Internet DRAFT - draft-fairhurst-tsvwg
draft-fairhurst-tsvwg
TSVWG Working Group G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Standards Track April 12, 2014
Expires: October 14, 2014
Network Transport Circuit Breakers
draft-fairhurst-tsvwg-00
Abstract
This note explains what is meant by the term "transport circuit
breaker" in the context of an Internet tunnel service.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Types of Circuit-Breaker . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Designing a Circuit-Breaker (What makes a good circuit
breaker?) . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Basic Function . . . . . . . . . . . . . . . . . . . . . 6
4. Examples of Circuit Breakers . . . . . . . . . . . . . . . . 6
4.1. A fast-trip Circuit Breaker . . . . . . . . . . . . . . . 6
4.1.1. A fast-trip RTP Circuit Breaker . . . . . . . . . . . 7
4.2. A Slow-trip Circuit Breaker . . . . . . . . . . . . . . . 7
4.3. A Managed Circuit Breaker . . . . . . . . . . . . . . . . 8
4.3.1. A Managed Circuit Breaker for SAToP Pseudo-Wires . . 8
5. Examples where circuit breakers may not be needed. . . . . . 9
5.1. CBs and uni-directional Traffic . . . . . . . . . . . . . 9
5.2. CBs over pre-provisioned Capacity . . . . . . . . . . . . 9
5.3. CBs with CC Traffic . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. Revision Notes . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
A transport Circuit Breaker (CB) is an automatic mechanism that is
used to estimate congestion caused by a flow, and to terminate (or
significantly reduce the rate of) the flow when excessive congestion
is detected. This is a safety measure to prevent congestion collapse
(starvation of resources available to other flows), essential for an
Internet that is heterogeneous and for traffic that is hard to
predict in advance.
A CB is intended as a protection mechanism of last resort. Under
normal circumstances, a CB should not be triggered; It is designed to
protect things when there is overload. Just as people do not expect
the electrical circuit-breaker (or fuse) in their home to be
triggered, except when there is a wiring fault or a problem with an
electrical appliance.
Persistent congestion (also known as "congestion collapse") was a
feature of the early Internet of the 1980s. This resulted in excess
traffic starving other connection from access to the Internet. It
was countered by the requirement to use congestion control (CC) by
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the TCP transport protocol[Jacobsen88] [RFC1112]. These mechanisms
operate in Internet hosts to cause TCP connections to "back off"
during congestion. The introduction of CC in TCP (currently
documented in [RFC5681] ensured the stability of the Internet,
because it was able to detect congestion and promptly react. This
worked well while TCP was by far the dominant traffic in the
Internet, and most TCP flows were long-lived (ensuring that they
could detect and respond to congestion before the flows terminated).
This is no longer the case, and non-congestion controlled traffic,
such as UDP can form a significant proportion of the total traffic
traversing a link. The current Internet therefore requires that non-
congestion controlled traffic needs to be considered to avoid
congestion collapse.
There are important differences between a transport circuit-breaker
and a congestion-control method. Specifically, congestion control
(as implemented in TCP, SCTP, and DCCP) needs to operate on the
timescale on the order of a packet round-trip-time (RTT), the time
from sender to destination and return. Congestion control methods
may react to a single packet loss/marking and reduce the transmission
rate for each loss or congestion event. The goal is usually to limit
the maximum transmission rate that reflects the available capacity of
a network path. These methods typically operate on individual
traffic flows (e.g. a 5-tuple).
In contrast, CBs are recommended for traffic aggregates, e.g.traffic
sent using a network tunnel. Later sections provide examples of
cases where circuit-breakers may or may not be desirable.
A CB needs to be designed to trigger robustly when there is
persistent congestion. It will often operate on a much longer
timescale: many RTTs, possibly many 10s of seconds. This longer
period is needed to provide sufficient time for transports (or
applications) to adjust their rate following congestion, and for the
network load to stabilise after adjustment. A CB also needs to
decide if a reaction is required based on a series of successive
samples taken over a reasonably long period of time. This is to
ensure that a CB does not accidentally trigger following a single (or
even successive) congestion events (congestion events are what
triggers congestion control, and are to be regarded as normal on a
network link operating near its capacity).
1.1. Types of Circuit-Breaker
There are various forms of circuit breaker, which are differentiated
mainly on the timescale over which they are triggered, but also in
the intended protection they offer:
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o Fast-Trip Circuit Breakers: The relatively short timescale used by
this form of circuit breaker is intended to protect a flow or
related group of flows.
o Slow-Trip Circuit Breakers: This circuit breaker utilises a longer
timescale and is designed to protect traffic aggregates.
o Managed Circuit Breakers: Utilise the operations and management
functions that may be present in a managed service to implement a
circuit breaker.
Examples of each type of circuit breaker are provided in section 4.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Designing a Circuit-Breaker (What makes a good circuit breaker?)
Although circuit breakers have been talked about in the IETF for many
years, there has not yet been guidance on the cases where they are
need for or the design of circuit breaker mechanisms. This document
seeks to offer advise on these topics.
