TSVWG Working Group                                         G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Best Current Practice                    April 04, 2016
Expires: October 6, 2016

                   Network Transport Circuit Breakers


   This document explains what is meant by the term "network transport
   Circuit Breaker" (CB).  It describes the need for circuit breakers
   for network tunnels and applications when using non-congestion-
   controlled traffic, and explains where circuit breakers are, and are
   not, needed.  It also defines requirements for building a circuit
   breaker and the expected outcomes of using a circuit breaker within
   the Internet.

Status of This Memo

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   This Internet-Draft will expire on October 6, 2016.

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   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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Types of Circuit Breaker  . . . . . . . . . . . . . . . .   6
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Design of a Circuit-Breaker (What makes a good circuit
       breaker?) . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Functional Components . . . . . . . . . . . . . . . . . .   7
     3.2.  Other network topologies  . . . . . . . . . . . . . . . .  10
       3.2.1.  Use with a multicast control/routing protocol . . . .  10
       3.2.2.  Use with control protocols supporting pre-provisioned
               capacity  . . . . . . . . . . . . . . . . . . . . . .  11
       3.2.3.  Unidirectional Circuit Breakers over Controlled Paths  12
   4.  Requirements for a Network Transport Circuit Breaker  . . . .  12
   5.  Examples of Circuit Breakers  . . . . . . . . . . . . . . . .  15
     5.1.  A Fast-Trip Circuit Breaker . . . . . . . . . . . . . . .  15
       5.1.1.  A Fast-Trip Circuit Breaker for RTP . . . . . . . . .  16
     5.2.  A Slow-trip Circuit Breaker . . . . . . . . . . . . . . .  17
     5.3.  A Managed Circuit Breaker . . . . . . . . . . . . . . . .  17
       5.3.1.  A Managed Circuit Breaker for SAToP Pseudo-Wires  . .  17
       5.3.2.  A Managed Circuit Breaker for Pseudowires (PWs) . . .  18
   6.  Examples where circuit breakers may not be needed.  . . . . .  19
     6.1.  CBs over pre-provisioned Capacity . . . . . . . . . . . .  19
     6.2.  CBs with tunnels carrying Congestion-Controlled Traffic .  19
     6.3.  CBs with Uni-directional Traffic and no Control Path  . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   10. Revision Notes  . . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   The term "Circuit Breaker" originates in electricity supply, and has
   nothing to do with network circuits or virtual circuits.  In
   electricity supply, a Circuit Breaker is intended as a protection
   mechanism of last resort.  Under normal circumstances, a Circuit
   Breaker ought not to be triggered; it is designed to protect the
   supply network and attached equipment when there is overload.  People
   do not expect an electrical circuit-breaker (or fuse) in their home
   to be triggered, except when there is a wiring fault or a problem
   with an electrical appliance.

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   In networking, the Circuit Breaker (CB) principle can be used as a
   protection mechanism of last resort to avoid persistent excessive
   congestion impacting other flows that share network capacity.
   Persistent congestion was a feature of the early Internet of the
   1980s.  This resulted in excess traffic starving other connections
   from access to the Internet.  It was countered by the requirement to
   use congestion control (CC) in the Transmission Control Protocol
   (TCP) [Jacobsen88].  These mechanisms operate in Internet hosts to
   cause TCP connections to "back off" during congestion.  The addition
   of a congestion control to TCP (currently documented in [RFC5681]
   ensured the stability of the Internet, because it was able to detect
   congestion and promptly react.  This was effective in an Internet
   where most TCP flows were long-lived (ensuring that they could detect
   and respond to congestion before the flows terminated).  Although TCP
   was by far the dominant traffic, this is no longer the always the
   case, and non-congestion-controlled traffic, including many
   applications using the User Datagram Protocol (UDP), can form a
   significant proportion of the total traffic traversing a link.  The
   current Internet therefore requires that non-congestion-controlled
   traffic is considered to avoid persistent excessive congestion.

   A network transport Circuit Breaker is an automatic mechanism that is
   used to continuously monitor a flow or aggregate set of flows.  The
   mechanism seeks to detect when the flow(s) experience persistent
   excessive congestion.  When this is detected, a Circuit Breaker
   terminates (or significantly reduce the rate of) the flow(s).  This
   is a safety measure to prevent starvation of network resources
   denying other flows from access to the Internet.  Such measures are
   essential for an Internet that is heterogeneous and for traffic that
   is hard to predict in advance.  Avoiding persistent excessive
   congestion is important to reduce the potential for "Congestion
   Collapse" [RFC2914].

   There are important differences between a transport Circuit Breaker
   and a congestion control method.  Congestion control (as implemented
   in TCP, SCTP, and DCCP) operates on a timescale on the order of a
   packet round-trip-time (RTT), the time from sender to destination and
   return.  Congestion at a network link can also be detected using
   Explicit Congestion Notification (ECN) [RFC3168], which allows the
   network to signal congestion by marking ECN-capable packets with a
   Congestion Experienced (CE) mark.  Both loss and reception of CE-
   marked packets are treated as congestion events.  Congestion control
   methods are able to react to a congestion event by continuously
   adapting to reduce their transmission rate.  The goal is usually to
   limit the transmission rate to a maximum rate that reflects a fair
   use of the available capacity across a network path.  These methods
   typically operate on individual traffic flows (e.g., a 5-tuple that
   includes the IP addresses, protocol, and ports).

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   In contrast, Circuit Breakers are recommended for non-congestion-
   controlled Internet flows and for traffic aggregates, e.g., traffic
   sent using a network tunnel.  They operate on timescales much longer
   than the packet RTT, and trigger under situations of abnormal
   (excessive) congestion.  People have been implementing what this
   document characterizes as circuit breakers on an ad hoc basis to
   protect Internet traffic.  This document therefore provides guidance
   on how to deploy and use these mechanisms.  Later sections provide
   examples of cases where circuit-breakers may or may not be desirable.

   A Circuit Breaker needs to measure (meter) some portion of the
   traffic to determine if the network is experiencing congestion and
   needs to be designed to trigger robustly when there is persistent
   excessive congestion.

