Network Working Group G. Fairhurst
Internet-Draft University of Aberdeen
Intended status: Informational M. Welzl
Expires: May 27, 2016 University of Oslo
November 24, 2015
The Benefits of using Explicit Congestion Notification (ECN)
draft-ietf-aqm-ecn-benefits-08
Abstract
The goal of this document is to describe the potential benefits when
applications use a transport that enables Explicit Congestion
Notification (ECN). The document outlines the principal gains in
terms of increased throughput, reduced delay and other benefits when
ECN is used over a network path that includes equipment that supports
Congestion Experienced (CE) marking. It also discusses challenges
for successful deployment of ECN. It does not propose new algorithms
to use ECN, nor does it describe the details of implementation of ECN
in endpoint devices (Internet hosts), routers or other network
devices.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 27, 2016.
Copyright Notice
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Benefit of using ECN to avoid Congestion Loss . . . . . . . . 5
2.1. Improved Throughput . . . . . . . . . . . . . . . . . . . 5
2.2. Reduced Head-of-Line Blocking . . . . . . . . . . . . . . 6
2.3. Reduced Probability of RTO Expiry . . . . . . . . . . . . 6
2.4. Applications that do not Retransmit Lost Packets . . . . 7
2.5. Making Incipient Congestion Visible . . . . . . . . . . . 8
2.6. Opportunities for new Transport Mechanisms . . . . . . . 8
3. Network Support for ECN . . . . . . . . . . . . . . . . . . . 9
3.1. The ECN Field . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Forwarding ECN-Capable IP Packets . . . . . . . . . . . . 10
3.3. Enabling ECN in Network Devices . . . . . . . . . . . . . 10
3.4. Co-existance of ECN and non-ECN flows . . . . . . . . . . 11
3.5. Bleaching and Middlebox Requirements to deploy ECN . . . 11
3.6. Tunneling ECN and the use of ECN by Lower Layer Networks 12
4. Using ECN across the Internet . . . . . . . . . . . . . . . . 12
4.1. Partial Deployment . . . . . . . . . . . . . . . . . . . 13
4.2. Detecting whether a Path Really Supports ECN . . . . . . 13
4.3. Detecting ECN Receiver Feedback Cheating . . . . . . . . 14
5. Summary: Enabling ECN in Network Devices and Hosts . . . . . 14
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Revision Information . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Internet Transports (such as TCP and SCTP) are implemented in
endpoints (Internet hosts) and are designed to detect and react to
network congestion. Congestion may be detected by loss of an IP
packet or, if Explicit Congestion Notification (ECN) [RFC3168] is
enabled, by the reception of a packet with a Congestion Experienced
(CE) marking in the IP header. Both of these are treated by
transports as indications of congestion. ECN may also be enabled by
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other transports: UDP applications that provide congestion control
may enable ECN when they are able to correctly process the ECN
signals [ID.RFC5405.bis] (e.g., ECN with RTP [RFC6679]).
Active Queue Management (AQM) [RFC7567] is a class of techniques that
can be used by network devices (a router, middlebox, or other device
that forwards packets through the network) to manage the size of
queues in network buffers.
A network device that does not support AQM typically uses a drop-tail
policy to drop excess IP packets when its queue becomes full. The
discard of packets is treated by transport protocols as a signal that
indicates congestion on the end-to-end network path. End-to-end
transports, such as TCP, can cause a low level of loss while seeking
to share capacity with other flows. Although losses are not always
due to congestion (loss may be due to link corruption, receiver-
overrun, etc) end points have to conservatively presume that all loss
is potentially due to congestion and reduce their rate. Observed
loss therefore results in a congestion control reaction by the
transport to reduce the maximum rate permitted by the sending
endpoint.
ECN makes it possible for the network to signal the presence of
incipient congestion without incurring packet loss, it lets the
network deliver some packets to an application that would otherwise
have been dropped if the application or transport did not support
ECN. This packet loss reduction 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 packet loss reduction from
receiving CE-marking instead of dropping packets. Several benefits
reduce latency (e.g., reduced Head-of-Line Blocking).
