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Keywords: Congestion control, CC, UDP, Tunnel, Encapsulation, Transport Protocol, Congestion Control
Internet Engineering Task Force (IETF) G. Fairhurst
Request for Comments: 8084 University of Aberdeen
BCP: 208 March 2017
Category: Best Current Practice
ISSN: 2070-1721
Network Transport Circuit Breakers
Abstract
This document explains what is meant by the term "network transport
Circuit Breaker". It describes the need for Circuit Breakers (CBs)
for network tunnels and applications when using non-congestion-
controlled traffic and explains where CBs are, and are not, needed.
It also defines requirements for building a CB and the expected
outcomes of using a CB within the Internet.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8084.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Fairhurst Best Current Practice [Page 1]
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Table of Contents
1. Introduction ....................................................2
1.1. Types of CBs ...............................................5
2. Terminology .....................................................6
3. Design of a CB (What makes a good CB?) ..........................6
3.1. Functional Components ......................................6
3.2. Other Network Topologies ...................................9
3.2.1. Use with a Multicast Control/Routing Protocol ......10
3.2.2. Use with Control Protocols Supporting
Pre-provisioned Capacity ...........................11
3.2.3. Unidirectional CBs over Controlled Paths ...........11
4. Requirements for a Network Transport CB ........................12
5. Examples of CBs ................................................15
5.1. A Fast-Trip CB ............................................15
5.1.1. A Fast-Trip CB for RTP .............................16
5.2. A Slow-Trip CB ............................................16
5.3. A Managed CB ..............................................17
5.3.1. A Managed CB for SAToP Pseudowires .................17
5.3.2. A Managed CB for Pseudowires (PWs) .................18
6. Examples in Which CBs May Not Be Needed ........................19
6.1. CBs over Pre-provisioned Capacity .........................19
6.2. CBs with Tunnels Carrying Congestion-Controlled Traffic ...19
6.3. CBs with Unidirectional Traffic and No Control Path .......20
7. Security Considerations ........................................20
8. References .....................................................22
8.1. Normative References ......................................22
8.2. Informative References ....................................22
Acknowledgments ...................................................24
Author's Address ..................................................24
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
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 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 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
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was countered by the requirement to use congestion control (CC) in
the Transmission Control Protocol (TCP) [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]) ensured the stability of
the Internet, because it was able to detect congestion and promptly
react. This was effective in an Internet where most TCP flows were
long lived (ensuring that they could detect and respond to congestion
before the flows terminated). Although TCP was, by far, the dominant
traffic, this is no longer the always the case, and non-congestion-
controlled traffic, including many applications using the User
Datagram Protocol (UDP), can form a significant proportion of the
total traffic traversing a link. To avoid persistent excessive
congestion, the current Internet therefore requires consideration of
the way that non-congestion-controlled traffic is forwarded.
A network transport 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 CB 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. Avoiding persistent excessive congestion is important to
reduce the potential for "Congestion Collapse" [RFC2914].
There are important differences between a transport CB and a
congestion control method. Congestion control (as implemented in
TCP, Stream Control Transmission Protocol (SCTP), and Datagram
Congestion Control Protocol (DCCP)) operates on a timescale on the
order of a packet Round-Trip Time (RTT): the time from sender to
destination and return. Congestion at a network link can also be
detected using Explicit Congestion Notification (ECN) [RFC3168],
which allows the network to signal congestion by marking ECN-capable
packets with a Congestion Experienced (CE) mark. Both loss and
reception of CE-marked packets are treated as congestion events.
Congestion control methods are able to react to a congestion event by
continuously adapting to reduce their transmission rate. The goal is
usually to limit the transmission rate to a maximum rate that
reflects a fair use of the available capacity across a network path.
These methods typically operate on individual traffic flows (e.g., a
5-tuple that includes the IP addresses, protocol, and ports).
In contrast, CBs 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)
Fairhurst Best Current Practice [Page 3]
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congestion. People have been implementing what this document
characterizes as 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 CBs
may or may not be desirable.
A 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 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 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 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 CB is not accidentally
triggered following a single (or even successive) congestion
event(s).
Once triggered, the 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 CB.
