draft-ietf-rtgwg-backoff-algo-06.txt   draft-ietf-rtgwg-backoff-algo-07.txt 
Network Working Group B. Decraene Network Working Group B. Decraene
Internet-Draft Orange Internet-Draft Orange
Intended status: Standards Track S. Litkowski Intended status: Standards Track S. Litkowski
Expires: April 25, 2018 Orange Business Service Expires: June 13, 2018 Orange Business Service
H. Gredler H. Gredler
RtBrick Inc RtBrick Inc
A. Lindem A. Lindem
Cisco Systems Cisco Systems
P. Francois P. Francois
C. Bowers C. Bowers
Juniper Networks, Inc. Juniper Networks, Inc.
October 22, 2017 December 10, 2017
SPF Back-off algorithm for link state IGPs SPF Back-off algorithm for link state IGPs
draft-ietf-rtgwg-backoff-algo-06 draft-ietf-rtgwg-backoff-algo-07
Abstract Abstract
This document defines a standard algorithm to back-off link-state IGP This document defines a standard algorithm to back-off link-state IGP
SPF computations. Shortest Path First (SPF) computations.
Having one standard algorithm improves interoperability by reducing Having one standard algorithm improves interoperability by reducing
the probability and/or duration of transient forwarding loops during the probability and/or duration of transient forwarding loops during
the IGP convergence when the IGP reacts to multiple temporally close the IGP convergence when the IGP reacts to multiple temporally close
IGP events. IGP events.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2018. This Internet-Draft will expire on June 13, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. High level goals . . . . . . . . . . . . . . . . . . . . . . 3 2. High level goals . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions and parameters . . . . . . . . . . . . . . . . . 4 3. Definitions and parameters . . . . . . . . . . . . . . . . . 4
4. Principles of SPF delay algorithm . . . . . . . . . . . . . . 5 4. Principles of SPF delay algorithm . . . . . . . . . . . . . . 5
5. Specification of the SPF delay state machine . . . . . . . . 5 5. Specification of the SPF delay state machine . . . . . . . . 5
5.1. States . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.1. States . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. States Transitions . . . . . . . . . . . . . . . . . . . 6 5.2. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. FSM Events . . . . . . . . . . . . . . . . . . . . . . . 7 5.3. States Transitions . . . . . . . . . . . . . . . . . . . 6
5.4. FSM Events . . . . . . . . . . . . . . . . . . . . . . . 7
6. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Partial Deployment . . . . . . . . . . . . . . . . . . . . . 9 7. Partial Deployment . . . . . . . . . . . . . . . . . . . . . 10
8. Impact on micro-loops . . . . . . . . . . . . . . . . . . . . 10 8. Impact on micro-loops . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Security considerations . . . . . . . . . . . . . . . . . . . 10 10. Security considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
12.1. Normative References . . . . . . . . . . . . . . . . . . 10 12.1. Normative References . . . . . . . . . . . . . . . . . . 11
12.2. Informative References . . . . . . . . . . . . . . . . . 11 12.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Link state IGPs, such as IS-IS [ISO10589-Second-Edition] and OSPF Link state IGPs, such as IS-IS [ISO10589-Second-Edition] and OSPF
[RFC2328], perform distributed route computation on all routers in [RFC2328], perform distributed route computation on all routers in
the area/level. In order to have consistent routing tables across the area/level. In order to have consistent routing tables across
the network, such distributed computation requires that all routers the network, such distributed computation requires that all routers
have the same version of the network topology (Link State DataBase have the same version of the network topology (Link State DataBase
(LSDB)) and perform their computation at the same time. (LSDB)) and perform their computation at the same time.
