Internet Engineering Task Force (IETF) T. Morin, Ed.

Internet Engineering Task Force (IETF) T. Morin, Ed.
Request for Comments: 7899 S. Litkowski
Updates: 6514 Orange
Category: Standards Track K. Patel
ISSN: 2070-1721 Cisco Systems
Z. Zhang
R. Kebler
J. Haas
Juniper Networks
June 2016

Multicast VPN State Damping

Abstract

This document describes procedures to damp Multicast VPN (MVPN)
routing state changes and control the effect of the churn due to the
multicast dynamicity in customer sites. The procedures described in
this document are applicable to BGP-based multicast VPN and help
avoid uncontrolled control-plane load increase in the core routing
infrastructure. The new procedures proposed were inspired by BGP
unicast route damping principles that have been adapted to multicast.

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 7841.

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/rfc7899.

Morin, et al. Standards Track [Page 1]

RFC 7899 MVPN Damping June 2016

Copyright Notice

Copyright (c) 2016 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Existing Mechanisms . . . . . . . . . . . . . . . . . . . . . 5
4.1. Rate-Limiting Multicast Control Traffic . . . . . . . . . 5
4.2. Existing PIM, IGMP, and MLD Timers . . . . . . . . . . . 6
4.3. BGP Route Damping . . . . . . . . . . . . . . . . . . . . 6
5. Procedures for Multicast State Damping . . . . . . . . . . . 7
5.1. PIM Procedures . . . . . . . . . . . . . . . . . . . . . 7
5.2. Procedures for Multicast VPN State Damping . . . . . . . 10
6. Procedures for P-Tunnel State Damping . . . . . . . . . . . . 12
6.1. Damping MVPN P-Tunnel Change Events . . . . . . . . . . . 12
6.2. Procedures for Ethernet VPNs . . . . . . . . . . . . . . 13
7. Operational Considerations . . . . . . . . . . . . . . . . . 13
7.1. Enabling Multicast Damping . . . . . . . . . . . . . . . 13
7.2. Troubleshooting and Monitoring . . . . . . . . . . . . . 13
7.3. Default and Maximum Values . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18

Morin, et al. Standards Track [Page 2]

RFC 7899 MVPN Damping June 2016

1. Introduction

In a multicast VPN [RFC6513] deployed with BGP-based procedures
[RFC6514], when receivers in VPN sites join and leave a given
multicast group or channel through multicast membership control
protocols (Internet Group Management Protocol (IGMP) [RFC3376] and
Multicast Listener Discovery (MLD) [RFC3810]), multicast routing
protocols accordingly adjust multicast routing states and P-multicast
tree states to forward or prune multicast traffic to these receivers.
Similar challenges arise in the context of the multicast
specification for Virtual Private LAN Service (VPLS) [RFC7117].

In VPN contexts, providing isolation between customers of a shared
infrastructure is a core requirement resulting in stringent
expectations with regard to risks of denial-of-service attacks.

By nature, multicast memberships change based on the behavior of
multicast applications running on end hosts. Hence, the frequency of
membership changes can legitimately be much higher than the typical
churn of unicast routing states.

Therefore, mechanisms need to be put in place to ensure that the load
put on the BGP control plane, and on the P-tunnel setup control
plane, remains under control regardless of the frequency at which
multicast membership changes are made by end hosts.

This document describes procedures inspired by existing BGP route
damping [RFC2439] that are aimed at offering means to set an upper
bound to the amount of processing for the MVPN control-plane
protocols: more precisely, the BGP control plane in [RFC6514], the
P-tunnel control-plane protocol in the contexts of [RFC6514], and the
multicast specification for VPLS [RFC7117]. The goal of setting this
upper bound is pursued simultaneous with the goal of preserving the
service provided (delivering the multicast stream as requested by
Customer Edge devices), although at the expense of a minimal increase
of average bandwidth use in the provider network). The upper bound
to the control-plane load due to the processing of a given multicast
state is controlled indirectly via configurable parameters.

Section 16 of [RFC6514] specifically spells out the need for damping
the activity of C-multicast and Leaf Auto-discovery routes and
outlines how to do it by "delaying the advertisement of withdrawals
of C-multicast routes". This specification provides appropriate
detail on how to implement this approach and how to provide control
to the operator; for this reason, it is an update to [RFC6514].

