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Versions: (draft-jholland-cb-assisted-cc) 00

Mboned                                                        J. Holland
Internet-Draft                                 Akamai Technologies, Inc.
Intended status: Standards Track                      September 26, 2019
Expires: March 29, 2020


              Circuit Breaker Assisted Congestion Control
                     draft-jholland-mboned-cbacc-00

Abstract

   This document specifies Circuit Breaker Assisted Congestion Control
   (CBACC).  CBACC enables fast-trip Circuit Breakers by publishing rate
   metadata about multicast channels from senders to intermediate
   network nodes or receivers.  The circuit breaker behavior is defined
   as a supplement to receiver driven congestion control systems, to
   preserve network health if receivers subscribe to a volume of traffic
   that exceeds capacity policies or capability for a network or
   receiver.

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 March 29, 2020.

Copyright Notice

   Copyright (c) 2019 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



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   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
     1.1.  Background and Terminology  . . . . . . . . . . . . . . .   3
   2.  Circuit Breaker Behavior  . . . . . . . . . . . . . . . . . .   4
     2.1.  Functional Components . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Ingress Meter . . . . . . . . . . . . . . . . . . . .   4
       2.1.2.  Egress Meter  . . . . . . . . . . . . . . . . . . . .   4
       2.1.3.  Communication Method  . . . . . . . . . . . . . . . .   5
       2.1.4.  Measurement Function  . . . . . . . . . . . . . . . .   5
       2.1.5.  Trigger Function  . . . . . . . . . . . . . . . . . .   6
       2.1.6.  Reaction  . . . . . . . . . . . . . . . . . . . . . .   7
       2.1.7.  Feedback Control Mechanism  . . . . . . . . . . . . .   7
     2.2.  States  . . . . . . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  Interface State . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Flow State  . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Implementation Design Considerations  . . . . . . . . . .   8
       2.3.1.  Oversubscription Thresholds . . . . . . . . . . . . .   8
       2.3.2.  Fairness Functions  . . . . . . . . . . . . . . . . .   9
   3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Module  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  YANG Module Names Registry  . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     5.1.  Metadata Security . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Denial of Service . . . . . . . . . . . . . . . . . . . .  11
       5.2.1.  State Overload  . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Overjoining  . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document defines Circuit Breaker Assisted Congestion Control
   (CBACC).  CBACC defines a Network Transport Circuit Breaker (CB), as
   described by [RFC8084].

   The CB behavior defined in this document uses bit-rate metadata about
   multicast data streams, coupled with policy, capacity, and load
   information at a network node, to prune multicast channels so that



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   the node's aggregate capacity is not exceeded by the subscribed
   channels.

   To communicate the required metadata, this document defines a YANG
   [RFC7950] module that augments the DORMS
   [I-D.draft-jholland-mboned-dorms-00] YANG module.  DORMS provides a
   mechanism for senders to publish metadata about the multicast streams
   they're sending through a RESTCONF service, so that receivers or
   forwarding nodes can discover and consume the metadata with a set of
   standard methods.  The metadata MAY be communicated to receivers or
   forwarding nodes by some other method, but the definition of any
   alternative methods is out of scope for this document.

   The CB behavior defined in this document matches the description
   provided in Section 3.2.3 of [RFC8084] of a unidirectional CB over a
   controlled path.  The control messages from that description are
   composed of the messages containing the metadata required for
   operation of the CB.

   CBACC is designed to supplement protocols that use multicast IP and
   rely on well-behaved receivers to achieve congestion control.
   Examples of congestion control systems fitting this description
   include [PLM], [RLM], [RLC], [FLID-DL], [SMCC], and WEBRC [RFC3738].

   CBACC addresses a problem with "overjoining" by untrusted receivers.

   In an overjoining condition, receivers (either malicious,
   misconfigured, or with implementation errors) subscribe to multicast
   channels but do not respond appropriately to congestion.  When
   sufficient multicast traffic is available for subscription by such
   receivers, this can overload any network.

   The overjoining problem is relevant to misbehaving receivers for both
   receiver-driven and feedback-driven congestion control strategies, as
   described in Section 4.1 of [RFC8085].

   Overjoining attacks and the challenges they present are discussed in
   more detail in Appendix A.

   CBACC offers a solution for the recommendation in Section 4 of
   [RFC8085] that circuit breaker solutions be used even where
   congestion control is optional.

1.1.  Background and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP



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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Circuit Breaker Behavior

2.1.  Functional Components

   This section maps the functional components described in Section 3.1
   of [RFC8084] to the operational components of the CBACC CB defined by
   this document.