In general, when the network is stable, there is a desire to compute In general, when the network is stable, there is a desire to compute
a new SPF as soon as a failure is detected in order to quickly route a new Shortest Path First (SPF) as soon as a failure is detected in
around the failure. However, when the network is experiencing order to quickly route around the failure. However, when the network
multiple temporally close failures over a short period of time, there is experiencing multiple temporally close failures over a short
is a conflicting desire to limit the frequency of SPF computations. period of time, there is a conflicting desire to limit the frequency
Indeed, this allows a reduction in control plane resources used by of SPF computations. Indeed, this allows a reduction in control
IGPs and all protocols/subsystems reacting on the attendant route plane resources used by IGPs and all protocols/subsystems reacting on
change, such as LDP, RSVP-TE, BGP, Fast ReRoute computations, FIB the attendant route change, such as LDP [RFC5036], RSVP-TE [RFC3209],
updates... This also reduces the churn on routers and in the network BGP [RFC4271], Fast ReRoute computations (e.g. Loop Free Alternates
and, in particular, reduces the side effects such as micro-loops that (LFA) [RFC5286], FIB updates... This also reduces the churn on
ensue during IGP convergence. 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. To allow for this, IGPs implement an SPF back-off algorithm.
However, different implementations have choosen different algorithms. However, different implementations have choosen different algorithms.
Hence, in a multi-vendor network, it's not possible to ensure that Hence, in a multi-vendor network, it's not possible to ensure that
all routers trigger their SPF computation after the same delay. This all routers trigger their SPF computation after the same delay. This
situation increases the average differential delay between routers situation increases the average and maximum differential delay
completing their SPF computation. It also increases the probability between routers completing their SPF computation. It also increases
that different routers compute their FIBs based on different LSDB the probability that different routers compute their FIBs based on
versions. Both factors increase the probability and/or duration of different LSDB versions. Both factors increase the probability and/
micro-loops. or duration of micro-loops as discussed in Section 8.
To allow multi-vendor networks to have all routers delay their SPF To allow multi-vendor networks to have all routers delay their SPF
computations for the same duration, this document specifies a computations for the same duration, this document specifies a
standard algorithm. Optionally, implementations may offer standard algorithm. Optionally, implementations may also offer
alternative algorithms. alternative algorithms.
2. High level goals 2. High level goals
The high level goals of this algorithm are the following: The high level goals of this algorithm are the following:
o Very fast convergence for a single event (e.g., link failure). o Very fast convergence for a single event (e.g., link failure).
o Paced fast convergence for multiple temporally close IGP events o Paced fast convergence for multiple temporally close IGP events
while IGP stability is considered acceptable. while IGP stability is considered acceptable.
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computation and SPF computation. computation and SPF computation.
SPF_DELAY: The delay between the first IGP event triggering a new SPF_DELAY: The delay between the first IGP event triggering a new
routing table computation and the start of that routing table routing table computation and the start of that routing table
computation. It can take the following values: computation. It can take the following values:
INITIAL_SPF_DELAY: A very small delay to quickly handle link INITIAL_SPF_DELAY: A very small delay to quickly handle link
failure, e.g., 0 milliseconds. failure, e.g., 0 milliseconds.
SHORT_SPF_DELAY: A small delay to have a fast convergence in case of SHORT_SPF_DELAY: A small delay to have a fast convergence in case of
a single component failure (node, SRLG..), e.g., 50-100 a single failure (node, SRLG..), e.g., 50-100 milliseconds.
milliseconds.
LONG_SPF_DELAY: A long delay when the IGP is unstable, e.g., 2 LONG_SPF_DELAY: A long delay when the IGP is unstable, e.g., 2
seconds. Note that this allows the IGP network to stabilize. seconds. Note that this allows the IGP network to stabilize.
TIME_TO_LEARN_INTERVAL: This is the maximum duration typically needed TIME_TO_LEARN_INTERVAL: This is the maximum duration typically needed
to learn all the IGP events related to a single component failure 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 (e.g., router failure, SRLG failure), e.g., 1 second. It's mostly
dependent on failure detection time variation between all routers dependent on failure detection time variation between all routers
that are adjacent to the failure. Additionally, it may depend on the that are adjacent to the failure. Additionally, it may depend on the
different IGP implementations across the network, related to different IGP implementations/parameters across the network, related
origination and flooding of their link state advertisements. to origination and flooding of their link state advertisements.