Morin, et al. Standards Track [Page 3]

RFC 7899 MVPN Damping June 2016

The base principle of this specification is described in Section 3.
Existing mechanisms that could be relied upon are discussed in
Section 4. Section 5 details the procedures introduced by this
specification.

Section 6 provides specific details related to the damping of
multicast VPNs P-tunnel state.

Finally, Section 7 discusses operational considerations related to
the proposed mechanism.

2. Terminology

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 RFC 2119 [RFC2119].

This document reuses terminology from [RFC7761] and [RFC6514].

In this specification, damping of a multicast state will be said to
be "active" or "inactive". Note that in [RFC2439], the term used for
a unicast route that is dampened is "suppressed", but we will avoid
this term in this specification given that the proposed solution
consists in holding active a damped multicast state.

3. Overview

The procedures described in this document allow the network operator
to configure multicast VPN PEs (Provider Edge routers) so that they
can delay the propagation of multicast state prune messages between
PEs when faced with a rate of multicast state dynamicity exceeding a
certain configurable threshold. Assuming that the number of
multicast states that can be created by a receiver is bounded,
delaying the propagation of multicast state pruning results in
setting up an upper bound to the average frequency at which the
router will send state updates to an upstream router.

From the point of view of a downstream router, such as a CE (Customer
Edge router), this approach has no impact: the multicast routing
state changes that it solicits to its PE will be honored without any
additional delay. Indeed, the propagation of Joins is not impacted
by the procedures specified here, and having the upstream router
delay state prune propagation to its own upstream router does not
affect what traffic is sent to the downstream router. In particular,
the amount of bandwidth used on the PE-CE link downstream to a PE
applying this damping technique is not increased.

Morin, et al. Standards Track [Page 4]

RFC 7899 MVPN Damping June 2016

This approach increases the average bandwidth utilization on a link
upstream to a PE applying this technique, such as a PE-PE link:
indeed, a given multicast flow will be forwarded for a longer time
than if no damping was applied. That said, it is expected that this
technique will meet the goals of protecting the multicast routing
infrastructure control plane without a significant average increase
of bandwidth; for instance, damping events happening at a frequency
higher than one event per X seconds can be done without increasing by
more than X seconds the time during which a multicast flow is present
on a link.

That said, simulation of the exponential decay algorithm shows that
the multicast state churn can be drastically reduced without
significantly increasing the duration for which multicast traffic is
forwarded. Hence, using this technique will efficiently protect the
multicast routing infrastructure control plane against the issues
described here without a significant average increase of bandwidth.
The exception will be a scenario with strict constraints on multicast
bandwidth, where extending the time a multicast flow is forwarded
would result in congestion.

To be practical, such a mechanism requires configurability. In
particular, means are required to control when damping is triggered
and to allow delaying the pruning of a multicast state for a time
increasing with the churn of this multicast state. This will let the
operator control the trade-off made between minimizing the dynamicity
and reducing bandwidth consumption.

4. Existing Mechanisms

This section describes mechanisms that could be considered to address
the issue but that end up appearing as not suitable or not efficient
enough.

4.1. Rate-Limiting Multicast Control Traffic

The Protocol Independent Multicast - Sparse Mode (PIM-SM)
specification [RFC7761] examines multicast security threats and,
among other things, the risk of denial-of-service attacks described
in Section 1. A mechanism relying on rate-limiting PIM messages is
proposed in Section 5.3.3 of [RFC4609] but has the identified
drawbacks of impacting the service delivered and having side-effects
on legitimate users.

Morin, et al. Standards Track [Page 5]

RFC 7899 MVPN Damping June 2016

4.2. Existing PIM, IGMP, and MLD Timers

In the context of PIM multicast routing protocols [RFC7761], a
mechanism exists that may offer a form of de facto damping of
multicast states, under some conditions. Indeed, when active, the
prune override mechanism consists in having a PIM upstream router
introduce a delay ("prune override interval") before taking into
account a PIM Prune message sent by a downstream neighbor.

