Link State protocols SPF trigger and delay algorithm impact on IGP micro-loops
draft-ietf-rtgwg-spf-uloop-pb-statement-07
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8541.
|
|
|---|---|---|---|
| Authors | Stephane Litkowski , Bruno Decraene , Martin Horneffer | ||
| Last updated | 2018-11-29 (Latest revision 2018-05-23) | ||
| Replaces | draft-litkowski-rtgwg-spf-uloop-pb-statement | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Chris Bowers | ||
| Shepherd write-up | Show Last changed 2018-05-28 | ||
| IESG | IESG state | Became RFC 8541 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Martin Vigoureux | ||
| Send notices to | Chris Bowers <chrisbowers.ietf@gmail.com> |
draft-ietf-rtgwg-spf-uloop-pb-statement-07
Routing Area Working Group S. Litkowski
Internet-Draft Orange Business Service
Intended status: Informational B. Decraene
Expires: November 24, 2018 Orange
M. Horneffer
Deutsche Telekom
May 23, 2018
Link State protocols SPF trigger and delay algorithm impact on IGP
micro-loops
draft-ietf-rtgwg-spf-uloop-pb-statement-07
Abstract
A micro-loop is a packet forwarding loop that may occur transiently
among two or more routers in a hop-by-hop packet forwarding paradigm.
In this document, we are trying to analyze the impact of using
different Link State IGP implementations in a single network, with
respect to micro-loops. The analysis is focused on the SPF delay
algorithm.
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."
This Internet-Draft will expire on November 24, 2018.
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Copyright Notice
Copyright (c) 2018 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/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem statement . . . . . . . . . . . . . . . . . . . . . . 4
3. SPF trigger strategies . . . . . . . . . . . . . . . . . . . 5
4. SPF delay strategies . . . . . . . . . . . . . . . . . . . . 5
4.1. Two steps SPF delay . . . . . . . . . . . . . . . . . . . 6
4.2. Exponential backoff . . . . . . . . . . . . . . . . . . . 6
5. Mixing strategies . . . . . . . . . . . . . . . . . . . . . . 7
6. Benefits of standardized SPF delay behavior . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Link State IGP protocols are based on a topology database on which
the SPF (Shortest Path First) algorithm is run to find a consistent
set of non-looping routing paths.
Specifications like IS-IS ([RFC1195]) propose some optimizations of
the route computation (See Appendix C.1) but not all the
implementations follow those non-mandatory optimizations.
We will call "SPF triggers", the events that would lead to a new SPF
computation based on the topology.
Link State IGP protocols, like OSPF ([RFC2328]) and IS-IS
([RFC1195]), are using multiple timers to control the router behavior
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in case of churn: SPF delay, PRC delay, LSP generation delay, LSP
flooding delay, LSP retransmission interval...
Some of those timers (values and behavior) are standardized in
protocol specifications, while some are not. The SPF computation
related timers have generally remained unspecified.
For non standardized timers, implementations are free to implement it
in any way. For some standardized timer, we can also see that rather
than using static configurable values for such timer, implementations
may offer dynamically adjusted timers to help controlling the churn.
We will call "SPF delay", the timer that exists in most
implementations that specifies the required delay before running SPF
computation after a SPF trigger is received.
A micro-loop is a packet forwarding loop that may occur transiently
among two or more routers in a hop-by-hop packet forwarding paradigm.
We can observe that these micro-loops are formed when two routers do
not update their Forwarding Information Base (FIB) for a certain
prefix at the same time. The micro-loop phenomenon is described in
[I-D.ietf-rtgwg-microloop-analysis].
Two micro-loop mitigation techniques have been defined by IETF.
[RFC6976] has not been widely implemented, presumably due to the
complexity of the technique. [I-D.ietf-rtgwg-uloop-delay]) has been
implemented. However, it does not prevent all micro-loops that can
occur for a given topology and failure scenario.