   A Circuit Breaker trigger will often utilize a series of successive
   sample measurements metered at an ingress point and an egress point
   (either of which could be a transport endpoint).  The trigger needs
   to operate on a timescale much longer than the path round trip time
   (e.g., seconds to possibly many tens of seconds).  This longer period
   is needed to provide sufficient time for transport congestion control
   (or applications) to adjust their rate following congestion, and for
   the network load to stabilize after any adjustment.  Congestion
   events can be common when a congestion-controlled transport is used
   over a network link operating near capacity.  Each event results in
   reduction in the rate of the transport flow experiencing congestion.
   The longer period seeks to ensure that a Circuit Breaker does not
   accidentally trigger following a single (or even successive)
   congestion events.

   Once triggered, the Circuit Breaker needs to provide a control
   function (called the "reaction").  This removes traffic from the
   network, either by disabling the flow or by significantly reducing
   the level of traffic.  This reaction provides the required protection
   to prevent persistent excessive congestion being experienced by other
   flows that share the congested part of the network path.

   Section 4 defines requirements for building a Circuit Breaker.

   The operational conditions that cause a Circuit Breaker to trigger
   ought to be regarded as abnormal.  Examples of situations that could
   trigger a Circuit Breaker include:

   o  anomalous traffic that exceeds the provisioned capacity (or whose
      traffic characteristics exceed the threshold configured for the
      Circuit Breaker);

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   o  traffic generated by an application at a time when the provisioned
      network capacity is being utilised for other purposes;

   o  routing changes that cause additional traffic to start using the
      path monitored by the Circuit Breaker;

   o  misconfiguration of a service/network device where the capacity
      available is insufficient to support the current traffic

   o  misconfiguration of an admission controller or traffic policer
      that allows more traffic than expected across the path monitored
      by the Circuit Breaker.

   Other mechanisms could also be available to network operators to
   detect excessive congestion (e.g., an observation of excessive
   utilisation for a port on a network device).  Utilising such
   information, operational mechanisms could react to reduce network
   load over a shorter timescale than those of a network transport
   Circuit Breaker.  The role of the Circuit Breaker over such paths
   remains as a method of last resort.  Because it acts over a longer
   timescale, the Circuit Breaker ought to trigger only when other
   reactions did not succeed in reducing persistent excessive

   In many cases, the reason for triggering a Circuit Breaker will not
   be evident to the source of the traffic (user, application, endpoint,
   etc).  A Circuit Breaker can be used to limit traffic from
   applications that are unable, or choose not, to use congestion
   control, or where the congestion control properties of the traffic
   cannot be relied upon (e.g., traffic carried over a network tunnel).
   In such circumstances, it is all but impossible for the Circuit
   Breaker to signal back to the impacted applications.  In some cases
   applications could therefore have difficulty in determining that a
   Circuit Breaker has triggered, and where in the network this

   Application developers are therefore advised, where possible, to
   deploy appropriate congestion control mechanisms.  An application
   that uses congestion control will be aware of congestion events in
   the network.  This allows it to regulate the network load under
   congestion, and is expected to avoid triggering a network Circuit
   Breaker.  For applications that can generate elastic traffic, this
   will often be a preferred solution.

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1.1.  Types of Circuit Breaker

   There are various forms of network transport circuit breaker.  These
   are differentiated mainly on the timescale over which they are
   triggered, but also in the intended protection they offer:

   o  Fast-Trip Circuit Breakers: The relatively short timescale used by
      this form of circuit breaker is intended to provide protection for
      network traffic from a single flow or related group of flows.

   o  Slow-Trip Circuit Breakers: This circuit breaker utilizes a longer
      timescale and is designed to protect network traffic from
      congestion by traffic aggregates.

   o  Managed Circuit Breakers: Utilize the operations and management
      functions that might be present in a managed service to implement
      a circuit breaker.

   Examples of each type of circuit breaker are provided in section 4.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Design of a Circuit-Breaker (What makes a good circuit breaker?)

   Although circuit breakers have been talked about in the IETF for many
   years, there has not yet been guidance on the cases where circuit
   breakers are needed or upon the design of circuit breaker mechanisms.
   This document seeks to offer advice on these two topics.

   Circuit Breakers are RECOMMENDED for IETF protocols and tunnels that
   carry non-congestion-controlled Internet flows and for traffic
   aggregates.  This includes traffic sent using a network tunnel.
   Designers of other protocols and tunnel encapsulations also ought to
   consider the use of these techniques as a last resort to protect
   traffic that shares the network path being used.

   This document defines the requirements for design of a Circuit
   Breaker and provides examples of how a Circuit Breaker can be
   constructed.  The specifications of individual protocols and tunnel
   encapsulations need to detail the protocol mechanisms needed to
   implement a Circuit Breaker.

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   Section 3.1 describes the functional components of a circuit breaker
   and section 3.2 defines requirements for implementing a Circuit

3.1.  Functional Components

   The basic design of a Circuit Breaker involves communication between
   an ingress point (a sender) and an egress point (a receiver) of a
   network flow or set of flows.  A simple picture of operation is
   provided in figure 1.  This shows a set of routers (each labelled R)
   connecting a set of endpoints.

   A Circuit Breaker is used to control traffic passing through a subset
   of these routers, acting between the ingress and a egress point
   network devices.  The path between the ingress and egress could be
   provided by a tunnel or other network-layer technique.  One expected
   use would be at the ingress and egress of a service, where all
   traffic being considered terminates beyond the egress point, and
   hence the ingress and egress carry the same set of flows.