This mechanism has not been designed specifically for the purpose of
damping multicast state, but as a means to allow PIM to operate on
multi-access networks. See Section 4.3.3 of [RFC7761]. However,
when active, this mechanism will prevent a downstream router from
producing multicast routing protocol messages that would cause, for a
given multicast state, the upstream router to send to its own
upstream router multicast routing protocol messages at a rate higher
than 1/[JP_Override_Interval]. This provides a form of de facto
damping.

Similarly, the IGMP and MLD multicast membership control protocols
can provide a similar behavior under the right conditions.

These mechanisms are not considered suitable to meet the goals
spelled out in Section 1, the main reasons being that:

o when enabled, these mechanisms require additional bandwidth on the
local link on which the effect of a prune is delayed (in our case,
the PE-CE link);

o when enabled, these mechanisms require disabling explicit tracking
(see Section 4.3.3 of [RFC7761]), even though enabling this
feature may otherwise be desired;

o on certain implementations, these mechanisms are incompatible with
behaviors that cannot be turned off (e.g., implementation applying
a fast-leave behavior on interfaces with only two neighbors);

o they do not provide a suitable level of configurability; and

o they do not provide a way to discriminate between multicast flows
based on estimation of their dynamicity.

4.3. BGP Route Damping

The procedures defined in [RFC2439] and [RFC7196] for BGP route flap
damping are useful for operators who want to control the impact of
unicast route churn on the routing infrastructure and offer a
standardized set of parameters to control damping.

Morin, et al. Standards Track [Page 6]

RFC 7899 MVPN Damping June 2016

These procedures are not directly relevant in a multicast context for
the following reasons:

o they are not specified for multicast routing protocol in general,
and

o even in contexts where BGP routes are used to carry multicast
routing states (e.g., [RFC6514]), these procedures do not allow
the implementation of the principle described in this document;
the main reason being that a damped route becomes suppressed while
the target behavior would be to keep advertising when damping is
triggered on a multicast route.

However, the set of parameters standardized to control the thresholds
of the exponential decay mechanism can be relevantly reused. This is
the approach proposed for the procedures described in this document
(Section 5). Motivations for doing so are to help the network
operator deploy this feature based on consistent configuration
parameters and to obtain predictable results without the drawbacks of
relying on rate-limiting multicast control protocol exchanges (as is
exposed in Section 4.1) or on the use of existing PIM/IGMP timers (as
is exposed in Section 4.2).

5. Procedures for Multicast State Damping

5.1. PIM Procedures

This section describes procedures for multicast state damping
satisfying the goals spelled out in Section 1. This section
describes procedures for (S,G) states in the PIM-SM protocol
[RFC7761]; they apply unchanged for such states created based on
multicast group management protocols (IGMP [RFC3376], MLD [RFC3810])
on downstream interfaces. The same procedures are applied to (*,G)
states in the context of PIM-SM Any-Source Multicast (ASM) groups
(damping is not applied to (S,G,Rpt) Prune state).

The following notions of [RFC2439] are reused in these procedures:

figure-of-merit: A number reflecting the current estimation of
recent past activity of an (S,G) multicast routing state, which
increases based on routing events related to this state and
decreases between these events following an exponential decay
function (see below); the activation or inactivation of damping on
the state is based on this number. This number is associated with
the upstream state machine for (S,G) and is initialized to a value
of zero on state creation.

Morin, et al. Standards Track [Page 7]

RFC 7899 MVPN Damping June 2016

exponential decay function: A mathematical function as defined in
Section 2.3 of [RFC2439] (ignoring the first paragraph of the
section, as it does not apply here).

decay-half-life: The duration used to control how fast the
exponential decay of the *figure-of-merit* is; this parameter of
the exponential decay function is the time duration during which
the *figure-of-merit* will be reduced by half when in the absence
of a routing event (configurable parameter).

cutoff-threshold: The value of the *figure-of-merit* over which
damping is applied (configurable parameter).

reuse-threshold: The value of the *figure-of-merit* under which
damping stops being applied (configurable parameter).

In addition to these values, a configurable *increment-factor*
parameter is introduced that controls by how much the *figure-of-
merit* is incremented on multicast state update events.

Section 7.3 proposes default and maximum values for the configurable
parameters.

