Network Working Group B. Decraene
Internet-Draft Orange
Intended status: Standards Track S. Litkowski
Expires: June 13, 2018 Orange Business Service
H. Gredler
RtBrick Inc
A. Lindem
Cisco Systems
P. Francois
C. Bowers
Juniper Networks, Inc.
December 10, 2017
SPF Back-off algorithm for link state IGPs
draft-ietf-rtgwg-backoff-algo-07
Abstract
This document defines a standard algorithm to back-off link-state IGP
Shortest Path First (SPF) computations.
Having one standard algorithm improves interoperability by reducing
the probability and/or duration of transient forwarding loops during
the IGP convergence when the IGP reacts to multiple temporally close
IGP events.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on June 13, 2018.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. High level goals . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions and parameters . . . . . . . . . . . . . . . . . 4
4. Principles of SPF delay algorithm . . . . . . . . . . . . . . 5
5. Specification of the SPF delay state machine . . . . . . . . 5
5.1. States . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. States Transitions . . . . . . . . . . . . . . . . . . . 6
5.4. FSM Events . . . . . . . . . . . . . . . . . . . . . . . 7
6. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Partial Deployment . . . . . . . . . . . . . . . . . . . . . 10
8. Impact on micro-loops . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Security considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Link state IGPs, such as IS-IS [ISO10589-Second-Edition] and OSPF
[RFC2328], perform distributed route computation on all routers in
the area/level. In order to have consistent routing tables across
the network, such distributed computation requires that all routers
have the same version of the network topology (Link State DataBase
(LSDB)) and perform their computation at the same time.
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In general, when the network is stable, there is a desire to compute
a new Shortest Path First (SPF) as soon as a failure is detected in
order to quickly route around the failure. However, when the network
is experiencing multiple temporally close failures over a short
period of time, there is a conflicting desire to limit the frequency
of SPF computations. Indeed, this allows a reduction in control
plane resources used by IGPs and all protocols/subsystems reacting on
the attendant route change, such as LDP [RFC5036], RSVP-TE [RFC3209],
BGP [RFC4271], Fast ReRoute computations (e.g. Loop Free Alternates
(LFA) [RFC5286], FIB updates... This also reduces the churn on
routers and in the network and, in particular, reduces the side
effects such as micro-loops [RFC5715] that ensue during IGP
convergence.
To allow for this, IGPs implement an SPF back-off algorithm.
However, different implementations have choosen different algorithms.
Hence, in a multi-vendor network, it's not possible to ensure that
all routers trigger their SPF computation after the same delay. This
situation increases the average and maximum differential delay
between routers completing their SPF computation. It also increases
the probability that different routers compute their FIBs based on
different LSDB versions. Both factors increase the probability and/
or duration of micro-loops as discussed in Section 8.
To allow multi-vendor networks to have all routers delay their SPF
computations for the same duration, this document specifies a
standard algorithm. Optionally, implementations may also offer
alternative algorithms.
2. High level goals
The high level goals of this algorithm are the following:
o Very fast convergence for a single event (e.g., link failure).
o Paced fast convergence for multiple temporally close IGP events
while IGP stability is considered acceptable.
o Delayed convergence when IGP stability is problematic. This will
allow the IGP and related processes to conserve resources during
the period of instability.
o Always try to avoid different SPF_DELAY timers values across
different routers in the area/level. Even though not all routers
will receive IGP messages at the same time, due to differences
both in the distance from the originator of the IGP event and in
flooding implementations.
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3. Definitions and parameters
IGP events: The reception or origination of an IGP LSDB change
requiring a new routing table computation. Examples are a topology
change, a prefix change, a metric change on a link or prefix... Note
that locally triggering a routing table computation is not considered
as an IGP event since other IGP routers are unaware of this
occurrence.
Routing table computation: Computation of the routing table, by the
IGP, using the IGP LSDB. No distinction is made between the type of
computation performed. e.g., full SPF, incremental SPF, Partial Route
Computation (PRC). The type of computation is a local consideration.
This document may interchangeably use the terms routing table
computation and SPF computation.
