wdiff rfc8084.original rfc8084.txt

TSVWG Working Group
Internet Engineering Task Force (IETF) G. Fairhurst
Internet-Draft
Request for Comments: 8084 University of Aberdeen
Intended status:
BCP: 208 March 2017
Category: Best Current Practice April 04, 2016
Expires: October 6, 2016
ISSN: 2070-1721

Network Transport Circuit Breakers
draft-ietf-tsvwg-circuit-breaker-15

Abstract

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

Status of This Memo

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This Internet-Draft will expire on October 6, 2016.
http://www.rfc-editor.org/info/rfc8084.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 ....................................................2
1.1. Types of Circuit Breaker . . . . . . . . . . . . . . . . 6 CBs ...............................................5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 .....................................................6
3. Design of a Circuit-Breaker CB (What makes a good circuit
breaker?) . . . . . . . . . . . . . . . . . . . . . . . . . . 6 CB?) ..........................6
3.1. Functional Components . . . . . . . . . . . . . . . . . . 7 ......................................6
3.2. Other network topologies . . . . . . . . . . . . . . . . 10 Network Topologies ...................................9
3.2.1. Use with a multicast control/routing protocol . . . . 10 Multicast Control/Routing Protocol ......10
3.2.2. Use with control protocols supporting pre-provisioned
capacity . . . . . . . . . . . . . . . . . . . . . . 11 Control Protocols Supporting
Pre-provisioned Capacity ...........................11
3.2.3. Unidirectional Circuit Breakers CBs over Controlled Paths 12 ...........11
4. Requirements for a Network Transport Circuit Breaker . . . . 12 CB ........................12
5. Examples of Circuit Breakers . . . . . . . . . . . . . . . . 15 CBs ................................................15
5.1. A Fast-Trip Circuit Breaker . . . . . . . . . . . . . . . 15 CB ............................................15
5.1.1. A Fast-Trip Circuit Breaker CB for RTP . . . . . . . . . 16 .............................16
5.2. A Slow-trip Circuit Breaker . . . . . . . . . . . . . . . 17 Slow-Trip CB ............................................16
5.3. A Managed Circuit Breaker . . . . . . . . . . . . . . . . 17 CB ..............................................17
5.3.1. A Managed Circuit Breaker CB for SAToP Pseudo-Wires . . 17 Pseudowires .................17
5.3.2. A Managed Circuit Breaker CB for Pseudowires (PWs) . . . 18 .................18
6. Examples where circuit breakers may not be needed. . . . . . 19 in Which CBs May Not Be Needed ........................19
6.1. CBs over pre-provisioned Pre-provisioned Capacity . . . . . . . . . . . . 19 .........................19
6.2. CBs with tunnels carrying Tunnels Carrying Congestion-Controlled Traffic . 19 ...19
6.3. CBs with Uni-directional Unidirectional Traffic and no No Control Path . . 20 .......20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20 ........................................20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
10. Revision Notes . . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. .....................................................22
8.1. Normative References . . . . . . . . . . . . . . . . . . 25
11.2. ......................................22
8.2. Informative References . . . . . . . . . . . . . . . . . 25 ....................................22
Acknowledgments ...................................................23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 27 ..................................................23

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 (CB) is intended as a
protection mechanism of last resort. Under normal circumstances, a Circuit
Breaker
CB 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 CB (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 (CB) 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]. [Jacobson88]. 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] [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
long lived (ensuring that they could detect and respond to congestion
before the flows terminated). Although TCP
was was, by far far, the dominant
traffic, this is no longer the always the case, and non-congestion-controlled 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 To avoid persistent excessive
congestion, the current Internet therefore requires consideration of
the way that non-congestion-controlled traffic is considered to avoid persistent excessive congestion. forwarded.

A network transport Circuit Breaker CB 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 CB terminates (or significantly reduce
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. Avoiding persistent excessive congestion is important to
reduce the potential for "Congestion Collapse" [RFC2914].