In multi-vendor networks, using different implementations of a link
state protocol may favor micro-loops creation during the convergence
process due to discrepancies of timers. Service Providers are
already aware to use similar timers (values and behavior) for all the
network as a best practice, but sometimes it is not possible due to
limitations of implementations.
This document will present why it sounds important for service
providers to have consistent implementations of Link State protocols
across vendors. We are particularly analyzing the impact of using
different Link State IGP implementations in a single network in
regards of micro-loops. The analysis is focused on the SPF delay
algorithm.
[I-D.ietf-rtgwg-backoff-algo] defines a solution that satisfies this
problem statement and this document captures the reasoning of the
provided solution.
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2. Problem statement
S ---- E
| |
10 | | 10
| |
D ---- A
| 2
Px
Figure 1 - Network topology suffering from micro-loops
In Figure 1, S uses primarily the SD link to reach the prefixes
behind D (Px). When the SD link fails, the IGP convergence occurs.
If S converges before E, S will forward the traffic to Px through E,
but as E has not converged yet, E will loop back traffic to S,
leading to a micro-loop.
The micro-loop appears due to the asynchronous convergence of nodes
in a network when an event occurs.
Multiple factors (or a combination of these factors) may increase the
probability for a micro-loop to appear:
o the delay of failure notification: the more E is advised of the
failure later than S, the more a micro-loop may have a chance to
appear.
o the SPF delay: most implementations support a delay for the SPF
computation to try to catch as many events as possible. If S uses
an SPF delay timer of x msec and E uses an SPF delay timer of y
msec and x < y, E would start converging after S leading to a
potential micro-loop.
o the SPF computation time: mostly a matter of CPU power and
optimizations like incremental SPF. If S computes its SPF faster
than E, there is a chance for a micro-loop to appear. CPUs are
today fast enough to consider SPF computation time as negligible
(on the order of milliseconds in a large network).
o the SPF computation order: an SPF trigger can be common to
multiple IGP areas or levels (e.g., IS-IS Level1/Level2) or for
multiple address families with multi-topologies. There is no
specified order for SPF computation today and it is implementation
dependent. In such scenarios, if the order of SPF computation
done in S and E for each area/level/topology/SPF-algorithm is
different, there is a possibility for a micro-loop to appear.
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o the RIB and FIB prefix insertion speed or ordering. This is
highly dependent on the implementation.
This document will focus on analysis of the SPF delay behavior and
associated triggers.
3. SPF trigger strategies
Depending on the change advertised in LSPDU or LSA, the topology may
be affected or not. An implementation may avoid running the SPF
computation (and may only run IP reachability computation instead) if
the advertised change does not affect the topology.
Different strategies exists to trigger the SPF computation:
1. An implementation may always run a full SPF for any type of
change.
2. An implementation may run a full SPF only when required. For
example, if a link fails, a local node will run an SPF for its
local LSP update. If the LSP from the neighbor (describing the
same failure) is received after SPF has started, the local node
can decide that a new full SPF is not required as the topology
has not change.
3. If the topology does not change, an implementation may only
recompute the IP reachability.
As noted in Section 1, SPF optimizations are not mandatory in
specifications. This has led to the implementation of different
strategies.
4. SPF delay strategies
Implementations of link state routing protocols use different
strategies to delay the SPF computation. The two most common SPF
delay behaviors are the following:
1. Two phase SPF delay.
2. Exponential backoff delay.
Those behavior will be explained in the next sections.
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4.1. Two steps SPF delay
The SPF delay is managed by four parameters:
o Rapid delay: amount of time to wait before running SPF, after the
initial SPF trigger event.
o Rapid runs: the number of consecutive SPF runs that can use the
rapid delay. When the number is exceeded, the delay moves to the
slow delay value.
o Slow delay: amount of time to wait before running SPF.
o Wait time: amount of time to wait without receiving SPF trigger
events before going back to the rapid delay.