HOLDDOWN_INTERVAL: The time required with no received IGP events HOLDDOWN_INTERVAL: The time required with no received IGP events
before considering the IGP to be stable again and allowing the before considering the IGP to be stable again and allowing the
SPF_DELAY to be restored to INITIAL_SPF_DELAY. e.g., 3 seconds. 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
SPF_TIMER: The Finite State Machine (FSM) abstract timer that uses the TIME_TO_LEARN_INTERVAL.
the computed SPF delay. Upon expiration, the Route Table Computation
(as defined above) is performed.
4. Principles of SPF delay algorithm 4. Principles of SPF delay algorithm
For this first IGP event, we assume that there has been a single 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 simple change in the network which can be taken into account using a
single routing computation (e.g., link failure, prefix (metric) single routing computation (e.g., link failure, prefix (metric)
change) and we optimize for very fast convergence, delaying the change) and we optimize for very fast convergence, delaying the
routing computation by INITIAL_SPF_DELAY. Under this assumption, routing computation by INITIAL_SPF_DELAY. Under this assumption,
there is no benefit in delaying the routing computation. In a there is no benefit in delaying the routing computation. In a
typical network, this is the most common type of IGP event. Hence, typical network, this is the most common type of IGP event. Hence,
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failure routing table in a single route computation. In this failure routing table in a single route computation. In this
situation, we delay the routing computation by SHORT_SPF_DELAY. situation, we delay the routing computation by SHORT_SPF_DELAY.
If IGP events are still received after TIME_TO_LEARN_INTERVAL from If IGP events are still received after TIME_TO_LEARN_INTERVAL from
the initial IGP event received in QUIET state, then the network is the initial IGP event received in QUIET state, then the network is
presumably experiencing multiple independent failures. In this case, presumably experiencing multiple independent failures. In this case,
while waiting for network stability, the computations are delayed for while waiting for network stability, the computations are delayed for
a longer time represented by LONG_SPF_DELAY. This SPF delay is kept a longer time represented by LONG_SPF_DELAY. This SPF delay is kept
until no IGP events are received for HOLDDOWN_INTERVAL. until no IGP events are received for HOLDDOWN_INTERVAL.
Note that previous SPF delay algorithms used to count the number of Note that previously implemented SPF delay algorithms counted the
SPF computations. However, as all routers may receive the IGP events number of SPF computations. However, as all routers may receive the
at different times, we cannot assume that all routers will perform IGP events at different times, we cannot assume that all routers will
the same number of SPF computations or that they will schedule them perform the same number of SPF computations or that they will
at the same time. For example, assuming that the SPF delay is 50 ms, schedule them at the same time. For example, assuming that the SPF
router R1 may receive 3 IGP events (E1, E2, E3) in those 50 ms and delay is 50 ms, router R1 may receive 3 IGP events (E1, E2, E3) in
hence will perform a single routing computation. While another those 50 ms and hence will perform a single routing computation.
router R2 may only receive 2 events (E1, E2) in those 50 ms and hence While another router R2 may only receive 2 events (E1, E2) in those
will schedule another routing computation when receiving E3. That's 50 ms and hence will schedule another routing computation when
why this document uses a time (TIME_TO_LEARN) from the initial event receiving E3. That's why this document uses a time
detection/reception as opposed to counting the number of SPF (TIME_TO_LEARN_INTERVAL) from the initial event detection/reception
computations to determine when the IGP is unstable. as opposed to counting the number of SPF computations to determine
when the IGP is unstable.
5. Specification of the SPF delay state machine 5. Specification of the SPF delay state machine
5.1. States 5.1. States
This section describes the state machine. The naming and semantics This section describes the state machine. The naming and semantics
of each state corresponds directly to the SPF delay used for IGP of each state corresponds directly to the SPF delay used for IGP
events received in that state. Three states are defined: events received in that state. Three states are defined:
QUIET: This is the initial state, when no IGP events have occured for QUIET: This is the initial state, when no IGP events have occured for
at least HOLDDOWN_INTERVAL since the previous routing table at least HOLDDOWN_INTERVAL since the previous routing table
computation. The state is meant to handle link failures very computation. The state is meant to handle link failures very
quickly. quickly.