The use of ECN is indicated in the ECN field [RFC3168], carried in
the packet header of all IPv4 and IPv6 packets. This field may be
set to one of four values shown in Table 1. The not-ECT codepoint
'00' indicates a packet that is not using ECN. The ECT(0) codepoint
'01' and the ECT(1) codepoint '10' both indicate that the transport
protocol using the IP layer supports the use of ECN. The CE
codepoint '11' is set by an ECN-capable network device to indicate
congestion to the transport endpoint.
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+-----+-----+---------+
| ECN FIELD | Name |
+-----+-----+---------+
| 0 | 0 | Not-ECT |
| 0 | 1 | ECT(1) |
| 1 | 0 | ECT(0) |
| 1 | 1 | CE |
+-----+-----+---------+
Table 1: The ECN Field in the IP Packet Header (based on [RFC3168]).
When an application uses a transport that enables use of ECN
[RFC3168], the transport layer sets the ECT(0) or ECT(1) codepoint in
the IP header of packets that it sends. This indicates to network
devices that they may mark, rather than drop the ECN-capable IP
packets. An ECN-capable network device can then signal incipient
congestion (network queueing) at a point before a transport
experiences congestion loss or high queuing delay. The marking is
generally performed as the result of various AQM algorithms
[RFC7567], where the exact combination of AQM/ECN algorithms does not
need to be known by the transport endpoints.
The focus of the document is on usage of ECN by transport and
application layer flows, not its implementation in endpoint hosts, or
in routers and other network devices.
1.1. Terminology
The following terms are used:
AQM: Active Queue Management.
CE: Congestion Experienced, a codepoint value '11' marked in the ECN
field of the IP packet header.
ECN-capable IP Packet : A packet where the ECN field is set to a non-
zero ECN value (i.e., with a ECT(0), ECT(1), or the CE codepoint).
ECN-capable network device : An ECN-capable network device may
forward, drop, or queue an ECN-capable packet and may choose to CE-
mark this packet when there is incipient congestion.
ECN-capable transport/application : A transport that sends ECN-
capable IP Packets, and monitors reception of the ECN field and
generates appropriate feedback to control the rate of the sending
endpoint.
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to provide end-to-end congestion control.
ECN field: A 2-bit field specified for use explicit congestion
signalling in the IPv4 and IPv6 packet headers.
Endpoint: An Internet host that terminates a transport protocol
connection across an Internet path.
Incipient Congestion: The detection of congestion when it is
starting, perhaps by a network device noting that the arrival rate
exceeds the forwarding rate.
Network device: A router, middlebox, or other device that forwards IP
packets through the network.
non-ECN-capable: A network device or endpoint that does not interpret
the ECN field. Such a device is not permitted to change the ECN
codepoint.
not-ECN-capable IP Packet: An IP packet with the ECN field set to a
value of zero ('00'). A not-ECN-capable packet may be forwarded,
dropped or queued by a network device.
2. Benefit of using ECN to avoid Congestion Loss
An ECN-capable network device is expected to CE-mark an ECN-capable
IP packet when an AQM method detects incipient congestion, rather
than to drop the packet [RFC7567]. An application can benefit from
this marking in several ways:
2.1. Improved Throughput
ECN seeks to avoid the inefficiency of dropping data that has already
made it across at least part of the network path.
ECN can improve the throughput of an application, although this
increase in throughput 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
would be 0.02 (i.e., 1 in 50 packets). This translates into an
approximate 2% throughput gain if ECN is enabled. (Note that in
heavy congestion, packet loss may be unavoidable with, or without,
ECN.)
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2.2. Reduced Head-of-Line Blocking
Many Internet transports provide in-order delivery of received data
segments to the applications they support. For these applications,
use of ECN can reduce the delay that can result when these
applications experience packet loss.