The operational conditions that cause a CB to trigger ought to be
regarded as abnormal. Examples of situations that could trigger a CB
include:
o anomalous traffic that exceeds the provisioned capacity (or whose
traffic characteristics exceed the threshold configured for the
CB);
o traffic generated by an application at a time when the provisioned
network capacity is being utilized for other purposes;
o routing changes that cause additional traffic to start using the
path monitored by the CB;
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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 CB.
Other mechanisms could also be available to network operators to
detect excessive congestion (e.g., an observation of excessive
utilization for a port on a network device). Utilizing such
information, operational mechanisms could react to reduce network
load over a shorter timescale than those of a network transport CB.
The role of the CB over such paths remains as a method of last
resort. Because it acts over a longer timescale, the CB ought to be
triggered only when other reactions did not succeed in reducing
persistent excessive congestion.
In many cases, the reason for triggering a CB will not be evident to
the source of the traffic (user, application, endpoint, etc.). A CB
can be used to limit traffic from applications that are unable, or
choose not, to use congestion control or 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 CB to signal back to
the impacted applications. In some cases, applications could
therefore have difficulty in determining that a CB has 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 CB. For
applications that can generate elastic traffic, this will often be a
preferred solution.
1.1. Types of CBs
There are various forms of network transport CBs. These are
differentiated mainly on the timescale over which they are triggered,
but also in the intended protection they offer:
o Fast-Trip CBs: The relatively short timescale used by this form of
CB is intended to provide protection for network traffic from a
single flow or related group of flows.
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o Slow-Trip CBs: This CB utilizes a longer timescale and is designed
to protect network traffic from congestion by traffic aggregates.
o Managed CBs: Utilize the operations and management functions that
might be present in a managed service to implement a CB.
Examples of each type of CB are provided in Section 4.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Design of a CB (What makes a good CB?)
Although CBs have been talked about in the IETF for many years, there
has not yet been guidance on the cases where CBs are needed or upon
the design of CB mechanisms. This document seeks to offer advice on
these two topics.
CBs are RECOMMENDED for IETF protocols and tunnels that carry non-
congestion-controlled Internet flows and for traffic aggregates.
This includes traffic sent using a network tunnel. Designers of
other protocols and tunnel encapsulations also ought to consider the
use of these techniques as a last resort to protect traffic that
shares the network path being used.
This document defines the requirements for the design of a CB and
provides examples of how a CB can be constructed. The specifications
of individual protocols and tunnel encapsulations need to detail the
protocol mechanisms needed to implement a CB.
Section 3.1 describes the functional components of a CB and
Section 3.2 defines requirements for implementing a CB.
3.1. Functional Components
The basic design of a 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 1. This shows a set of routers (each labeled R) connecting a
set of endpoints.
A 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
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be at the ingress and egress of a service, where all traffic being
considered terminates beyond the egress 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 some cases, the trigger
and measurement functions could alternatively be located at other
locations (e.g., at a network operations center).
In the context of a 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 2), implemented as components within a transport
protocol.
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+----------+ +----------+
| Ingress | +-+ +-+ +-+ | Egress |
| Endpoint +->+R+--+R+--+R+--+ Endpoint |
+--+----+--+ +-+ +-+ +-+ +----+-----+
^ | |
| +--+------+ +----+----+
| | Ingress | | Egress |
| | Meter | | Meter |
| +----+----+ +----+----+
| | |
| +--- +----+ |
| | Measure +<-----------------+
| +----+----+ Reported
| | Egress
| +----+----+ Measurement
| | Trigger |
| +----+----+
| |
+---<--+
Reaction
Figure 2: An endpoint CB implemented at the sender (ingress)
and receiver (egress).
The set of components needed to implement a 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 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 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.
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3. A method that communicates the measured values at the ingress and
egress to the CB Measurement function. This could use several
methods including sending return measurement packets (or control
messages) from a receiver to a trigger function at the sender; an
implementation using Operations, Administration and Management
(OAM); or sending an in-band signaling datagram to the trigger
function. This could also be implemented purely as a control-
plane function, e.g., using a software-defined network
controller.
4. A measurement function that combines the ingress and egress
measurements to assess the present level of network congestion.
(For example, the loss rate for each measurement interval could
be deduced from calculating the difference between ingress and
egress counter values.) Note the method does not require high
accuracy for the period of the measurement interval (or therefore
the measured value, since isolated and/or infrequent loss events
need to be disregarded).