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SHORT_WAIT: State entered when an IGP event has been received in SHORT_WAIT: State entered when an IGP event has been received in
QUIET state. This state is meant to handle single component failure QUIET state. This state is meant to handle single component failure
requiring multiple IGP events (e.g., node, SRLG). requiring multiple IGP events (e.g., node, SRLG).
LONG_WAIT: State reached after TIME_TO_LEARN_INTERVAL. In other LONG_WAIT: State reached after TIME_TO_LEARN_INTERVAL. In other
words, state reached after TIME_TO_LEARN_INTERVAL in state words, state reached after TIME_TO_LEARN_INTERVAL in state
SHORT_WAIT. This state is meant to handle multiple independent SHORT_WAIT. This state is meant to handle multiple independent
component failures during periods of IGP instability. component failures during periods of IGP instability.
5.2. States Transitions 5.2. Timers
The FSM is initialized to the QUIET_STATE with all three timers SPF_TIMER: The Finite State Machine (FSM) abstract timer that uses
deactivated. The following diagram describes briefly the state the computed SPF delay. Upon expiration, the Route Table Computation
transitions. (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.
+-------------------+ +-------------------+
| |<-------------------+ +---->| |<-------------------+
| QUIET | | | | QUIET | |
| |<---------+ | +-----| |<---------+ |
+-------------------+ | | 7: +-------------------+ | |
| | | SPF_TIMER | | |
| | | expiration | | |
| 1: IGP event | | | 1: IGP event | |
| | | | | |
v | | v | |
+-------------------+ | | +-------------------+ | |
+---->| | | | +---->| | | |
| | SHORT_WAIT |----->----+ | | | SHORT_WAIT |----->----+ |
+-----| | | +-----| | |
2: +-------------------+ 6: HOLDDOWN_TIMER | 2: +-------------------+ 6: HOLDDOWN_TIMER |
IGP event | expiration | IGP event | expiration |
| | 8: SPF_TIMER | |
| | expiration | |
| 3: LEARN_TIMER | | 3: LEARN_TIMER |
| expiration | | expiration |
| | | |
v | v |
+-------------------+ | +-------------------+ |
+---->| | | +---->| | |
| | LONG_WAIT |------------>-------+ | | LONG_WAIT |------------>-------+
+-----| | +-----| |
4: +-------------------+ 5: HOLDDOWN_TIMER 4: +-------------------+ 5: HOLDDOWN_TIMER
IGP event expiration IGP event expiration
9: SPF_TIMER expiration
Figure 1: State Machine Figure 1: State Machine
5.3. FSM Events 5.4. FSM Events
This section describes the events and the actions performed in This section describes the events and the actions performed in
response. response.
Event 1: IGP event, while in QUIET_STATE. Transition 1: IGP event, while in QUIET_STATE.
Actions on event 1: Actions on event 1:
o If SPF_TIMER is not already running, start it with value o If SPF_TIMER is not already running, start it with value
INITIAL_SPF_DELAY. INITIAL_SPF_DELAY.
o Start LEARN_TIMER with TIME_TO_LEARN_INTERVAL. o Start LEARN_TIMER with TIME_TO_LEARN_INTERVAL.
o Start HOLDDOWN_TIMER with HOLDDOWN_INTERVAL. o Start HOLDDOWN_TIMER with HOLDDOWN_INTERVAL.
o Transition to SHORT_WAIT state. o Transition to SHORT_WAIT state.
Event 2: IGP event, while in SHORT_WAIT. Transition 2: IGP event, while in SHORT_WAIT.
Actions on event 2: Actions on event 2:
o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL. o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.
o If SPF_TIMER is not already running, start it with value o If SPF_TIMER is not already running, start it with value
SHORT_SPF_DELAY. SHORT_SPF_DELAY.
o Remain in current state. o Remain in current state.
