Network Working Group J. Moy
Request for Comments: 2329 Ascend Communications, Inc.
Category: Informational April 1998

OSPF Standardization Report

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

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

Abstract

This memo documents how the requirements for advancing a routing
protocol to Full Standard, set out in [Ref2], have been met for
OSPFv2.

Please send comments to ospf@gated.cornell.edu.

Table of Contents

1 IntrodUCtion ........................................... 2
2 Modifications since Draft Standard status .............. 3
2.1 Point-to-MultiPoint interface .......................... 4
2.2 Cryptographic Authentication ........................... 5
3 Updated implementation and deployment eXPerience ....... 5
4 Protocol Security ...................................... 7
References ............................................. 8
Security Considerations ................................ 8
Author's Address ....................................... 8
Full Copyright Statement ............................... 9

1. Introduction

OSPFv2, herein abbreviated simply as OSPF, is an IPv4 routing
protocol documented in [Ref8]. OSPF is a link-state routing
protocol. It is designed to be run internal to a single Autonomous
System. Each OSPF router maintains an identical database describing
the Autonomous System's topology. From this database, a routing
table is calculated by constructing a shortest-path tree. OSPF
features include the following:

o OSPF responds quickly to topology changes, expending a minimum
of network bandwidth in the process.

o Support for CIDR addressing.

o OSPF routing exchanges can be authenticated, providing routing
security.

o Equal-cost multipath.

o An area routing capability is provided, enabling an Autonomous
system to be split into a two level hierarchy to further reduce
the amount of routing protocol traffic.

o OSPF allows import of external routing information into the
Autonomous System, including a tagging feature that can be
exploited to exchange extra information at the AS boundary (see
[Ref7]).

An analysis of OSPF together with a more detailed description of
OSPF features was originally provided in [Ref6], as a part of
promoting OSPF to Draft Standard status. The analysis of OSPF
remains unchanged. Two additional major features have been developed
for OSPF since the protocol achieved Draft Standard status: the
Point-to-MultiPoint interface and Cryptographic Authentication.
These features are described in Sections 2.1 and 2.2 respectively of
this memo.

The OSPF MIB is documented in [Ref4]. It is currently at Draft
Standard status.

2. Modifications since Draft Standard status

OSPF became a Draft Standard with the release of RFC1583 [Ref3].
Implementations of the new specification in [Ref8] are backward-
compatible with RFC 1583. The differences between the two documents
are described in the Appendix Gs of [Ref1] and [Ref8]. These
differences are listed briefly below. Two major features were also
added, the Point-to-MultiPoint interface and Cryptographic
Authentication, which are described in separate sections.

o Configuration requirements for OSPF area address ranges have
been relaxed to allow greater flexibility in area assignment.
See Section G.3 of [Ref1] for details.

o The OSPF flooding algorithm was modified to a) improve database
convergence in networks with low speed links b) resolve a
problem where unnecessary LSA retransmissions could occur as a
result of differing clock granularities, c) remove race
conditions between the flooding of MaxAge LSAs and the Database
Exchange process, d) clarify the use of the MinLSArrival
constant, and e) rate-limit the response to less recent LSAs
received via flooding. See Sections G.4 and G.5 of [Ref1] and
Section G.1 of [Ref8] for details.

o To resolve the long-standing confusion regarding representation
of point-to-point links in OSPF, the specification now
optionally allows advertisement of a stub link to a point-to-
point link's subnet, ala RIP. See Section G.6 of [Ref1].

o Several problems involving advertising the same external route
from multiple areas were found and fixed, as described in
Section G.7 of [Ref1] and Section G.2 of [Ref8]. Without the
fixes, persistent routing loops could form in certain such
configurations. Note that one of the fixes was not backward-
compatible, in that mixing routers implementing the fixes with
those implementing just RFC1583 could cause loops not present
in an RFC1583-only configuration. This caused an
RFC1583Compatibility global configuration parameter to be added,
as described in Section C.1 of [Ref1].

o In order to deal with high delay links, retransmissions of
initial Database Description packets no longer reset an OSPF
adjacency.

o In order to detect link MTU mismatches, which can cause problems
both in IP forwarding and in the OSPF routing protocol itself,
MTU was added to OSPF's Database Description packets.
Neighboring routers refuse to bring up an OSPF adjacency unless
they agree on their common link's MTU.

o The TOS routing option was deleted from OSPF. However, for
backward compatibility the formats of OSPF's various LSAs remain
unchanged, maintaining the ability to specify TOS metrics in
router-LSAs, summary-LSAs, ASBR-summary-LSAs, and AS-external-
LSAs.

o OSPF's routing table lookup algorithm was changed to reflect
current practice. The "best match" routing table entry is now
always selected to be the one providing the most specific
(longest) match. See Section G.4 of [Ref8] for details.