5. A trigger function that determines whether the measurements
indicate persistent excessive congestion. This function defines
an appropriate threshold for determining that there is persistent
excessive congestion between the ingress and egress. This
preferably considers a rate or ratio, rather than an absolute
value (e.g., more than 10% loss, but other methods could also be
based on the rate of transmission as well as the loss rate). The
CB is triggered when the threshold is exceeded in multiple
measurement intervals (e.g., 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 when the CB is
triggered. This seeks to automatically remove the traffic
causing persistent excessive congestion.
7. A feedback control mechanism that triggers when either the
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
A CB can be deployed in networks with topologies different from that
presented in Figures 1 and 2. This section describes examples of
such usage and possible places where functions can be implemented.
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3.2.1. Use with a Multicast Control/Routing Protocol
+----------+ +--------+ +----------+
| Ingress | +-+ +-+ +-+ | Egress | | Egress |
| Endpoint +->+R+--+R+--+R+--+ Router |--+ Endpoint +->+
+----+-----+ +-+ +-+ +-+ +---+--+-+ +----+-----+ |
^ ^ ^ ^ | ^ | |
| | | | | | | |
+----+----+ + - - - < - - - - + | +----+----+ | Reported
| Ingress | multicast Prune | | Egress | | Ingress
| Meter | | | Meter | | Measurement
+---------+ | +----+----+ |
| | |
| +----+----+ |
| | Measure +<--+
| +----+----+
| |
| +----+----+
multicast | | Trigger |
Leave | +----+----+
Message | |
+----<----+
Figure 3: An example of a multicast CB controlling the end-to-end
path between an ingress endpoint and an egress endpoint.
Figure 3 shows one example of how a multicast CB could be implemented
at a pair of multicast endpoints (e.g., to implement a Fast-Trip 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 endpoints.
In 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 and compares the ingress and
egress measurements to determine if the CB ought to be triggered.
This measurement has to be robust to loss (see the previous section).
If the CB is triggered, it generates a multicast leave message for
the egress (e.g., an IGMP or MLD message sent to the last-hop
router), which causes the upstream router to cease forwarding traffic
to the egress endpoint [RFC1112].
Fairhurst Best Current Practice [Page 10]
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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 [RFC7761]). Each egress operates
autonomously, and the CB "reaction" is executed by the multicast
control plane (e.g., by PIM) requiring no explicit signaling by the
CB along the communication path used for the control messages. Note
there is no direct communication with the ingress; hence, a triggered
CB only controls traffic downstream of the first-hop multicast
router. It does not stop traffic flowing from the sender to the
first-hop router; this is common practice for multicast deployment.
The method could also be used with a multicast tunnel or subnetwork
(e.g., Section 5.2, Section 5.3), where a meter at the ingress
generates additional control messages to carry the measurement data
towards the egress where the egress metering is implemented.
3.2.2. Use with Control Protocols Supporting Pre-provisioned Capacity
Some paths are provisioned using a control protocol, e.g., flows
provisioned using the Multiprotocol Label Switching (MPLS) services,
paths provisioned using the Resource Reservation Protocol (RSVP),
networks utilizing Software-Defined Network (SDN) functions, or
admission-controlled Differentiated Services. Figure 1 shows one
expected use case, where in this usage a separate device could be
used to perform the measurement and trigger functions. The reaction
generated by the trigger could take the form of a network-control
message sent to the ingress and/or other network elements causing
these elements to react to the CB. Examples of this type of use are
provided in Section 5.3.
3.2.3. Unidirectional CBs over Controlled Paths
A CB can be used to control 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 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.
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4. Requirements for a Network Transport CB
The requirements for implementing a 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 from the traffic flow.
3. An egress meter can also count ECN [RFC3168] 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,
[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 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).
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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 three of the measurements within the
triggering 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 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.
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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 or out of band. The use of in-band communication is
RECOMMENDED when either design would be possible. The preferred
CB design is one that triggers when it fails to receive
measurement reports that indicate an absence of congestion, in
contrast to relying on the successful transmission of a
"congested" signal back to the sender. (The feedback signal
could itself be lost under congestion).
