bit describes the router's willingness to receive and for-
ward Opaque-LSAs as specified in this document.
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A.2 Opaque LSA
Opaque LSAs are the Type 9, 10 and 11 link-state advertisements. These
advertisements are originated by any router and may be used directly
by OSPF or indirectly by other protocols such as BGP wishing to dis-
tribute information throughout the OSPF domain. The primary function
of the Opaque LSA is to provide for future extensibility to OSPF.
The data contained in the Opaque LSA consists of some number of octets
padded to 32-bit alignment. Like any other LSA, the Opaque LSA uses
the link-state database distribution mechanism for flooding this
information throughout the topology. However, the Opaque LSA has a
flooding scope associated with it so that the scope of flooding may be
link-local (type 9), area-local (type 10) or the entire OSPF routing
domain (type 11).
Origination of Opaque LSAs are unique to the application using it.
Section 3 of this document describes the flooding procedures for the
Opaque LSA.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 9, 10 or 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Opaque Information |
+ +
| ... |
Link-State Type
The link-state type identifies the flooding scope (or range) of
the topology to which this LSA may be distributed to. The follow-
ing explains the flooding scope of each of the link-state types.
o A value of 9 denotes a link-local scope. Opaque LSAs with a
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link-local scope are not flooded beyond the local (sub)network.
o A value of 10 denotes an area-local scope. Opaque LSAs with a
area-local scope are not flooded beyond the area that they are
originated into.
o A value of 11 denotes that the LSA is flooded throughout the
Autonomous System (e.g., has the same scope as type-5 LSAs).
Opaque LSAs with AS-wide scope are not flooded into stub areas.
Syntax Of The Opaque LSA's Link-State ID
The link-state ID of the Opaque LSA is divided into an Opaque
Type field (the first 8 bits) and an Opaque ID (the remaining 24
bits). Opaque type values in the range of 128-255 are reserved
for private and experimental use.
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