 +--------+                                                   +--------+
 |Endpoint|                                                   |Endpoint|
 +--+-----+          >>> circuit breaker traffic >>>          +--+-----+
    |                                                            |
    | +-+  +-+  +---------+  +-+  +-+  +-+  +--------+  +-+  +-+ |
    +-+R+--+R+->+ Ingress +--+R+--+R+--+R+--+ Egress |--+R+--+R+-+
      +++  +-+  +------+--+  +-+  +-+  +-+  +-----+--+  +++  +-+
       |         ^     |                          |      |
       |         |  +--+------+            +------+--+   |
       |         |  | Ingress |            | Egress  |   |
       |         |  | Meter   |            | Meter   |   |
       |         |  +----+----+            +----+----+   |
       |         |       |                      |        |
  +-+  |         |  +----+----+                 |        |  +-+
  |R+--+         |  | Measure +<----------------+        +--+R|
  +++            |  +----+----+      Reported               +++
   |             |       |           Egress                  |
   |             |  +----+----+      Measurement             |
+--+-----+       |  | Trigger +                           +--+-----+
|Endpoint|       |  +----+----+                           |Endpoint|
+--------+       |       |                                +--------+

   Figure 1: A CB controlling the part of the end-to-end path between an
   ingress point and an egress point.  (Note: In some cases, the trigger

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   and measurement functions could alternatively be located at other
   locations (e.g., at a network operations centre.)

   In the context of a Circuit Breaker, the ingress and egress functions
   could be implemented in different places.  For example, they could be
   located in network devices at a tunnel ingress and at the tunnel
   egress.  In some cases, they could be located at one or both network
   endpoints (see figure 2), implemented as components within a
   transport protocol.

    +----------+                 +----------+
    | Ingress  |  +-+  +-+  +-+  | Egress   |
    | Endpoint +->+R+--+R+--+R+--+ Endpoint |
    +--+----+--+  +-+  +-+  +-+  +----+-----+
       ^    |                         |
       | +--+------+             +----+----+
       | | Ingress |             | Egress  |
       | | Meter   |             | Meter   |
       | +----+----+             +----+----+
       |      |                       |
       | +--- +----+                  |
       | | Measure +<-----------------+
       | +----+----+      Reported
       |      |           Egress
       | +----+----+      Measurement
       | | Trigger |
       | +----+----+
       |      |

   Figure 2: An endpoint CB implemented at the sender (ingress) and
   receiver (egress).

   The set of components needed to implement a Circuit Breaker are:

   1.  An ingress meter (at the sender or tunnel ingress) that records
       the number of packets/bytes sent in each measurement interval.
       This measures the offered network load for a flow or set of
       flows.  For example, the measurement interval could be many
       seconds (or every few tens of seconds or a series of successive
       shorter measurements that are combined by the Circuit Breaker
       Measurement function).

   2.  An egress meter (at the receiver or tunnel egress) that records
       the number/bytes received in each measurement interval.  This
       measures the supported load for the flow or set of flows, and

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       could utilize other signals to detect the effect of congestion
       (e.g., loss/congestion marking [RFC3168] experienced over the
       path).  The measurements at the egress could be synchronised
       (including an offset for the time of flight of the data, or
       referencing the measurements to a particular packet) to ensure
       any counters refer to the same span of packets.

   3.  A method that communicates the measured values at the ingress and
       egress to the Circuit Breaker Measurement function.  This could
       use several methods including: Sending return measurement packets
       (or control messages) from a receiver to a trigger function at
       the sender; an implementation using Operations, Administration
       and Management (OAM); or sending an in-band signalling datagram
       to the trigger function.  This could also be implemented purely
       as a control plane function, e.g., using a software-defined
       network controller.

   4.  A measurement function that combines the ingress and egress
       measurements to assess the present level of network congestion.
       (For example, the loss rate for each measurement interval could
       be deduced from calculating the difference between ingress and
       egress counter values.)  Note the method does not require high
       accuracy for the period of the measurement interval (or therefore
       the measured value, since isolated and/or infrequent loss events
       need to be disregarded.)

   5.  A trigger function that determines whether the measurements
       indicate persistent excessive congestion.  This function defines
       an appropriate threshold for determining that there is persistent
       excessive congestion between the ingress and egress.  This
       preferably considers a rate or ratio, rather than an absolute
       value (e.g., more than 10% loss, but other methods could also be
       based on the rate of transmission as well as the loss rate).  The
       Circuit Breaker is triggered when the threshold is exceeded in
       multiple measurement intervals (e.g., 3 successive measurements).
       Designs need to be robust so that single or spurious events do
       not trigger a reaction.

   6.  A reaction that is applied at the Ingress when the Circuit
       Breaker is triggered.  This seeks to automatically remove the
       traffic causing persistent excessive congestion.

   7.  A feedback control mechanism that triggers when either the
       receive or ingress and egress measurements are not available,
       since this also could indicate a loss of control packets (also a
       symptom of heavy congestion or inability to control the load).

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3.2.  Other network topologies

   A Circuit Breaker can be deployed in networks with topologies
   different to that presented in figures 1 and 2.  This section
   describes examples of such usage, and possible places where functions
   can be implemented.

3.2.1.  Use with a multicast control/routing protocol

    +----------+                 +--------+  +----------+
    | Ingress  |  +-+  +-+  +-+  | Egress |  |  Egress  |
    | Endpoint +->+R+--+R+--+R+--+ Router |--+ Endpoint +->+
    +----+-----+  +-+  +-+  +-+  +---+--+-+  +----+-----+  |
         ^         ^    ^    ^       |  ^         |        |
         |         |    |    |       |  |         |        |
    +----+----+    + - - - < - - - - +  |    +----+----+   | Reported
    | Ingress |      multicast Prune    |    | Egress  |   | Ingress
    | Meter   |                         |    | Meter   |   | Measurement
    +---------+                         |    +----+----+   |
                                        |         |        |
                                        |    +----+----+   |
                                        |    | Measure +<--+
                                        |    +----+----+
                                        |         |
                                        |    +----+----+
                              multicast |    | Trigger |
                              Leave     |    +----+----+
                              Message   |         |

   Figure 3: An example of a multicast CB controlling the end-to-end
   path between an ingress endpoint and an egress endpoint.

   Figure 3 shows one example of how a multicast Circuit Breaker could
   be implemented at a pair of multicast endpoints (e.g., to implement a
   Fast-Trip Circuit Breaker, Section 5.1).  The ingress endpoint (the
   sender that sources the multicast traffic) meters the ingress load,
   generating an ingress measurement (e.g., recording timestamped packet
   counts), and sends this measurement to the multicast group together
   with the traffic it has measured.