In Band: An in-band control method SHOULD assume that loss of
control messages is an indication of potential congestion on
the path, and repeated loss ought to cause the CB to be
triggered. This design has the advantage that it provides
fate-sharing of the traffic flow(s) and the control
communications. This fate-sharing property is weaker when
some or all of the measured traffic is sent using a path that
differs from the path taken by the control traffic (e.g.,
where traffic and control messages follow a different path
due to use of equal-cost multipath routing, traffic
engineering, or tunnels for specific types of traffic).
Out of Band: An out-of-band control method SHOULD NOT trigger 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; 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 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 and/or to regard a lack of control
traffic as an indication that excessive congestion could be
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being experienced [RFC8085]. If control message 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 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 CBs
There are multiple types of CB that could be defined for use in
different deployment cases. There could be cases where a flow
becomes controlled by multiple CBs (e.g., when the traffic of an end-
to-end flow is carried in a tunnel within the network). This section
provides examples of different types of CB.
5.1. A Fast-Trip CB
[RFC2309] discusses the dangers of congestion unresponsive flows and
states that "all UDP-based streaming applications should incorporate
effective congestion avoidance mechanisms." Some applications do not
use a full-featured transport (TCP, SCTP, DCCP). These applications
(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 [RFC8085].
Such mechanisms can be designed to react on much shorter timescales
than a 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 CB is the most responsive form of 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).
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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 complement end-to-end congestion control methods. A shorter
response time enables these mechanisms to triggers before other forms
of CB (e.g., CBs operating on traffic aggregates at a point along the
network path).
5.1.1. A Fast-Trip CB for RTP
A set of Fast-Trip CB methods have been specified for use together by
a Real-time Transport Protocol (RTP) flow using the RTP/AVP Profile
[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 CBs. An RTP Fast-Trip CB is
therefore implemented as a fail-safe that, when triggered, will
terminate RTP traffic.
The sending endpoint monitors reception of in-band RTP Control
Protocol (RTCP) reception report blocks, as contained in sender
report (SR) or receiver report (RR) packets, that convey reception
quality feedback information. This is used to measure (congestion)
loss, possibly in combination with ECN [RFC6679].
The CB action (shutdown of the flow) triggers when any of the
following trigger conditions are true:
1. An RTP CB triggers on reported lack of progress.
2. An RTP CB triggers when no receiver reports messages are
received.
3. An RTP 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 CB includes the notion of Media Usability. This CB is
triggered when the quality of the transported media falls below
some required minimum acceptable quality.
5.2. A Slow-Trip CB
A Slow-Trip CB could be implemented in an endpoint or network device.
This type of CB is much slower at responding to congestion than a
Fast-Trip CB. This is expected to be more common.
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One example where a Slow-Trip CB 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 [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 CB
A managed CB is implemented in the signaling protocol or management
plane that relates to the traffic aggregate being controlled. This
type of CB is typically applicable when the deployment is within a
"controlled environment".
A CB requires more than the ability to determine that a network path
is forwarding data or to measure the rate of a path -- which are
often normal network operational functions. There is an additional
need to determine a metric for congestion on the path and to trigger
a reaction when a threshold is crossed that indicates persistent
excessive congestion.
The control messages can use either in-band or out-of-band
communications.
5.3.1. A Managed CB for SAToP Pseudowires
Section 8 of [RFC4553], SAToP Pseudowire Emulation Edge-to-Edge
(PWE3), describes an example of a managed CB for isochronous flows.
If such flows were to run over a pre-provisioned (e.g., Multiprotocol
Label Switching, MPLS) infrastructure, then it could be expected that
the PW would not experience congestion, because a flow is not
expected to either increase (or decrease) their rate. If, instead,
PW traffic is multiplexed with other traffic over the general
Internet, it could experience congestion. [RFC4553] states: "If
SAToP PWs run over a PSN providing best-effort service, they SHOULD
monitor packet loss in order to detect 'severe congestion'." The
currently recommended measurement period is 1 second, and the trigger
operates when there are more than three measured Severely Errored
Seconds (SES) within a period. [RFC4553] goes on to state that "If
such a condition is detected, a SAToP PW ought to shut down
bi-directionally for some period of time...".
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The concept was that when the packet-loss ratio (congestion) level
increased above a threshold, the PW was, by default, disabled. This
use case considered fixed-rate transmission, where the PW had no
reasonable way to shed load.
The trigger needs to be set at a rate at which the PW is likely to
experience a serious problem, possibly making the service
noncompliant. At this point, triggering the CB 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 now need to
trigger this intervention.