On reception of updated multicast membership or routing information
on a downstream interface I for a given (S,G) state, which results in
a change of the state of the PIM downstream state machine (see
Section 4.5.3 of [RFC7761]), a router implementing these procedures
MUST:

o apply procedures of [RFC7761] unchanged, for everything relating
to what multicast traffic ends up being sent on downstream
interfaces, including interface I

o update the *figure-of-merit* following the exponential decay
algorithm

o increase the *figure-of-merit* for the (S,G) by the *increment-
factor*

o update the damping state for the (S,G) state: damping becomes
active on the state if the recomputed *figure-of-merit* is
strictly above the configured *cutoff-threshold*:

* if damping remains inactive on (S,G) state, update the upstream
state machine as usual (as per Section 4.5.7 of [RFC7761]).

Morin, et al. Standards Track [Page 8]

RFC 7899 MVPN Damping June 2016

* if damping becomes active for the (S,G) state:

+ if the received message has caused the upstream state
machine to transition to Joined state, update the upstream
state machine for (S,G) applying usual PIM procedures in
Section 4.5.7 of [RFC7761] and including sending a PIM Join
to the upstream neighbor

+ if the received message has caused the upstream state
machine to transition to NotJoined state, do not update the
upstream state machine for (S,G)

+ hold the upstream state machine in Joined state until the
reuse threshold is reached: for the purpose of updating this
state machine, events that may result in updating the state
based on [RFC7761] SHOULD be ignored until the *reuse-
threshold* is reached. The effect is that in the meantime,
while PIM Join messages may be sent as refreshes to the
upstream neighbor, no PIM Prune message will be sent.

* if damping was already active, do not update the upstream state
machine for (S,G); the upstream state machine was frozen after
processing the previous message.

Once the *figure-of-merit* for (S,G) damping state decays to a value
strictly below the configured *reuse-threshold*, the upstream state
machine for (S,G) is recomputed based on states of downstream state
machines, eventually leading to a PIM Join or Prune message to be
sent to the upstream neighbor.

Given the specificity of multicast applications, it is REQUIRED for
the implementation to let the operator configure the *decay-half-
life* in seconds, rather than in minutes.

This specification does not impose the use of a particular technique
to update the *figure-of-merit* following the exponential decay
controlled by the configured *decay-half-life*. For instance, the
same techniques as the ones described in [RFC2439] can be applied.
The only requirement is that the *figure-of-merit* has to be updated
prior to increasing it and that its decay below the *reuse-threshold*
has to be reacted upon in a timely manner: in particular, if the
recomputation is done with a fixed time granularity, this granularity
should be low enough to not significantly delay the inactivation of
damping on a multicast state beyond what the operator wanted to
configure (e.g., for a *decay-half-life* of 10s, recomputing the
*figure-of-merit* each minute would result in a multicast state
remaining damped for a much longer time than specified).

Morin, et al. Standards Track [Page 9]

RFC 7899 MVPN Damping June 2016

PIM implementations typically follow the suggestion from Section 4.1
of [RFC7761] that:

implementations will only maintain state when it is relevant to
forwarding operations - for example, the 'NoInfo' state might be
assumed from the lack of other state information, rather than
being held explicitly.

To properly implement damping procedures, an implementation MUST keep
an explicit (S,G) state as long as damping is active on an (S,G).
Once an (S,G) state expires, and damping becomes inactive on this
state, its associated *figure-of-merit* and damping state are removed
as well.

Note that these procedures:

o do not impact PIM procedures related to refreshes or expiration of
multicast routing states: PIM Prune messages triggered by the
expiration of the (S,G) keep-alive timer are not suppressed or
delayed, and the reception of Join messages not causing transition
of state on the downstream interface does not lead to incrementing
the *figure-of-merit*;

o do not impact the PIM Assert mechanism: in particular, PIM Prune
messages triggered by a change of the PIM Assert winner on the
upstream interface are not suppressed or delayed;

o do not impact PIM Prune messages that are sent when the RPF
neighbor is updated for a given multicast flow; and

o do not impact PIM Prune messages that are sent in the context of
switching between a Rendezvous Point Tree and a Shortest Path
Tree.

Note also that no action is triggered based on the reception of PIM
Prune messages (or corresponding IGMP/MLD messages) that relate to
non-existing (S,G) state: in particular, no *figure-of-merit* or
damping state is created in this case.

