Network Working Group M. Welzl
Internet-Draft University of Oslo
Intended status: Informational G. Fairhurst
Expires: September 10, 2015 University of Aberdeen
March 09, 2015
The Benefits to Applications of using Explicit Congestion Notification
(ECN)
draft-welzl-ecn-benefits-02
Abstract
This document describes the potential benefits to applications when
they enable Explicit Congestion Notification (ECN). It outlines the
principal gains in terms of increased throughput, reduced delay and
other benefits when ECN is used over network paths that include
equipment that supports ECN-marking. The focus of this document is
on usage of ECN, not its implementation in hosts, routers and other
network devices.
Status of This Memo
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This Internet-Draft will expire on September 10, 2015.
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
1. Introduction
Internet Transports (such as TCP and SCTP) have two ways to detect
congestion: the loss of a packet and, if Explicit Congestion
Notification (ECN) [RFC3168] is enabled, by reception of a packet
with a Congestion Experienced (CE)-marking in the IP header. Both of
these are treated by transports as indications of (potential)
congestion. ECN may also be enabled by other transports: UDP
applications may enable ECN when they are able to correctly process
the ECN signals [RFC5405] (e.g. ECN with RTP [RFC6679]).
A network device (router, middlebox, or other device that forward
packets through the network) that does not support AQM, typically
uses a drop-tail policy to discard excess IP packets when its queue
becomes full. The discard of this packet serves as a signal to the
end-to-end transport that there may be congestion on the network path
being used. This triggers a congestion control reaction to reduce
the maximum rate permitted by the sending endpoint.
When an application uses a transport that enables the use of ECN, the
transport layer sets the ECT(0) or ECT(1) codepoint in the IP header
of packets that it sends. This indicate to network devices that they
may mark, rather than drop, packets in periods of congestion. This
marking is generally performed by Active Queue Management (AQM)
[RFC2309.bis] and may be the result of various AQM algorithms, where
the exact combination of AQM/ECN algorithms is generally not known by
the transport endpoints. The focus of this document is on usage of
ECN, not its implementation in hosts, routers and other network
devices.
ECN makes it possible for the network to signal congestion without
packet loss. This lets the network deliver some packets to an
application that would otherwise have been dropped. This reduction
in packet loss is the most obvious benefit of ECN, but it is often
relatively modest. However, enabling ECN can also result in a number
of beneficial side-effects, some of which may be much more
significant than the immediate reduction in packet loss from ECN-
marking instead of dropping packets.
The remainder of this document discusses the potential for ECN to
positively benefit an application without making specific assumptions
about configuration or implementation.
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[RFC3168] describes a method in which a router, sets the CE codepoint
of an ECN-Capable packet at the time that the network device would
otherwise have dropped the packet. While it has often been assumed
that network devices mark packets at the same level of congestion at
which they would otherwise have dropped them (e.g., when a queue
reaches an AQM threshold), separate configuration of the drop and
mark thresholds is known to be supported in some network devices and
this is recommended in [RFC2309.bis]. Some benefits of ECN that are
discussed rely upon routers marking packets at a lower level of
congestion before they would otherwise drop packets [KH13].
Some benefits are also only realised when the transport endpoint
behaviour is also updated, this is discussed further in Section 5.
2. ECN Deployment
For an application to use ECN requires that the endpoint first
enables ECN within the transport. This requires network devices
along the path to at least pass IP packets that set ECN codepoints,
and do not drop packets because these codepoints are used. This is
the recommended behaviour for network devices [RFC2309.bis].
Applications and transports (such as TCP or SCTP) can be designed to
fall-back to not using ECN when they discover they are using a path
that does not allow use of ECN (e.g., a firewall or other network
device configured to drop the ECN codepoint).
XXX NOTE: A future revision could include some words and reference a
paper on the current state of network support for transparently
passing the ECN codepoints.