There are important differences between a transport Circuit Breaker CB and a
congestion control method. Congestion control (as implemented in
TCP, SCTP, Stream Control Transmission Protocol (SCTP), and DCCP) Datagram
Congestion Control Protocol (DCCP)) operates on a timescale on the
order of a packet round-trip-time (RTT), 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 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).

In contrast, Circuit Breakers CBs are recommended for non-congestion-
controlled 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 CBs 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 CBs
may or may not be desirable.

A Circuit Breaker CB 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 CB 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 RTT (e.g., seconds
to possibly many tens of seconds). This longer period is needed to
provide sufficient time for transport congestion control
(or applications) 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 CB is not accidentally trigger
triggered following a single (or even successive) congestion events.
event(s).

Once triggered, the Circuit Breaker CB 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. CB.

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

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

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

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

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

In many cases, the reason for triggering a Circuit Breaker CB will not be evident to
the source of the traffic (user, application, endpoint,
etc). etc.). A Circuit Breaker CB
can be used to limit traffic from applications that are unable, or
choose not, to use congestion
control, control or where in cases in which 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 CB to signal back to
the impacted applications. In some cases cases, applications could
therefore have difficulty in determining that a
Circuit Breaker CB has triggered, been 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 it is expected to avoid triggering a network Circuit
Breaker. CB. For
applications that can generate elastic traffic, this will often be a
preferred solution.

1.1. Types of Circuit Breaker CBs

There are various forms of network transport circuit breaker. CBs. 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: CBs: The relatively short timescale used by this form of circuit breaker
CB is intended to provide protection for network traffic from a
single flow or related group of flows.

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

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

Examples of each type of circuit breaker CB are provided in section 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 CB (What makes a good circuit breaker?) CB?)

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

Circuit Breakers

CBs are RECOMMENDED for IETF protocols and tunnels that carry non-congestion-controlled 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 the design of a Circuit
Breaker CB and
provides examples of how a Circuit Breaker CB can be constructed. The specifications
of individual protocols and tunnel encapsulations need to detail the
protocol mechanisms needed to implement a Circuit Breaker. CB.

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

3.1. Functional Components

The basic design of a Circuit Breaker CB 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
Figure 1. This shows a set of routers (each labelled labeled R) connecting a
set of endpoints.

A Circuit Breaker CB 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 point; 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 Note in some cases, the trigger
and measurement functions could alternatively be located at other
locations (e.g., at a network operations centre.) center).

In the context of a Circuit Breaker, CB, 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 Figure 2), implemented as components within a transport
protocol.

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

The set of components needed to implement a Circuit Breaker CB 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 CB 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 it
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 synchronized
(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 CB Measurement function. This could use several
methods including: Sending 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 signaling datagram to the trigger
function. This could also be implemented purely as a control 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.) 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
CB is triggered when the threshold is exceeded in multiple
measurement intervals (e.g., 3 three 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 ingress when the Circuit
Breaker CB 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).

3.2. Other network topologies Network Topologies

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

3.2.1. Use with a multicast control/routing protocol Multicast Control/Routing Protocol

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 CB could be implemented
at a pair of multicast endpoints (e.g., to implement a Fast-Trip Circuit Breaker, CB,
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 it 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). endpoints.

In this figure, Figure 3, each endpoint includes a meter that performs a local
egress load measurement. An endpoint also extracts the received
ingress measurement from the traffic, traffic and compares the ingress and
egress measurements to determine if the Circuit Breaker CB ought to be triggered.
This measurement has to be robust to loss (see the previous section).
If the Circuit Breaker CB is triggered, it generates a multicast leave message for
the egress (e.g., an IGMP or MLD message sent to the last hop 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 CB "reaction" is executed by the multicast
control plane (e.g., by PIM) requiring no explicit
signalling signaling by the Circuit Breaker
CB along the communication path used for the control messages. Note: Note
there is no direct communication with the Ingress, and hence ingress; hence, a triggered Circuit Breaker
CB only controls traffic downstream of the first hop first-hop multicast
router. It does not stop traffic flowing from the sender to the first hop
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 Control Protocols Supporting Pre-provisioned Capacity

Some paths are provisioned using a control protocol, e.g., flows
provisioned using the Multi-Protocol Multiprotocol Label Switching (MPLS) services,
paths provisioned using the resource reservation protocol Resource Reservation Protocol (RSVP),
networks utilizing Software Defined 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 network-control
message sent to the ingress and/or other network elements causing
these elements to react to the Circuit Breaker. CB. Examples of this type of use are
provided in section Section 5.3.

