One document matched: draft-ietf-tsvwg-circuit-breaker-09.xml
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<front>
<title abbrev="">Network Transport Circuit Breakers</title>
<author fullname="Godred Fairhurst" initials="G." surname="Fairhurst">
<organization>University of Aberdeen</organization>
<address>
<postal>
<street>School of Engineering</street>
<street>Fraser Noble Building</street>
<city>Aberdeen</city>
<region>Scotland</region>
<code>AB24 3UE</code>
<country>UK</country>
</postal>
<email>gorry@erg.abdn.ac.uk</email>
<uri>http://www.erg.abdn.ac.uk</uri>
</address>
</author>
<date day="18" month="November" year="2015" />
<area>Transport</area>
<workgroup>TSVWG Working Group</workgroup>
<keyword></keyword>
<keyword></keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<!-- text starts here -->
<section title="Introduction" toc="include">
<t><xref target="RFC2309"></xref> discusses the dangers of
congestion-unresponsive flows and also 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 <xref
target="ID-ietf-tsvwg-RFC5405.bis"></xref>. Such mechanisms can be
designed to react on much shorter timescales than a Circuit Breaker,
that only observes a traffic envelope. Congestion-control mechanisms can
also interact with an application to more effectively control its
sending rate. However, not all traffic is known to respond to the onset
of congestion.</t>
<t>A network transport Circuit Breaker (CB) is an automatic mechanism
that is used to continuously monitor a flow or aggregate set of flows to
detect when the flow(s) experience persistent excessive congestion. When
this is detected the Circuit Breaker terminates (or significantly
reduces 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.</t>
<t>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.</t>
<t>In networking, the Circuit Breaker principle can be used as a
protection mechanism of last resort to avoid persistent excessive
congestion impacting other flows that share network capacity. Persistent
excessive congestion was a feature of the early Internet of the 1980s.
This resulted in excess traffic starving another connection from access
to the Internet. It was countered by the requirement to use congestion
control (CC) by the Transmission Control Protocol (TCP) <xref
target="Jacobsen88"></xref>. These mechanisms operate in Internet hosts
to cause TCP connections to "back off" during congestion. The
introduction of a congest control in TCP (currently documented in <xref
target="RFC5681"></xref> 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
non-congestion-controlled traffic to be considered to avoid persistent
excessive congestion impacting other flows. This is expected to also
help reduce the potential for "Congestion Collapse" <xref
target="RFC2914"></xref>.</t>
<t>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
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.</t>
<t>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.</t>
<t>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.</t>
<t><xref target="Require"></xref> defines requirements for building a
Circuit Breaker.</t>
<t>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:<list style="symbols">
<t>anomalous traffic that exceeds the provisioned capacity (or whose
traffic characteristics exceed the threshold configured for the
Circuit Breaker);</t>
<t>traffic generated by an application at a time when the
provisioned network capacity is being utilised for other
purposes;</t>
<t>routing changes that cause additional traffic to start using the
path monitored by the Circuit Breaker;</t>
<t>misconfiguration of a service/network device where the capacity
available is insufficient to support the current traffic
aggregate;</t>
<t>misconfiguration of an admission controller or traffic policer
that allows more traffic than expected across the path monitored by
the Circuit Breaker.</t>
</list></t>
<t>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.</t>
<t>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.</t>
<section title="Types of Circuit Breaker">
<t>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:<list
style="symbols">
<t>Fast-Trip Circuit Breakers: The relatively short timescale used
by this form of Circuit Breaker is intended to provide protection
for network traffic of a single non-responsive flow or related
group of non-responsive flows.</t>
<t>Slow-Trip Circuit Breakers: This Circuit Breaker utilizes a
longer timescale and is designed to protect network traffic from
congestion by non-responsive traffic aggregates.</t>
<t>Managed Circuit Breakers: Utilize the operations and management
functions that might be present in a managed service to implement
a Circuit Breaker.</t>
</list>Examples of each type of Circuit Breaker are provided in
section 4.</t>
</section>
</section>
<section title="Terminology" toc="include">
<t>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 <xref
target="RFC2119"></xref>.</t>
</section>
<section title="Design of a Circuit-Breaker (What makes a good circuit breaker?)"