5.2. Procedures for Multicast VPN State Damping

The procedures described in Section 5.1 can be applied in the Virtual
Routing and Forwarding (VRF) PIM-SM implementation (in the "C-PIM
instance"), with the corresponding action to suppressing the emission
of a Prune(S,G) message being to not withdraw the C-multicast Source
Tree Join (C-S,C-G) BGP route. An implementation of [RFC6513]
relying on the use of PIM to carry C-multicast routing information
MUST support this technique to be compliant with this specification.

Morin, et al. Standards Track [Page 10]

RFC 7899 MVPN Damping June 2016

In the context of [RFC6514], where BGP is used to distribute
C-multicast routing information, the following procedure is proposed
as an alternative to the procedures in Section 5.1 and consists in
applying damping in the BGP implementation based on existing BGP
damping mechanisms applied to C-multicast Source Tree Join routes and
Shared Tree Join routes (and as well to Leaf A-D routes - see
Section 6) and modified to implement the behavior described in
Section 3 along the following guidelines:

o not withdrawing (instead of not advertising) damped routes;

o providing means to configure the *decay-half-life* in seconds if
that option is not already available; and

o using parameters for the exponential decay that are specific to
multicast based on default values and multicast-specific
configuration.

While these procedures would typically be implemented on PE routers,
in a context where BGP Route Reflectors (RRs) [RFC4456] are used it
can be considered useful to also be able to apply damping on RRs as
well to provide additional protection against activity created behind
multiple PEs. Additionally, for MVPN Inter-AS deployments, it can be
needed to protect one Autonomous System (AS) from the dynamicity of
multicast VPN routing events from other ASes.

The choice to implement damping based on BGP routes or the procedures
described in Section 5.1 is up to the implementor, but at least one
of the two MUST be implemented. In the perspective of allowing
damping to be done on RRs and Autonomous System Border Routers
(ASBRs), implementing the BGP approach is recommended.

When not all routers in a deployment have the capability to drop
traffic coming from the wrong PE (as spelled out in Section 9.1.1 of
[RFC6513]), then the withdrawal of a C-multicast route resulting from
a change in the Upstream Multicast Hop or Upstream Multicast PE
SHOULD NOT be damped. An implementation of this specification MUST
do at least one of the two following things:

o not damp these withdrawals by default, and/or

o provide a tuning knob to disable the damping of these withdrawals.

Additionally, in such a deployment context, it is RECOMMENDED not to
enable any multicast VPN route damping on RRs and ASBRs since these
types of equipment cannot distinguish the event having caused a
C-multicast to be withdrawn.

Morin, et al. Standards Track [Page 11]

RFC 7899 MVPN Damping June 2016

Note well that it is out of scope of this section to consider the
application of these damping techniques on MVPN BGP routes other than
C-multicast routes.

6. Procedures for P-Tunnel State Damping

6.1. Damping MVPN P-Tunnel Change Events

When selective P-tunnels are used (see Section 7 of [RFC6513]), the
effect of updating the upstream state machine for a given (C-S,C-G)
state on a PE connected to multicast receivers is not only to
generate activity to propagate C-multicast routing information to the
source connected PE, but also to possibly trigger changes related to
the P-tunnels carrying (C-S,C-G) traffic. Protecting the provider
network from an excessive amount of change in the state of P-tunnels
is required, and this section details how this can be done.

A PE implementing these procedures for MVPN MUST damp Leaf A-D routes
in the same manner as it would for C-multicast routes (see
Section 5.2).

A PE implementing these procedures for MVPN MUST damp the activity
related to removing itself from a P-tunnel. Possible ways to do so
depend on the type of P-tunnel, and local implementation details are
left up to the implementor.

The following is proposed as an example of how the above can be
achieved:

o For P-tunnels implemented with the PIM protocol, this consists in
applying multicast state damping techniques described in
Section 5.1 to the P-PIM instance, at least for (S,G) states
corresponding to P-tunnels.

o For P-tunnels implemented with multipoint LDP (mLDP), this
consists in applying damping techniques completely similar to the
one described in Section 5 but generalized to apply to mLDP
states.

o For root-initiated P-tunnels (P-tunnels implemented with the
Point-to-Multipoint (P2MP) RSVP-TE, or relying on ingress
replication), no particular action needs to be implemented to damp
P-tunnels membership, if the activity of Leaf A-D route themselves
is damped.

