Network Transport Circuit Breakers
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This is an older version of an Internet-Draft that was ultimately published as RFC 8084.
|RFC stream||Internet Engineering Task Force (IETF)|
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|Stream||WG state||Submitted to IESG for Publication|
|Document shepherd||David L. Black|
|Shepherd write-up||Show Last changed 2015-09-28|
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|Responsible AD||Spencer Dawkins|
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TSVWG Working Group G. Fairhurst Internet-Draft University of Aberdeen Intended status: Best Current Practice March 20, 2016 Expires: September 21, 2016 Network Transport Circuit Breakers draft-ietf-tsvwg-circuit-breaker-14 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 September 21, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Fairhurst Expires September 21, 2016 [Page 1] Internet-Draft March 2016 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 . . 18 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 . 20 6.3. CBs with Uni-directional Traffic and no Control Path . . 20 7. Security Considerations . . . . . . . . . . . . . . . . . . . 21 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 10. Revision Notes . . . . . . . . . . . . . . . . . . . . . . . 22 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 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. Fairhurst Expires September 21, 2016 [Page 2] Internet-Draft March 2016 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). Fairhurst Expires September 21, 2016 [Page 3] Internet-Draft March 2016 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); Fairhurst Expires September 21, 2016 [Page 4] Internet-Draft March 2016 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; 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 congestion. 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 happened. 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. Fairhurst Expires September 21, 2016 [Page 5] Internet-Draft March 2016 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", "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. Fairhurst Expires September 21, 2016 [Page 6] Internet-Draft March 2016 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 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| +--------+ | | +--------+ +---<---+ 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 Fairhurst Expires September 21, 2016 [Page 7] Internet-Draft March 2016 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 | | +----+----+ | | +---<--+ 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) 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 Fairhurst Expires September 21, 2016 [Page 8] Internet-Draft March 2016 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). Fairhurst Expires September 21, 2016 [Page 9] Internet-Draft March 2016 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). Fairhurst Expires September 21, 2016 [Page 10] Internet-Draft March 2016 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. Fairhurst Expires September 21, 2016 [Page 11] Internet-Draft March 2016 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. Fairhurst Expires September 21, 2016 [Page 12] Internet-Draft March 2016 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 7-10). 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. 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.) 10. 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). 11. The normal reaction to a trigger SHOULD disable all traffic that contributed to congestion (otherwise, see requirements 12,13). 12. 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). Fairhurst Expires September 21, 2016 [Page 13] Internet-Draft March 2016 13. 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. 14. 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. 15. 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. 16. 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/ Fairhurst Expires September 21, 2016 [Page 14] Internet-Draft March 2016 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. 17. 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. 18. 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 Fairhurst Expires September 21, 2016 [Page 15] Internet-Draft March 2016 (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 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 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 [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. Fairhurst Expires September 21, 2016 [Page 16] Internet-Draft March 2016 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 transported media falls below some required minimum acceptable quality. 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 communications. Fairhurst Expires September 21, 2016 [Page 17] Internet-Draft March 2016 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, 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. Fairhurst Expires September 21, 2016 [Page 18] Internet-Draft March 2016 [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 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. 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 congestion. 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 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 protects network traffic using a non- pre-provisioned path from a failure caused by additional network load resulting when Circuit Breaker traffic is routed over this path. Fairhurst Expires September 21, 2016 [Page 19] Internet-Draft March 2016 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? 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 could not be possible). Some IP-based applications might be 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 traffic. 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 that has dedicated capacity and does not share this capacity with other elastic Internet flows (i.e., flows that vary their rate). One way to mitigate the impact on other flows that share the same capacity 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. Fairhurst Expires September 21, 2016 [Page 20] Internet-Draft March 2016 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 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 Fairhurst Expires September 21, 2016 [Page 21] Internet-Draft March 2016 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. 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 appreciated. 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. Fairhurst Expires September 21, 2016 [Page 22] Internet-Draft March 2016 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. 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). Fairhurst Expires September 21, 2016 [Page 23] Internet-Draft March 2016 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. 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. Requirement 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. 11. References Fairhurst Expires September 21, 2016 [Page 24] Internet-Draft March 2016 11.1. 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>. 11.2. Informative References [ID-ietf-pals-congcons] Stein, YJ., Black, D., and B. Briscoe, "Pseudowire Congestion Considerations (Work-in-Progress)", 2015. [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>. Fairhurst Expires September 21, 2016 [Page 25] Internet-Draft March 2016 [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>. [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, <http://www.rfc-editor.org/info/rfc4601>. [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>. [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>. Fairhurst Expires September 21, 2016 [Page 26] Internet-Draft March 2016 [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 UK Email: firstname.lastname@example.org URI: http://www.erg.abdn.ac.uk Fairhurst Expires September 21, 2016 [Page 27]