toc="include">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>Section 3.1 describes the functional components of a Circuit Breaker
and section 3.2 defines requirements for implementing a Circuit
Breaker.</t>
<section title="Functional Components" toc="include">
<t>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.</t>
<t>In this example, 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.</t>
<figure>
<artwork><![CDATA[
+--------+ +--------+
|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
]]></artwork>
</figure>
<t>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.)</t>
<t>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), e.g., implemented as components
within a transport protocol.</t>
<t><figure>
<artwork><![CDATA[
+----------+ +----------+
| Ingress | +-+ +-+ +-+ | Egress |
| Endpoint +->+R+--+R+--+R+--+ Endpoint |
+--+----+--+ +-+ +-+ +-+ +----+-----+
^ | |
| +--+------+ +----+----+
| | Ingress | | Egress |
| | Meter | | Meter |
| +----+----+ +----+----+
| | |
| +--- +----+ |
| | Measure +<-----------------+
| +----+----+ Reported
| | Egress
| +----+----+ Measurement
| | Trigger |
| +----+----+
| |
+---<--+
Reaction
]]></artwork>
</figure></t>
<t>Figure 2: An endpoint CB implemented at the sender (ingress) and
receiver (egress).</t>
<t>The set of components needed to implement a Circuit Breaker
are:</t>
<t><list style="numbers">
<t>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).</t>
<t>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.</t>
<t>The measured values (measurements) 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 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.</t>
<t>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 that methods do 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.</t>
<t>A trigger function determines whether the measurements indicate
persistent excessive congestion. This function defines an
appropriate trigger interval and 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
measurements within the triggering interval <xref
target="RFC4553"></xref>). Designs need to be robust so that
single or spurious events do not trigger a reaction.</t>
<t>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.</t>
<t>A method for control communication control between the
components that provides appropriate security and is robust when
ingress and egress measurements are not available.</t>
</list></t>
</section>
</section>
<section anchor="Require"
title="Requirements for a Network Transport Circuit Breaker">
<t>The requirements for implementing a Circuit Breaker are:</t>
<t><list style="symbols">
<t>A Circuit Breaker REQUIRED to define a measurement function to
measure the level of congestion or loss. This does not have to
detect individual packet loss, but MUST specify a way to know that
packets have been lost/marked from the traffic flow. If Explicit
Congestion Notification (ECN) is enabled <xref
target="RFC3168"></xref>, an egress meter MAY also count the number
of ECN congestion marks/event per measurement interval, but even if
ECN is used, loss MUST still be measured, since this better reflects
the impact of persistent excessive congestion. In this context, loss
represents a reliable indication of congestion, as opposed to the
finer-grain marking of incipient congestion that can be provided via
ECN.</t>
<t>A Circuit Breaker is REQUIRED to define the period over each
measurement is made by the Circuit Breaker measurement function. The
measurement period MUST be longer than the time that current
congestion control mechanisms need to reduce their rate following
detection of congestion. This is important because end-to-end
congestion control mechanisms 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. A
sufficiently long period is needed to avoid additionally penalizing
flows with a long path RTT. The type of Circuit Breaker will
determine how long this measurement period needs to be, but it needs
to be significantly longer than the RTT experienced by the Circuit
Breaker itself.</t>
<t>If necessary, the measurement period MAY combine successive
individual meter samples from the ingress and egress to ensure
observation 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.)</t>
<t>A Circuit Breaker is REQUIRED to define the triggering interval.
