RFC7196
Internet Engineering Task Force (IETF) C. Pelsser Request for Comments: 7196 R. Bush Category: Standards Track Internet Initiative Japan ISSN: 2070-1721 K. Patel
Cisco Systems
P. Mohapatra
Sproute Networks
O. Maennel
Loughborough University
May 2014
Making Route Flap Damping Usable
Abstract
Route Flap Damping (RFD) was first proposed to reduce BGP churn in routers. Unfortunately, RFD was found to severely penalize sites for being well connected because topological richness amplifies the number of update messages exchanged. Many operators have turned RFD off. Based on experimental measurement, this document recommends adjusting a few RFD algorithmic constants and limits in order to reduce the high risks with RFD. The result is damping a non-trivial amount of long-term churn without penalizing well-behaved prefixes' normal convergence process.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7196.
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Contents
Introduction
Route Flap Damping (RFD) was first proposed (see [RIPE178] and RFC2439) and subsequently implemented to reduce BGP churn in routers. Unfortunately, RFD was found to severely penalize sites for being well connected because topological richness amplifies the number of update messages exchanged, see [MAO2002]. Subsequently, many operators turned RFD off; see [RIPE378]. Based on the measurements of [PELSSER2011], [RIPE580] now recommends that RFD is usable with some changes to the parameters. Based on the same measurements, this document recommends adjusting a few RFD algorithmic constants and limits. The result is damping of a non- trivial amount of long-term churn without penalizing well-behaved prefixes' normal convergence process.
Very few prefixes are responsible for a large amount of the BGP messages received by a router; see [HUSTON2006] and [PELSSER2011]. For example, the measurements in [PELSSER2011] showed that only 3% of
the prefixes were responsible for 36% percent of the BGP messages at a router with real feeds from a Tier-1 provider and an Internet Exchange Point during a one-week experiment. Only these very frequently flapping prefixes should be damped. The values recommended in Section 6 achieve this. Thus, RFD can be enabled, and some churn reduced.
The goal is to, with absolutely minimal change, ameliorate the danger of current RFD implementations and use. It is not a panacea, nor is it a deep and thorough approach to flap reduction.
Suggested Reading
It is assumed that the reader understands BGP RFC4271 and Route Flap Damping RFC2439. This work is based on the measurements in the paper [PELSSER2011]. A survey of Japanese operators' use of RFD and their desires is reported in [RFD-SURVEY].
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 RFC2119 only when they appear in all upper case. They may also appear in lower or mixed case as English words, without normative meaning.
RFD Parameters
The following RFD parameters are common to all implementations. Some may be tuned by the operator, some not. There is currently no consensus on a single set of default values.
+--------------------------+----------+-------+---------+
| Parameter | Tunable? | Cisco | Juniper |
+--------------------------+----------+-------+---------+
| Withdrawal | No | 1,000 | 1,000 |
| Re-Advertisement | No | 0 | 1,000 |
| Attribute Change | No | 500 | 500 |
| Suppress Threshold | Yes | 2,000 | 3,000 |
| Half-Life (min.) | Yes | 15 | 15 |
| Reuse Threshold | Yes | 750 | 750 |
| Max Suppress Time (min.) | Yes | 60 | 60 |
+--------------------------+----------+-------+---------+
Note: Values without units specified are dimensionless constants.
Table 1: Default RFD Parameters of Juniper and Cisco
Suppress Threshold versus Churn
By turning RFD back on with the values recommended in Section 6, churn is reduced. Moreover, with these values, prefixes going through normal convergence are generally not damped.
[PELSSER2011] estimates that, with a suppress threshold of 6,000, the BGP update rate is reduced by 19% compared to a situation without RFD enabled. [PELSSER2011] studies the number of prefixes damped over a week between September 29, 2010 and October 6, 2010. With this 6,000 suppress threshold, 90% fewer prefixes are damped compared to use of a 2,000 threshold. That is, far fewer well-behaved prefixes are damped.
Setting the suppress threshold to 12,000 leads to very few damped prefixes (0.22% of the prefixes were damped with a threshold of 12,000 in the experiments in [PELSSER2011], yielding an average hourly update reduction of 11% compared to not using RFD).
+---------------+-------------+--------------+----------------------+ | Suppress | Damped | % of Table | Update Rate (one- | | Threshold | Prefixes | Damped | hour bins) | +---------------+-------------+--------------+----------------------+ | 2,000 | 43,342 | 13.16% | 53.11% | | 4,000 | 11,253 | 3.42% | 74.16% | | 6,000 | 4,352 | 1.32% | 81.03% | | 8,000 | 2,104 | 0.64% | 84.85% | | 10,000 | 1,286 | 0.39% | 87.12% | | 12,000 | 720 | 0.22% | 88.74% | | 14,000 | 504 | 0.15% | 89.97% | | 16,000 | 353 | 0.11% | 91.01% | | 18,000 | 311 | 0.09% | 91.88% | | 20,000 | 261 | 0.08% | 92.69% | +---------------+-------------+--------------+----------------------+
Note: the current default Suppress Threshold (2,000) is overly
agressive.