Morin, et al. Standards Track [Page 12]

RFC 7899 MVPN Damping June 2016

o Another possibility is to base the decision to join or not join
the P-tunnel to which a given (C-S,C-G) is bound and to advertise
or not advertise a Leaf A-D route related to (C-S,C-G) based on
whether or not a C-multicast Source Tree Join route is being
advertised for (C-S,C-G) rather than by relying on the state of
the C-PIM Upstream state machine for (C-S,C-G).

6.2. Procedures for Ethernet VPNs

Specifications exist to support or optimize multicast and broadcast
in the context of Ethernet VPNs [RFC7117] relying on the use of
Selective P-Multicast Service Interface (S-PMSI) and P-tunnels. For
the same reasons as for IP multicast VPNs, an implementation of
[RFC7117] MUST follow the procedures described in Section 6.1 to be
compliant with this specification.

7. Operational Considerations

7.1. Enabling Multicast Damping

In the context of multicast VPNs, these procedures would be enabled
on PE routers. Additionally, in the case of C-multicast routing
based on BGP extensions ([RFC6514]), these procedures can be enabled
on ASBRs and RRs.

7.2. Troubleshooting and Monitoring

Implementing the damping mechanisms described in this document should
be complemented by appropriate tools to observe and troubleshoot
damping activity.

Complementing the existing interface providing information on
multicast states with information on eventual damping of
corresponding states (e.g., Multicast Routing Information Base (MRIB)
states) is RECOMMENDED for C-multicast routing states and P-tunnel
states.

7.3. Default and Maximum Values

Considering that, by design, multicast streams will be delivered
unchanged to the end user independent of the value chosen for the
configurable parameters, and that the only trade-off being made is an
increase of bandwidth use, the default and maximum values do not have
to be perfectly tuned.

This section proposes default and maximum values that are
conservative, so as to not significantly impact network dimensioning
but still prevent multicast state churn going beyond what can be

Morin, et al. Standards Track [Page 13]

RFC 7899 MVPN Damping June 2016

considered a reasonably low churn for a multicast state (see below
for illustrations in order of magnitude of the effect of these
values).

The following values are RECOMMENDED to be adopted as default values:

o *increment-factor*: 1000

o *cutoff-threshold*: 3000

o *decay-half-life*: 10s

o *reuse-threshold*: 1500

For unicast damping, it is common to set an upper bound to the time
during which a route is suppressed. In the case of multicast state
damping, which relies on not withdrawing a damped route, it may be
desirable to avoid a situation where a multicast flow would keep
flowing in a portion of the network for a very long time in the
absence of receivers.

The proposed default maximum value for the *figure-of-merit* is
20x*increment-factor*, i.e., 20000 with the proposed default
*increment-factor* of 1000.

As illustrations, with these values:

o a multicast state updated less frequently than once every 6 s will
not be damped at all;

o a multicast state changing once per second for 3 s, and then not
changing, will not be damped;

o a multicast state changing once per second for 4 s, and then not
changing, will be damped after the fourth change for approximately
13 s;

o a multicast state changing twice per second for 15 s, and then not
changing, will be damped after the fourth change for approximately
50 s; and

o a multicast state changing at a fast pace for a long time will
reach the maximum of *figure-of-merit*; once the activity on this
state stops, corresponding traffic may still flow in the network
for approximately 37 s before dampening stops being active.

Morin, et al. Standards Track [Page 14]

RFC 7899 MVPN Damping June 2016

The following values are proposed as maximums:

o *decay-half-life*: 60 s

o *cutoff-threshold*: 50000

More aggressive protection against the risk of denial of service can
be achieved by increasing the *increment-factor* or the
*decay-half-life*, or by reducing the *cutoff-threshold* and/or
*reuse-threshold*.

8. Security Considerations

The procedures defined in this document do not introduce additional
security issues not already present in the contexts addressed and
actually aim at addressing some of the identified risks without
introducing as much denial-of-service risk as some of the mechanisms
already defined.