SPF_DELAY: The delay between the first IGP event triggering a new
routing table computation and the start of that routing table
computation. It can take the following values:
INITIAL_SPF_DELAY: A very small delay to quickly handle link
failure, e.g., 0 milliseconds.
SHORT_SPF_DELAY: A small delay to have a fast convergence in case of
a single failure (node, SRLG..), e.g., 50-100 milliseconds.
LONG_SPF_DELAY: A long delay when the IGP is unstable, e.g., 2
seconds. Note that this allows the IGP network to stabilize.
TIME_TO_LEARN_INTERVAL: This is the maximum duration typically needed
to learn all the IGP events related to a single component failure
(e.g., router failure, SRLG failure), e.g., 1 second. It's mostly
dependent on failure detection time variation between all routers
that are adjacent to the failure. Additionally, it may depend on the
different IGP implementations/parameters across the network, related
to origination and flooding of their link state advertisements.
HOLDDOWN_INTERVAL: The time required with no received IGP events
before considering the IGP to be stable again and allowing the
SPF_DELAY to be restored to INITIAL_SPF_DELAY. e.g., 3 seconds. The
HOLDDOWN_INTERVAL MUST be defaulted or configured to be longer than
the TIME_TO_LEARN_INTERVAL.
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4. Principles of SPF delay algorithm
For this first IGP event, we assume that there has been a single
simple change in the network which can be taken into account using a
single routing computation (e.g., link failure, prefix (metric)
change) and we optimize for very fast convergence, delaying the
routing computation by INITIAL_SPF_DELAY. Under this assumption,
there is no benefit in delaying the routing computation. In a
typical network, this is the most common type of IGP event. Hence,
it makes sense to optimize this case.
If subsequent IGP events are received in a short period of time
(TIME_TO_LEARN_INTERVAL), we then assume that a single component
failed, but that this failure requires the knowledge of multiple IGP
events in order for IGP routing to converge. Under this assumption,
we want fast convergence since this is a normal network situation.
However, there is a benefit in waiting for all IGP events related to
this single component failure so that the IGP can compute the post-
failure routing table in a single route computation. In this
situation, we delay the routing computation by SHORT_SPF_DELAY.
If IGP events are still received after TIME_TO_LEARN_INTERVAL from
the initial IGP event received in QUIET state, then the network is
presumably experiencing multiple independent failures. In this case,
while waiting for network stability, the computations are delayed for
a longer time represented by LONG_SPF_DELAY. This SPF delay is kept
until no IGP events are received for HOLDDOWN_INTERVAL.
Note that previously implemented SPF delay algorithms counted the
number of SPF computations. However, as all routers may receive the
IGP events at different times, we cannot assume that all routers will
perform the same number of SPF computations or that they will
schedule them at the same time. For example, assuming that the SPF
delay is 50 ms, router R1 may receive 3 IGP events (E1, E2, E3) in
those 50 ms and hence will perform a single routing computation.
While another router R2 may only receive 2 events (E1, E2) in those
50 ms and hence will schedule another routing computation when
receiving E3. That's why this document uses a time
(TIME_TO_LEARN_INTERVAL) from the initial event detection/reception
as opposed to counting the number of SPF computations to determine
when the IGP is unstable.
5. Specification of the SPF delay state machine
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5.1. States
This section describes the state machine. The naming and semantics
of each state corresponds directly to the SPF delay used for IGP
events received in that state. Three states are defined:
QUIET: This is the initial state, when no IGP events have occured for
at least HOLDDOWN_INTERVAL since the previous routing table
computation. The state is meant to handle link failures very
quickly.
SHORT_WAIT: State entered when an IGP event has been received in
QUIET state. This state is meant to handle single component failure
requiring multiple IGP events (e.g., node, SRLG).
LONG_WAIT: State reached after TIME_TO_LEARN_INTERVAL. In other
words, state reached after TIME_TO_LEARN_INTERVAL in state
SHORT_WAIT. This state is meant to handle multiple independent
component failures during periods of IGP instability.
5.2. Timers
SPF_TIMER: The Finite State Machine (FSM) abstract timer that uses
the computed SPF delay. Upon expiration, the Route Table Computation
(as defined in Section 3) is performed.