   Routers along a multicast path forward the multicast traffic
   (including the ingress measurement) to all active endpoint receivers.
   Each last hop (egress) router forwards the traffic to one or more
   egress endpoint(s).

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   In this figure, each endpoint includes a meter that performs a local
   egress load measurement.  An endpoint also extracts the received
   ingress measurement from the traffic, and compares the ingress and
   egress measurements to determine if the Circuit Breaker ought to be
   triggered.  This measurement has to be robust to loss (see previous
   section).  If the Circuit Breaker is triggered, it generates a
   multicast leave message for the egress (e.g., an IGMP or MLD message
   sent to the last hop router), which causes the upstream router to
   cease forwarding traffic to the egress endpoint [RFC1112].

   Any multicast router that has no active receivers for a particular
   multicast group will prune traffic for that group, sending a prune
   message to its upstream router.  This starts the process of releasing
   the capacity used by the traffic and is a standard multicast routing
   function (e.g., using Protocol Independent Multicast Sparse Mode
   (PIM-SM) routing protocol [RFC4601]).  Each egress operates
   autonomously, and the Circuit Breaker "reaction" is executed by the
   multicast control plane (e.g., by PIM) requiring no explicit
   signalling by the Circuit Breaker along the communication path used
   for the control messages.  Note: there is no direct communication
   with the Ingress, and hence a triggered Circuit Breaker only controls
   traffic downstream of the first hop multicast router.  It does not
   stop traffic flowing from the sender to the first hop router; this is
   common practice for multicast deployment.

   The method could also be used with a multicast tunnel or subnetwork
   (e.g., Section 5.2, Section 5.3), where a meter at the ingress
   generates additional control messages to carry the measurement data
   towards the egress where the egress metering is implemented.

3.2.2.  Use with control protocols supporting pre-provisioned capacity

   Some paths are provisioned using a control protocol, e.g., flows
   provisioned using the Multi-Protocol Label Switching (MPLS) services,
   paths provisioned using the resource reservation protocol (RSVP),
   networks utilizing Software Defined Network (SDN) functions, or
   admission-controlled Differentiated Services.  Figure 1 shows one
   expected use case, where in this usage a separate device could be
   used to perform the measurement and trigger functions.  The reaction
   generated by the trigger could take the form of a network control
   message sent to the ingress and/or other network elements causing
   these elements to react to the Circuit Breaker.  Examples of this
   type of use are provided in section Section 5.3.

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3.2.3.  Unidirectional Circuit Breakers over Controlled Paths

   A Circuit Breaker can be used to control uni-directional UDP traffic,
   providing that there is a communication path that can be used for
   control messages to connect the functional components at the Ingress
   and Egress.  This communication path for the control messages can
   exist in networks for which the traffic flow is purely
   unidirectional.  For example, a multicast stream that sends packets
   across an Internet path and can use multicast routing to prune flows
   to shed network load.  Some other types of subnetwork also utilize
   control protocols that can be used to control traffic flows.

4.  Requirements for a Network Transport Circuit Breaker

   The requirements for implementing a Circuit Breaker are:

   1.   There needs to be a communication path for control messages to
        carry measurement data from the ingress meter and from the
        egress meter to the point of measurement.  (Requirements 16-18
        relate to the transmission of control messages.)

   2.   A CB is REQUIRED to define a measurement period over which the
        CB Measurement function measures the level of congestion or
        loss.  This method does not have to detect individual packet
        loss, but MUST have a way to know that packets have been lost/
        marked from the traffic flow.

   3.   An egress meter can also count ECN [RFC3168] congestion marks as
        a part of measurement of congestion, but in this case, loss MUST
        also be measured to provide a complete view of the level of
        congestion.  For tunnels,
        [ID-ietf-tsvwg-tunnel-congestion-feedback] describes a way to
        measure both loss and ECN-marking; these measurements could be
        used on a relatively short timescale to drive a congestion
        control response and/or aggregated over a longer timescale with
        a higher trigger threshold to drive a CB.  Subsequent bullet
        items in this section discuss the necessity of using a longer
        timescale and a higher trigger threshold.

   4.   The measurement period used by a CB Measurement function MUST be
        longer than the time that current Congestion Control algorithms
        need to reduce their rate following detection of congestion.
        This is important because end-to-end Congestion Control
        algorithms require at least one RTT to notify and adjust the
        traffic when congestion is experienced, and congestion
        bottlenecks can share traffic with a diverse range of RTTs.  The
        measurement period is therefore expected to be significantly
        longer than the RTT experienced by the CB itself.

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   5.   If necessary, a CB MAY combine successive individual meter
        samples from the ingress and egress to ensure observation of an
        average measurement over a sufficiently long interval.  (Note
        when meter samples need to be combined, the combination needs to
        reflect the sum of the individual sample counts divided by the
        total time/volume over which the samples were measured.
        Individual samples over different intervals can not be directly
        combined to generate an average value.)

   6.   A CB MUST be constructed so that it does not trigger under light
        or intermittent congestion (see requirements 7-9).

   7.   A CB is REQUIRED to define a threshold to determine whether the
        measured congestion is considered excessive.

   8.   A CB is REQUIRED to define the triggering interval, defining the
        period over which the trigger uses the collected measurements.
        CBs need to trigger over a sufficiently long period to avoid
        additionally penalizing flows with a long path RTT (e.g., many
        path RTTs).

   9.   A CB MUST be robust to multiple congestion events.  This usually
        will define a number of measured persistent congestion events
        per triggering period.  For example, a CB MAY combine the
        results of several measurement periods to determine if the CB is
        triggered (e.g., it is triggered when persistent excessive
        congestion is detected in 3 of the measurements within the
        triggering interval).

   10.  The normal reaction to a trigger SHOULD disable all traffic that
        contributed to congestion (otherwise, see requirements 11,12).