2.1.1.  Ingress Meter

   The metadata provides an ingress meter in the form of an advertised
   maximum data bit-rate, namely the "max-bits-per-second" field in the
   YANG model in Section 3.  This is a self-report by the sender about
   the maximum amount of traffic a sender will send within the interval
   given by the "data-rate-window" field, which is the measurement
   interval.

   The sender MUST NOT send more data for a data stream than the amount
   of data implied by its advertised data rate within any measurement
   window, and it's RECOMMENDED for the sender to provide some margin to
   account for forwarding bursts.  If an egress node observes a higher
   data rate within any measurement window, it MAY circuit-break that
   flow immediately.

2.1.2.  Egress Meter

   The node implementing the CB behavior has access to several pieces of
   information that can be used as relevant egress metrics:

   1.  Physical capacity limits on each interface.

   2.  Configured capacity limits for multicast traffic for each
       interface, if any.

   3.  The observed received data rates of subscribed multicast channels
       with CBACC metadata.

   4.  The observed received data rates of subscribed multicast channels
       without CBACC metadata.

   5.  The observed received data rates of competing non-multicast
       traffic.

   6.  The loss rate for subscribed multicast channels, when available.
       The loss rate is only sometimes observable at an egress node; for



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       example, when using AMBI [I-D.draft-jholland-mboned-ambi-03], or
       when the data stream carries a protocol that is known to the
       egress node by some out of band means, and whose traffic can be
       monitored for loss.  When available, the loss rates may be used.

   Note that any on-path router can be considered an egress node for
   purposes of this CB, even though it may be forwarding traffic
   downstream, and even though other egress points may also be operating
   a downstream CB that covers the same data stream.  Components in the
   receiving devices, such as an operating system or browser can also
   act as an egress node, as can a receiving application.

2.1.3.  Communication Method

   CBACC operates at an egress node, so the egress metrics in
   Section 2.1.2 are available through system calls, or by communication
   with various locally deployable system monitoring applications.  Any
   suitable application that provides the necessary egress meter is
   appropriate.

   The communication path defined in this document for the information
   from the ingress meter is the use of DORMS
   [I-D.draft-jholland-mboned-dorms-00].  Other methods MAY be used as
   well, but are out of scope for this document.

2.1.4.  Measurement Function

   The measurement function maintains a few values for each interface,
   computed from the egress and ingress meter values:

   1.  The aggregate advertised maximum bit-rate capacity consumed by
       CBACC data streams.  This is the sum of the max-bit-rate values
       in the CBACC metadata for all data streams subscribed through an
       interface

   2.  An oversubscription threshold for each interface.  The
       oversubscription threshold will be determined differently for CBs
       in different contexts.  In some network devices, it might be as
       simple as an administratively configured absolute value or
       proportion of an interface's capacity.  For other situations,
       like a CB operating in a context with loss visibility, it could
       be a dynamically changing value that grows when data streams are
       successfully subscribed and receiving data without loss, and
       shrinks as loss is observed across subscribed data streams.  The
       oversubscription threshold calculation could also incorporate
       other information like out-of-band path capacity measurements
       with [PathChirp], if available.




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       This document covers some non-normative examples of valid
       oversubscription threshold functions in Section 2.3.1, but in
       general, the oversubscription threshold is the primary parameter
       that different CBs in different contexts can tune to provide the
       safety guarantees necessary for their context.

2.1.5.  Trigger Function

   The trigger function fires when the aggregate advertised maximum bit-
   rate exceeds the oversubscription threshold for any interface.

   When oversubscribed, the trigger function changes the states of
   subscribed channels to "blocked" until the aggregate subscribed bit-
   rate is below the oversubscription threshold again.

2.1.5.1.  Fairness and Inter-flow Ordering

   The trigger function orders the monitored flows according to a
   fairness function, and blocks flows in order as needed to ensure that
   only a safe level of bandwidth can be consumed by subscribed flows.
   The fairness function can be different for CBs in different contexts.

   Flows from a single sender MUST be ordered according to their
   priority field from the CBACC metadata when compared with each other.
   Between-sender flows and flows from the same sender with the same
   priority are ordered according to the fairness function.  Where flows
   from the same sender have a priority order that conflicts with the
   ordering the fairness function would use, it's appropriate to treat
   those out of order flows from the sender as an aggregate flow for
   between-sender flow comparisons.  (TBD: the aggregation algorithm
   probably needs more explaining and good examples.)