Table 2: Damped Prefixes vs. Churn, from [PELSSER2011]
Maximum Penalty
It is important to understand that the parameters shown in Table 1 and the implementation's sampling rate impose an upper bound on the penalty value, which we can call the 'computed maximum penalty'.
In addition, BGP implementations have an internal constant, which we will call the 'maximum penalty', and the current computed penalty may not exceed it.
Recommendations
Use of the following values is recommended:
Router Maximum Penalty: The internal constant for the maximum
penalty value MUST be raised to at least 50,000.
Default Configurable Parameters: In order not to break existing
operational configurations, existing BGP implementations, including the examples in Table 1, SHOULD NOT change their default values.
Minimum Suppress Threshold: Operators that want damping that is much
less destructive than the current damping, but still somewhat aggressive, SHOULD configure the Suppress Threshold to no less than 6,000.
Conservative Suppress Threshold: Conservative operators SHOULD
configure the Suppress Threshold to no less than 12,000.
Calculate But Do Not Damp: Implementations MAY have a test mode
where the operator can see the results of a particular configuration without actually damping any prefixes. This will allow for fine-tuning of parameters without losing reachability.
Security Considerations
It is well known that an attacker can generate false flapping to cause a victim's prefix(es) to be damped.
As the recommendations merely change parameters to more conservative values, there should be no increase in risk. In fact, the parameter change to more conservative values should slightly mitigate the false-flap attack.
Acknowledgments
Nate Kushman initiated this work some years ago. Ron Bonica, Seiichi Kawamura, and Erik Muller contributed useful suggestions.
References
Normative References
[MAO2002] Mao, Z., Govidan, R., Varghese, G., and R. Katz, "Route
Flap Damping Exacerbates Internet Routing Convergence", In
Proceedings of SIGCOMM, August 2002,
<http://conferences.sigcomm.org/sigcomm/2002/papers/
routedampening.pdf>.
[PELSSER2011]
Pelsser, C., Maennel, O., Mohapatra, P., Bush, R., and K.
Patel, "Route Flap Damping Made Usable", PAM 2011: Passive
and Active Measurement Conference, March 2011,
<http://pam2011.gatech.edu/papers/pam2011--Pelsser.pdf>.
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC2439 Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
Flap Damping", RFC 2439, November 1998.
RFC4271 Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RIPE378] Smith, P. and P. Panigl, "RIPE Routing Working Group
Recommendations On Route-flap Damping", RIPE 378, May
2006, <http://www.ripe.net/ripe/docs/ripe-378>.
Informative References
[HUSTON2006]
Huston, G., "2005 - A BGP Year in Review", RIPE 52, 2006,
<http://meetings.ripe.net/ripe-52/presentations/
ripe52-plenary-bgp-review.pdf>.
[RFD-SURVEY]
Tsuchiya, S., Kawamura, S., Bush, R., and C. Pelsser,
"Route Flap Damping Deployment Status Survey", Work in
Progress, June 2012.
[RIPE178] Barber, T., Doran, S., Karrenberg, D., Panigl, C., and J.
Schmitz, "RIPE Routing-WG Recommendation for Coordinated
Route-flap Damping Parameters", RIPE 178, February 1998,
<http://www.ripe.net/ripe/docs/ripe-178>.
[RIPE580] Bush, R., Pelsser, C., Kuhne, M., Maennel, O., Mohapatra,
P., Patel, K., and R. Evans, "RIPE Routing Working Group
Recommendation for Route Flap Damping", RIPE 580, January
2013, <http://www.ripe.net/ripe/docs/ripe-580>.
Authors' Addresses
Cristel Pelsser Internet Initiative Japan Jinbocho Mitsui Buiding, 1-105 Kanda-Jinbocho, Chiyoda-ku, Tokyo 101-0051 JP
Phone: +81 3 5205 6464 EMail: [email protected]
Randy Bush Internet Initiative Japan 5147 Crystal Springs Bainbridge Island, Washington 98110 US
EMail: [email protected]
Keyur Patel Cisco Systems 170 W. Tasman Drive San Jose, CA 95134 US
EMail: [email protected]
Pradosh Mohapatra Sproute Networks 41529 Higgins Way Fremont, CA 94539 US
EMail: [email protected]
Olaf Maennel Loughborough University Department of Computer Science - N.2.03 Loughborough UK
Phone: +44 115 714 0042 EMail: [email protected]