   11.  The reaction MUST be much more severe than that of a Congestion
        Control algorithm (such as TCP's congestion control [RFC5681] or
        TCP-Friendly Rate Control, TFRC [RFC5348]), because the CB
        reacts to more persistent congestion and operates over longer
        timescales (i.e., the overload condition will have persisted for
        a longer time before the CB is triggered).

   12.  A reaction that results in a reduction SHOULD result in reducing
        the traffic by at least an order of magnitude.  A response that
        achieves the reduction by terminating flows, rather than
        randomly dropping packets, will often be more desirable to users
        of the service.  A CB that reduces the rate of a flow, MUST
        continue to monitor the level of congestion and MUST further
        react to reduce the rate if the CB is again triggered.

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   13.  The reaction to a triggered CB MUST continue for a period that
        is at least the triggering interval.  Operator intervention will
        usually be required to restore a flow.  If an automated response
        is needed to reset the trigger, then this needs to not be
        immediate.  The design of an automated reset mechanism needs to
        be sufficiently conservative that it does not adversely interact
        with other mechanisms (including other CB algorithms that
        control traffic over a common path).  It SHOULD NOT perform an
        automated reset when there is evidence of continued congestion.

   14.  A CB trigger SHOULD be regarded as an abnormal network event.
        As such, this event SHOULD be logged.  The measurements that
        lead to triggering of the CB SHOULD also be logged.

   15.  The control communication needs to carry measurements
        (requirement 1) and, in some uses, also needs to transmit
        trigger messages to the ingress.  This control communication may
        be in-band or out-of-band.  The use of in-band communication is
        RECOMMENDED when either design would be possible.  The preferred
        CB design is one that triggers when it fails to receive
        measurement reports that indicate an absence of congestion, in
        contrast to relying on the successful transmission of a
        "congested" signal back to the sender.  (The feedback signal
        could itself be lost under congestion).

        in-Band:  An in-band control method SHOULD assume that loss of
           control messages is an indication of potential congestion on
           the path, and repeated loss ought to cause the CB to be
           triggered.  This design has the advantage that it provides
           fate-sharing of the traffic flow(s) and the control
           communications.  This fate-sharing property is weaker when
           some or all of the measured traffic is sent using a path that
           differs from the path taken by the control traffic (e.g.,
           where traffic and control messages follow a different path
           due to use of equal-cost multipath routing, traffic
           engineering, or tunnels for specific types of traffic).

        Out-of-Band:  An out-of-band control method SHOULD NOT trigger
           CB reaction when there is loss of control messages (e.g., a
           loss of measurements).  This avoids failure amplification/
           propagation when the measurement and data paths fail
           independently.  A failure of an out-of-band communication
           path SHOULD be regarded as abnormal network event and be
           handled as appropriate for the network, e.g., this event
           SHOULD be logged, and additional network operator action
           might be appropriate, depending on the network and the
           traffic involved.

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   16.  The control communication MUST be designed to be robust to
        packet loss.  A control message can be lost if there is a
        failure of the communication path used for the control messages,
        loss is likely to also be experienced during congestion/
        overload.  This does not imply that it is desirable to provide
        reliable delivery (e.g., over TCP), since this can incur
        additional delay in responding to congestion.  Appropriate
        mechanisms could be to duplicate control messages to provide
        increased robustness to loss, or/and to regard a lack of control
        traffic as an indication that excessive congestion could be
        being experienced [ID-ietf-tsvwg-RFC5405.bis].  If control
        messages traffic are sent over a shared path, it is RECOMMENDED
        that this control traffic is prioritized to reduce the
        probability of loss under congestion.  Control traffic also
        needs to be considered when provisioning a network that uses a
        Circuit Breaker.

   17.  There are security requirements for the control communication
        between endpoints and/or network devices (Section 7).  The
        authenticity of the source and integrity of the control messages
        (measurements and triggers) MUST be protected from off-path
        attacks.  When there is a risk of on-path attack, a
        cryptographic authentication mechanism for all control/
        measurement messages is RECOMMENDED.

5.  Examples of Circuit Breakers

   There are multiple types of Circuit Breaker that could be defined for
   use in different deployment cases.  There could be cases where a flow
   become controlled by multiple Circuit Breakers (e.g., when the
   traffic of an end-to-end flow is carried in a tunnel within the
   network).  This section provides examples of different types of
   Circuit Breaker:

5.1.  A Fast-Trip Circuit Breaker

   [RFC2309] discusses the dangers of congestion-unresponsive flows and
   states that "all UDP-based streaming applications should incorporate
   effective congestion avoidance mechanisms".  Some applications do not
   use a full-featured transport (TCP, SCTP, DCCP).  These applications
   (e.g., using UDP and its UDP-Lite variant) need to provide
   appropriate congestion avoidance.  Guidance for applications that do
   not use congestion-controlled transports is provided in
   [ID-ietf-tsvwg-RFC5405.bis].  Such mechanisms can be designed to
   react on much shorter timescales than a Circuit Breaker, that only
   observes a traffic envelope.  Congestion control methods can also
   interact with an application to more effectively control its sending

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   A fast-trip Circuit Breaker is the most responsive form of Circuit
   Breaker.  It has a response time that is only slightly larger than
   that of the traffic that it controls.  It is suited to traffic with
   well-understood characteristics (and could include one or more
   trigger functions specifically tailored the type of traffic for which
   it is designed).  It is not suited to arbitrary network traffic and
   could be unsuitable for traffic aggregates, since it could
   prematurely trigger (e.g., when the combined traffic from multiple
   congestion-controlled flows leads to short-term overload).

   Although the mechanisms can be implemented in RTP-aware network
   devices, these mechanisms are also suitable for implementation in
   endpoints (e.g., as a part of the transport system) where they can
   also compliment end-to-end congestion control methods.  A shorter
   response time enables these mechanisms to triggers before other forms
   of Circuit Breaker (e.g., Circuit Breakers operating on traffic
   aggregates at a point along the network path).