This is the period over which the trigger uses the collected
measurements.</t>
<t>A Circuit Breaker is REQUIRED to define a threshold to determine
whether the measurements indicate that congestion is excessive. This
SHOULD be constructed so that it does not trigger under light or
intermittent congestion and MUST be robust to multiple congestion
events per triggering period. For example, a Circuit Breaker is
expected to monitor over several measurement periods to determine
whether the Circuit Breaker is to be triggered. (e.g., triggered
when persistent excessive congestion is detected in at least 3 of
the measurement periods within the triggering interval).</t>
<t>Once triggered, the Circuit Breaker MUST react decisively by
disabling or significantly reducing traffic at the source (e.g.,
ingress). The reaction needs to be much more severe than that of a
congestion control mechanism (such as TCP's congestion control <xref
target="RFC5681"></xref> or TCP-Friendly Rate Control, TFRC <xref
target="RFC5348"></xref>), because the Circuit Breaker reacts to
more persistent excessive congestion and operates over longer
timescales (i.e., the overload condition will have persisted for a
longer time before the Circuit Breaker is triggered).</t>
<t>The default response to a trigger SHOULD cause the ingress to
disable all of the traffic flows managed by the Circuit Breaker.</t>
<t>A reaction that instead results in a reduction SHOULD reduce the
traffic by at least an order of magnitude. A response that achieves
the reduction by terminating flows, rather than uniformally dropping
packets across multiple flows, 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.</t>
<t>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
mechanisms that control traffic over a common path). It SHOULD NOT
perform an automated reset when there is evidence of continued
congestion.</t>
<t>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.</t>
<t>A Circuit Breaker needs a communication path for control between
the ingress and the egress meters and other components. The source
and integrity of control information (measurements and triggers)
MUST be protected from off-path attacks (<xref
target="sec"></xref>). When there is a risk of on-path attack, a
cryptographic authentication mechanism for all control/measurement
messages is RECOMMENDED (<xref target="sec"></xref>).</t>
<t>Control communication can 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. <list style="hanging">
<t hangText="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 (e.g., failure to
receive measurement reports) ought to cause the Circuit Breaker
to be triggered. (Because the feedback signal could itself be
lost under congestion, this needs to confirm the absence of
congestion, rather than relying on the successful transmission
of a "congested" signal back to the sender.) This design has the
advantage that it provides fate-sharing of the traffic flow(s)
and the control communications.</t>
<t hangText="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 measurement reports). 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.</t>
</list></t>
<t>The Circuit Breaker MUST be designed to be robust to loss of
control messages 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 duplicate control messages over time 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 <xref
target="ID-ietf-tsvwg-RFC5405.bis"></xref>.</t>
<t>The volume of control traffic ought to be considered when
provisioning a network that uses a Circuit Breaker.</t>
</list></t>
</section>
<section title="Other network topologies">
<t>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.</t>
<section title="Use with a multicast control/routing protocol">
<t><figure>
<artwork><![CDATA[
+----------+ +--------+ +----------+
| Ingress | +-+ +-+ +-+ | Egress | | Egress |
| Endpoint +->+R+--+R+--+R+--+ Router |--+ Endpoint +->+
+----+-----+ +-+ +-+ +-+ +---+--+-+ +----+-----+ |
^ ^ ^ ^ | ^ | |
| | | | | | | |
+----+----+ + - - - < - - - - + | +----+----+ | Reported
| Ingress | multicast Prune | | Egress | | Ingress
| Meter | | | Meter | | Measurement
+---------+ | +----+----+ |
| | |
| +----+----+ |
| | Measure +<--+
| +----+----+
| |
| +----+----+
multicast | | Trigger |
Leave | +----+----+
Message | |
+----<----+
]]></artwork>
</figure>Figure 3: An example of a multicast CB controlling the
end-to-end path between an ingress endpoint and an egress
endpoint.</t>
<t>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, <xref target="FCB"></xref>). 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.</t>
<t>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).</t>
<t>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 multicast router), which causes the upstream
multicast router to cease forwarding traffic to the egress
endpoint.</t>
<t>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: In
this example, there is no direct communication back to 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.</t>
<t>The method could also be used with a multicast tunnel or subnetwork
(e.g., <xref target="SCB"></xref>, <xref target="MCB"></xref>), where
a meter at the ingress generates additional control messages to carry
the measurement data towards the point where the egress metering is
implemented.</t>
</section>
<section title="Use with control protocols supporting pre-provisioned capacity">
<t>Some network 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.</t>
<t>Figure 1 shows one expected use case, where in this usage a