For an application at an endpoint to gain benefit from ECN, network
devices need to enable ECN marking. Not all network devices along
the path need to enable ECN, for the application to benefit. Any
network devices that does not set a CE-codepoint can be expected to
drop packets under congestion. Applications that experience
congestion in such endpoints do not see any benefit from using ECN,
but would see benefit if the congestion were to occur within a
network device that did support ECN.
ECN can be incrementally deployed in the general Internet. The IETF
has provided guidance on configuration and usage in [RFC2309.bis].
ECN may also be deployed within a controlled environment, for example
within a data centre or within a well-managed private network. In
this case, the use of ECN may be tuned to the specific use-case. An
example is Datacenter TCP (DCTCP) [AL10].
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Deployment needs also to consider the requirements for processing ECN
at tunnel endpoints of network tunnels, and guidance on the treatment
of ECN is provided in [RFC6040]. Further guidance on the
encapsulation and use of ECN by non-IP network devices is provided in
[ID.ECN-Encap].
3. Benefit of using ECN to avoid congestion loss
When packet loss is a result of (mild) congestion, an ECN-enabled
router may CE-mark, rather than drop an ECN-enabled packet. An
application can benefit from this marking in several ways:
3.1. Improved Throughput
ECN can improve the throughput performance of an application,
although this increase in throughput offered by ECN is often not the
most significant gain.
When an application uses a light to moderately loaded network path,
the number of packets that are dropped due to congestion is small.
Using an example from Table 1 of [RFC3649], for a standard TCP sender
with a Round Trip Time, RTT, of 0.1 seconds, a packet size of 1500
bytes and an average throughput of 1 Mbps, the average packet drop
ratio is 0.02. This translates into an approximate 2% throughput
gain if ECN is enabled. In heavy congestion, packet loss may be
unavoidable with, or without, ECN [RFC2309.bis].
3.2. Reduced Head-of-Line Blocking
Many transports provide in-order delivery of received data segments
to the applications they support. This requires that the transport
stalls (or waits) for all data that was sent ahead of a particular
segment to be correctly received before it can forward any later
data. This is the usual requirement for TCP and SCTP. PR-SCTP
[RFC3758], UDP, and DCCP [RFC4340] provide a transport that does not
have this requirement.
Delaying data to provide in-order transmission to an application
results in latency when segments are dropped as indications of
congestion. The congestive loss creates a delay of at least one RTT
for a loss event before data can be delivered to an application. We
call this Head-of-Line (HOL) blocking.
In contrast, using ECN can remove the resulting delay following a
loss that is a result of congestion:
o First, the application receives the data normally - this also
avoids dropping data that has already made it across the network
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path. It avoids the additional delay of waiting for recovery of
the lost segment when using a reliable transport.
o Second, the transport receiver notes that it has received CE-
marked packets, and then requests the sender to make an
appropriate congestion-response to reduce the maximum transmission
rate for future traffic.
3.3. Reduced Probability of RTO Expiry
In some situations, ECN can help reduce the chance of a
retransmission timer expiring (e.g., expiry of the TCP or SCTP
retransmission timeout, RTO [RFC5681]). When an application sends a
burst of segments and then becomes idle (either because the
application has no further data to send or the network prevents
sending further data - e.g., flow or congestion control at the
transport layer), the last segment of the burst may be lost. It is
often not possible to recover this last segment (or last few
segments) using standard methods such as Fast Recovery [RFC5681],
since the receiver generates no feedback because it is unaware that
the lost segments were actually sent.
In addition to avoiding HOL blocking, this allows the transport to
avoid the consequent loss of state about the network path it is
using, which would have arisen had there been a retransmission
timeout. Typical impacts of a transport timeout are to reset path
estimates such as the RTT, the congestion window, and possibly other
transport state that can reduce the performance of the transport
until it again adapts to the path.
Avoiding timeouts can hence improve the throughput of the
application. This benefits applications that send intermittent
bursts of data, and rely upon timer-based recovery of packet loss.
It can be especially significant when ECN is used on TCP SYN/ACK
packets as specified in [RFC5562] where the RTO interval may be large
because in this case TCP cannot base the timeout period on prior RTT
measurements from the same connection.