Example: Rapid delay = 50msec, Rapid runs = 3, Slow delay = 1sec,
Wait time = 2sec
SPF delay time
^
|
|
SD- | x xx x
|
|
|
RD- | x x x x
|
+---------------------------------> Events
| | | | || | |
< wait time >
Figure 2 - Two phase delay algorithm
4.2. Exponential backoff
The algorithm has two modes: the fast mode and the backoff mode. In
the fast mode, the SPF delay is usually delayed by a very small
amount of time (fast reaction). When an SPF computation has run in
the fast mode, the algorithm automatically moves to the backoff mode
(a single SPF run is authorized in the fast mode). In the backoff
mode, the SPF delay is increasing exponentially at each run. When
the network becomes stable, the algorithm moves back to the fast
mode. The SPF delay is managed by four parameters:
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o First delay: amount of time to wait before running SPF. This
delay is used only when SPF is in fast mode.
o Incremental delay: amount of time to wait before running SPF.
This delay is used only when SPF is in backoff mode and increments
exponentially at each SPF run.
o Maximum delay: maximum amount of time to wait before running SPF.
o Wait time: amount of time to wait without events before going back
to the fast mode.
Example: First delay = 50msec, Incremental delay = 50msec, Maximum
delay = 1sec, Wait time = 2sec
SPF delay time
^
MD- | xx x
|
|
|
|
|
| x
|
|
|
| x
|
FD- | x x x
ID |
+---------------------------------> Events
| | | | || | |
< wait time >
FM->BM -------------------->FM
Figure 3 - Exponential delay algorithm
5. Mixing strategies
In Figure 1, we consider a flow of packet from S to D. We consider
that S is using optimized SPF triggering (Full SPF is triggered only
when necessary), and two steps SPF delay (rapid=150ms,rapid-runs=3,
slow=1s). As implementation of S is optimized, Partial Reachability
Computation (PRC) is available. We consider the same timers as SPF
for delaying PRC. We consider that E is using a SPF trigger strategy
that always compute a Full SPF for any change, and uses the
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exponential backoff strategy for SPF delay (start=150ms, inc=150ms,
max=1s)
We also consider the following sequence of events:
o t0=0 ms: a prefix is declared down in the network. We consider
this event to happen at time=0.
o 200ms: the prefix is declared as up.
o 400ms: a prefix is declared down in the network.
o 1000ms: S-D link fails.
+--------+--------------------+------------------+------------------+
| Time | Network Event | Router S events | Router E events |
+--------+--------------------+------------------+------------------+
| t0=0 | Prefix DOWN | | |
| 10ms | | Schedule PRC (in | Schedule SPF (in |
| | | 150ms) | 150ms) |
| | | | |
| | | | |
| 160ms | | PRC starts | SPF starts |
| 161ms | | PRC ends | |
| 162ms | | RIB/FIB starts | |
| 163ms | | | SPF ends |
| 164ms | | | RIB/FIB starts |
| 175ms | | RIB/FIB ends | |
| 178ms | | | RIB/FIB ends |
| | | | |
| 200ms | Prefix UP | | |
| 212ms | | Schedule PRC (in | |
| | | 150ms) | |
| 214ms | | | Schedule SPF (in |
| | | | 150ms) |
| | | | |
| | | | |
| 370ms | | PRC starts | |
| 372ms | | PRC ends | |
| 373ms | | | SPF starts |
| 373ms | | RIB/FIB starts | |
| 375ms | | | SPF ends |
| 376ms | | | RIB/FIB starts |
| 383ms | | RIB/FIB ends | |
| 385ms | | | RIB/FIB ends |
| | | | |
| 400ms | Prefix DOWN | | |
| 410ms | | Schedule PRC (in | Schedule SPF (in |
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| | | 300ms) | 300ms) |
| | | | |
| | | | |
| | | | |
| | | | |
| 710ms | | PRC starts | SPF starts |
| 711ms | | PRC ends | |
| 712ms | | RIB/FIB starts | |
| 713ms | | | SPF ends |
| 714ms | | | RIB/FIB starts |
| 716ms | | RIB/FIB ends | RIB/FIB ends |
| | | | |
| 1000ms | S-D link DOWN | | |
| 1010ms | | Schedule SPF (in | Schedule SPF (in |
| | | 150ms) | 600ms) |
| | | | |
| | | | |
| 1160ms | | SPF starts | |
| 1161ms | | SPF ends | |
| 1162ms | Micro-loop may | RIB/FIB starts | |
| | start from here | | |
| 1175ms | | RIB/FIB ends | |
| | | | |
| | | | |
| | | | |
| | | | |
| 1612ms | | | SPF starts |
| 1615ms | | | SPF ends |
| 1616ms | | | RIB/FIB starts |
| 1626ms | Micro-loop ends | | RIB/FIB ends |
+--------+--------------------+------------------+------------------+
Table 1 - Route computation when S and E use the different behaviors
and multiple events appear
In the Table 1, we can see that due to discrepancies in the SPF
management, after multiple events of a different type, the values of
the SPF delay are completely misaligned between node S and node E,
leading to the creation of micro-loops.