HOLDDOWN_TIMER: The Finite State Machine (FSM) abstract timer that is
(re)started whan an IGP event is received and set to
HOLDDOWN_INTERVAL. Upon expiration, the FSM is moved to the QUIET
state.
LEARN_TIMER: The Finite State Machine (FSM) abstract timer that is
started when an IGP event is recevied while the FSM is in the QUIET
state. Upon expiration, the FSM is moved to the LONG_WAIT state.
5.3. States Transitions
The FSM is initialized to the QUIET state with all three timers
timers (SPF_TIMER, HOLDDOWN_TIMER, LEARN_TIMER) deactivated.
The events which may change the FSM states are an IGP event or the
expiration of one timer (SPF_TIMER, HOLDDOWN_TIMER, LEARN_TIMER).
The following diagram briefly describes the state transitions.
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+-------------------+
+---->| |<-------------------+
| | QUIET | |
+-----| |<---------+ |
7: +-------------------+ | |
SPF_TIMER | | |
expiration | | |
| 1: IGP event | |
| | |
v | |
+-------------------+ | |
+---->| | | |
| | SHORT_WAIT |----->----+ |
+-----| | |
2: +-------------------+ 6: HOLDDOWN_TIMER |
IGP event | expiration |
8: SPF_TIMER | |
expiration | |
| 3: LEARN_TIMER |
| expiration |
| |
v |
+-------------------+ |
+---->| | |
| | LONG_WAIT |------------>-------+
+-----| |
4: +-------------------+ 5: HOLDDOWN_TIMER
IGP event expiration
9: SPF_TIMER expiration
Figure 1: State Machine
5.4. FSM Events
This section describes the events and the actions performed in
response.
Transition 1: IGP event, while in QUIET_STATE.
Actions on event 1:
o If SPF_TIMER is not already running, start it with value
INITIAL_SPF_DELAY.
o Start LEARN_TIMER with TIME_TO_LEARN_INTERVAL.
o Start HOLDDOWN_TIMER with HOLDDOWN_INTERVAL.
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o Transition to SHORT_WAIT state.
Transition 2: IGP event, while in SHORT_WAIT.
Actions on event 2:
o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.
o If SPF_TIMER is not already running, start it with value
SHORT_SPF_DELAY.
o Remain in current state.
Transition 3: LEARN_TIMER expiration.
Actions on event 3:
o Transition to LONG_WAIT state.
Transition 4: IGP event, while in LONG_WAIT.
Actions on event 4:
o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.
o If SPF_TIMER is not already running, start it with value
LONG_SPF_DELAY.
o Remain in current state.
Transition 5: HOLDDOWN_TIMER expiration, while in LONG_WAIT.
Actions on event 5:
o Transition to QUIET state.
Transition 6: HOLDDOWN_TIMER expiration, while in SHORT_WAIT.
Actions on event 6:
o Deactivate LEARN_TIMER.
o Transition to QUIET state.
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Transition 7: SPF_TIMER expiration, while in QUIET.
Actions on event 7:
o Compute SPF.
o Remain in current state.
Transition 8: SPF_TIMER expiration, while in SHORT_WAIT.
Actions on event 8:
o Compute SPF.
o Remain in current state.
Transition 9: SPF_TIMER expiration, while in LONG_WAIT.
Actions on event 9:
o Compute SPF.
o Remain in current state.
6. Parameters
All the parameters MUST be configurable [I-D.ietf-isis-yang-isis-cfg]
[I-D.ietf-ospf-yang] at the protocol instance granularity. They MAY
be configurable at the area/level granularity. All the delays
(INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY,
TIME_TO_LEARN_INTERVAL, HOLDDOWN_INTERVAL) SHOULD be configurable at
the millisecond granularity. They MUST be configurable at least at
the tenth of second granularity. The configurable range for all the
parameters SHOULD at least be from 0 milliseconds to 60 seconds.
This document does not propose default values for the parameters
because these values are expected to be context dependent.
Implementations are free to propose their own default values.
In order to satisfy the goals stated in Section 2, operators are
RECOMMENDED to configure delay intervals such that SPF_INITIAL_DELAY
<= SPF_SHORT_DELAY and SPF_SHORT_DELAY <= SPF_LONG_DELAY.
