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Versions: (draft-fairhurst-tsvwg-circuit-breaker) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 8084

TSVWG Working Group                                         G. Fairhurst
Internet-Draft                                    University of Aberdeen
Intended status: Best Current Practice                 December 22, 2015
Expires: June 24, 2016


                   Network Transport Circuit Breakers
                  draft-ietf-tsvwg-circuit-breaker-11

Abstract

   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

   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 http://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 June 24, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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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  . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Design of a Circuit-Breaker (What makes a good circuit
       breaker?) . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Functional Components . . . . . . . . . . . . . . . . . .   6
   4.  Requirements for a Network Transport Circuit Breaker  . . . .   9
   5.  Other network topologies  . . . . . . . . . . . . . . . . . .  13
     5.1.  Use with a multicast control/routing protocol . . . . . .  13
     5.2.  Use with control protocols supporting pre-provisioned
           capacity  . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  Unidirectional Circuit Breakers over Controlled Paths . .  15
   6.  Examples of Circuit Breakers  . . . . . . . . . . . . . . . .  15
     6.1.  A Fast-Trip Circuit Breaker . . . . . . . . . . . . . . .  15
       6.1.1.  A Fast-Trip Circuit Breaker for RTP . . . . . . . . .  16
     6.2.  A Slow-trip Circuit Breaker . . . . . . . . . . . . . . .  16
     6.3.  A Managed Circuit Breaker . . . . . . . . . . . . . . . .  17
       6.3.1.  A Managed Circuit Breaker for SAToP Pseudo-Wires  . .  17
       6.3.2.  A Managed Circuit Breaker for Pseudowires (PWs) . . .  18
   7.  Examples where circuit breakers may not be needed.  . . . . .  18
     7.1.  CBs over pre-provisioned Capacity . . . . . . . . . . . .  19
     7.2.  CBs with tunnels carrying Congestion-Controlled Traffic .  19
     7.3.  CBs with Uni-directional Traffic and no Control Path  . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   11. Revision Notes  . . . . . . . . . . . . . . . . . . . . . . .  21
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     12.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

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.  Just
   as people do not expect the 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 connection
   from access to the Internet.  It was countered by the requirement to
   use congestion control (CC) by 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 worked well while TCP was by far
   the dominant traffic in the Internet, and most TCP flows were long-
   lived (ensuring that they could detect and respond to congestion
   before the flows terminated).  This is no longer the case, and non-
   congestion-controlled traffic, including many applications of 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 needs to be
   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 and when this is detected to terminate (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 prevention is
   important to reduce the potential for "Congestion Collapse"
   [RFC2914].

   There are important differences between a transport circuit-breaker
   and a congestion control method.  Specifically, congestion control
   (as implemented in TCP, SCTP, and DCCP) operates on the timescale on
   the order of a packet round-trip-time (RTT), the time from sender to
   destination and return.  Congestion control methods are able to react
   to a single packet loss/marking and continuously adapt to reduce the
   transmission rate for each loss or congestion event.  The goal is
   usually to limit the maximum transmission rate to a 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).

   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



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   than the packet RTT, and trigger under situations of abnormal
   excessive congestion.  People have been implementing what this draft
   characterizes as circuit breakers on an ad hoc basis to protect
   Internet traffic, this draft 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) 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 transports (or applications)
   to adjust their rate following congestion, and for the network load
   to stabilize after any adjustment.  This is to ensure that a Circuit
   Breaker does not accidentally trigger following a single (or even
   successive) congestion events (congestion events are what triggers
   congestion control, and are to be regarded as normal on a network
   link operating near its capacity).  Once triggered, a control
   function needs to remove 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
   should 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);

   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
      aggregate;



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

   In many cases the reason for triggering a Circuit Breaker will not be
   evident to the source of the traffic (user, application, endpoint,
   etc).  In contrast, an application that uses congestion control will
   generate elastic traffic that may be expected to regulate the load it
   introduces under congestion.  This will therefore often be a
   preferred solution for applications that can respond to congestion
   signals or that can use a congestion-controlled transport.

   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 their 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, and it may further be the
   case that applications may have some difficulty determining that a
   Circuit Breaker has in fact been tripped, and where in the network
   this happened.  Application developers are advised to avoid these
   circumstances, where possible, by deploying appropriate congestion
   control mechanisms.

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.








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2.  Terminology

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

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

3.1.  Functional Components

   The basic design of a transport 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
   Circuit Breaker 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.






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 +--------+                                                   +--------+
 |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|
+--------+       |       |                                +--------+
                 +---<---+
                  Reaction


   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
   and measure 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.















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


   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) 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) records the
       number/bytes received in each measurement interval.  This
       measures the supported load for the flow or set of flows, and
       could utilize other signals to detect the effect of congestion
       (e.g., loss/marking 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.  The measured values at the ingress and egress are communicated to
       the Circuit Breaker Measurement function.  This could use several
       methods including: Sending return measurement packets from a



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       receiver to a trigger function at the sender; An implementation
       using Operations, Administration and Management (OAM); or be
       sending another 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.  The measurement function 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 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
       transport 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 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).

