RFC2370 - The OSPF Opaque LSA Option

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　　Network Working Group R. Coltun
Request for Comments: 2370 FORE Systems
See Also: 2328 July 1998
Category: Standards Track

The OSPF Opaque LSA Option

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 (1998). All Rights Reserved.

Table Of Contents

1.0 Abstract ................................................. 1
2.0 Overview ................................................. 2
2.1 Organization Of This Document ............................ 2
2.2 Acknowledgments .......................................... 3
3.0 The Opaque LSA ........................................... 3
3.1 Flooding Opaque LSAs ..................................... 4
3.2 Modifications To The Neighbor State Machine .............. 5
4.0 Protocol Data Structures ................................. 6
4.1 Additions To The OSPF Neighbor Structure ................. 6
5.0 Management Considerations ................................ 7
6.0 Security Considerations .................................. 9
7.0 IANA Considerations ...................................... 10
8.0 References ............................................... 10
9.0 Author's Information ..................................... 11
Appendix A: OSPF Data Formats ................................ 12
A.1 The Options Field ........................................ 12
A.2 The Opaque LSA ........................................... 13
Appendix B: Full Copyright Statment .......................... 15

1.0 Abstract

This memo defines enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs. Opaque
LSAs provide a generalized mechanism to allow for the future
extensibility of OSPF. Opaque LSAs consist of a standard LSA header
followed by application-specific information. The information field

may be used directly by OSPF or by other applications. Standard OSPF
link-state database flooding mechanisms are used to distribute Opaque
LSAs to all or some limited portion of the OSPF topology.

2.0 Overview

Over the last several years the OSPF routing protocol [OSPF] has been
widely deployed throughout the Internet. As a result of this
deployment and the evolution of networking technology, OSPF has been
extended to support many options; this evolution will obviously
continue.

This memo defines enhancements to the OSPF protocol to support a new
class of link-state advertisements (LSA) called Opaque LSAs. Opaque
LSAs provide a generalized mechanism to allow for the future
extensibility of OSPF. The information contained in Opaque LSAs may
be used directly by OSPF or indirectly by some application wishing to
distribute information throughout the OSPF domain. For example, the
OSPF LSA may be used by routers to distribute IP to link-layer
address resolution information (see [ARA] for more information). The
exact use of Opaque LSAs is beyond the scope of this memo.

Opaque LSAs consist of a standard LSA header followed by a 32-bit
qaligned application-specific information field. Like any other LSA,
the Opaque LSA uses the link-state database distribution mechanism
for flooding this information throughout the topology. The link-
state type field of the Opaque LSA identifies the LSA's range of
topological distribution. This range is referred to as the Flooding
Scope.

It is envisioned that an implementation of the Opaque option provides
an application interface for 1) encapsulating application-specific
information in a specific Opaque type, 2) sending and receiving
application-specific information, and 3) if required, informing the
application of the change in validity of previously received
information when topological changes are detected.

2.1 Organization Of This Document

This document first defines the three types of Opaque LSAs followed
by a description of OSPF packet processing. The packet processing
sections include modifications to the flooding procedure and to the
neighbor state machine. Appendix A then gives the packet formats.

2.2 Acknowledgments

The author would like to thank Dennis Ferguson, Acee Lindem, John
Moy, Sandra Murphy, Man-Kit Yeung, Zhaohui "Jeffrey" Zhang and the
rest of the OSPF Working 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. Opaque
LSAs consist of a standard LSA header followed by a 32-bit aligned
application-specific information field. Standard link-state database
flooding mechanisms are used for distribution of Opaque LSAs. The
range of topological distribution (i.e., the flooding scope) of an
Opaque LSA is identified by its link-state type. This section
documents the flooding of Opaque LSAs.

The flooding scope associated with each Opaque link-state type is
defined as follows.

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 borders of their associated area.

o Link-state type 11 denotes that the LSA is flooded throughout
the Autonomous System (AS). The flooding scope of type-11
LSAs are equivalent to the flooding scope of AS-external (type-5)
LSAs. 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 by routers into their connected
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.

The link-state ID of the Opaque LSA is divided into an Opaque type
field (the first 8 bits) and a type-specific ID (the remaining 24
bits). The packet format of the Opaque LSA is given in Appendix A.
Section 7.0 describes Opaque type allocation and assignment.

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 (e.g., a type-11 (domain-wide) Opaque LSA
is not accepted in a stub area). The flooding scope effects both the

synchronization of the link-state database and the flooding
procedure.

The following describes the modifications to these procedures that
are necessary to insure conformance to 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. Section 13 of [OSPF] describes the
OSPF flooding procedure. The following describes the Opaque LSA's
type-specific flooding restrictions.

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 target neighbor), the Opaque LSA must not be flooded
out that interface (or to that neighbor). An implementation
should keepk 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 (upon reception) is not
the same as the area associated with the target 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 LSA is flooded throughout the
AS) and the target interface is associated with a stub area the
Opaque LSA must not be flooded out the interface. A type-11
Opaque LSA that is received on an interface associated with a
stub area must be discarded and not acknowledged (the
neighboring router has flooded the LSA in error).