When setting (default) values, one SHOULD consider the customers and
their application requirements, the computational power of the
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routers, the size of the network, and, in particular, the number of
IP prefixes advertised in the IGP, the frequency and number of IGP
events, the number of protocols reactions/computations triggered by
IGP SPF (e.g., BGP, PCEP, Traffic Engineering CSPF, Fast ReRoute
computations).
Note that some or all of these factors may change over the life of
the network. In case of doubt, it's RECOMMENDED to play it safe and
start with safe, i.e., longer timers.
For the standard algorithm to be effective in mitigating micro-loops,
it is RECOMMENDED that all routers in the IGP domain, or at least all
the routers in the same area/level, have exactly the same configured
values.
7. Partial Deployment
In general, the SPF delay algorithm is only effective in mitigating
micro-loops if it is deployed, with the same parameters, on all
routers, in the IGP domain or, at least, all routers in an IGP area/
level. The impact of partial deployment is based on the particular
event, topology, and the SPF algorithm(s) used on other routers in
the IGP area/level. In cases where the previous SPF algorithm was
implemented uniformly, partial deployment will increase the frequency
and duration of micro-loops. Hence, it is RECOMMENDED that all
routers in the IGP domain or at least within the same area/level be
migrated to the SPF algorithm described herein at roughly the same
time.
Note that this is not a new consideration as over times, network
operators have changed SPF delay parameters in order to accommodate
new customer requirements for fast convergence, as permitted by new
software and hardware. They may also have progressively replaced an
implementation with a given SPF delay algorithm by another
implementation with a different one.
8. Impact on micro-loops
Micro-loops during IGP convergence are due to a non-synchronized or
non-ordered update of the forwarding information tables (FIB)
[RFC5715] [RFC6976] [I-D.ietf-rtgwg-spf-uloop-pb-statement]. FIBs
are installed after multiple steps such as flooding of the IGP event
across the network, SPF wait time, SPF computation, FIB distribution
across line cards, and FIB update. This document only addresses the
first contribution. This standardized procedure reduces the
probability and/or duration of micro-loops when IGPs experience
multiple temporally close events. It does not prevent all micro-
loops. However, it is beneficial and is less complex and costly to
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implement when compared to full solutions such as [RFC5715] or
[RFC6976].
9. IANA Considerations
No IANA actions required.
10. Security considerations
The algorithm presented in this document does not compromise IGP
security. An attacker having the ability to generate IGP events
would be able to delay the IGP convergence time. The LONG_SPF_DELAY
state may help mitigate the effects of Denial-of-Service (DOS)
attacks generating many IGP events.
11. Acknowledgements
We would like to acknowledge Les Ginsberg, Uma Chunduri, Mike Shand
and Alexander Vainshtein for the discussions and comments related to
this document.
12. References
12.1. Normative References
[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>.
12.2. Informative References
[I-D.ietf-isis-yang-isis-cfg]
Litkowski, S., Yeung, D., Lindem, A., Zhang, Z., and L.
Lhotka, "YANG Data Model for IS-IS protocol", draft-ietf-
isis-yang-isis-cfg-19 (work in progress), November 2017.
[I-D.ietf-ospf-yang]
Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
"Yang Data Model for OSPF Protocol", draft-ietf-ospf-
yang-09 (work in progress), October 2017.
[I-D.ietf-rtgwg-spf-uloop-pb-statement]
Litkowski, S., Decraene, B., and M. Horneffer, "Link State
protocols SPF trigger and delay algorithm impact on IGP
micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-05
(work in progress), December 2017.
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[ISO10589-Second-Edition]
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)", ISO/
IEC 10589:2002, Second Edition, Nov 2002.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <https://www.rfc-editor.org/info/rfc5036>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <https://www.rfc-editor.org/info/rfc6976>.
Authors' Addresses
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
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Stephane Litkowski
Orange Business Service
Email: stephane.litkowski@orange.com
Hannes Gredler
RtBrick Inc
Email: hannes@rtbrick.com
Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 27513
USA
Email: acee@cisco.com
Pierre Francois
Email: pfrpfr@gmail.com
Chris Bowers
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: cbowers@juniper.net
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