3.2.3. Unidirectional Circuit Breakers CBs over Controlled Paths

A Circuit Breaker CB can be used to control uni-directional unidirectional UDP traffic, providing
that there is a communication path that can be used for control
messages to connect the functional components at the Ingress ingress and Egress.
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 CB

The requirements for implementing a Circuit Breaker CB 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 it MUST have a way to know that packets have been lost/
marked
lost/marked from the traffic flow.

3. An egress meter can also count ECN [RFC3168] congestion Congestion
Experienced (CE) 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]
[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 end-to-end
RTTs. The measurement period is therefore expected to be
significantly longer than the RTT experienced by the CB itself.

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 cannot be directly
combined to generate an average value.)

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

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

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

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

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

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

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

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

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

15. The control communication needs to carry measurements
(requirement 1) and, in some uses, also needs to transmit
trigger messages to the ingress. This control communication may
be in-band in or out-of-band. 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:

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:

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

16. The control communication MUST be designed to be robust to
packet loss. A control message can be lost if there is a
failure of the communication path used for the control messages,
loss is likely to also to 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 loss and/or to regard a lack of control
traffic as an indication that excessive congestion could be
being experienced [ID-ietf-tsvwg-RFC5405.bis]. [RFC8085]. If control
messages message traffic are 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. CB.

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

5. Examples of Circuit Breakers CBs

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

5.1. A Fast-Trip Circuit Breaker CB

[RFC2309] discusses the dangers of congestion-unresponsive congestion unresponsive flows and
states that "all UDP-based streaming applications should incorporate
effective congestion avoidance mechanisms". mechanisms." Some applications do not
use a full-featured transport (TCP, SCTP, DCCP). These applications
(e.g., using UDP and its UDP-Lite variant) need to provide
appropriate congestion avoidance. Guidance for applications that do
not use congestion-controlled transports is provided in
[ID-ietf-tsvwg-RFC5405.bis]. [RFC8085].
Such mechanisms can be designed to react on much shorter timescales
than a Circuit Breaker, CB, 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 Fast-trip CB is the most responsive form of Circuit
Breaker. CB. 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 complement end-to-end congestion control methods. A shorter
response time enables these mechanisms to triggers before other forms
of Circuit Breaker CB (e.g., Circuit Breakers CBs operating on traffic aggregates at a point along the
network path).

5.1.1. A Fast-Trip Circuit Breaker CB for RTP

A set of fast-trip Circuit Breaker Fast-Trip CB methods have been specified for use together by
a Real-time Transport Protocol (RTP) flow using the RTP/AVP Profile [RTP-CB].
[RFC8083]. 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 CBs. An RTP Circuit Breaker Fast-Trip CB is
therefore implemented as a fail-safe that that, when triggered triggered, will
terminate RTP traffic.

The sending endpoint monitors reception of in-band RTP Control
Protocol (RTCP) reception report blocks, as contained in SR sender
report (SR) or RR receiver report (RR) packets, that convey reception
quality feedback information. This is used to measure (congestion)
loss, possibly in combination with ECN [RFC6679].

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

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

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

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

5.2. A Slow-trip Circuit Breaker Slow-Trip CB

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

One example where a slow-trip Circuit Breaker Slow-Trip CB is needed is where flows or traffic-aggregates 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 [RFC8085], 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 CB

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

A Circuit Breaker CB requires more than the ability to determine that a network path
is forwarding data, 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.

