Networking Working Group P. Psenak, Ed.
Internet-Draft C. Filsfils
Intended status: Informational S. Litkowski
Expires: September 26, 2022 Cisco Systems
D. Voyer
Bell Canada
A. Dhamija
Rakuten
March 25, 2022
IGP Unreachable Prefix Announcement
draft-ppsenak-lsr-igp-ureach-prefix-announce-00
Abstract
In the presence of summarization, there is a need to signal loss of
reachability to an individual prefix covered by the summary in order
to enable fast convergence away from paths to the node which owns the
prefix which is no longer reachable. This document describes how to
use existing protocol mechanisms in IS-IS and OSPF to advertise such
prefix reachability loss.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 26, 2022.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . . 3
2.1. Advertisement of UPA in IS-IS . . . . . . . . . . . . . . 3
2.2. Propagation of UPA in IS-IS . . . . . . . . . . . . . . . 4
3. Supporting UPA in OSPF . . . . . . . . . . . . . . . . . . . 4
3.1. Advertisement of UPA in OSPF . . . . . . . . . . . . . . 5
3.2. Propagation of UPA in OSPF . . . . . . . . . . . . . . . 5
4. Deployment Considerations for UPA . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Link-state IGP protocols like IS-IS and OSPF are primarily used to
distribute routing information between routers belonging to a single
Autonomous System (AS) and to calculate the reachability for IPv4 or
IPv6 prefixes advertised by the individual nodes inside the AS. Each
node advertises the state of its local adjacencies, connected
prefixes, capabilities, etc. The collection of these states from all
the routers inside the area form a link-state database (LSDB) that
describes the topology of the area and holds additional state
information about the prefixes, router capabilities, etc.
The growth of networks running a link-state routing protocol results
in the addition of more state which leads to scalability and
convergence challenges. The organization of networks into levels/
areas and IGP domains helps limit the scope of link-state information
within certain boundaries. However, the state related to prefix
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reachability often requires propagation across a multi-area/ level
and/or multi-domain IGP network. Techniques such as summarization
have been used traditionally to address the scale challenges
associated with advertising prefix state outside of the local area/
domain. However, this results in suppression of the individual
prefix state that is useful for triggering fast-convergence
mechanisms outside of the IGPs - e.g., BGP PIC Edge [I-D.ietf-rtgwg-
bgp-pic].
This document describes how the use of existing protocol mechanisms
can support the necessary functionality without the need for any
protocol extensions. The functionality being described is called
Unreachable Prefix Announcement (UPA).
2. Supporting UPA in IS-IS
[RFC5305] defines the encoding for advertising IPv4 prefixes using 4
octets of metric information. Section 4 specifies:
"If a prefix is advertised with a metric larger then MAX_PATH_METRIC
(0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered
during the normal SPF computation. This allows advertisement of a
prefix for purposes other than building the normal IP routing table.
"
Similarly, [RFC5308] defines the encoding for advertising IPv6
prefixes using 4 octets of metric information. Section 2 states:
"...if a prefix is advertised with a metric larger than
MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered
during the normal Shortest Path First (SPF) computation. This will
allow advertisement of a prefix for purposes other than building the
normal IPv6 routing table."
This functionality can be used to advertise a prefix (IPv4 or IPv6)
in a manner which indicates that reachability has been lost - and to
do so without requiring all nodes in the network to be upgraded to
support the functionality.
2.1. Advertisement of UPA in IS-IS
Existing nodes in a network which receive UPA advertisements will
ignore them. This allows flooding of such advertisements to occur
without the need to upgrade all nodes in a network.
Recognition of the advertisement as UPA is only required on routers
which have a use case for this information. Area Border Routers
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(ABRs), which would be responsible for propagating UPA advertisements
into other areas would need to recognize such advertisements.
As per the definitions referenced in the preceding section, any
prefix advertisement with a metric value greater than 0xFE000000 can
be used for purposes other than normal routing calculations. Such an
advertisement can be interpreted by the receiver as a UPA.
Optionally, an implementation may use local configuration to limit
the set of metric values which will be interpreted as UPA. The only
restriction is that such values MUST be greater than 0xFE000000.
