| TSVWG Working Group | G. Fairhurst |
| Internet-Draft | University of Aberdeen |
| Intended status: Best Current Practice | October 17, 2015 |
| Expires: April 19, 2016 |
Network Transport Circuit Breakers
draft-ietf-tsvwg-circuit-breaker-06
This document explains what is meant by the term "network transport Circuit Breaker" (CB). It describes the need for circuit breakers when using network tunnels, and other non-congestion controlled applications, 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.
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A network transport Circuit Breaker (CB) is an automatic mechanism that is used to estimate congestion caused by a flow, and to terminate (or significantly reduce the rate of) the flow when persistent congestion is detected. 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 prevention is important to reduce the potential for "Congestion Collapse" [RFC2914].
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.
In networking, the Circuit Breaker principle can be used as a protection mechanism of last resort to avoid persistent 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] [RFC1112]. These mechanisms operate in Internet hosts to cause TCP connections to "back off" during congestion. The introduction of a Congestion Controller in 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 congestion.
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 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 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. 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 congestion. This means 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.
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). These measurements need to be taken over a reasonably long period of time. 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 congestion being experienced by other flows that share the congested part of the network path.
Section 4 defines requirements for building a 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:
Examples of each type of circuit breaker are provided in section 4.
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].
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 to provide 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.
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.
+--------+ +--------+
|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.
+----------+ +----------+
| 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:
The requirements for implementing a Circuit Breaker are:
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.
+----------+ +--------+ +----------+
| 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 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).
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.
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.
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.
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:
Applications ought to use a full-featured transport (TCP, SCTP, DCCP), and if not, application (e.g. those using UDP and its UDP-Lite variant [RFC3828])they need to provide appropriate congestion avoidance. [RFC2309] discusses the dangers of congestion-unresponsive flows and states that "all UDP-based streaming applications should incorporate effective congestion avoidance mechanisms". 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. These 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 fro traffic aggregates, since it could prematurely trigger (e.g., when multiple congestion-controlled flows lead to short-term overload).
These mechanisms are suitable for implementation in endpoints, 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).
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 sender monitors reception of 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:
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 Internet service provider or enterprise network), especially when the tunnel could need to cross a customer access router.
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 congestion.
[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 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.
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.
A Circuit Breaker is not required for a single Congestion Controller-controlled flow using TCP, SCTP, TFRC, etc. In these cases, the Congestion Control methods are already designed to prevent persistent congestion.
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 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.
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 (or some other Congestion Controller-controlled) traffic: 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 Controller mechanism does not necessarily prevent persistent congestion. For instance, most Congestion Controller 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 CC-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 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 that persistent congestion occurs this protects other network traffic that shares capacity with the tunnel traffic.
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 congestion.
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.
Mechanisms need to be implemented to prevent attacks on the network control information that would result in Denial of Service (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 packets with values that could prematurely trigger a circuit breaker resulting in DoS. 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.
Transmission of network control information 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. 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].
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.
This document makes no request from IANA.
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 and Andrew McGregor. This work was part-funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700).
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.
WG Draft 01
Incorporates feedback after Dallas IETF TSVWG meeting. This version is thought ready for WGLC comments.
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.
| [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. |
| [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. |