3.4. Applications that do not retransmit lost packets
Some latency-critical applications use transports that do not
retransmit lost packets, yet these applications may be able to adjust
the sending rate in the presence of congestion. Examples of such
applications include UDP-based services that carry Voice over IP
(VoIP), interactive video, or real-time data. The performance of
many such applications degrades rapidly with increasing packet loss,
and many therefore employ loss-hiding mechanisms (e.g., packet
forward error correction, or data duplication) to mitigate the effect
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of congestion loss on the application. However, such mechanisms add
complexity and can themselves consume additional network capacity
reducing the capacity for application data and contributing to the
path latency when congestion is experienced.
By decoupling congestion control from loss, ECN can allow the
transports supporting these applications to reduce their rate before
the application experiences loss from congestion, especially when the
congestion is mild and the application/transport can react promptly
to reception of a CE-marked packet. Because this reduces the
negative impact of using loss-hiding mechanisms, ECN can have a
direct positive impact on the quality experienced by the users of
these applications.
4. Benefit from Early Congestion Detection
An application can further benefit from using ECN, when the network
devices are configured such that they mark packets at a lower level
of congestion before they would otherwise have dropped packets from
queue overflow:
4.1. Avoiding Capacity Overshoot
Internet transports do not know apriori how much capacity exists
along a network path. Transports therefore try to measure the
capacity available to an application by probing the network path with
increasing traffic to the point where they detect the onset of
congestion (such as TCP or SCTP Slow Start).
ECN can help capacity probing algorithms from significantly exceeding
the bottleneck capacity of a network path. Since a transport that
enables ECN can receive congestion signals before there is
significant congestion, an early-marking method in network devices
can help a transport respond before it induces significant congestion
with resultant loss to itself or other applications sharing a common
bottleneck. For example, an application/transport can avoid
incurring significant congestion during Slow Start, or a bulk
application that tries to increase its rate as fast as possible, may
quickly detect the presence of congestion, causing it to promptly
reduce its rate.
Use of ECN is more effective than transport mechanisms such as
Limited Slow-Start [RFC3742] because it provides direct information
about the state of the network path. An ECN-enabled application/
transport that probes for capacity can reduce its rate as soon as it
discovers CE-marked packets are received, and before the applications
increases its rate to the point where it builds a queue in a network
device that induces congestion loss. This benefits an application
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seeking to increase its rate - but perhaps more significantly, it
eliminates the often unwanted loss and queueing delay that otherwise
may be inflicted on flows that share a common bottleneck.
4.2. Making Congestion Visible
A characteristic of using ECN is that it exposes the presence of
congestion on a network path to the transport and network layers.
This information could be used for monitoring the performance of the
path, and could be used to directly meter the amount of congestion
that has been encountered upstream on a path; metering packet loss is
harder. This is used by Congestion Exposure (CoNex) [RFC6789].
A network flow that only experiences CE-marks and no loss implies
that the sending endpoint is experiencing only congestion and not
other sources of packet loss (e.g., link corruption or loss in
middleboxes). The converse is not true - a flow may experience a
mixture of ECN-marks and loss when there is only congestion or when
there is a combination of packet loss and congestion [RFC2309.bis].
Recording the presence of CE-marked packets can therefore provide
information about the performance of the network path.
5. Other forms of ECN-Marking/Reactions
The ECN mechanism defines both how packets are CE-marked and how
transports need to react to reception of marked packets. This
section describes the benefits when updated methods are used.
Benefit has been noted when packets are CE-marked earlier than they
would otherwise be dropped, using an instantaneous queue, and if the
receiver provides precise feedback about the number of packet marks
encountered, a better sender behavior is possible. This has been
shown by Datacenter TCP (DCTCP) [AL10].
Precise feedback about the number of packet marks encountered is
supported by RTP over UDP [RFC6679] and proposed for SCTP [ST14] and
TCP [KU13]. An underlying assumption of DCTCP is that it is deployed
in confined environments such as a datacenter. It is currently
unknown whether or how such behaviour could be introduced into the
Internet.