   A CB implementation SHOULD provide mechanisms for administrative
   controls to configure explicit biases, as this may be necessary to
   support Service Level Agreements for specific events or providers, or
   to blacklist or de-prioritize channels with historically known
   misbehavior.

   Subject to the above constraints, where possible the default fairness
   behavior SHOULD favor streams with many receivers over streams with
   few receivers, and streams with a low bit-rate over streams with a
   high bit-rate.  For example, when receiver count is known, a good
   fairness metric is max-bandwidth divided by receiver-count.
   (Receiver count in some networks can be known through technologies
   such as the experimental PIM extension for population count described
   in [RFC6807], or other custom signaling methods.)





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   An overview of some other approaches to appropriate fairness metrics
   is given in Section 2.3 of [RFC5166].

2.1.6.  Reaction

   When the trigger function fires and a subscribed channel becomes
   blocked, the reaction depends on whether it's an upstream interface
   or a downstream interface.

   If a channel is blocked on one downstream interface, it may still be
   unblocked on other downstream interfaces.  When this is the case,
   traffic is simply not forwarded along blocked interfaces, even though
   clients might still be joined.

   When a channel is blocked on all downstream interfaces, or when the
   upstream interface is oversubscribed, the channel is pruned so that
   data no longer arrives from the network on the upstream interface, by
   a PIM prune (Section 3.5 of [RFC7761]), or a "leave" operation with
   IGMP, MLD, or another multicast signaling mechanism.

   Once initially circuit-broken, a flow SHOULD remain circuit-broken
   for no less than 3 minutes, even if space clears up, to ensure
   downstream subscriptions will notice and respond.  (3 minutes is
   chosen to exceed the default maximum lifetime of 2 minutes that can
   occur if an IGMP responder suddenly stops operation, and ceases
   responding to IGMP queries with membership reports.)

   When enough capacity is available for a circuit-broken stream to be
   unblocked and the circuit-breaker hold-down time is expired, the
   flows SHOULD be unblocked according to the priority order.

2.1.7.  Feedback Control Mechanism

   The metadata should be refreshed as needed to maintain up to date
   values.  When using DORMS and RESTCONF, the HTTP Cache Control
   headers provide valid refresh time properties from the server, and
   SHOULD be used if present.  If No-Cache is used, the default refresh
   timing SHOULD be 30 seconds plus a random value between 0 and 10
   seconds.

2.2.  States

2.2.1.  Interface State

   A CB holds the following state for each interface, for both the
   inbound and outbound directions on that interface:





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   o  aggregate bandwidth: The sum of the bandwidths of all non-
      circuit-broken CBACC flows which transit this interface in this
      direction.

   o  bandwidth limit: The maximum aggregate CBACC advertised bandwidth
      allowed, not including circuit-broken flows.

      When reducing the bandwidth limit due to congestion, the circuit
      breaker SHOULD NOT reduce the limit by more than half its value in
      10 seconds, and SHOULD use a smoothing function to reduce the
      limit gradually over time.

      It is RECOMMENDED that no more than half the capacity for a link
      be allocated to CBACC flows if the link might be shared with TCP
      or other traffic that is responsive to congestion.

2.2.2.  Flow State

   Data streams with CBACC metadata have a state for the upstream
   interface through which the stream is joined:

   o  'subscribed' Indicates that the circuit breaker is subscribed
      upstream to the flow and forwarding packets through zero or more
      egress interfaces.

   o  'pruned' Indicates that the flow has been circuit-broken.  A
      request to unsubscribe from the flow has been sent upstream, e.g.
      a PIM prune (Section 3.5 of [RFC7761]) or a "leave" operation via
      IGMP, MLD, or another group membership management mechanism.

   Data streams also have a per-interface state for downstream
   interfaces with subscribers, where the data is being forwarded.  It's
   one of:

   o  'forwarding' Indicates that the flow is a non-circuit-broken flow
      in steady state, forwarding packets downstream.

   o  'blocked' Indicates that data packets for this flow are NOT
      forwarded downstream via this interface.

2.3.  Implementation Design Considerations

2.3.1.  Oversubscription Thresholds

   TBD.






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2.3.2.  Fairness Functions

   TBD.

3.  YANG Module

3.1.  Tree Diagram

   module: ietf-cbacc
     augment /dorms:metadata/dorms:sender/dorms:group/dorms:udp-stream:
       +--rw cbacc!
          +--rw max-bits-per-second    uint32
          +--rw max-mss?               uint16
          +--rw data-rate-window?      uint32
          +--rw priority?              uint16


3.2.  Module

<CODE BEGINS> file ietf-cbacc@2019-09-26.yang
module ietf-cbacc {
    yang-version 1.1;

    namespace "urn:ietf:params:xml:ns:yang:ietf-cbacc";
    prefix "ambi";

    import ietf-dorms {
        prefix "dorms";
        reference "I-D.jholland-mboned-dorms";
    }

    organization "IETF";

    contact
        "Author:   Jake Holland
                   <mailto:jholland@akamai.com>
        ";

    description
        "This module contains the definition for the AMBI anchor
         message data type.