Event 3: LEARN_TIMER expiration. Transition 3: LEARN_TIMER expiration.
Actions on event 3: Actions on event 3:
o Transition to LONG_WAIT state. o Transition to LONG_WAIT state.
Event 4: IGP event, while in LONG_WAIT. Transition 4: IGP event, while in LONG_WAIT.
Actions on event 4: Actions on event 4:
o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL. o Reset HOLDDOWN_TIMER to HOLDDOWN_INTERVAL.
o If SPF_TIMER is not already running, start it with value o If SPF_TIMER is not already running, start it with value
LONG_SPF_DELAY. LONG_SPF_DELAY.
o Remain in current state. o Remain in current state.
Event 5: HOLDDOWN_TIMER expiration, while in LONG_WAIT. Transition 5: HOLDDOWN_TIMER expiration, while in LONG_WAIT.
Actions on event 5: Actions on event 5:
o Transition to QUIET state. o Transition to QUIET state.
Event 6: HOLDDOWN_TIMER expiration, while in SHORT_WAIT. Transition 6: HOLDDOWN_TIMER expiration, while in SHORT_WAIT.
Actions on event 6: Actions on event 6:
o Deactivate LEARN_TIMER. o Deactivate LEARN_TIMER.
o Transition to QUIET state. o Transition to QUIET state.
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 6. Parameters
All the parameters MUST be configurable [I-D.ietf-isis-yang-isis-cfg] 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 [I-D.ietf-ospf-yang] at the protocol instance granularity. They MAY
be configurable at the area/level granularity. All the delays be configurable at the area/level granularity. All the delays
(INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY, (INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY,
TIME_TO_LEARN_INTERVAL, HOLDDOWN_INTERVAL) SHOULD be configurable at TIME_TO_LEARN_INTERVAL, HOLDDOWN_INTERVAL) SHOULD be configurable at
the millisecond granularity. They MUST be configurable at least at the millisecond granularity. They MUST be configurable at least at
the tenth of second granularity. The configurable range for all the the tenth of second granularity. The configurable range for all the
parameters SHOULD at least be from 0 milliseconds to 60 seconds. parameters SHOULD at least be from 0 milliseconds to 60 seconds.
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start with safe, i.e., longer timers. start with safe, i.e., longer timers.
For the standard algorithm to be effective in mitigating micro-loops, 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 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 the routers in the same area/level, have exactly the same configured
values. values.
7. Partial Deployment 7. Partial Deployment
In general, the SPF delay algorithm is only effective in mitigating In general, the SPF delay algorithm is only effective in mitigating
micro-loops if it is deployed on all routers in the IGP domain or, at micro-loops if it is deployed, with the same parameters, on all
least, all routers in an IGP area/level. The impact of partial routers, in the IGP domain or, at least, all routers in an IGP area/
deployment is based on the particular event, topology, and the SPF level. The impact of partial deployment is based on the particular
algorithm(s) used on other routers in the IGP area/level. In cases event, topology, and the SPF algorithm(s) used on other routers in
where the previous SPF algorithm was implemented uniformly, partial the IGP area/level. In cases where the previous SPF algorithm was
deployment will increase the frequency and duration of micro-loops. implemented uniformly, partial deployment will increase the frequency
Hence, it is RECOMMENDED that all routers in the IGP domain or at and duration of micro-loops. Hence, it is RECOMMENDED that all
least within the same area/level be migrated to the SPF algorithm routers in the IGP domain or at least within the same area/level be
described herein at roughly the same time. migrated to the SPF algorithm described herein at roughly the same
time.
Note that this is not a new consideration as over times, network Note that this is not a new consideration as over times, network
operators have changed SPF delay parameters in order to accommodate operators have changed SPF delay parameters in order to accommodate
new customer requirements for fast convergence, as permitted by new new customer requirements for fast convergence, as permitted by new
software and hardware. They may also have progressively replaced an software and hardware. They may also have progressively replaced an
implementation with a given SPF delay algorithm by another implementation with a given SPF delay algorithm by another
implementation with a different one. implementation with a different one.