Packet loss may occur for various reasons. One cause arises when an
AQM scheme drops a packet as a signal of incipient congestion.
Whatever the cause of loss, a missing packet needs to trigger a
congestion control response. A reliable transport also triggers
retransmission to recover the lost data. For a transport providing
in-order delivery, this requires that the transport receiver stalls
(or waits) for all data that was sent ahead of a lost segment to be
correctly received before it can forward any later data to the
application. A loss therefore creates a delay of at least one RTT
after a loss event before data can be delivered to an application.
We call this Head-of-Line (HOL) blocking. This is the usual
requirement for TCP and SCTP. (PR-SCTP [RFC3758], UDP
[RFC0768][ID.RFC5405.bis], and DCCP [RFC4340] provide a transport
that does not provide re-ordering).
By enabling ECN, a transport continues to receive in-order data when
there is incipient congestion, and can pass this data to the
receiving application. Use of ECN avoids the additional reordering
delay in a reliable transport. The sender still needs to make an
appropriate congestion-response to reduce the maximum transmission
rate for future traffic, which usually will require a reduction in
the sending rate [ID.RFC5405.bis].)
2.3. Reduced Probability of RTO Expiry
Some patterns of packet loss can result in a Retransmission Time Out
(RTO), which causes a sudden and significant change in the allowed
rate at which a transport/application can forward packets. Because
ECN provides an alternative to drop for network devices to signal
incipient congestion, this can reduce the probability of loss and
hence reduce the likelihood of RTO expiry.
Internet transports/applications generally use a RTO timer as a last
resort to detect and recover loss [ID.RFC5405.bis] [RFC5681]).
Specifically, a RTO timer detects loss of a packet that is not
followed by other packets, such as at the end of a burst of data
segments or when an application 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). This loss of the last segment (or last few
segments) of a traffic burst is also known as a "tail loss".
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Standard transport recovery methods, such as Fast Recovery (
[RFC5681], are often unable to recover from a tail loss. This is
because the endpoint receiver is unaware that the lost segments were
actually sent, and therefore generates no feedback [Fla13].
Retransmission of these segments therefore relies on expiry of a
transport retransmission timer. This timer is also used to detect a
lack of forwarding along a path. Expiry of the RTO therefore results
in the consequent loss of state about the network path being used.
This typically includes resetting path estimates such as the RTT, re-
initialising the congestion window, and possibly updates to other
transport state. This can reduce the performance of the transport
until it again adapts to the path.
An ECN-capable network device cannot eliminate the possibility of
tail loss, because a drop may occur due to a traffic burst exceeding
the instantaneous available capacity of a network buffer or as a
result of the AQM algorithm (overload protection mechanisms, etc
[RFC7567]). However, an ECN-capable network device that observes
incipient congestion may be expected to buffer the IP packets of an
ECN-capable flow and set a CE-mark in one or more packet(s), rather
than triggering packet drop. Setting a CE-mark signals incipient
congestion without forcing the transport/application to enter
retransmission timeout. This reduces application-level latency and
can improve the throughput for applications that send intermittent
bursts of data.
The benefit of avoiding retransmission loss is expected to be
significant when ECN is used on TCP SYN/ACK packets [RFC5562] where
the RTO interval may be large because TCP cannot base the timeout
period on prior RTT measurements from the same connection.
2.4. Applications that do not Retransmit Lost Packets
A transport that enables ECN can receive timely congestion signals
without the need to retransmit packets each time it receives a
congestion signal.
Some latency-critical applications do not retransmit lost packets,
yet may be able to adjust their sending rate following detection of
incipient 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 the transport/application may
therefore employ mechanisms (e.g., packet forward error correction,
data duplication, or media codec error concealment) to mitigate the
immediate effect of congestion loss on the application. Some
mechanisms consume additional network capacity, some require
additional processing and some contribute additional path latency
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when congestion is experienced. By decoupling congestion control
from loss, ECN can allow transports that support these applications
to reduce their rate before the application experiences loss from
congestion. This can reduce the negative impact of triggering loss-
hiding mechanisms with a direct positive impact on the quality
experienced by the users of these applications.