An operator-based response to the triggering of a CB provides an
opportunity for other action to restore the service quality (e.g., by
shedding other loads or assigning additional capacity) or to
consciously avoid reacting to the trigger while engineering a
solution to the problem. This could require the trigger function to
send a control message to a third location (e.g., a network
operations center, NOC) that is responsible for operation of the
tunnel ingress, rather than the tunnel ingress itself.
5.3.2. A Managed 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.
[RFC7893] 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 [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 Time Division
Multiplex, TDM [RFC4553] [RFC5086] [RFC5087]) have the potential to
harm congestion-responsive traffic or to contribute to excessive
congestion because inelastic PWs do not adjust their transmission
rate in response to congestion. [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 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 [RFC7893] for further discussion.)
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6. Examples in Which CBs May Not Be Needed
A 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 Capacity
One common question is whether a 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 CB will
hence only be triggered when there is noncompliant traffic. It could
be argued that this event ought never to happen -- but it could also
be argued that the CB equally ought never to be triggered. If a CB
were to be implemented, it will provide an appropriate response, if
persistent congestion occurs in an operational network.
Implementing a CB 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. It
also protects network traffic from a failure when CB traffic is
(re)routed to cause additional network load on a non-pre-provisioned
path.
6.2. CBs with Tunnels Carrying Congestion-Controlled Traffic
IP-based traffic is generally assumed to be congestion controlled,
i.e., it is assumed that the transport protocols generating IP-based
traffic at the sender already employ mechanisms that are sufficient
to address congestion on the path. Therefore, a question arises when
people deploy a tunnel that is thought to 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 CB?
TCP (and SCTP) traffic in a tunnel is expected to reduce the
transmission rate when network congestion is detected. Other
transports (e.g., using UDP) can employ mechanisms that are
sufficient to address congestion on the path [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.
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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 CB will not trigger. This is the
expected case in this context -- so implementing a CB ought not to
reduce performance of the tunnel, but in the event that persistent
excessive congestion occurs, the CB protects other network traffic
that shares capacity with the tunnel traffic.
6.3. CBs with Unidirectional Traffic and No Control Path
A one-way forwarding path could have no associated communication path
for sending control messages; therefore, it cannot be controlled
using a 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 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 information (or protocol
messages) across the network. Timely operation of a CB depends on
the choice of measurement period. If the receiver has an interval
that is overly long, then the responsiveness of the CB decreases.
This impacts the ability of the CB to detect and react to congestion.
If the interval is too short, the CB could trigger prematurely
resulting in insufficient time for other mechanisms to act and
potentially resulting in unnecessary disruption to the service.
A CB could potentially be exploited by an attacker to mount a Denial-
of-Service (DoS) attack against the traffic being controlled by the
CB. Therefore, mechanisms need to be implemented to prevent attacks
on the network-control information that would result in DoS.
The authenticity of the source and integrity of the control messages
(measurements and triggers) MUST be protected from off-path attacks.
Without protection, it could be trivial for an attacker to inject
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fake or modified control/measurement messages (e.g., indicating high
packet loss rates) causing a CB to trigger and 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, the value of
additional protection from DoS attacks will therefore lead to a
requirement to authenticate all control messages.
Transmission of network-control messages consumes network capacity.
This control traffic needs to be considered in the design of a CB and
could potentially add to network congestion. If this traffic is sent
over a shared path, it is RECOMMENDED that this control traffic be
prioritized to reduce the probability of loss under congestion.
Control traffic also needs to be considered when provisioning a
network that uses a CB.
The 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 CB MUST therefore evaluate whether the
particular CB mechanism has new security implications.
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8. References
8.1. Normative References
[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>.
[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
[CONGESTION-FEEDBACK]
Wei, X., Zhu, L., and L. Deng, "Tunnel Congestion
Feedback", Work in Progress,
draft-ietf-tsvwg-tunnel-congestion-feedback-04,
January 2017.
[Jacobson88]
Jacobson, V., "Congestion Avoidance and Control", SIGCOMM
Symposium proceedings on Communications architectures
and protocols, August 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>.
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[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>.
[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
<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>.
[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>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <http://www.rfc-editor.org/info/rfc7761>.
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[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", 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
United Kingdom
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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