Internet DRAFT - draft-ppsenak-lsr-igp-pfx-reach-loss
draft-ppsenak-lsr-igp-pfx-reach-loss
Networking Working Group P. Psenak, Ed.
Internet-Draft L. Ginsberg
Intended status: Informational Cisco Systems
Expires: 8 September 2022 D. Voyer
Bell Canada
7 March 2022
IGP Prefix Reachability Loss Anouncement
draft-ppsenak-lsr-igp-pfx-reach-loss-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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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Supporting PRLA in IS-IS . . . . . . . . . . . . . . . . . . 3
2.1. Advertisement of PRLA in IS-IS . . . . . . . . . . . . . 3
2.2. Propagation of PRLA in IS-IS . . . . . . . . . . . . . . 4
3. Supporting PRLA in OSPF . . . . . . . . . . . . . . . . . . . 4
3.1. Advertisement of PRLA in OSPF . . . . . . . . . . . . . . 5
3.2. Propagation of PRLA in OSPF . . . . . . . . . . . . . . . 5
4. Deployment Considerations for PRLA . . . . . . . . . . . . . 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
reachability often requires propagation across a multi-area/ level
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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
Prefix Reachability Loss Announcement (PRLA).
2. Supporting PRLA 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 PRLA in IS-IS
Existing nodes in a network which receive PRLA advertisements will
ignore them. This allows flooding of such advertisements to occur
without the need to upgrade all nodes in a network.
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Recognition of the advertisement as PRLA is only required on routers
which have a use case for this information. Area Border Routers
(ABRs), which would be responsible for propagating PRLA
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 PRLA.
Optionally, an implementation may use local configuration to limit
the set of metric values which will be interpreted as PRLA. The only
restriction is that such values MUST be greater than 0xFE000000.
2.2. Propagation of PRLA in IS-IS
ISIS L1/L2 routers may wish to advertise received PRLAs into other
areas (upwards and/or downwards). When propagating PRLAs the
original metric value MUST be preserved. The cost to reach the
originator of the received PRLA MUST NOT be considered when
readvertising the PRLA.
3. Supporting PRLA 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:
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"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."
3.1. Advertisement of PRLA 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 PRLA.
Existing nodes in a network which receive PRLA advertisements will
propagate it following existing standard procedures defined by OSPF.
OSPF Area Border Routers (ABRs), which would be responsible for
propagating PRLA advertisements into other areas would need to
recognize such advertisements.
3.2. Propagation of PRLA in OSPF
OSPF ABRs may wish to advertise received PRLAs into other connected
areas. When doing so, the original LSInfinity metric value in PRLA
MUST be preserved. The cost to reach the originator of the received
PRLA MUST NOT be considered when readvertising the PRLA to connected
areas.
4. Deployment Considerations for PRLA
The economy provided by the use of summary advertisements diminishes
in the presence of PRLA. It is therefore recommended that
implementations limit the number of PRLA advertisements which can be
originated at a given time. This implies that PRLA can be used to
signal the loss of reachablity to a modest number of nodes - but it
is not a good tool to signal the loss of many nodes simultaneously.
The intent of PRLA 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. PRLA advertisements should
therefore be withdrawn after a modest amount of time, that would
provides sufficient time for PRLA to be flooded network-wide and
acted upon by receiving nodes, but limits the presence of PRLA in the
network to a short time period. The time the PRLA is kept in the
network SHOULD also reflect the intended use-case for which the PRLA
was advertised.
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As PRLA 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, PRLAs are advertised in LSPs
dedicated to this type of advertisement. This will minimize the
number of LSPs which need to be updated when PRLAs 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.
5. IANA Considerations
This document makes no requests to IANA.
6. Security Considerations
The use of PRLAs introduces the possibility that an attacker could
inject a false, but apparently valid, PRLA. 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
TBD
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)", November
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>.
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[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>.
[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
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Les Ginsberg
Cisco Systems
821 Alder Drive
Milpitas, CA 95035
United States of America
Email: ginsberg@cisco.com
Daniel Voyer
Bell Canada
Email: daniel.voyer@bell.ca
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