The basic design of a circuit breaker involves communication between
the sender and receiver of a network flow. It is assumed that a
sender can control the rate of the flow, but the effect of congestion
can only be measured at the corresponding receiver (after loss/
marking is experienced across the end-to-end path). The receiver
therefore needs to be responsible for either measuring the level of
congestion (and returning this measure to the sender to inform a
trigger) or for detecting excessive congestion (returning the trigger
to the sender). Whether the trigger is generated at the receiver or
based on measurements returned to the sender, the result of the
trigger (the circuit-breaker action) needs to be applied at the
sender.
The set of components needed to implement a circuit breaker are:
o There MUST be a control path from the receiver to the sender.
Ideally the CB should trigger if this control path fails. That
is, the feedback indicating a congested period is designed so that
the sender triggers the CB action when it fails to receive reports
from the receiver that indicate an absence of congestion, rather
than relying on the successful transmission of a "congested"
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signal back to the sender. (The feedback signal could itself be
lost under congestion collapse).
o A CB MUST define a measurement period over which the receiver
measures the level of congestion. This method does not have to
detect individual packet loss, but MUST have a way to know that
packets have been lost/marked from the traffic flow. If ECN is
enabled, a receiver MAY also count the number of Explicit
Congestion Notification (ECN)[RFC3168] marks per measurement
interval, but even if ECN is used, the loss MUST still be
measured, since this better reflects the impact of excessive
congestion. The type of CB will determine how long this
measurement period needs to be. The minimum time must be
significantly longer than the time that current CC algorithms need
to reduce their rate following detection of congestion (i.e. many
path RTTs).
o A CB MUST define a threshold to determine whether the measured
congestion is considered excessive.
o A CB MUST define a period over which the trigger uses collected
measurements.
o A CB MUST be robust to multiple congestion events. This usually
will define a number of measured excessive 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. triggered when excessive congestion is detected in 3
measurements within the triggering interval).
o A triggered CB MUST react decisively by reducing traffic at the
source (e.g. tunnel egress). A CB SHOULD be constructed so that
it does not trigger under light or intermittent congestion, hence
the response when triggered needs to be much more severe than that
of a CC algorithm. By default, a CB SHOULD disable the flow, it
could alternatively significantly reduce the rate of the flow it
controls.
o Triggering a CB SHOULD result in a response that continues for a
period of time. This by default SHOULD be at least the triggering
interval. Manual operator intervention MAY be required to restore
the flow. If an automated response is needed to restore the flow,
then this MUST NOT be immediate.
o When a CB is triggered, it SHOULD be regarded as an abnormal
network event. As such, this event SHOULD be logged. The
measurements that lead to triggering of the CB SHOULD also be
logged.
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3.1. Basic Function
This section provides one example of a suitable method to measure
congestion:
1. A sender or a tunnel ingress records the number of packets/bytes
sent in each measurement interval. The measurement interval
could be every few seconds.
2. The receiver or tunnel egress also records the number/bytes
received (at ) in each measurement interval.
3. The receiver periodically returns the measured values. (This
could be using Operations and Management (OAM), or an in-band
signalling datagram).
4. Using the ingress and egress measurements, the loss rate for each
measurement interval can be deduced from calculating the
difference between these two counter values. Note that accurate
measurement intervals are not typically important, since isolated
loss events need to be disregard. An appropriate threshold for
determining excessive congestion needs to be set (e.g. more than
10% loss, but other methods could also be based on the rate of
transmission as well as the loss rate).
5. The transport circuit breaker is triggered when the threshold is
exceeded in multiple measurement intervals (e.g. 3 successive
measurements). This design is to be robust to single or spurious
events resulting in a trigger.
6. The design may also trigger loss when it does not receive
receiver measurements for 3 successive measurement periods - this
may indicate a loss of control packets.
4. Examples of Circuit Breakers
This section provides examples of different types of circuit breaker.
There are multiple types of circuit breaker that may be defined for
use in different deployment cases:
4.1. A fast-trip Circuit Breaker
A fast-trip circuit breaker is the most responsive It has a response
time that is only slightly larger than that of the traffic it
controls. It is suited to traffic with well-understood
characteristics. It is not be suited to arbitrary network traffic,
since it may prematurely trigger (e.g. when multiple congestion-
controlled flows lead to short-term overload).
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4.1.1. A fast-trip RTP Circuit Breaker
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
:[RTP-CB] . It is expected that, in the absence of severe congestion,
all RTP applications running on best-effort IP networks will be able
to run without triggering these circuit breakers.
The RTP congestion control specification is therefore implemented as
a fail-safe.
The sender monitors reception of RTCP Reception Report (RR or XRR)
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) is triggered 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 uses a TFRC-style check and set a hard upper limit to
the long-term RTP throughput (over many RTTs).
4. An RTP CB includes the notion of Media Usability. This circuit
breaker is triggered when the quality of the transported media
falls below some required minimum acceptable quality.
4.2. A Slow-trip Circuit Breaker
It is expected that most circuit breakers will be slower at
responding to loss.