2.5. Making Incipient 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
allowing information to be collected about the presence of incipient
congestion.
Recording the presence of CE-marked packets can provide information
about the current congestion level experienced on a network path. A
network flow that only experiences CE-marking and no loss implies
that the sending endpoint is experiencing only congestion. A network
flow may also experience loss (e.g., due to queue overflow, AQM
methods that protect other flows, link corruption or loss in
middleboxes). When a mixture of CE-marking and packet loss is
experienced, transports and measurements need to assume there is
congestion [RFC7567]. An absence of CE-marks therefore does not
indicate a path has not experienced congestion.
The reception of CE-marked packets can be used to monitor the level
of congestion by a transport/application or a network operator. For
example, ECN measurements are used by Congestion Exposure (ConEx)
[RFC6789]. In contrast, metering packet loss is harder.
2.6. Opportunities for new Transport Mechanisms
ECN can enable design and deployment of new algorithms in network
devices and Internet transports. Internet transports need to regard
both loss and CE-marking as an indication of congestion. However,
while the amount of feedback provided by drop ought naturally to be
minimized, this is not the case for ECN. In contrast, an ECN-Capable
network device could provide richer (more frequent and fine-grained)
indication of its congestion state to the transport.
For any ECN-capable transport, the receiving endpoint needs to
provide feedback to the transport sender to indicate that CE-marks
have been received.[RFC3168] provides one method that signals once
each round trip time that CE-marked packets have been received.
A receiving endpoint may provide more detailed feedback to the
congestion controller at the sender (e.g., describing the set of
received ECN codepoints, or indicating each received CE-marked
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packet). Precise feedback about the number of CE-marks encountered
is supported by the Real Time Protocol (RTP) when used over UDP
[RFC6679] and has been proposed for SCTP [ST14] and TCP [ID.Acc.ECN].
More detailed feedback is expected to enable evolution of transport
protocols allowing the congestion control mechanism to make a more
appropriate decision on how to react to congestion. Designers of
transport protocols need to consider not only how network devices CE-
mark packets, but also how the control loop in the application/
transport reacts to reception of these CE-marked packets.
Benefit has been noted when packets are CE-marked early using an
instantaneous queue, and if the receiving endpoint provides feedback
about the number of packet marks encountered, an improved sender
behavior has been shown to be possible, e.g, Datacenter TCP (DCTCP)
[AL10]. DCTCP is targeted at controlled environments such as a
datacenter. This is work-in-progress and it is currently unknown
whether or how such behaviour could be safely introduced into the
Internet. Any update to an Internet transport protocol requires
careful consideration of the robustness of the behaviour when working
with endpoints or network devices that were not designed for the new
congestion reaction.
3. Network Support for ECN
For an application to use ECN requires that the endpoints first
enable ECN within the transport being used, but also for all network
devices along the path to at least forward IP packets that set a non-
zero ECN codepoint.
ECN can be deployed both in the general Internet and in controlled
environments:
o ECN can be incrementally deployed in the general Internet. The
IETF has provided guidance on configuration and usage in
[RFC7567].
o ECN may be deployed within a controlled environment, for example
within a data centre or within a well-managed private network.
This use of ECN may be tuned to the specific use-case. An example
is DCTCP [AL10] [ID.DCTCP].
Early experience of using ECN across the general Internet encountered
a number of operational difficulties when the network path either
failed to transfer ECN-capable packets or inappropriately changed the
ECN codepoints [BA11]. A recent survey reported a growing support
for network paths to pass ECN codepoints [TR15].
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The remainder of this section identifies what is needed for network
devices to effectively support ECN.