The same issue can also appear with only a single type of event as
shown below:
+--------+--------------------+------------------+------------------+
| Time | Network Event | Router S events | Router E events |
+--------+--------------------+------------------+------------------+
| t0=0 | Link DOWN | | |
| 10ms | | Schedule SPF (in | Schedule SPF (in |
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| | | 150ms) | 150ms) |
| | | | |
| | | | |
| 160ms | | SPF starts | SPF starts |
| 161ms | | SPF ends | |
| 162ms | | RIB/FIB starts | |
| 163ms | | | SPF ends |
| 164ms | | | RIB/FIB starts |
| 175ms | | RIB/FIB ends | |
| 178ms | | | RIB/FIB ends |
| | | | |
| 200ms | Link DOWN | | |
| 212ms | | Schedule SPF (in | |
| | | 150ms) | |
| 214ms | | | Schedule SPF (in |
| | | | 150ms) |
| | | | |
| | | | |
| 370ms | | SPF starts | |
| 372ms | | SPF ends | |
| 373ms | | | SPF starts |
| 373ms | | RIB/FIB starts | |
| 375ms | | | SPF ends |
| 376ms | | | RIB/FIB starts |
| 383ms | | RIB/FIB ends | |
| 385ms | | | RIB/FIB ends |
| | | | |
| 400ms | Link DOWN | | |
| 410ms | | Schedule SPF (in | Schedule SPF (in |
| | | 150ms) | 300ms) |
| | | | |
| | | | |
| 560ms | | SPF starts | |
| 561ms | | SPF ends | |
| 562ms | Micro-loop may | RIB/FIB starts | |
| | start from here | | |
| 568ms | | RIB/FIB ends | |
| | | | |
| | | | |
| 710ms | | | SPF starts |
| 713ms | | | SPF ends |
| 714ms | | | RIB/FIB starts |
| 716ms | Micro-loop ends | | RIB/FIB ends |
| | | | |
| 1000ms | Link DOWN | | |
| 1010ms | | Schedule SPF (in | Schedule SPF (in |
| | | 1s) | 600ms) |
| | | | |
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| | | | |
| | | | |
| | | | |
| 1612ms | | | SPF starts |
| 1615ms | | | SPF ends |
| 1616ms | Micro-loop may | | RIB/FIB starts |
| | start from here | | |
| 1626ms | | | RIB/FIB ends |
| | | | |
| | | | |
| | | | |
| | | | |
| 2012ms | | SPF starts | |
| 2014ms | | SPF ends | |
| 2015ms | | RIB/FIB starts | |
| 2025ms | Micro-loop ends | RIB/FIB ends | |
| | | | |
| | | | |
+--------+--------------------+------------------+------------------+
Table 2 - Route computation upon multiple link down events when S and
E use the different behaviors
6. Benefits of standardized SPF delay behavior
Using the same event sequence as in Table 1, we may expect fewer and/
or shorter micro-loops using a standardized SPF delay.
