< draft-ppsenak-lsr-igp-pfx-reach-loss-00.txt   draft-ppsenak-lsr-igp-ureach-prefix-announce-00.txt >
Networking Working Group P. Psenak, Ed. Networking Working Group P. Psenak, Ed.
Internet-Draft L. Ginsberg Internet-Draft C. Filsfils
Intended status: Informational Cisco Systems Intended status: Informational S. Litkowski
Expires: 8 September 2022 D. Voyer Expires: September 26, 2022 Cisco Systems
D. Voyer
Bell Canada Bell Canada
7 March 2022 A. Dhamija
Rakuten
March 25, 2022
IGP Prefix Reachability Loss Anouncement IGP Unreachable Prefix Announcement
draft-ppsenak-lsr-igp-pfx-reach-loss-00 draft-ppsenak-lsr-igp-ureach-prefix-announce-00
Abstract Abstract
In the presence of summarization, there is a need to signal loss of In the presence of summarization, there is a need to signal loss of
reachability to an individual prefix covered by the summary in order reachability to an individual prefix covered by the summary in order
to enable fast convergence away from paths to the node which owns the to enable fast convergence away from paths to the node which owns the
prefix which is no longer reachable. This document describes how to prefix which is no longer reachable. This document describes how to
use existing protocol mechanisms in IS-IS and OSPF to advertise such use existing protocol mechanisms in IS-IS and OSPF to advertise such
prefix reachability loss. prefix reachability loss.
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This Internet-Draft will expire on 8 September 2022. This Internet-Draft will expire on September 26, 2022.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Supporting PRLA in IS-IS . . . . . . . . . . . . . . . . . . 3 2. Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . . 3
2.1. Advertisement of PRLA in IS-IS . . . . . . . . . . . . . 3 2.1. Advertisement of UPA in IS-IS . . . . . . . . . . . . . . 3
2.2. Propagation of PRLA in IS-IS . . . . . . . . . . . . . . 4 2.2. Propagation of UPA in IS-IS . . . . . . . . . . . . . . . 4
3. Supporting PRLA in OSPF . . . . . . . . . . . . . . . . . . . 4 3. Supporting UPA in OSPF . . . . . . . . . . . . . . . . . . . 4
3.1. Advertisement of PRLA in OSPF . . . . . . . . . . . . . . 5 3.1. Advertisement of UPA in OSPF . . . . . . . . . . . . . . 5
3.2. Propagation of PRLA in OSPF . . . . . . . . . . . . . . . 5 3.2. Propagation of UPA in OSPF . . . . . . . . . . . . . . . 5
4. Deployment Considerations for PRLA . . . . . . . . . . . . . 5 4. Deployment Considerations for UPA . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . 6 8. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
Link-state IGP protocols like IS-IS and OSPF are primarily used to Link-state IGP protocols like IS-IS and OSPF are primarily used to
distribute routing information between routers belonging to a single distribute routing information between routers belonging to a single
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have been used traditionally to address the scale challenges have been used traditionally to address the scale challenges
associated with advertising prefix state outside of the local area/ associated with advertising prefix state outside of the local area/
domain. However, this results in suppression of the individual domain. However, this results in suppression of the individual
prefix state that is useful for triggering fast-convergence prefix state that is useful for triggering fast-convergence
mechanisms outside of the IGPs - e.g., BGP PIC Edge [I-D.ietf-rtgwg- mechanisms outside of the IGPs - e.g., BGP PIC Edge [I-D.ietf-rtgwg-
bgp-pic]. bgp-pic].
This document describes how the use of existing protocol mechanisms This document describes how the use of existing protocol mechanisms
can support the necessary functionality without the need for any can support the necessary functionality without the need for any
protocol extensions. The functionality being described is called protocol extensions. The functionality being described is called
Prefix Reachability Loss Announcement (PRLA). Unreachable Prefix Announcement (UPA).
2. Supporting PRLA in IS-IS 2. Supporting UPA in IS-IS
[RFC5305] defines the encoding for advertising IPv4 prefixes using 4 [RFC5305] defines the encoding for advertising IPv4 prefixes using 4
octets of metric information. Section 4 specifies: octets of metric information. Section 4 specifies:
"If a prefix is advertised with a metric larger then MAX_PATH_METRIC "If a prefix is advertised with a metric larger then MAX_PATH_METRIC
(0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered
during the normal SPF computation. This allows advertisement of a during the normal SPF computation. This allows advertisement of a
prefix for purposes other than building the normal IP routing table. prefix for purposes other than building the normal IP routing table.