2.2. Propagation of UPA in IS-IS
ISIS L1/L2 routers may wish to advertise received UPAs into other
areas (upwards and/or downwards). When propagating UPAs the original
metric value MUST be preserved. The cost to reach the originator of
the received UPA MUST NOT be considered when readvertising the UPA.
3. Supporting UPA in OSPF
[RFC2328] Appendix B defines the following architectural constant for
OSPF:
"LSInfinity The metric value indicating that the destination
described by an LSA is unreachable. Used in summary-LSAs and AS-
external-LSAs as an alternative to premature aging (see
Section 14.1). It is defined to be the 24-bit binary value of all
ones: 0xffffff."
[RFC5340] Appendix B states:
"Architectural constants for the OSPF protocol are defined in
Appendix B of OSPFV2."
indicating that these same constants are applicable to OSPFv3.
[RFC2328] section 14.1. also describes the usage of LSInfinity as a
way to indicate loss of prefix reachability:
"Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its AS-
external-LSA from the routing domain via premature aging. This
procedure is preferable to the alternative, which is to originate a
new LSA for the destination specifying a metric of LSInfinity."
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3.1. Advertisement of UPA in OSPF
Using the existing mechanism already defined in the standards, as
described in previous section, an advertisement of the inter-area or
external prefix inside OSPF or OSPFv3 LSA that has the age set to
value lower than MaxAge and metic set to LSInfinity can be
interpreted by the receiver as a UPA.
Existing nodes in a network which receive UPA advertisements will
propagate it following existing standard procedures defined by OSPF.
OSPF Area Border Routers (ABRs), which would be responsible for
propagating UPA advertisements into other areas would need to
recognize such advertisements.
3.2. Propagation of UPA in OSPF
OSPF ABRs may wish to advertise received UPAs into other connected
areas. When doing so, the original LSInfinity metric value in UPA
MUST be preserved. The cost to reach the originator of the received
UPA MUST NOT be considered when readvertising the UPA to connected
areas.
4. Deployment Considerations for UPA
The intent of UPA is to provide an event driven signal of the
transition of a destination from reachable to unreachable. It is not
intended to advertise a persistent state. UPA advertisements should
therefore be withdrawn after a modest amount of time, that would
provides sufficient time for UPA to be flooded network-wide and acted
upon by receiving nodes, but limits the presence of UPA in the
network to a short time period. The time the UPA is kept in the
network SHOULD also reflect the intended use-case for which the UPA
was advertised.
As UPA advertisements in ISIS are advertised in existing Link State
PDUs (LSPs) and the unit of flooding in IS-IS is an LSP, it is
recommended that, when possible, UPAs are advertised in LSPs
dedicated to this type of advertisement. This will minimize the
number of LSPs which need to be updated when UPAs are advertised and
withdrawn.
In OSPF and OSPFv3, each inter-area and external prefix is advertised
in it's own LSA, so the above optimisation does not apply to OSPF.
It is also recommended that implementations limit the number of UPA
advertisements which can be originated at a given time.
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5. IANA Considerations
This document makes no requests to IANA.
6. Security Considerations
The use of UPAs introduces the possibility that an attacker could
inject a false, but apparently valid, UPA. However, the risk of this
occurring is no greater than the risk today of an attacker injecting
any other type of false advertisement .
The risks can be reduced by the use of existing security extensions
as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328][ and
[RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3.
7. Acknowledgements
The authors would like to thank Kamran Raza and Michael MacKenzie for
their contribution to the overall solution proposed in this document.
8. Normative References
[ISO10589]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", Nov 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
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[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Pribinova Street 10
Bratislava 81109
Slovakia
Email: ppsenak@cisco.com
Clarence Filsfils
Cisco Systems
Brussels
Belgium
Email: cfilsfil@cisco.com
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Stephane Litkowski
Cisco Systems
La Rigourdiere
Cesson Sevigne
France
Email: slitkows@cisco.com
Daniel Voyer
Bell Canada
Email: daniel.voyer@bell.ca
Amit Dhamija
Rakuten
Email: amit.dhamija@rakuten.com
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