2.1. Point-to-MultiPoint interface

The Point-to-MultiPoint interface was added as an alternative to
OSPF's NBMA interface when running OSPF over non-broadcast
subnets. Unlike the NBMA interface, Point-to-MultiPoint does not
require full mesh connectivity over the non-broadcast subnet.
Point-to-MultiPoint is less efficient than NBMA, but is easier
to configure (in fact, it can be self-configuring) and is more
robust than NBMA, tolerating all failures within the non-
broadcast subnet. For more information on the Point-to-
MultiPoint interface, see Section G.2 of [Ref1].

There are at least six independent implementations of the
Point-to-MultiPoint interface. Interoperability has been
demonstrated between at least two pairs of implementations:
between 3com and Bay Networks, and between cisco and Cascade.

2.2. Cryptographic Authentication

Non-trivial authentication was added to OSPF with the
development of the Cryptographic Authentication type. This
authentication type uses any keyed message digest algorithm,
with explicit instructions included for the use of MD5. For more
information on OSPF authentication, see Section 4.

There are at least three independent implementations of the OSPF
Cryptographic authentication type. Interoperability has been
demonstrated between the implementations from cisco and Cascade.

3. Updated implementation and deployment experience

When OSPF was promoted to Draft Standard Status, a report was issued
documenting current implementation and deployment experience (see
[Ref6]). That report is now quite dated. In an attempt to get more
current data, a questionnaire was sent to OSPF mailing list in
January 1996. Twelve responses were received, from 11 router vendors
and 1 manufacturer of test equipment. These responses represented 6
independent implementations. A tabulation of the results are
presented below.

Table 1 indicates the implementation, interoperability and
deployment of the major OSPF functions. The number in each column
represents the number of responses in the affirmative.

Imple- Inter-
Feature mented operated Deployed
_______________________________________________________
OSPF areas 10 10 10
Stub areas 10 10 9
Virtual links 10 9 8
Equal-cost multipath 10 7 8
NBMA support 9 8 7
CIDR addressing 8 5 6
OSPF MIB 8 5 5
Cryptographic auth. 3 2 1
Point-to-Multipoint ifc. 6 3 4

Table 1: Implementation of OSPF features

Table 2 indicates the size of the OSPF routing domains that vendors
have tested. For each size parameter, the number of responders and
the range of responses (minimum, mode, mean and maximum) are listed.

Parameter Responses Min Mode Mean Max
_________________________________________________________________
Max routers in domain 7 30 240 460 1600
Max routers in single area 7 20 240 380 1600
Max areas in domain 7 1 10 16 60
Max AS-external-LSAs 9 50 10K 10K 30K

Table 2: OSPF domain sizes tested

Table 3 indicates the size of the OSPF routing domains that vendors
have deployed in real networks. For each size parameter, the number
of responders and the range of responses (minimum, mode, mean and
maximum) are listed.

Parameter Responses Min Mode Mean Max
_________________________________________________________________
Max routers in domain 8 20 350 510 1000
Max routers in single area 8 20 100 160 350
Max areas in domain 7 1 15 23 60
Max AS-external-LSAs 6 50 1K 2K 5K

Table 3: OSPF domain sizes deployed

In an attempt to ascertain the extent to which OSPF is currently
deployed, vendors were also asked in January 1998 to provide
deployment estimates. Four vendors of OSPF routers responded, with a
total estimate of 182,000 OSPF routers in service, organized into
4300 separate OSPF routing domains.

4. Protocol Security

All 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 OSPF authentication types provide confidentiality. Nor
do they protect against traffic analysis. Key management is also not
addressed by the OSPF specification.

For more information, see Sections 8.1, 8.2, and Appendix D of
[Ref1].

References

[Ref1] Moy, J., "OSPF Version 2", RFC2178, July 1997.

[Ref2] Hinden, B., "Internet Routing Protocol Standardization
Criteria", RFC1264, October 1991.

[Ref3] Moy, J., "OSPF Version 2", RFC1583, March 1994.

[Ref4] Baker, F., and R. Coltun, "OSPF Version 2 Management
Information Base", RFC1850, November 1995.

[Ref5] Moy, J., "OSPF Protocol Analysis", RFC1245, August 1991.

[Ref6] Moy, J., "Experience with the OSPF Protocol", RFC1246,
August 1991.

[Ref7] Varadhan, K., Hares S., and Y. Rekhter, "BGP4/IDRP for IP--
-OSPF Interaction", RFC1745, December 1994.

[Ref8] Moy, J., "OSPF Version 2", STD 54, RFC2328, April 1998.

Security Considerations

Security considerations are addressed in Section 4 of this memo.

Author's Address

John Moy
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886

Phone: 978-952-1367
Fax: 978-392-2075
EMail: jmoy@casc.com

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