" "
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MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered
during the normal Shortest Path First (SPF) computation. This will during the normal Shortest Path First (SPF) computation. This will
allow advertisement of a prefix for purposes other than building the allow advertisement of a prefix for purposes other than building the
normal IPv6 routing table." normal IPv6 routing table."
This functionality can be used to advertise a prefix (IPv4 or IPv6) This functionality can be used to advertise a prefix (IPv4 or IPv6)
in a manner which indicates that reachability has been lost - and to 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 do so without requiring all nodes in the network to be upgraded to
support the functionality. support the functionality.
2.1. Advertisement of PRLA in IS-IS 2.1. Advertisement of UPA in IS-IS
Existing nodes in a network which receive PRLA advertisements will Existing nodes in a network which receive UPA advertisements will
ignore them. This allows flooding of such advertisements to occur ignore them. This allows flooding of such advertisements to occur
without the need to upgrade all nodes in a network. without the need to upgrade all nodes in a network.
Recognition of the advertisement as PRLA is only required on routers Recognition of the advertisement as UPA is only required on routers
which have a use case for this information. Area Border Routers which have a use case for this information. Area Border Routers
(ABRs), which would be responsible for propagating PRLA (ABRs), which would be responsible for propagating UPA advertisements
advertisements into other areas would need to recognize such into other areas would need to recognize such advertisements.
advertisements.
As per the definitions referenced in the preceding section, any As per the definitions referenced in the preceding section, any
prefix advertisement with a metric value greater than 0xFE000000 can prefix advertisement with a metric value greater than 0xFE000000 can
be used for purposes other than normal routing calculations. Such an be used for purposes other than normal routing calculations. Such an
advertisement can be interpreted by the receiver as a PRLA. advertisement can be interpreted by the receiver as a UPA.
Optionally, an implementation may use local configuration to limit Optionally, an implementation may use local configuration to limit
the set of metric values which will be interpreted as PRLA. The only the set of metric values which will be interpreted as UPA. The only
restriction is that such values MUST be greater than 0xFE000000. restriction is that such values MUST be greater than 0xFE000000.
2.2. Propagation of PRLA in IS-IS 2.2. Propagation of UPA in IS-IS
ISIS L1/L2 routers may wish to advertise received PRLAs into other ISIS L1/L2 routers may wish to advertise received UPAs into other
areas (upwards and/or downwards). When propagating PRLAs the areas (upwards and/or downwards). When propagating UPAs the original
original metric value MUST be preserved. The cost to reach the metric value MUST be preserved. The cost to reach the originator of
originator of the received PRLA MUST NOT be considered when the received UPA MUST NOT be considered when readvertising the UPA.
readvertising the PRLA.
3. Supporting PRLA in OSPF 3. Supporting UPA in OSPF
[RFC2328] Appendix B defines the following architectural constant for [RFC2328] Appendix B defines the following architectural constant for
OSPF: OSPF:
"LSInfinity The metric value indicating that the destination "LSInfinity The metric value indicating that the destination
described by an LSA is unreachable. Used in summary-LSAs and AS- described by an LSA is unreachable. Used in summary-LSAs and AS-
external-LSAs as an alternative to premature aging (see external-LSAs as an alternative to premature aging (see
Section 14.1). It is defined to be the 24-bit binary value of all Section 14.1). It is defined to be the 24-bit binary value of all
ones: 0xffffff." ones: 0xffffff."
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[RFC2328] section 14.1. also describes the usage of LSInfinity as a [RFC2328] section 14.1. also describes the usage of LSInfinity as a
way to indicate loss of prefix reachability: way to indicate loss of prefix reachability:
"Premature aging can also be used when, for example, one of the "Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its AS- reachable. In this circumstance, the router can flush its AS-
external-LSA from the routing domain via premature aging. This external-LSA from the routing domain via premature aging. This
procedure is preferable to the alternative, which is to originate a procedure is preferable to the alternative, which is to originate a
new LSA for the destination specifying a metric of LSInfinity." new LSA for the destination specifying a metric of LSInfinity."
3.1. Advertisement of PRLA in OSPF 3.1. Advertisement of UPA in OSPF
Using the existing mechanism already defined in the standards, as Using the existing mechanism already defined in the standards, as
described in previous section, an advertisement of the inter-area or described in previous section, an advertisement of the inter-area or
external prefix inside OSPF or OSPFv3 LSA that has the age set to external prefix inside OSPF or OSPFv3 LSA that has the age set to
value lower than MaxAge and metic set to LSInfinity can be value lower than MaxAge and metic set to LSInfinity can be
interpreted by the receiver as a PRLA. interpreted by the receiver as a UPA.