5.1.1.  A Fast-Trip Circuit Breaker for RTP

   A set of fast-trip Circuit Breaker methods have been specified for
   use together by a Real-time Transport Protocol (RTP) flow using the
   RTP/AVP Profile [RTP-CB].  It is expected that, in the absence of
   severe congestion, all RTP applications running on best-effort IP
   networks will be able to run without triggering these Circuit
   Breakers.  A fast-trip RTP Circuit Breaker is therefore implemented
   as a fail-safe that when triggered will terminate RTP traffic.

   The sending endpoint monitors reception of in-band RTP Control
   Protocol (RTCP) reception report blocks, as contained in SR or RR
   packets, that convey reception quality feedback information.  This is
   used to measure (congestion) loss, possibly in combination with ECN

   The Circuit Breaker action (shutdown of the flow) is triggered when
   any of the following trigger conditions are true:

   1.  An RTP Circuit Breaker triggers on reported lack of progress.

   2.  An RTP Circuit Breaker triggers when no receiver reports messages
       are received.

   3.  An RTP Circuit Breaker triggers when the long-term RTP throughput
       (over many RTTs) exceeds a hard upper limit determined by a
       method that resembles TCP-Friendly Rate Control (TFRC).

   4.  An RTP Circuit Breaker includes the notion of Media Usability.
       This Circuit Breaker is triggered when the quality of the

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       transported media falls below some required minimum acceptable

5.2.  A Slow-trip Circuit Breaker

   A slow-trip Circuit Breaker could be implemented in an endpoint or
   network device.  This type of Circuit Breaker is much slower at
   responding to congestion than a fast-trip Circuit Breaker.  This is
   expected to be more common.

   One example where a slow-trip Circuit Breaker is needed is where
   flows or traffic-aggregates use a tunnel or encapsulation and the
   flows within the tunnel do not all support TCP-style congestion
   control (e.g., TCP, SCTP, TFRC), see [ID-ietf-tsvwg-RFC5405.bis]
   section 3.1.3.  A use case is where tunnels are deployed in the
   general Internet (rather than "controlled environments" within an
   Internet service provider or enterprise network), especially when the
   tunnel could need to cross a customer access router.

5.3.  A Managed Circuit Breaker

   A managed Circuit Breaker is implemented in the signalling protocol
   or management plane that relates to the traffic aggregate being
   controlled.  This type of Circuit Breaker is typically applicable
   when the deployment is within a "controlled environment".

   A Circuit Breaker requires more than the ability to determine that a
   network path is forwarding data, or to measure the rate of a path -
   which are often normal network operational functions.  There is an
   additional need to determine a metric for congestion on the path and
   to trigger a reaction when a threshold is crossed that indicates
   persistent excessive congestion.

   The control messages can use either in-band or out-of-band

5.3.1.  A Managed Circuit Breaker for SAToP Pseudo-Wires

   [RFC4553], SAToP Pseudo-Wires (PWE3), section 8 describes an example
   of a managed Circuit Breaker for isochronous flows.

   If such flows were to run over a pre-provisioned (e.g., Multi-
   Protocol Label Switching, MPLS) infrastructure, then it could be
   expected that the Pseudowire (PW) would not experience congestion,
   because a flow is not expected to either increase (or decrease) their
   rate.  If, instead, PW traffic is multiplexed with other traffic over
   the general Internet, it could experience congestion.  [RFC4553]
   states: "If SAToP PWs run over a PSN providing best-effort service,

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   they SHOULD monitor packet loss in order to detect "severe
   congestion".  The currently recommended measurement period is 1
   second, and the trigger operates when there are more than three
   measured Severely Errored Seconds (SES) within a period.  If such a
   condition is detected, a SAToP PW ought to shut down bidirectionally
   for some period of time...".

   The concept was that when the packet loss ratio (congestion) level
   increased above a threshold, the PW was by default disabled.  This
   use case considered fixed-rate transmission, where the PW had no
   reasonable way to shed load.

   The trigger needs to be set at the rate that the PW was likely to
   experience a serious problem, possibly making the service non-
   compliant.  At this point, triggering the Circuit Breaker would
   remove the traffic preventing undue impact on congestion-responsive
   traffic (e.g., TCP).  Part of the rationale, was that high loss
   ratios typically indicated that something was "broken" and ought to
   have already resulted in operator intervention, and therefore need to
   trigger this intervention.

   An operator-based response to triggering of a Circuit Breaker
   provides an opportunity for other action to restore the service
   quality, e.g., by shedding other loads or assigning additional
   capacity, or to consciously avoid reacting to the trigger while
   engineering a solution to the problem.  This could require the
   trigger function to send a control message to a third location (e.g.,
   a network operations centre, NOC) that is responsible for operation
   of the tunnel ingress, rather than the tunnel ingress itself.

5.3.2.  A Managed Circuit Breaker for Pseudowires (PWs)

   Pseudowires (PWs) [RFC3985] have become a common mechanism for
   tunneling traffic, and could compete for network resources both with
   other PWs and with non-PW traffic, such as TCP/IP flows.

   [ID-ietf-pals-congcons] discusses congestion conditions that can
   arise when PWs compete with elastic (i.e., congestion responsive)
   network traffic (e.g, TCP traffic).  Elastic PWs carrying IP traffic
   (see [RFC4488]) do not raise major concerns because all of the
   traffic involved responds, reducing the transmission rate when
   network congestion is detected.

   In contrast, inelastic PWs (e.g., a fixed bandwidth Time Division
   Multiplex, TDM) [RFC4553] [RFC5086] [RFC5087]) have the potential to
   harm congestion responsive traffic or to contribute to excessive
   congestion because inelastic PWs do not adjust their transmission
   rate in response to congestion.  [ID-ietf-pals-congcons] analyses TDM

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   PWs, with an initial conclusion that a TDM PW operating with a degree
   of loss that could result in congestion-related problems is also
   operating with a degree of loss that results in an unacceptable TDM
   service.  For that reason, the document suggests that a managed
   Circuit Breaker that shuts down a PW when it persistently fails to
   deliver acceptable TDM service is a useful means for addressing these
   congestion concerns.  (See Appendix A of [ID-ietf-pals-congcons] for
   further discussion.)