separate device could 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 <xref
target="MCB"></xref>.</t>
</section>
<section title="Unidirectional Circuit Breakers over Controlled Paths">
<t>A Circuit Breaker can be used to control uni-directional traffic
where the traffic does not itself provide congestion control (e.g.,
there is no feedback of congestion information at the transport or
higher layers), 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.</t>
</section>
</section>
<section title="Examples of Circuit Breakers">
<t>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:</t>
<section anchor="FCB" title="A Fast-Trip Circuit Breaker">
<t>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).</t>
<t>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 transport system), where they can
also compliment end-to-end congestion control mechanism. 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).</t>
<section title="A Fast-Trip Circuit Breaker for RTP">
<t>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 <xref target="RTP-CB"></xref>. 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.</t>
<t>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
<xref target="RFC6679"></xref>.</t>
<t>The Circuit Breaker reaction (shutdown of the flow) is triggered
when any of the following trigger conditions are true:</t>
<t><list style="numbers">
<t>An RTP Circuit Breaker triggers on reported lack of
progress.</t>
<t>An RTP Circuit Breaker triggers when no receiver reports
messages are received.</t>
<t>An RTP Circuit Breaker uses a TFRC-style check and sets a
hard upper limit to the long-term RTP throughput (over many
RTTs).</t>
<t>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.</t>
</list></t>
</section>
</section>
<section anchor="SCB" title="A Slow-trip Circuit Breaker">
<t>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.</t>
<t>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 <xref
target="ID-ietf-tsvwg-RFC5405.bis"></xref> 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.</t>
</section>
<section anchor="MCB" title="A Managed Circuit Breaker">
<t>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".</t>
<t>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.</t>
<t>The control messages can use either in-band or out-of-band
communications.</t>
<section title="A Managed Circuit Breaker for SAToP Pseudo-Wires">
<t><xref target="RFC4553"></xref>, SAToP Pseudo-Wires (PWE3),
section 8 describes an example of a managed Circuit Breaker for
isochronous flows.</t>
<t>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. <xref target="RFC4553"></xref> 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...".</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="A Managed Circuit Breaker for Pseudowires (PWs)">
<t>Pseudowires (PWs) <xref target="RFC3985"></xref> 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.</t>
<t><xref target="ID-ietf-pals-congcons"></xref> 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 <xref target="RFC4488"></xref>) do not
raise major concerns because all of the traffic involved responds,
reducing the transmission rate when network congestion is
detected.</t>
<t>In contrast, inelastic PWs (e.g., a fixed bandwidth Time Division
Multiplex, TDM) <xref target="RFC4553"></xref> <xref
target="RFC5086"></xref> <xref target="RFC5087"></xref>) 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. <xref
target="ID-ietf-pals-congcons"></xref> 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.</t>
</section>
</section>
</section>
<section title="Examples where circuit breakers may not be needed. ">
<t>A Circuit Breaker is not required for a single congestion-controlled
flow using TCP, SCTP, TFRC, etc. In these cases, the congestion control
mechanisms are already designed to prevent persistent excessive
congestion.</t>
<section title="CBs over pre-provisioned Capacity">
<t>One common question is whether a Circuit Breaker is needed when a
tunnel is deployed in a private network with pre-provisioned
capacity.</t>
<t>In this case, compliant traffic that does not exceed the
provisioned capacity ought not to result in persistent excessive
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
excessive congestion occurs in an operational network.</t>
<t>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.</t>
</section>
<section title="CBs with tunnels carrying Congestion-Controlled Traffic">
<t>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 <xref
target="ID-ietf-tsvwg-RFC5405.bis"></xref>. A question therefore
arises when people deploy a tunnel that is thought to only carry an
aggregate of TCP (or some other congestion control) traffic: Is there
advantage in this case in using a Circuit Breaker?</t>
<t>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
excessive 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 an
appropriately configured Circuit Breaker will not reduce performance
of the tunnel, but in the event that persistent excessive congestion
occurs this protects other network traffic that shares capacity with
the tunnel traffic.</t>
</section>
<section title="CBs with Uni-directional Traffic and no Control Path">
<t>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 and
respond to congestion indications).</t>
<t>A way to mitigate the impact on other flows when capacity could be
shared is to manage the traffic envelope by using ingress
policing.§</t>
<t>Supporting this type of traffic in the general Internet requires
operator monitoring to detect and respond to persistent excessive
congestion.</t>
</section>
</section>