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;
the O-bit is not set in packets other than Database Description

packets. Then, in the next step of the Database Exchange process,
Opaque LSAs are included in the Database summary list that is 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
Section 13.3 of [OSPF], Opaque LSAs are only placed on the link-state
retransmission lists of opaque-capable neighbors. However, when send
ing 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 this action is changed
to the following.

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 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 area (see Section 3.6 of
[OSPF]).

Type-9 Opaque LSAs are omitted from the Database summary
list if the interface associated with the neighbor is
not the interface associated with the Opaque LSA (as
noted upon reception).

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 10.6 and
10.8 of [OSPF].

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
implementation. In addition to the data structures listed below, this
specification references 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
reception of the neighbor's Database Description packets, and
determines whether Opaque LSAs are flooded to the neighbor. For a
more detailed explanation of the flooding of the Opaque LSA see
section 3 of this document.

5.0 Management Considerations

This section identifies the current OSPF MIB [OSPFMIB] capabilities
that are applicable to the Opaque option and lists the additional
management information which is required for its support.

Opaque LSAs are types 9, 10 and 11 link-state advertisements. The
link-state ID of the Opaque LSA is divided into an Opaque type field
(the first 8 bits) and a type-specific ID (the remaining 24 bits).
The packet format of the Opaque LSA is given in Appendix A. The
range of topological distribution (i.e., the flooding scope) of an
Opaque LSA is identified by its link-state type.

o Link-State type 9 Opaque LSAs have a link-local scope. Type-9
Opaque LSAs are flooded on a single local (sub)network but are
not flooded beyond the local (sub)network.

o Link-state type 10 Opaque LSAs have an area-local scope. Type-10
Opaque LSAs are flooded throughout a single area but are not
flooded beyond the borders of the associated area.

o Link-state type 11 Opaque LSAs have an Autonomous-System-wide
scope. The flooding scope of type-11 LSAs are equivalent to the
flooding scope of AS-external (type-5) LSAs.

The OSPF MIB provides a number of objects that can be used to manage
and monitor an OSPF router's Link-State Database. The ones that are
relevant to the Opaque option are as follows.

The ospfGeneralGroup defines two objects for keeping track of newly
originated and newly received LSAs (ospfOriginateNewLsas and
ospfRxNewLsas respectively).

The OSPF MIB defines a set of optional traps. The ospfOriginateLsa
trap signifies that a new LSA has been originated by a router and
the ospfMaxAgeLsa trap signifies that one of the LSAs in the
router's link-state database has aged to MaxAge.

The ospfAreaTable describes the configured parameters and
cumulative statistics of the router's attached areas. This table
includes a count of the number of LSAs contained in the area's
link-state database (ospfAreaLsaCount), and a sum of the LSA's LS
checksums contained in this area (ospfAreaLsaCksumSum). This sum
can be used to determine if there has been a change in a router's
link-state database, and to compare the link-state database of two
routers.

The ospfLsdbTable describes the OSPF Process's link-state database
(excluding AS-external LSAs). Entries in this table are indexed
with an Area ID, a link-state type, a link-state ID and the
originating router's Router ID.

The management objects that are needed to support the Opaque option
are as follows.

An Opaque-option-enabled object is needed to indicate if the Opaque
option is enabled on the router.

The origination and reception of new Opaque LSAs should be
reflected in the counters ospfOriginateNewLsas and ospfRxNewLsas
(inclusive for types 9, 10 and 11 Opaque LSAs).

If the OSPF trap option is supported, the origination of new Opaque
LSAs and purging of MaxAge Opaque LSAs should be reflected in the
ospfOriginateLsa and ospfMaxAgeLsa traps (inclusive for types 9, 10
and 11 Opaque LSAs).

The number of type-10 Opaque LSAs should be reflected in
ospfAreaLsaCount; the checksums of type-10 Opaque LSAs should be
included in ospfAreaLsaChksumSum.

Type-10 Opaque LSAs should be included in the ospfLsdbTable. Note
that this table does not include a method of examining the Opaque
type field (in the Opaque option this is a sub-field of the link-
state ID).

Up until now, LSAs have not had a link-local scope so there is no
method of requesting the number of, or examining the LSAs that are
associated with a specific OSPF interface. A new group of
management objects are required to support type-9 Opaque LSAs.
These objects should include a count of type-9 Opaque LSAs, a
checksum sum and a table for displaying the link-state database for
type-9 Opaque LSAs on a per-interface basis. Entries in this table
should be indexed with an Area ID, interface's IP address, Opaque
type, link-state ID and the originating router's Router ID.

Prior to the introduction of type-11 Opaque LSAs, AS-External
(type-5) LSAs have been the only link-state types which have an
Autonomous-System-wide scope. A new group of objects are required
to support type-11 Opaque LSAs. These objects should include a
count of type-11 Opaque LSAs, a type-11 checksum sum and a table
for displaying the type-11 link-state database. Entries in this
table should be indexed with the Opaque type, link-state ID and the

originating router's Router ID. The type-11 link-state database
table will allow type-11 LSAs to be displayed once for the router
rather than once in each non-stub area.