6.  Examples where circuit breakers may not be needed.

   A Circuit Breaker is not required for a single congestion-controlled
   flow using TCP, SCTP, TFRC, etc.  In these cases, the congestion
   control methods are already designed to prevent persistent excessive

6.1.  CBs over pre-provisioned Capacity

   One common question is whether a Circuit Breaker is needed when a
   tunnel is deployed in a private network with pre-provisioned

   In this case, compliant traffic that does not exceed the provisioned
   capacity ought not to result in persistent congestion.  A Circuit
   Breaker will hence only be triggered when there is non-compliant
   traffic.  It could be argued that this event ought never to happen -
   but it could also be argued that the Circuit Breaker equally ought
   never to be triggered.  If a Circuit Breaker were to be implemented,
   it will provide an appropriate response if persistent congestion
   occurs in an operational network.

   Implementing a Circuit Breaker will not reduce the performance of the
   flows, but in the event that persistent excessive congestion occurs
   it protects network traffic that shares network capacity with these
   flows.  It also protects network traffic from a failure when Circuit
   Breaker traffic is (re)routed to cause additional network load on a
   non-pre-provisioned path.

6.2.  CBs with tunnels carrying Congestion-Controlled Traffic

   IP-based traffic is generally assumed to be congestion-controlled,
   i.e., it is assumed that the transport protocols generating IP-based
   traffic at the sender already employ mechanisms that are sufficient
   to address congestion on the path.  A question therefore arises when
   people deploy a tunnel that is thought to only carry an aggregate of
   TCP traffic (or traffic using some other congestion control method):
   Is there advantage in this case in using a Circuit Breaker?

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   TCP (and SCTP) traffic in a tunnel is expected to reduce the
   transmission rate when network congestion is detected.  Other
   transports (e.g, using UDP) can employ mechanisms that are sufficient
   to address congestion on the path [ID-ietf-tsvwg-RFC5405.bis].
   However, even if the individual flows sharing a tunnel each implement
   a congestion control mechanism, and individually reduce their
   transmission rate when network congestion is detected, the overall
   traffic resulting from the aggregate of the flows does not
   necessarily avoid persistent congestion.  For instance, most
   congestion control mechanisms require long-lived flows to react to
   reduce the rate of a flow.  An aggregate of many short flows could
   result in many flows terminating before they experience congestion.
   It is also often impossible for a tunnel service provider to know
   that the tunnel only contains congestion-controlled traffic (e.g.,
   Inspecting packet headers might not be possible).  Some IP-based
   applications might not implement adequate mechanisms to address
   congestion.  The important thing to note is that if the aggregate of
   the traffic does not result in persistent excessive congestion
   (impacting other flows), then the Circuit Breaker will not trigger.
   This is the expected case in this context - so implementing a Circuit
   Breaker ought not to reduce performance of the tunnel, but in the
   event that persistent excessive congestion occurs the Circuit Breaker
   protects other network traffic that shares capacity with the tunnel

6.3.  CBs with Uni-directional Traffic and no Control Path

   A one-way forwarding path could have no associated communication path
   for sending control messages, and therefore cannot be controlled
   using a Circuit Breaker (compare with Section 3.2.3).

   A one-way service could be provided using a path with dedicated pre-
   provisioned capacity that is not shared with other elastic Internet
   flows (i.e., flows that vary their rate).  A forwarding path could
   also be shared with other flows.  One way to mitigate the impact of
   traffic on the other flows is to manage the traffic envelope by using
   ingress policing.  Supporting this type of traffic in the general
   Internet requires operator monitoring to detect and respond to
   persistent excessive congestion.

7.  Security Considerations

   All Circuit Breaker mechanisms rely upon coordination between the
   ingress and egress meters and communication with the trigger
   function.  This is usually achieved by passing network control
   information (or protocol messages) across the network.  Timely
   operation of a Circuit Breaker depends on the choice of measurement
   period.  If the receiver has an interval that is overly long, then

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   the responsiveness of the Circuit Breaker decreases.  This impacts
   the ability of the Circuit Breaker to detect and react to congestion.
   If the interval is too short the Circuit Breaker could trigger
   prematurely resulting in insufficient time for other mechanisms to
   act, potentially resulting in unnecessary disruption to the service.

   A Circuit Breaker could potentially be exploited by an attacker to
   mount a Denial of Service (DoS) attack against the traffic being
   controlled by the Circuit Breaker.  Mechanisms therefore need to be
   implemented to prevent attacks on the network control information
   that would result in DoS.

   The authenticity of the source and integrity of the control messages
   (measurements and triggers) MUST be protected from off-path attacks.
   Without protection, it could be trivial for an attacker to inject
   fake or modified control/measurement messages (e.g., indicating high
   packet loss rates) causing a Circuit Breaker to trigger and to
   therefore mount a DoS attack that disrupts a flow.

   Simple protection can be provided by using a randomized source port,
   or equivalent field in the packet header (such as the RTP SSRC value
   and the RTP sequence number) expected not to be known to an off-path
   attacker.  Stronger protection can be achieved using a secure
   authentication protocol to mitigate this concern.

   An attack on the control messages is relatively easy for an attacker
   on the control path when the messages are neither encrypted nor
   authenticated.  Use of a cryptographic authentication mechanism for
   all control/measurement messages is RECOMMENDED to mitigate this
   concern, and would also provide protection from off-path attacks.
   There is a design trade-off between the cost of introducing
   cryptographic security for control messages and the desire to protect
   control communication.  For some deployment scenarios the value of
   additional protection from DoS attack will therefore lead to a
   requirement to authenticate all control messages.

   Transmission of network control messages consumes network capacity.
   This control traffic needs to be considered in the design of a
   Circuit Breaker and could potentially add to network congestion.  If
   this traffic is sent over a shared path, it is RECOMMENDED that this
   control traffic is prioritized to reduce the probability of loss
   under congestion.  Control traffic also needs to be considered when
   provisioning a network that uses a Circuit Breaker.