<section anchor="sec" title="Security Considerations" toc="include">
<t>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.</t>
<t>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.</t>
<t>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 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.</t>
<t>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.</t>
<t>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 <xref target="Require"></xref>.</t>
<t>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.</t>
</section>
<section title="IANA Considerations" toc="include">
<t>This document makes no request from IANA.</t>
</section>
<section title="Acknowledgments">
<t>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).</t>
</section>
<section title="Revision Notes">
<t>XXX RFC-Editor: Please remove this section prior to publication
XXX</t>
<t>Draft 00</t>
<t>This was the first revision. Help and comments are greatly
appreciated.</t>
<t>Draft 01</t>
<t>Contained clarifications and changes in response to received
comments, plus addition of diagram and definitions. Comments are
welcome.</t>
<t>WG Draft 00</t>
<t>Approved as a WG work item on 28th Aug 2014.</t>
<t>WG Draft 01</t>
<t>Incorporates feedback after Dallas IETF TSVWG meeting. This version
is thought ready for WGLC comments. Definitions of abbreviations.</t>
<t>WG Draft 02</t>
<t>Minor fixes for typos. Rewritten security considerations section.</t>
<t>WG Draft 03</t>
<t>Updates following WGLC comments (see TSV mailing list). Comments from
C Perkins; D Black and off-list feedback.</t>
<t>A clear recommendation of intended scope.</t>
<t>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).</t>
<t>WG Draft 04</t>
<t>Added section on PW CB for TDM - a newly adopted draft (D.
Black).</t>
<t>WG Draft 05</t>
<t>Added clarifications requested during AD review.</t>
<t>WG Draft 06</t>
<t>Fixed some remaining typos.</t>
<t>Update following detailed review by Bob Briscoe, and comments by D.
Black.</t>
<t>WG Draft 07</t>
<t>Additional update following review by Bob Briscoe.</t>
<t>WG Draft 08</t>
<t>Updated text on the response to lack of meter measurements with
managed Circuit Breakers. Additional comments from Eliot Lear (APPs
area).</t>
<t>WG Draft 09</t>
<t>Updated text on applications from Eliot Lear. Additional feedback
from Bob Briscoe. Comments from David Black and Mirja Kuehlewind.
Resulted in change of terminology to describe this as reacting to
"persistent excessive congestion", and more consistent use of
"congestion control mechanisms". The requirements section was reordered
and repetition removed to ease reading. Moved text on value of CC to
front of document.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<!-- -->
<references title="Normative References">
<?rfc sortrefs="yes"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3168.xml"
?>
<reference anchor="ID-ietf-tsvwg-RFC5405.bis">
<front>
<title>UDP Usage Guidelines (Work-in-Progress)</title>
<author fullname="Lars" initials="L" surname="Eggert">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="Gorry" initials="G" surname="Fairhurst">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="Greg" initials="G" surname="Shepherd">
<organization></organization>
</author>
<date year="2015" />
</front>
</reference>
</references>
<references title="Informative References">
<reference anchor="Jacobsen88">
<front>
<title>Congestion Avoidance and Control", SIGCOMM Symposium
proceedings on Communications architectures and protocols</title>
<author fullname="Jacobson, V.">
<organization>European Telecommunication Standards, Institute
(ETSI)</organization>
</author>
<date month="August" year="1998" />
</front>
</reference>
<reference anchor="RTP-CB">
<front>
<title>Multimedia Congestion Control: Circuit Breakers for Unicast
RTP Sessions</title>
<author fullname="C. S. Perkins" surname="Perkins">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="V. Singh" surname="Singh">
<organization></organization>
</author>
<date month="February" year="2014" />
</front>
</reference>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.1112.xml"
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<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2309.xml"
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<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.2914.xml"
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<?rfc ?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.3985.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.4488.xml"?>
<?rfc ?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.4553.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5086.xml"
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<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5087.xml"
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<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5348.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5681.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6679.xml"?>
<reference anchor="ID-ietf-pals-congcons">
<front>
<title>Pseudowire Congestion Considerations
(Work-in-Progress)</title>
<author fullname="Yaakov" initials="YJ" surname="Stein">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="David" initials="D" surname="Black">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="Bob" initials="B" surname="Briscoe">
<organization></organization>
</author>
<date year="2015" />
</front>
</reference>
<?rfc ?>
</references>
</back>
</rfc>
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