4.  Requirements for a Network Transport Circuit Breaker

   The requirements for implementing a Circuit Breaker are:

   o  There needs to be a communication path used for control messages
      from the ingress meter and the egress meter to the point of
      measurement.  The Circuit Breaker MUST trigger if there is a
      failure of the communication path used for the control messages.
      That is, the feedback indicating a congested period needs to be
      designed so that the Circuit Breaker is triggered when it fails to
      receive measurement reports that indicate an absence of



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      congestion, rather than relying on the successful transmission of
      a "congested" signal back to the sender.  (The feedback signal
      could itself be lost under congestion).

   o  A Circuit Breaker is REQUIRED to define a measurement period over
      which the Circuit Breaker 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.

   o  An egress meter can also count Explicit Congestion Notification
      (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 Circuit Breaker.  Subsequent bullet
      items in this section discuss the necessity of using a longer
      timescale and a higher trigger threshold.

   o  The measurement period used by a Circuit Breaker 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 to experienced congestion, 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 Circuit Breaker itself.

   o  If necessary, MAY combine successive individual meter samples from
      the ingress and egress to ensure observation of an average 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.)

   o  A Circuit Breaker is REQUIRED to define a threshold to determine
      whether the measured congestion is considered excessive.

   o  A Circuit Breaker is REQUIRED to define the triggering interval,
      defining the period over which the trigger uses the collected
      measurements.  Circuit Breakers need to trigger over a




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      sufficiently long period to avoid additionally penalizing flows
      with a long path RTT (e.g., many path RTTs).

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

   o  A Circuit Breaker SHOULD be constructed so that it does not
      trigger under light or intermittent congestion.

   o  The default response to a trigger SHOULD disable all traffic that
      contributed to congestion.

   o  Once triggered, the Circuit Breaker MUST react decisively by
      disabling or significantly reducing traffic at the source (e.g.,
      ingress).

   o  The reaction needs to 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 Circuit Breaker reacts to more persistent congestion and
      operates over longer timescales (i.e., the overload condition will
      have persisted for a longer time before the Circuit Breaker is
      triggered).

   o  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 Circuit Breaker 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 Circuit Breaker is again triggered.

   o  The reaction to a triggered Circuit Breaker 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 Circuit Breaker
      algorithms that control traffic over a common path).  It SHOULD
      NOT perform an automated reset when there is evidence of continued
      congestion.




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   o  When a Circuit Breaker is triggered, it SHOULD be regarded as an
      abnormal network event.  As such, this event SHOULD be logged.
      The measurements that lead to triggering of the Circuit Breaker
      SHOULD also be logged.

   o  A Circuit Breaker requires control communication between endpoints
      and/or network devices.  The source and integrity of control
      messages (measurements and triggers) MUST be protected from off-
      path attacks (Section 8).  When there is a risk of on-path attack,
      a cryptographic authentication mechanism for all control/
      measurement messages is RECOMMENDED (Section 8).

   o  The circuit breaker MUST be designed to be robust to packet loss
      that can 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 may be being experienced
      [ID-ietf-tsvwg-RFC5405.bis].

   o  The control communication may be in-band or out-of-band.  In-band
      communication is RECOMMENDED when either design would be possible.
      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.

      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 Circuit Breaker
         to be triggered.  This design has the advantage that it
         provides fate-sharing of the traffic flow(s) and the control
         communications.

      Out-of-Band:  An out-of-band control method SHOULD NOT trigger
         Circuit Breaker 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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5.  Other network topologies

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

5.1.  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 6.1).  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.

   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 the PIM-SM routing protocol).  Each egress
   operates autonomously, and the circuit breaker "reaction" is executed
   by the multicast control plane (e.g., by the PIM multicast routing
   protocol), 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 router.  It does not stop traffic flowing from the sender
   to the first hop router; this is however the common practice for
   multicast deployment.

   The method could also be used with a multicast tunnel or subnetwork
   (e.g., Section 6.2, Section 6.3), where a meter at the ingress
   generates additional control messages to carry the measurement data
   towards the egress where the egress metering is implemented.

5.2.  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,
   path 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 6.3.







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5.3.  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.

6.  Examples of Circuit Breakers

   There are multiple types of Circuit Breaker that could be defined for
   use in different deployment cases.  This section provides examples of
   different types of circuit breaker:

6.1.  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".  All applications ought
   to use a full-featured transport (TCP, SCTP, DCCP), and if not, an
   application (e.g., those using UDP and its UDP-Lite variant) needs 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
   rate.

   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
   may be unsuitable for traffic aggregates, since it could prematurely
   trigger (e.g., when multiple congestion-controlled flows lead to
   short-term overload).