6. Conclusion
Network devices should enable ECN and people configuring host stacks
should also enable ECN. These are pre-requisites to allow
applications to gain the benefits of ECN.
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Application developers should where possible use transports that
enable the benefits of ECN. Applications that directly use UDP need
to provide support to implement the functions required for ECN. Once
enabled, an application that uses a transport that supports ECN will
experience the benefits of ECN as network deployment starts to enable
ECN. The application does not need to be rewritten to gain these
benefits.
Table 1 summarizes some of these benefits.
+---------+-----------------------------------------------------+
| Section | Benefit |
+---------+-----------------------------------------------------+
| 2.1 | Improved Throughput |
| 2.2 | Reduced Head-of-Line |
| 2.3 | Reduced Probability of RTO Expiry |
| 2.4 | Applications that do not retransmit lost packets |
| 3.1 | Avoiding Capacity Overshoot |
| 3.2 | Making Congestion Visible |
+---------+-----------------------------------------------------+
Table 1: Summary of Key Benefits from using ECN
7. Acknowledgements
The authors were part-funded by the European Community under its
Seventh Framework Programme through the Reducing Internet Transport
Latency (RITE) project (ICT-317700). The views expressed are solely
those of the authors.
8. IANA Considerations
XXRFC ED - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
9. Security Considerations
This document introduces no new security considerations. Each RFC
listed in this document discusses the security considerations of the
specification it contains.
10. Revision Information
RFC-Ed please remove this section prior to publication.
Revision 00 was the first WG draft.
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Revision 01 includes updates to complete sections and improve
readability. Added section 2.
Comments are welcome to the authors or via the IETF AQM or TSVWG
mailing lists.
11. References
11.1. Normative References
[RFC2309.bis]
Baker, F. and G. Fairhurst, "IETF Recommendations
Regarding Active Queue Management", Internet-draft draft-
ietf-aqm-recommendation-06, October 2014.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
11.2. Informative References
[AL10] Alizadeh, M., Greenberg, A., Maltz, D., Padhye, J., Patel,
P., Prabhakar, B., Sengupta, S., and M. Sridharan, "Data
Center TCP (DCTCP)", SIGCOMM 2010, August 2010.
[ID.ECN-Encap]
Briscoe, B., Kaippallimalil, J., and P. Thaler,
"Guidelines for Adding Congestion Notification to
Protocols that Encapsulate IP , IETF work-in-progress:
draft-ietf-tsvwg-ecn-encap-guidelines", .
[KH13] Khademi, N., Ros, D., and M. Welzl, "The New AQM Kids on
the Block: Much Ado About Nothing?", University of Oslo
Department of Informatics technical report 434, October
2013.
[KU13] Kuehlewind, M. and R. Scheffenegger, "Problem Statement
and Requirements for a More Accurate ECN Feedback",
Internet-draft draft-ietf-tcpm-accecn-reqs-04.txt, October
2013.
[RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks",
RFC 2884, July 2000.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, December 2003.
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[RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large
Congestion Windows", RFC 3742, March 2004.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, May 2004.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC5405] Eggert, Lars. and Gorry. Fairhurst, "Unicast UDP Usage
Guidelines for Application Designers", November 2008.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, June
2009.
[RFC5681] "TCP Congestion Control", .
[RFC6040] "Tunnelling of Explicit Congestion Notification", .
[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.
[RFC6789] Briscoe, B., Woundy, R., and A. Cooper, "Congestion
Exposure (ConEx) Concepts and Use Cases", RFC 6789,
December 2012.
[ST14] Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", Internet-draft
draft-stewart-tsvwg-sctpecn-05.txt, January 2014.
Authors' Addresses
Michael Welzl
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Phone: +47 22 85 24 20
Email: michawe@ifi.uio.no
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Godred Fairhurst
University of Aberdeen
School of Engineering, Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
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