         Copyright (c) 2019 IETF Trust and the persons identified as
         authors of the code.  All rights reserved.

         Redistribution and use in source and binary forms, with or
         without modification, is permitted pursuant to, and subject
         to the license terms contained in, the Simplified BSD License



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         set forth in Section 4.c of the IETF Trust's Legal Provisions
         Relating to IETF Documents
         (http://trustee.ietf.org/license-info).

         This version of this YANG module is part of
         draft-jholland-mboned-cbacc, [TBD: change]
         see the internet draft itself for full legal notices.";

    revision 2019-07-31 {
        description "Initial revision as an extension.";
        reference
          "";
    }

    augment
      "/dorms:metadata/dorms:sender/dorms:group/dorms:udp-stream" {
        description "Definition of the manifest stream providing
            integrity info for the data stream";

      container cbacc {
        presence "cbacc-enabled flow";
        description "Information to enable fast-trip circuit breakers";
        leaf max-bits-per-second {
            type uint32;
            mandatory true;
            description "Maximum bitrate for this stream, in Kilobits
                of IP packet data (including headers) of native
                multicast traffic per second";
        }
        leaf max-mss {
            type uint16;
            default 1400;
            description "Maximum payload size, in bytes";
        }
        leaf data-rate-window {
            type uint32;
            default 2000;
            description "Time window over which data rate is guaranteed,
                in milliseconds.";
            /* TBD: range limits? */
        }
        leaf priority {
            type uint16;
            default 256;
            description "The relative preference level for keeping this
                flow compared to other flows from this sender (higher
                value is more preferred to keep)";
        }



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      }
    }
}

<CODE ENDS>

4.  IANA Considerations

4.1.  YANG Module Names Registry

   This document adds one YANG module to the "YANG Module Names"
   registry maintained at <https://www.iana.org/assignments/yang-
   parameters>.  The following registrations are made, per the format in
   Section 14 of [RFC6020]:

         name:      ietf-cbacc
         namespace: urn:ietf:params:xml:ns:yang:ietf-cbacc
         prefix:    cbacc
         reference: I-D.draft-jholland-mboned-cbacc

5.  Security Considerations

5.1.  Metadata Security

   Be sure to authenticate the metadata.  See DORMS security
   considerations, and don't accept unauthenticated metadata if using an
   alternative means.

5.2.  Denial of Service

5.2.1.  State Overload

   Since CBACC flows require state, it may be possible for a set of
   receivers and/or senders, possibly acting in concert, to generate
   many flows in an attempt to overflow the circuit breakers' state
   tables.

   It is permissible for a network node to behave as a CBACC circuit
   breaker for some CBACC flows while treating other CBACC flows as non-
   CBACC, as part of a load balancing strategy for the network as a
   whole, or simply as defense against this concern when the number of
   monitored flows exceeds some threshold.

   The same techniques described in Section 3.1 of [RFC4609] can be used
   to help mitigate this attack, for much the same reasons.  It is
   RECOMMENDED that network operators implement measures to mitigate
   such attacks.




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6.  Acknowledgements

   Many thanks to Devin Anderson, Ben Kaduk, Cheng Jin, Scott Brown,
   Miroslav Ponec, Bob Briscoe, Lenny Giuliani, and Christian Worm
   Mortensen for their thoughtful comments and contributions.

7.  References

7.1.  Normative References

   [I-D.draft-jholland-mboned-ambi-03]
              Holland, J. and K. Rose, "Asymmetric Manifest Based
              Integrity", draft-jholland-mboned-ambi-03 (work in
              progress), September 2019.

   [I-D.draft-jholland-mboned-dorms-00]
              Holland, J., "Discovery Of Restconf Metadata for Source-
              specific multicast", draft-jholland-mboned-dorms-00 (work
              in progress), August 2019.

   [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>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8084]  Fairhurst, G., "Network Transport Circuit Breakers",
              BCP 208, RFC 8084, DOI 10.17487/RFC8084, March 2017,
              <https://www.rfc-editor.org/info/rfc8084>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [FLID-DL]  Byers, J., Horn, G., Luby, M., Mitzenmacher, M., Shaver,
              W., and IEEE, "FLID-DL: congestion control for layered
              multicast", DOI 10.1109/JSAC.2002.803998, n.d.,
              <https://ieeexplore.ieee.org/document/1038584>.