3.1. The ECN Field
The current IPv4 and IPv6 specifications assign usage of 2 bits in
the IP header to carry the ECN codepoint. This 2-bit field was
reserved in [RFC2474] and assigned in [RFC3168].
[RFC4774] discusses some of the issues in defining alternate
semantics for the ECN field, and specifies requirements for a safe
coexistence in an Internet that could include routers that do not
understand the defined alternate semantics.
Some network devices were configured to use a routing hash that
included the set of 8 bits forming the now deprecated Type of Service
(ToS) field [RFC1349]. The present use of this field assigns 2 of
these bits to carry the ECN field. This is incompatible with use in
a routing hash, because it could lead to IP packets that carry a CE-
mark being routed over a different path to those packets that carried
an ECT mark. The resultant reordering would impact the performance
of transport protocols (such as TCP or SCTP) and UDP-based
applications that are senstive to reordering. A network device that
conforms to this older specification needs to be updated to the
current specifications [RFC2474] to support ECN. Configuraton of
network devices must note that the ECN field may be updated by any
ECN-capable network device along a path.
3.2. Forwarding ECN-Capable IP Packets
Not all network devices along a path need to be ECN-capable (i.e.,
perform CE-marking). However, all network devices need to be
configured not to drop packets solely because the ECT(0) or ECT(1)
codepoints are used.
Any network device that does not perform CE-marking of an ECN-capable
packet can be expected to drop these packets under congestion.
Applications that experience congestion at these network devices do
not see any benefit from enabling ECN. However, they may see benefit
if the congestion were to occur within a network device that did
support ECN.
3.3. Enabling ECN in Network Devices
Network devices should use an AQM algorithm that CE-marks ECN-capable
traffic when making decisions about the response to congestion
[RFC7567]. An ECN method should set a CE-mark on ECN-capable packets
in the presence of incipient congestion. A CE-marked packet will be
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interpreted as an indication of incipient congestion by the transport
endpoints.
There is opportunity to design an AQM method for an ECN-capable
network device that differs from an AQM method designed to drop
packets. [RFC7567] states that the network device should allow this
behaviour to be configurable.
[RFC3168] describes a method in which a network device sets the CE-
mark at the time that the network device would otherwise have dropped
the packet. While it has often been assumed that network devices
should CE-mark packets at the same level of congestion at which they
would otherwise have dropped them, [RFC7567] recommends that network
devices allow independent configuration of the settings for AQM
dropping and ECN marking. Such separate configuration of the drop
and mark policies is supported in some network devices.
3.4. Co-existance of ECN and non-ECN flows
Network devices need to be able to forward all IP flows and provide
appropriate treatment for both ECN and non-ECN traffic.
The design considerations for an AQM scheme supporting ECN needs to
consider the impact of queueing during incipient congestion. For
example, a simple AQM scheme could choose to queue ECN-capable and
non-ECN capable flows in the same queue with an ECN scheme that CE-
mark packets during incipient congestion. The CE-marked packets that
remain in the queue during congestion can continue to contribute to
queueing delay. In contrast, non-ECN-capable packets would normally
be dropped by an AQM scheme under incipient congestion. This
difference in queueing is one motivation for consideration of more
advanced AQM schemes, and may provide an incentive for enabling flow
isolation using scheduling [RFC7567]. The IETF is defining methods
to evaluate the suitability of AQM schemes for deployment in the
general Internet [ID.AQM.eval].
3.5. Bleaching and Middlebox Requirements to deploy ECN
Network devices should not be configured to change the ECN codepoint
in the packets that they forward, except to set the CE-codepoint to
signal incipient congestion.
Cases have been noted where an endpoint sends a packet with a non-
zero ECN mark, but the packet is received by the remote endpoint with
a zero ECN codepoint [TR15]. This could be a result of a policy that
erases or "bleaches" the ECN codepoint values at a network edge
(resetting the codepoint to zero). Bleaching may occur for various
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reasons (including normalising packets to hide which equipment
supports ECN). This policy prevents use of ECN by applications.