One example where a circuit breaker is needed is where flows or
traffic-aggregates use a tunnel or encapsulation and the flows within
the tunnel do not all support TCP-style congestion control (e.g. TCP,
SCTP, TFRC), see [RFC5405] section 3.1.3. The usual case where this
is needed is when tunnels are deployed in the general Internet
(rather than "controlled environments" within an ISP or Enterprise),
especially when the tunnel may need to cross a customer access
router.
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4.3. A Managed Circuit Breaker
This type of circuit breaker is implemented in the signalling
protocol or management plane that relates to the traffic aggregate
being controlled. This type of circuit breaker is typically
applicable when the deployment is within a "controlled environment".
4.3.1. A Managed Circuit Breaker for SAToP Pseudo-Wires
[RFC4553], SAToP Pseudo-Wires (PWE3), section 8 describes an example
of a managed circuit breaker for isochronous flows.
If such flows were to run over a pre-provisioned (e.g. MPLS)
infrastructure, then it may be expected that the Pseudo-Wire (PW)
would not experience congestion, because a flow is not expected to
either increase (or decrease) their rate. If instead Pseudo-Wire
traffic is multiplexed with other traffic over the general Internet,
it could experience congestion. [RFC4553] states: "If SAToP PWs run
over a PSN providing best-effort service, they SHOULD monitor packet
loss in order to detect "severe congestion". The currently
recommended measurement period is 1 second, and the trigger operates
when there are more than three measured Severely Errored Seconds
(SES) within a period.
If such a condition is detected, a SAToP PW should shut down
bidirectionally for some period of time..." The concept was that when
the packet loss ratio (congestion) level increased above a threshold,
the PW was by default disabled. This use case considered fixed-rate
transmission, where the PW had no reasonable way to shed load.
The trigger needs to be set at the rate the PW was likely have a
serious problem, possibly making the service non-compliant. At this
point triggering the CB would remove the traffic prevent undue impact
congestion-responsive traffic (e.g., TCP). Part of the rationale,
was that high loss ratios typically indicated that something was
"broken" and should have already resulted in operator intervention,
and should trigger this intervention. An operator-based response
provides opportunity for other action to restore the service quality,
e.g. by shedding other loads or assigning additional capacity, or to
consciously avoid reacting to the trigger while engineering a
solution to the problem. This may require the trigger to be sent to
a third location (e.g. a network operations centre, NOC) responsible
for operation of the tunnel ingress, rather than the tunnel ingress
itself.
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5. Examples where circuit breakers may not be needed.
A CB is not required for a single CC-controlled flow using TCP, SCTP,
TFRC, etc. In these cases, the CC methods are designed to prevent
congestion collapse.
5.1. CBs and uni-directional Traffic
A CB can not be used to control uni-directional UDP traffic. The
lack of feedback prevents automated triggering of the CB. Supporting
this type of traffic in the general Internet requires operator
monitoring to detect and respond to congestion collapse or the use of
dedicated capacity - e.g. Using per-provisioned MPLS services, RSVP,
or admission-controlled Differentiated Services.
5.2. 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
should not result in congestion. The CB will hence only be triggered
when there is non-compliant traffic. It could be argued that this
event should never happen - but it may also be argued that the CB
equally should never be triggered. If a CB were to be implemented,
it would provide an appropriate response should this excessive
congestion occur in an operational network.
5.3. CBs with CC 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 [RFC5405]. A question therefore
arises when people deploy a tunnel that is thought to only carry an
aggregate of TCP (or some other CC-controlled) traffic: Is there
advantage in this case in using a CB? For sure, traffic in a such a
tunnel will respond to congestion. However, the answer to the
question is not obvious, because the overall traffic formed by an
aggregate of flows that implement a CC mechanism does not necessarily
prevent congestion collapse. For instance, most CC mechanisms
require long-lived flows to react to reduce the rate of a flow, an
aggregate of many short flows may result in many terminating before
they experience congestion. It is also often impossible for a tunnel
service provider to know that the tunnel only contains CC-controlled
traffic (e.g. Inspecting packet headers may not be possible). The
important thing to note is that if the aggregate of the traffic does
not result in persistent congestion (impacting other flows), then the
CB will not trigger. This is the expected case in this context - so
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implementing a CB will not reduce performance of the tunnel, but
offers protection should congestion collapse occur.
6. Security Considerations
This section will describe security considerations.
7. IANA Considerations
This document makes no request from IANA.
8. Acknowledgments
There are many people who have discussed and described the issues
that have motivated this draft.
9. Revision Notes
RFC-Editor: Please remove this section prior to publication
Draft 00
This was the first revision. Help and comments are greatly
appreciated.
10. References
10.1. Normative References
[Jacobsen88]
European Telecommunication Standards, Institute (ETSI),
"Congestion Avoidance and Control", SIGCOMM Symposium
proceedings on Communications architectures and
protocols", August 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, November
2008.
[RTP-CB] and , "Multimedia Congestion Control: Circuit Breakers for
Unicast RTP Sessions", February 2014.
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10.2. Informative References
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC4553] Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
Division Multiplexing (TDM) over Packet (SAToP)", RFC
4553, June 2006.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, November 2010.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, August 2012.
Author's Address
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen, Scotland AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk
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