+--------+--------------------+------------------+------------------+
| Time | Network Event | Router S events | Router E events |
+--------+--------------------+------------------+------------------+
| t0=0 | Prefix DOWN | | |
| 10ms | | Schedule PRC (in | Schedule PRC (in |
| | | 150ms) | 150ms) |
| | | | |
| | | | |
| 160ms | | PRC starts | PRC starts |
| 161ms | | PRC ends | |
| 162ms | | RIB/FIB starts | PRC ends |
| 163ms | | | RIB/FIB starts |
| 175ms | | RIB/FIB ends | |
| 176ms | | | RIB/FIB ends |
| | | | |
| 200ms | Prefix UP | | |
| 212ms | | Schedule PRC (in | |
| | | 150ms) | |
| 213ms | | | Schedule PRC (in |
| | | | 150ms) |
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| | | | |
| | | | |
| 370ms | | PRC starts | PRC starts |
| 372ms | | PRC ends | |
| 373ms | | RIB/FIB starts | PRC ends |
| 374ms | | | RIB/FIB starts |
| 383ms | | RIB/FIB ends | |
| 384ms | | | RIB/FIB ends |
| | | | |
| 400ms | Prefix DOWN | | |
| 410ms | | Schedule PRC (in | Schedule PRC (in |
| | | 300ms) | 300ms) |
| | | | |
| | | | |
| | | | |
| | | | |
| 710ms | | PRC starts | PRC starts |
| 711ms | | PRC ends | PRC ends |
| 712ms | | RIB/FIB starts | |
| 713ms | | | RIB/FIB starts |
| 716ms | | RIB/FIB ends | RIB/FIB ends |
| | | | |
| 1000ms | S-D link DOWN | | |
| 1010ms | | Schedule SPF (in | Schedule SPF (in |
| | | 150ms) | 150ms) |
| | | | |
| | | | |
| 1160ms | | SPF starts | |
| 1161ms | | SPF ends | SPF starts |
| 1162ms | Micro-loop may | RIB/FIB starts | SPF ends |
| | start from here | | |
| 1163ms | | | RIB/FIB starts |
| 1175ms | | RIB/FIB ends | |
| 1177ms | Micro-loop ends | | RIB/FIB ends |
+--------+--------------------+------------------+------------------+
Table 3 - Route computation when S and E use the same standardized
behavior
As displayed above, there could be some other parameters like router
computation power, flooding timers that may also influence micro-
loops. In all the examples in this document comparing the SPF timer
behavior of router S and router E, we have made router E a bit slower
than router S. This can lead to micro-loops even when both S and E
use a common standardized SPF behavior. However, we expect that by
aligning implementations of the SPF delay, service providers may
reduce the number and the duration of micro-loops.
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7. Security Considerations
This document does not introduce any security consideration.
8. Acknowledgements
Authors would like to thank Mike Shand and Chris Bowers for their
useful comments.
9. IANA Considerations
This document has no action for IANA.
10. References
10.1. Normative References
[I-D.ietf-rtgwg-backoff-algo]
Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "SPF Back-off Delay algorithm
for link state IGPs", draft-ietf-rtgwg-backoff-algo-10
(work in progress), March 2018.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[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>.
10.2. Informative References
[I-D.ietf-rtgwg-microloop-analysis]
Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", draft-ietf-rtgwg-microloop-
analysis-01 (work in progress), October 2005.
[I-D.ietf-rtgwg-uloop-delay]
Litkowski, S., Decraene, B., Filsfils, C., and P.
Francois, "Micro-loop prevention by introducing a local
convergence delay", draft-ietf-rtgwg-uloop-delay-09 (work
in progress), November 2017.
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[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
Stephane Litkowski
Orange Business Service
Email: stephane.litkowski@orange.com
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Martin Horneffer
Deutsche Telekom
Email: martin.horneffer@telekom.de
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