8. Impact on micro-loops 8. Impact on micro-loops
Micro-loops during IGP convergence are due to a non-synchronized or Micro-loops during IGP convergence are due to a non-synchronized or
non-ordered update of the forwarding information tables (FIB) non-ordered update of the forwarding information tables (FIB)
[RFC5715] [RFC6976] [I-D.ietf-rtgwg-spf-uloop-pb-statement]. FIBs [RFC5715] [RFC6976] [I-D.ietf-rtgwg-spf-uloop-pb-statement]. FIBs
are installed after multiple steps such as SPF wait time, SPF are installed after multiple steps such as flooding of the IGP event
computation, FIB distribution, and FIB update. This document only across the network, SPF wait time, SPF computation, FIB distribution
addresses the first contribution. This standardized procedure across line cards, and FIB update. This document only addresses the
reduces the probability and/or duration of micro-loops when IGPs first contribution. This standardized procedure reduces the
experience multiple temporally close events. It does not prevent all probability and/or duration of micro-loops when IGPs experience
micro-loops. However, it is beneficial and is less complex and multiple temporally close events. It does not prevent all micro-
costly to implement when compared to full solutions such as [RFC5715] loops. However, it is beneficial and is less complex and costly to
or [RFC6976]. implement when compared to full solutions such as [RFC5715] or
[RFC6976].
9. IANA Considerations 9. IANA Considerations
No IANA actions required. No IANA actions required.
10. Security considerations 10. Security considerations
The algorithm presented in this document does not compromise IGP The algorithm presented in this document does not compromise IGP
security. An attacker having the ability to generate IGP events security. An attacker having the ability to generate IGP events
would be able to delay the IGP convergence time. The LONG_SPF_DELAY would be able to delay the IGP convergence time. The LONG_SPF_DELAY
skipping to change at page 11, line 10 skipping to change at page 11, line 39
[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>.
12.2. Informative References 12.2. Informative References
[I-D.ietf-isis-yang-isis-cfg] [I-D.ietf-isis-yang-isis-cfg]
Litkowski, S., Yeung, D., Lindem, A., Zhang, Z., and L. Litkowski, S., Yeung, D., Lindem, A., Zhang, Z., and L.
Lhotka, "YANG Data Model for IS-IS protocol", draft-ietf- Lhotka, "YANG Data Model for IS-IS protocol", draft-ietf-
isis-yang-isis-cfg-18 (work in progress), July 2017. isis-yang-isis-cfg-19 (work in progress), November 2017.
[I-D.ietf-ospf-yang] [I-D.ietf-ospf-yang]
Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
"Yang Data Model for OSPF Protocol", draft-ietf-ospf- "Yang Data Model for OSPF Protocol", draft-ietf-ospf-
yang-08 (work in progress), July 2017. yang-09 (work in progress), October 2017.
[I-D.ietf-rtgwg-spf-uloop-pb-statement] [I-D.ietf-rtgwg-spf-uloop-pb-statement]
Litkowski, S., Decraene, B., and M. Horneffer, "Link State Litkowski, S., Decraene, B., and M. Horneffer, "Link State
protocols SPF trigger and delay algorithm impact on IGP protocols SPF trigger and delay algorithm impact on IGP
micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-04 micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-05
(work in progress), May 2017. (work in progress), December 2017.
[ISO10589-Second-Edition] [ISO10589-Second-Edition]
International Organization for Standardization, 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)", ISO/ connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, Nov 2002. IEC 10589:2002, Second Edition, Nov 2002.
[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>.
[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 [RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>. 2010, <https://www.rfc-editor.org/info/rfc5715>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C., [RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <https://www.rfc-editor.org/info/rfc6976>. 2013, <https://www.rfc-editor.org/info/rfc6976>.
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