6.0 Security Considerations

There are two types of issues that need be addressed when looking at
protecting routing protocols from misconfigurations and malicious
attacks. The first is authentication and certification of routing
protocol information. The second is denial of service attacks
resulting from repetitive origination of the same router
advertisement or origination a large number of distinct
advertisements resulting in database overflow. Note that both of
these concerns exist independently of a router's support for the
Opaque option.

To address the authentication concerns, OSPF protocol exchanges are
authenticated. OSPF supports multiple types of authentication; the
type of authentication in use can be configured on a per network
segment basis. One of OSPF's authentication types, namely the
Cryptographic authentication option, is believed to be secure against
passive attacks and provide significant protection against active
attacks. When using the Cryptographic authentication option, each
router appends a "message digest" to its transmitted OSPF packets.
Receivers then use the shared secret key and received digest to
verify that each received OSPF packet is authentic.

The quality of the security provided by the Cryptographic
authentication option depends completely on the strength of the
message digest algorithm (MD5 is currently the only message digest
algorithm specified), the strength of the key being used, and the
correct implementation of the security mechanism in all communicating
OSPF implementations. It also requires that all parties maintain the
secrecy of the shared secret key. None of the standard OSPF
authentication types provide confidentiality. Nor do they protect
against traffic analysis. For more information on the standard OSPF
security mechanisms, see Sections 8.1, 8.2, and Appendix D of [OSPF].

[DIGI] describes the extensions to OSPF required to add digital
signature authentication to Link State data and to provide a
certification mechanism for router data. [DIGI] also describes the
added LSA processing and key management as well as a method for
migration from, or co-existence with, standard OSPF V2.

Repetitive origination of advertisements are addressed by OSPF by
mandating a limit on the frequency that new instances of any
particular LSA can be originated and accepted during the flooding
procedure. The frequency at which new LSA instances may be

originated is set equal to once every MinLSInterval seconds, whose
value is 5 seconds (see Section 12.4 of [OSPF]). The frequency at
which new LSA instances are accepted during flooding is once every
MinLSArrival seconds, whose value is set to 1 (see Section 13,
Appendix B and G.5 of [OSPF]).

Proper operation of the OSPF protocol requires that all OSPF routers
maintain an identical copy of the OSPF link-state database. However,
when the size of the link-state database becomes very large, some
routers may be unable to keep the entire database due to resource
shortages; we term this "database overflow". When database overflow
is anticipated, the routers with limited resources can be
accommodated by configuring OSPF stub areas and NSSAs. [OVERFLOW]
details a way of gracefully handling unanticipated database
overflows.

7.0 IANA Considerations

Opaque types are maintained by the IANA. Extensions to OSPF which
require a new Opaque type must be reviewed by the OSPF working group.
In the event that the OSPF working group has disbanded the review
shall be performed by a recommended Designated Expert.

Following the policies outlined in [IANA], Opaque type values in the
range of 0-127 are allocated through an IETF Consensus action and
Opaque type values in the range of 128-255 are reserved for private
and experimental use.

8.0 References

[ARA] Coltun, R., and J. Heinanen, "The OSPF Address Resolution
Advertisement Option", Work in Progress.

[DEMD] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.

[DIGI] Murphy, S., Badger, M., and B. Wellington, "OSPF with Digital
Signatures", RFC2154, June 1997.

[IANA] Narten, T., and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", Work in Progress.

[MOSPF] Moy, J., "Multicast Extensions to OSPF", RFC1584, March
1994.

[NSSA] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC1587,
March 1994.

[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.

[OSPFMIB] Baker, F., and R. Coltun, "OSPF Version 2 Management
Information Base", RFC1850, November 1995.

[OVERFLOW] Moy, J., "OSPF Database Overflow", RFC1765,
March 1995.

9.0 Author's Information

Rob Coltun
FORE Systems

Phone: (703) 245-4543
EMail: rcoltun@fore.com

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
capabilities, 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.

Six 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 Packets, Database Description packets or
link-state advertisements should ignore the capability and process
the packet/advertisement normally.

+------------------------------------+
| * | O | DC | EA | N/P | MC | E | * |
+------------------------------------+

The Options Field

E-bit
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
forward External-Attributes-LSAs, as specified in [EAL].

O-bit
This bit describes the router's willingness to receive and
forward Opaque-LSAs as specified in this document.

A.2 The Opaque LSA

Opaque LSAs are Type 9, 10 and 11 link-state advertisements. These
advertisements may be used directly by OSPF or indirectly by some
application wishing to distribute information throughout the OSPF
domain. The function of the Opaque LSA option is to provide for
future extensibility of OSPF.

Opaque LSAs contain some number of octets (of application-specific
data) 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). 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 of the Opaque LSA identifies the LSA's range of
topological distribution. This range is referred to as the Flooding
Scope. The following 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
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).
See section 7.0 of this document for a description of Opaque type
allocation and assignment.

Appendix B. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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The limited permissions granted above are perpetual and will not be
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