   The Circuit Breaker MUST be designed to be robust to packet loss that
   can also be experienced during congestion/overload.  Loss of control
   messages could be a side-effect of a congested network, but also
   could arise from other causes Section 4.

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   The security implications depend on the design of the mechanisms, the
   type of traffic being controlled and the intended deployment
   scenario.  Each design of a Circuit Breaker MUST therefore evaluate
   whether the particular Circuit Breaker mechanism has new security

8.  IANA Considerations

   This document makes no request from IANA.

9.  Acknowledgments

   There are many people who have discussed and described the issues
   that have motivated this document.  Contributions and comments
   included: Lars Eggert, Colin Perkins, David Black, Matt Mathis,
   Andrew McGregor, Bob Briscoe and Eliot Lear.  This work was part-
   funded by the European Community under its Seventh Framework
   Programme through the Reducing Internet Transport Latency (RITE)
   project (ICT-317700).

10.  Revision Notes

   XXX RFC-Editor: Please remove this section prior to publication XXX

   Draft 00

   This was the first revision.  Help and comments are greatly

   Draft 01

   Contained clarifications and changes in response to received
   comments, plus addition of diagram and definitions.  Comments are

   WG Draft 00

   Approved as a WG work item on 28th Aug 2014.

   WG Draft 01

   Incorporates feedback after Dallas IETF TSVWG meeting.  This version
   is thought ready for WGLC comments.  Definitions of abbreviations.

   WG Draft 02

   Minor fixes for typos.  Rewritten security considerations section.

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   WG Draft 03

   Updates following WGLC comments (see TSV mailing list).  Comments
   from C Perkins; D Black and off-list feedback.

   A clear recommendation of intended scope.

   Changes include: Improvement of language on timescales and minimum
   measurement period; clearer articulation of endpoint and multicast
   examples - with new diagrams; separation of the controlled network
   case; updated text on position of trigger function; corrections to
   RTP-CB text; clarification of loss v ECN metrics; checks against
   submission checklist 9use of keywords, added meters to diagrams).

   WG Draft 04

   Added section on PW CB for TDM - a newly adopted draft (D.  Black).

   WG Draft 05

   Added clarifications requested during AD review.

   WG Draft 06

   Fixed some remaining typos.

   Update following detailed review by Bob Briscoe, and comments by D.

   WG Draft 07

   Additional update following review by Bob Briscoe.

   WG Draft 08

   Updated text on the response to lack of meter measurements with
   managed circuit breakers.  Additional comments from Eliot Lear (APPs

   WG Draft 09

   Updated text on applications from Eliot Lear.  Additional feedback
   from Bob Briscoe.

   WG Draft 10

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   Updated text following comments by D Black including a rewritten ECN
   requirements bullet with of a reference to a tunnel measurement
   method in [ID-ietf-tsvwg-tunnel-congestion-feedback].

   WG Draft 11

   Minor corrections after second WGLC.

   WG Draft 12

   Update following Gen-ART, RTG, and OPS review comments.

   WG Draft 13

   Fixed a typo.

   WG Draft 14

   Update after IESG discussion, including:

   Reworded introduction.  Added definition of ECN.


   Addressed inconsistency between requirements for control messages. -
   Removed a "MUST" - following WG feedback on a anearlier version of
   the draft that "SHOULD" is more appropriate.

   Addressed comment about grouping requirements to help show they were
   inter-related.  This reordered some requirements.

   Reworded the security considerations.

   Corrections to wording to improve clarity.

   WG Draft 15 (incorporating pending corrections)

   Corrected /applications might be implement/applications might not

   Corrected /Inspecting packet headers could/Inspecting packet headers

   Removed Requirement 9, now duplicated (and renumbered remaining

   Added "(See Appendix A of [ID-ietf-pals-congcons] for further
   discussion.)" to end of 5.3.2 - missed comment.

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   Simplified a sentence in section 6.1, without intended change of

   Added a linking sentence to the second para of Section 6.3.

11.  References

11.1.  Normative References

              Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines (Work-in-Progress)", 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

11.2.  Informative References

              Stein, YJ., Black, D., and B. Briscoe, "Pseudowire
              Congestion Considerations (Work-in-Progress)", 2015.

              Wei, X., Zhu, L., and L. Dend, "Tunnel Congestion Feedback
              (Work-in-Progress)", 2015.

              European Telecommunication Standards, Institute (ETSI),
              "Congestion Avoidance and Control", SIGCOMM Symposium
              proceedings on Communications architectures and
              protocols", August 1998.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,

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   [RFC2309]  Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
              S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
              Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
              S., Wroclawski, J., and L. Zhang, "Recommendations on
              Queue Management and Congestion Avoidance in the
              Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,

   [RFC4488]  Levin, O., "Suppression of Session Initiation Protocol
              (SIP) REFER Method Implicit Subscription", RFC 4488,
              DOI 10.17487/RFC4488, May 2006,

   [RFC4553]  Vainshtein, A., Ed. and YJ. Stein, Ed., "Structure-
              Agnostic Time Division Multiplexing (TDM) over Packet
              (SAToP)", RFC 4553, DOI 10.17487/RFC4553, June 2006,

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601,
              DOI 10.17487/RFC4601, August 2006,

   [RFC5086]  Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T., and
              P. Pate, "Structure-Aware Time Division Multiplexed (TDM)
              Circuit Emulation Service over Packet Switched Network
              (CESoPSN)", RFC 5086, DOI 10.17487/RFC5086, December 2007,

   [RFC5087]  Stein, Y(J)., Shashoua, R., Insler, R., and M. Anavi,
              "Time Division Multiplexing over IP (TDMoIP)", RFC 5087,
              DOI 10.17487/RFC5087, December 2007,

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,

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Internet-Draft                                                April 2016

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <http://www.rfc-editor.org/info/rfc6679>.

   [RTP-CB]   Perkins, C. and V. Singh, "Multimedia Congestion Control:
              Circuit Breakers for Unicast RTP Sessions (draft-ietf-
              avtcore-rtp-circuit-breakers)", February 2014.

Author's Address

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, Scotland  AB24 3UE

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

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