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   [PathChirp]
              Ribeiro, V., Riedi, R., Baraniuk, R., Navratil, J.,
              Cottrell, L., Department of Electrical and Computer
              Engineering Rice University, and SLAC/SCS-Network
              Monitoring, Stanford University, "pathChirp: Efficient
              Available Bandwidth Estimation for Network Paths", 2003.

   [PLM]      Biersack, Institut EURECOM, A., "PLM: Fast Convergence for
              Cumulative Layered Multicast Transmission Schemes", 1999.

   [RFC3738]  Luby, M. and V. Goyal, "Wave and Equation Based Rate
              Control (WEBRC) Building Block", RFC 3738,
              DOI 10.17487/RFC3738, April 2004,
              <https://www.rfc-editor.org/info/rfc3738>.

   [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,
              <https://www.rfc-editor.org/info/rfc4609>.

   [RFC5166]  Floyd, S., Ed., "Metrics for the Evaluation of Congestion
              Control Mechanisms", RFC 5166, DOI 10.17487/RFC5166, March
              2008, <https://www.rfc-editor.org/info/rfc5166>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6807]  Farinacci, D., Shepherd, G., Venaas, S., and Y. Cai,
              "Population Count Extensions to Protocol Independent
              Multicast (PIM)", RFC 6807, DOI 10.17487/RFC6807, December
              2012, <https://www.rfc-editor.org/info/rfc6807>.

   [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, <https://www.rfc-editor.org/info/rfc7761>.

   [RLC]      Rizzo, L., Vicisano, L., and J. Crowcroft, "The RLC
              multicast congestion control algorithm", 1999.

   [RLM]      McCanne, S., Jacobson, V., Vetterli, M., University of
              California, Berkeley, and Lawrence Berkeley National
              Laboratory, "Receiver-driven Layered Multicast", 1995.




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   [SMCC]     Kwon, G., Byers, J., and Computer Science Department,
              Boston University, "Smooth Multirate Multicast Congestion
              Control", 2002.

Appendix A.  Overjoining

   [RFC8085] describes several remedies for unicast congestion control
   under UDP, even though UDP does not itself provide congestion
   control.  In general, any network node under congestion could in
   theory collect evidence that a unicast flow's sending rate is not
   responding to congestion, and would then be justified in circuit-
   breaking it.

   With multicast IP, the situation is different, especially in the
   presence of malicious receivers.  A well-behaved sender using a
   receiver-controlled congestion scheme such as WEBRC does not reduce
   its send rate in response to congestion, instead relying on receivers
   to leave the appropriate multicast groups.

   This leads to a situation where, when a network accepts inter-domain
   multicast traffic, as long as there are senders somewhere in the
   world with aggregate bandwidth that exceeds a network's capacity,
   receivers in that network can join the flows and overflow the network
   capacity.  A receiver controlled by an attacker could do this at the
   IGMP/MLD level without running the application layer protocol that
   participates in the receiver-controlled congestion control.

   A network might be able to detect and defend against the most naive
   version of such an attack by blocking end users that try to join too
   many flows at once.  However, an attacker can achieve the same effect
   by joining a few high-bandwidth flows, if those exist anywhere, and
   an attacker that controls a few machines in a network can coordinate
   the receivers so they join disjoint sets of non-responsive sending
   flows.

   This scenario will produce congestion in a middle node in the network
   that can't be easily detected at the edge where the IGMP/MLD join is
   accepted.  Thus, an attacker with a small set of machines in a target
   network can always trip a circuit breaker if present, or can induce
   excessive congestion among the bandwidth allocated to multicast.
   This problem gets worse as more multicast flows become available.

   Although the same can apply to non-responsive unicast traffic,
   network operators can assume that non-responsive sending flows are in
   violation of congestion control best practices, and can therefore cut
   off flows associated with the misbehaving senders.  By contrast, non-
   responsive multicast senders are likely to be well-behaved
   participants in receiver-controlled congestion control schemes.



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   However, receiver controlled congestion control schemes also show the
   most promise for efficient massive scale content distribution via
   multicast, provided network health can be ensured.  Therefore,
   mechanisms to mitigate overjoining attacks while still permitting
   receiver-controlled congestion control are necessary.

Author's Address

   Jake Holland
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144
   United States of America

   Email: jakeholland.net@gmail.com




































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