Existing nodes in a network which receive PRLA advertisements will Existing nodes in a network which receive UPA advertisements will
propagate it following existing standard procedures defined by OSPF. propagate it following existing standard procedures defined by OSPF.
OSPF Area Border Routers (ABRs), which would be responsible for OSPF Area Border Routers (ABRs), which would be responsible for
propagating PRLA advertisements into other areas would need to propagating UPA advertisements into other areas would need to
recognize such advertisements. recognize such advertisements.
3.2. Propagation of PRLA in OSPF 3.2. Propagation of UPA in OSPF
OSPF ABRs may wish to advertise received PRLAs into other connected OSPF ABRs may wish to advertise received UPAs into other connected
areas. When doing so, the original LSInfinity metric value in PRLA areas. When doing so, the original LSInfinity metric value in UPA
MUST be preserved. The cost to reach the originator of the received MUST be preserved. The cost to reach the originator of the received
PRLA MUST NOT be considered when readvertising the PRLA to connected UPA MUST NOT be considered when readvertising the UPA to connected
areas. areas.
4. Deployment Considerations for PRLA 4. Deployment Considerations for UPA
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 The intent of UPA is to provide an event driven signal of the
transition of a destination from reachable to unreachable. It is not transition of a destination from reachable to unreachable. It is not
intended to advertise a persistent state. PRLA advertisements should intended to advertise a persistent state. UPA advertisements should
therefore be withdrawn after a modest amount of time, that would therefore be withdrawn after a modest amount of time, that would
provides sufficient time for PRLA to be flooded network-wide and provides sufficient time for UPA to be flooded network-wide and acted
acted upon by receiving nodes, but limits the presence of PRLA in the upon by receiving nodes, but limits the presence of UPA in the
network to a short time period. The time the PRLA is kept 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 PRLA network SHOULD also reflect the intended use-case for which the UPA
was advertised. was advertised.
As PRLA advertisements in ISIS are advertised in existing Link State 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 PDUs (LSPs) and the unit of flooding in IS-IS is an LSP, it is
recommended that, when possible, PRLAs are advertised in LSPs recommended that, when possible, UPAs are advertised in LSPs
dedicated to this type of advertisement. This will minimize the dedicated to this type of advertisement. This will minimize the
number of LSPs which need to be updated when PRLAs are advertised and number of LSPs which need to be updated when UPAs are advertised and
withdrawn. withdrawn.
In OSPF and OSPFv3, each inter-area and external prefix is advertised 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. 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.
5. IANA Considerations 5. IANA Considerations
This document makes no requests to IANA. This document makes no requests to IANA.
6. Security Considerations 6. Security Considerations
The use of PRLAs introduces the possibility that an attacker could The use of UPAs introduces the possibility that an attacker could
inject a false, but apparently valid, PRLA. However, the risk of inject a false, but apparently valid, UPA. However, the risk of this
this occurring is no greater than the risk today of an attacker occurring is no greater than the risk today of an attacker injecting
injecting any other type of false advertisement . any other type of false advertisement .
The risks can be reduced by the use of existing security extensions The risks can be reduced by the use of existing security extensions
as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328][ and as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328][ and
[RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3. [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3.
7. Acknowledgements 7. Acknowledgements
TBD 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 8. Normative References
[ISO10589] International Organization for Standardization, [ISO10589]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain "Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in routeing information exchange protocol for use in
conjunction with the protocol for providing the conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", November connectionless-mode Network Service (ISO 8473)", Nov 2002.
2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998, DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>. <https://www.rfc-editor.org/info/rfc2328>.
skipping to change at page 7, line 46 skipping to change at page 7, line 38
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Authors' Addresses Authors' Addresses
Peter Psenak (editor) Peter Psenak (editor)
Cisco Systems Cisco Systems
Pribinova Street 10 Pribinova Street 10
Bratislava 81109 Bratislava 81109
Slovakia Slovakia
Email: ppsenak@cisco.com Email: ppsenak@cisco.com
Les Ginsberg
Clarence Filsfils
Cisco Systems Cisco Systems
821 Alder Drive Brussels
Milpitas, CA 95035 Belgium
United States of America
Email: ginsberg@cisco.com Email: cfilsfil@cisco.com
Stephane Litkowski
Cisco Systems
La Rigourdiere
Cesson Sevigne
France
Email: slitkows@cisco.com
Daniel Voyer Daniel Voyer
Bell Canada Bell Canada
Email: daniel.voyer@bell.ca Email: daniel.voyer@bell.ca
Amit Dhamija
Rakuten
Email: amit.dhamija@rakuten.com
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