   Although the mechanisms can be implemented in a RTP-aware network
   devices, these mechanisms are also suitable for implementation in
   endpoints (e.g., as a part of the tranport system), where they can
   also compliment end-to-end congestion control methods.  A shorter
   response time enables these mechanisms to triggers before other forms



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   of circuit breaker (e.g., circuit breakers operating on traffic
   aggregates at a point along the network path).

6.1.1.  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
   [RFC6679].

   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 uses a TFRC-style check and sets a hard
       upper limit to the long-term RTP throughput (over many RTTs).

   4.  An RTP Circuit Breaker includes the notion of Media Usability.
       This circuit breaker is triggered when the quality of the
       transported media falls below some required minimum acceptable
       quality.

6.2.  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 and 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



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   Internet service provider or enterprise network), especially when the
   tunnel could need to cross a customer access router.

6.3.  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
   communications.

6.3.1.  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 Pseudo-Wire 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, 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



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   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 provides 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 to be sent to a third location (e.g., a network
   operations centre, NOC) responsible for operation of the tunnel
   ingress, rather than the tunnel ingress itself.

6.3.2.  A Managed Circuit Breaker for Pseudowires (PWs)

   Pseudowires (PWs) [RFC3985] have become a common mechanism for
   tunneling traffic, and may 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
   PWs, with an initial conclusion that a TDM PW operating with a degree
   of loss that may result in congestion-related problems is also
   operating with a degree of loss that results in an unacceptable TDM
   service.  For that reason, the draft 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.

7.  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
   congestion.






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7.1.  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
   capacity.

   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.  A Circuit Breaker also could be used to protect other sharing
   network traffic from a failure that causes the Circuit Breaker
   traffic to be routed over a non-pre-provisioned path.

7.2.  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 [ID-ietf-tsvwg-RFC5405.bis].  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?

   For sure, traffic in a such a tunnel will respond to congestion.
   However, the answer to the question is not always obvious, because
   the overall traffic formed by an aggregate of flows that implement a
   congestion control mechanism does not necessarily prevent 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 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 could not be
   possible).  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 will not reduce performance of the tunnel, but in the event



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   that persistent excessive congestion occurs this protects other
   network traffic that shares capacity with the tunnel traffic.

7.3.  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 an automated process.  This service could be provided using a
   path that has dedicated capacity and does not share this capacity
   with other elastic Internet flows (i.e., flows that vary their rate).

   A way to mitigate the impact on other flows when capacity could be
   shared 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.

8.  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
   the responsiveness of the circuit breaker decreases.  This impacts
   the ability of the circuit breaker to detect and react to congestion.

   A Circuit Breaker could potentially be exploited by an attacker to
   mount a Denial of Service (DoS) attack against the traffic being
   measured.  Mechanisms therefore need to be implemented to prevent
   attacks on the network control information that would result in DoS.
   The source and integrity of control information (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.  This attack is relatively easy for an on-
   path attacker when the messages are neither encrypted nor
   authenticated.  When there is a risk of on-path attack, a
   cryptographic authentication mechanism for all control/measurement



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   messages is RECOMMENDED to mitigate this concern.  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.

   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
   implications.

9.  IANA Considerations

   This document makes no request from IANA.

10.  Acknowledgments

   There are many people who have discussed and described the issues
   that have motivated this draft.  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).

11.  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
   appreciated.




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   Draft 01

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

   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.

   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.
   Black.




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   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
   area).

   WG Draft 09

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

   WG Draft 10

   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.

12.  References

12.1.  Normative References

   [ID-ietf-tsvwg-RFC5405.bis]
              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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc3168>.

12.2.  Informative References

   [ID-ietf-pals-congcons]
              Stein, YJ., Black, D., and B. Briscoe, "Pseudowire
              Congestion Considerations (Work-in-Progress)", 2015.



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   [ID-ietf-tsvwg-tunnel-congestion-feedback]
              Wei, X., Zhu, L., and L. Dend, "Tunnel Congestion Feedback
              (Work-in-Progress)", 2015.

   [Jacobsen88]
              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,
              <http://www.rfc-editor.org/info/rfc1112>.

   [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,
              <http://www.rfc-editor.org/info/rfc2309>.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,
              <http://www.rfc-editor.org/info/rfc2914>.

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <http://www.rfc-editor.org/info/rfc3985>.

   [RFC4488]  Levin, O., "Suppression of Session Initiation Protocol
              (SIP) REFER Method Implicit Subscription", RFC 4488,
              DOI 10.17487/RFC4488, May 2006,
              <http://www.rfc-editor.org/info/rfc4488>.

   [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,
              <http://www.rfc-editor.org/info/rfc4553>.

   [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,
              <http://www.rfc-editor.org/info/rfc5086>.





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   [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,
              <http://www.rfc-editor.org/info/rfc5087>.

   [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,
              <http://www.rfc-editor.org/info/rfc5348>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
              <http://www.rfc-editor.org/info/rfc5681>.

   [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", February 2014.

Author's Address

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

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


















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