The protection provided relates to the control plane of the multicast
routing protocols, including the components implementing the routing
protocols and the components responsible for updating the multicast
forwarding plane.

The procedures described are meant to provide some level of
protection for the router on which they are enabled by reducing the
amount of routing state updates that it needs to send to its upstream
neighbor or peers but do not provide any reduction of the control-
plane load related to processing routing information from downstream
neighbors. Protecting routers from an increase in control-plane load
due to activity on downstream interfaces toward core routers (or in
the context of BGP-based MVPN C-multicast routing, BGP peers) relies
on the activation of damping on corresponding downstream neighbors
(or BGP peers) and/or at the edge of the network. Protecting routers
from an increase in control-plane load due to activity on customer-
facing downstream interfaces or downstream interfaces to routers in
another administrative domain is out of the scope of this document
and should use already defined mechanisms (see [RFC4609]).

To be effective, the procedures described here must be complemented
by configuration limiting the number of multicast states that can be
created on a multicast router through protocol interactions with
multicast receivers, neighbor routers in adjacent ASes, or in
multicast VPN contexts with multicast CEs. Note well that the two
mechanisms may interact: the state for which Prune has been requested
may still remain taken into account for some time if damping has been
triggered and hence result in an otherwise acceptable new state from
being successfully created.

Morin, et al. Standards Track [Page 15]

RFC 7899 MVPN Damping June 2016

Additionally, it is worth noting that these procedures are not meant
to protect against peaks of control-plane load but only address
averaged load. For instance, assuming a set of multicast states are
submitted to the same Join/Prune events, damping can prevent more
than a certain number of Join/Prune messages to be sent upstream in
the period of time that elapses between the reception of Join/Prune
messages triggering the activation of damping on these states and
when damping becomes inactive after decay.

9. References

9.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
Flap Damping", RFC 2439, DOI 10.17487/RFC2439, November
1998, <http://www.rfc-editor.org/info/rfc2439>.

[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<http://www.rfc-editor.org/info/rfc3376>.

[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<http://www.rfc-editor.org/info/rfc3810>.

[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <http://www.rfc-editor.org/info/rfc6513>.

[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<http://www.rfc-editor.org/info/rfc6514>.

[RFC7117] Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
C. Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, DOI 10.17487/RFC7117, February 2014,
<http://www.rfc-editor.org/info/rfc7117>.

Morin, et al. Standards Track [Page 16]

RFC 7899 MVPN Damping June 2016

[RFC7196] Pelsser, C., Bush, R., Patel, K., Mohapatra, P., and O.
Maennel, "Making Route Flap Damping Usable", RFC 7196,
DOI 10.17487/RFC7196, May 2014,
<http://www.rfc-editor.org/info/rfc7196>.

[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <http://www.rfc-editor.org/info/rfc7761>.

9.2. Informative References

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.

[RFC4609] Savola, P., Lehtonen, R., and D. Meyer, "Protocol
Independent Multicast - Sparse Mode (PIM-SM) Multicast
Routing Security Issues and Enhancements", RFC 4609,
DOI 10.17487/RFC4609, October 2006,
<http://www.rfc-editor.org/info/rfc4609>.

Acknowledgements

We would like to thank Bruno Decraene and Lenny Giuliano for
discussions that helped shape this proposal. We would also like to
thank Yakov Rekhter and Eric Rosen for their reviews and helpful
comments. Thanks to Wim Henderickx for his comments and support of
this proposal. Additionally, Martin Vigoureux, Gunter Van De Velde,
and Alvaro Retana provided useful comments to finalize the document.

Authors' Addresses

Thomas Morin (editor)
Orange
2, avenue Pierre Marzin
Lannion 22307
France

Email: [email protected]

Morin, et al. Standards Track [Page 17]

RFC 7899 MVPN Damping June 2016

Stephane Litkowski
Orange
France

Email: [email protected]

Keyur Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America

Email: [email protected]

Zhaohui Zhang
Juniper Networks Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America

Email: [email protected]

Robert Kebler
Juniper Networks Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America

Email: [email protected]

Jeffrey Haas
Juniper Networks

Email: [email protected]

Morin, et al. Standards Track [Page 18]