5.3.1. A Managed Circuit Breaker CB for SAToP Pseudo-Wires Pseudowires

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

If such flows were to run over a pre-provisioned (e.g., Multi-
Protocol Multiprotocol
Label Switching, MPLS) infrastructure, then it could be expected that
the Pseudowire (PW) 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". '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 [RFC4553] goes on to state that "If
such a condition is detected, a SAToP PW ought to shut down bidirectionally bi-
directionally for some period of time...".

The concept was that when the packet loss packet-loss ratio (congestion) level
increased above a threshold, the PW was was, by default 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 a rate that at which the PW was is likely to
experience a serious problem, possibly making the service non-
compliant.
noncompliant. At this point, triggering the Circuit Breaker CB would remove the
traffic preventing undue impact on congestion-responsive traffic
(e.g., TCP). Part of the rationale, rationale was that high loss high-loss ratios
typically indicated that something was "broken" and ought to have
already resulted in operator intervention, intervention and therefore now need to
trigger this intervention.

An operator-based response to the triggering of a Circuit Breaker CB provides an
opportunity for other action to restore the service
quality, e.g., quality (e.g., by
shedding other loads or assigning additional
capacity, 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, center, NOC) that is responsible for operation of the
tunnel ingress, rather than the tunnel ingress itself.

5.3.2. A Managed Circuit Breaker CB for Pseudowires (PWs)

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

[ID-ietf-pals-congcons]

[RFC7893] discusses congestion conditions that can arise when PWs
compete with elastic (i.e., congestion responsive) network traffic (e.g,
(e.g., TCP traffic). Elastic PWs carrying IP traffic (see [RFC4488]) [RFC4448])
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 fixed-bandwidth Time Division
Multiplex, TDM) TDM [RFC4553] [RFC5086] [RFC5087]) have the potential to
harm congestion responsive 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] [RFC7893] 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 CB that shuts down a PW
when it persistently fails to deliver acceptable TDM service is a
useful means for addressing these congestion concerns. (See
Appendix A of [ID-ietf-pals-congcons] [RFC7893] for further discussion.)

6. Examples where circuit breakers may not be needed. in Which CBs May Not Be Needed

A Circuit Breaker CB 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 Pre-provisioned Capacity

One common question is whether a Circuit Breaker CB 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 CB will
hence only be triggered when there is non-compliant noncompliant traffic. It could
be argued that this event ought never to happen - -- but it could also
be argued that the Circuit Breaker CB equally ought never to be triggered. If a Circuit Breaker CB
were to be implemented, it will provide an appropriate response response, if
persistent congestion occurs in an operational network.

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

6.2. CBs with tunnels carrying Tunnels Carrying Congestion-Controlled Traffic

IP-based traffic is generally assumed to be congestion-controlled, 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 Therefore, a question therefore arises when
people deploy a tunnel that is thought to only carry only an aggregate of
TCP traffic (or traffic using some other congestion control method):
Is there an advantage in this case in using a Circuit Breaker? CB?

TCP (and SCTP) traffic in a tunnel is expected to reduce the
transmission rate when network congestion is detected. Other
transports (e.g, (e.g., using UDP) can employ mechanisms that are
sufficient to address congestion on the path [ID-ietf-tsvwg-RFC5405.bis]. [RFC8085]. However,
even if the individual flows sharing a tunnel each implement a
congestion control mechanism, and individually reduce their
transmission rate when network congestion is detected, the overall
traffic resulting from the aggregate of the flows does not
necessarily avoid persistent congestion. For instance, most
congestion control mechanisms require long-lived flows to react to
reduce the rate of a flow. An aggregate of many short flows could
result in many flows terminating before they experience congestion.

