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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Suggested Reading . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  RFD Parameters  . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Suppress Threshold versus Churn . . . . . . . . . . . . . . .   4
   5.  Maximum Penalty . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6

1. 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.

1.1. 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].

2. 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.

3. 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

4. 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]

5. 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.

6. 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.

7. 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.

8. Acknowledgments

Nate Kushman initiated this work some years ago. Ron Bonica, Seiichi Kawamura, and Erik Muller contributed useful suggestions.

9. References

9.1. 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>.

9.2. 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: cristel@iij.ad.jp
   
   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   US

EMail:

          randy@psg.com
   
   Keyur Patel
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   US

EMail:

          keyupate@cisco.com
   
   Pradosh Mohapatra
   Sproute Networks
   41529 Higgins Way
   Fremont, CA  94539
   US

EMail:

          mpradosh@yahoo.com

Olaf Maennel
Loughborough University
Department of Computer Science - N.2.03
Loughborough
UK

   Phone: +44 115 714 0042
   EMail: o@maennel.net