When ECN-capable IP packets, marked as ECT(0) or ECT(1), are remarked
to non-ECN-capable (i.e., the ECN field is set to zero codepoint),
this could result in the packets being dropped by ECN-capable network
devices further along the path. This eliminates the advantage of
using of ECN.
A network device must not change a packet with a CE mark to a zero
codepoint, if the network device decides not to forward the packet
with the CE-mark, it has to instead drop the packet and not bleach
the marking. This is because a CE-marked packet has already received
ECN treatment in the network, and remarking it would then hide the
congestion signal from the receiving endpoint. This eliminates the
benefits of ECN. It can also slow down the response to congestion
compared to using AQM, because the transport will only react if it
later discovers congestion by some other mechanism.
Prior to RFC2474, a previous usage assigned the bits now forming the
ECN field as a part of the now deprecated Type of Service (ToS) field
[RFC1349]. A network device that conforms to this older
specification was allowed to remark or erase the ECN codepoints, and
such equipment needs to be updated to the current specifications to
support ECN.
3.6. Tunneling ECN and the use of ECN by Lower Layer Networks
Some networks may use ECN internally or tunnel ECN (e.g., for traffic
engineering or security). These methods need to ensure that the ECN-
field of the tunnel packets is handled correctly at the ingress and
egress of the tunnel. Guidance on the correct use 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].
4. Using ECN across the Internet
A receiving endpoint needs to report the loss it experiences when it
uses loss-based congestion control. So also, when ECN is enabled, a
receiving endpoint must correctly report the presence of CE-marks by
providing a mechanism to feed this congestion information back to the
sending endpoint , [RFC3168], [ID.RFC5405.bis], enabling the sender
to react to experienced congestion. This mechanism needs to be
designed to operate robustly across a wide range of Internet path
characteristics. This section describes partial deployment, how ECN-
enabled endpoints can continue to work effectively over a path that
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experiences misbehaving network devices or when an endpoint does not
correctly provide feedback of ECN congestion information.
4.1. Partial Deployment
Use of ECN is negotiated between the endpoints prior to using the
mechanism.
ECN has been designed to allow incremental partial deployment
[RFC3168]. Any network device can choose to use either ECN or some
other loss-based policy to manage its traffic. Similarly, transport/
application negotiation allows senders and receiving endpoints to
choose whether ECN will be used to manage congestion for a particular
network flow.
4.2. Detecting whether a Path Really Supports ECN
Internet transport and applications need to be robust to the variety
and sometimes varying path characteristics that are encountered in
the general Internet. They need to monitor correct forwarding of ECN
over the entire path and duration of a session.
To be robust, applications and transports need to be designed with
the expectation of heterogeneous forwarding (e.g., where some IP
packets are CE-marked by one network device, and some by another,
possibly using a different AQM algorithm, or when a combination of
CE-marking and loss-based congestion indications are used.
([ID.AQM.eval] describes methodologies for evaluating AQM schemes.)
A transport/application also needs to be robust to path changes. A
change in the set of network devices along a path could impact the
ability to effectively signal or use ECN across the path, e.g., when
a path changes to use a middlebox that bleaches ECN codepoints (see
Section 3.5).
A sending endpoint can check that any CE-marks applied to packets
received over the path are indeed delivered to the remote receiving
endpoint and that appropriate feedback is provided. (This could be
done by a sender setting known a CE codepoint for specific packets in
a network flow and then checking whether the remote endpoint
correctly reports these marks [ID.Fallback], [TR15].) If a sender
detects persistent misuse of ECN, it needs to fall back to using
loss-based recovery and congestion control. Guidance on a suitable
tranport reaction is provided in [ID.Fallback].