It is also often impossible for a tunnel service provider to know
that the tunnel only contains congestion-controlled traffic (e.g.,
Inspecting packet headers might not be possible). Some IP-based
applications might not implement adequate mechanisms to address
congestion. The important thing to note is that if the aggregate of
the traffic does not result in persistent excessive congestion
(impacting other flows), then the Circuit Breaker CB will not trigger. This is the
expected case in this context - -- so implementing a Circuit
Breaker CB ought not to
reduce performance of the tunnel, but in the event that persistent
excessive congestion occurs occurs, the Circuit Breaker CB protects other network traffic
that shares capacity with the tunnel traffic.

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

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

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

7. Security Considerations

All Circuit Breaker CB mechanisms rely upon coordination between the ingress and
egress meters and communication with the trigger function. This is
usually achieved by passing network control network-control information (or protocol
messages) across the network. Timely operation of a Circuit Breaker CB depends on
the choice of measurement period. If the receiver has an interval
that is overly long, then the responsiveness of the Circuit Breaker CB decreases.
This impacts the ability of the Circuit Breaker CB to detect and react to congestion.
If the interval is too short short, the Circuit Breaker CB could trigger prematurely
resulting in insufficient time for other mechanisms to
act, act and
potentially resulting in unnecessary disruption to the service.

A Circuit Breaker CB could potentially be exploited by an attacker to mount a Denial of Service Denial-
of-Service (DoS) attack against the traffic being controlled by the Circuit Breaker. Mechanisms therefore
CB. Therefore, mechanisms need to be implemented to prevent attacks
on the network control 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 CB to trigger and to therefore to 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 scenarios, the value of
additional protection from DoS attack attacks will therefore lead to a
requirement to authenticate all control messages.

Transmission of network control network-control messages consumes network capacity.
This control traffic needs to be considered in the design of a
Circuit Breaker CB and
could potentially add to network congestion. If this traffic is sent
over a shared path, it is RECOMMENDED that this control traffic is be
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. CB.

The Circuit Breaker CB 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 it 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 CB MUST therefore evaluate whether the
particular Circuit Breaker CB 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.

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).

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.

WG Draft 15 (incorporating pending corrections)

Corrected /applications might be implement/applications might not
implement/

Corrected /Inspecting packet headers could/Inspecting packet headers
might/

Removed Requirement 9, now duplicated (and renumbered remaining
items).

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

Simplified a sentence in section 6.1, without intended change of
meaning.

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

11. References

11.1.

8.1. 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.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <http://www.rfc-editor.org/info/rfc8085>.

8.2. 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]

[CONGESTION-FEEDBACK]
Wei, X., Zhu, L., and L. Dend, Deng, "Tunnel Congestion Feedback
(Work-in-Progress)", 2015.

[Jacobsen88]
European Telecommunication Standards, Institute (ETSI),
Feedback", Work in Progress,
draft-ietf-tsvwg-tunnel-congestion-feedback-03,
September 2016.

[Jacobson88]
Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
Symposium proceedings on Communications architectures
and
protocols", protocols, August 1998. 1988.

[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, DOI 10.17487/RFC1112, August 1989,
<http://www.rfc-editor.org/info/rfc1112>.

[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
<http://www.rfc-editor.org/info/rfc2309>.

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

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

[RFC4488] Levin, O., "Suppression

[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Session Initiation Protocol
(SIP) REFER Method Implicit Subscription", Ethernet over MPLS
Networks", RFC 4488, 4448, DOI 10.17487/RFC4488, May 10.17487/RFC4448, April 2006,
<http://www.rfc-editor.org/info/rfc4488>.
<http://www.rfc-editor.org/info/rfc4448>.

[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>.

[RTP-CB]

[RFC7893] Stein, Y(J)., Black, D., and B. Briscoe, "Pseudowire
Congestion Considerations", RFC 7893,
DOI 10.17487/RFC7893, June 2016,
<http://www.rfc-editor.org/info/rfc7893>.

[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions (draft-ietf-
avtcore-rtp-circuit-breakers)", February 2014. Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
<http://www.rfc-editor.org/info/rfc8083>.

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 partly
funded by the European Community under its Seventh Framework
Programme through the Reducing Internet Transport Latency (RITE)
project (ICT-317700).

Author's Address

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

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