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4.3. Detecting ECN Receiver Feedback Cheating
Appropriate feedback requires that the endpoint receiver does not try
to conceal reception of CE-marked packets in the ECN feedback
information provided to the sending endpoint [RFC7567]. Designers of
applications/transports are therefore encouraged to include
mechanisms that can detect this misbehavior. If a sending endpoint
detects that a receiver is not correctly providing this feedback, it
needs to fall back to using loss-based recovery instead of ECN.
5. Summary: Enabling ECN in Network Devices and Hosts
This section summarises the benefits of deploying and using ECN
within the Internet. It also provides a list of prerequisites to
achieve ECN deployment.
Application developers should where possible use transports that
enable ECN. Applications that directly use UDP need to provide
support to implement the functions required for ECN [ID.RFC5405.bis].
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 2 summarises the key benefits.
+---------+-----------------------------------------------------+
| Section | Benefit |
+---------+-----------------------------------------------------+
| 2.1 | Improved throughput |
| 2.2 | Reduced Head-of-Line blocking |
| 2.3 | Reduced probability of RTO Expiry |
| 2.4 | Applications that do not retransmit lost packets |
| 2.5 | Making incipient congestion visible |
| 2.6 | Opportunities for new transport mechanisms |
+---------+-----------------------------------------------------+
Table 2: Summary of Key Benefits
Network operators and people configuring network devices should
enable ECN [RFC7567].
Prerequisites for network devices (including IP routers) to enable
use of ECN include:
o A network device that updates the ECN field in IP packets must use
IETF-specified methods (see Section 3.1).
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o A network device may support alternate ECN semantics (see
Section 3.1).
o A network device must not choose a different network path solely
because a packet carries has a CE-codepoint set in the ECN Field,
CE-marked packets need to follow the same path as packets with an
ECT(0) or ECT(1) codepoint (see Section 3.1).Network devices need
to be configured not to drop packets solely because the ECT(0) or
ECT(1) codepoints are used (see Section 3.2).
o A network device must not change a packet with a CE mark to a not-
ECN-capable codepoint ('00'), if the network device decides not to
forward the packet with the CE-mark, it has to instead drop the
packet and not bleach the marking (see Section 3.5).
o An ECN-capable network device should correctly update the ECN
codepoint of ECN-capable packets in the presence of incipient
congestion (see Section 3.3).
o Network devices need to be able to forward both ECN-capable and
not-ECN-capable flows (see Section 3.4).
Prerequisites for network endpoints to enable use of ECN include:
o An application should use an Internet transport that can set and
receive ECN marks (see Section 4).
o An ECN-capable transport/application must return feedback
indicating congestion to the sending endpoint and perform an
appropriate congestion response (see Section 4).
o An ECN-capable transport/application should detect paths where
there is there is persistent misuse of ECN and fall back to not
sending ECT(0) or ECT(1) (see Section 4.2).
o Designers of applications/transports are encouraged to include
mechanisms that can detect and react appropriately to misbehaving
receivers that fail to report CE-marked packets (see Section 4.3).
6. 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.
The authors would like to thank the following people for their
comments on prior versions of this document: Bob Briscoe, David
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Collier-Brown, Colin Perkins, Richard Scheffenegger, Dave Taht, Wes
Eddy, Fred Baker, Mikael Abrahamsson, Mirja Kuehlewind, John Leslie,
and other members of the TSVWG and AQM working groups.
7. IANA Considerations
XX RFC Ed - PLEASE REMOVE THIS SECTION XXX
This memo includes no request to IANA.
8. Security Considerations
This document introduces no new security considerations. Each RFC
listed in this document discusses the security considerations of the
specification it contains.
9. Revision Information
XXX RFC-Ed please remove this section prior to publication.
Revision 00 was the first WG draft.
Revision 01 includes updates to complete all the sections and a
rewrite to improve readability. Added section 2. Author list
reversed, since Gorry has become the lead author. Corrections
following feedback from Wes Eddy upon review of an interim version of
this draft.
Note: Wes Eddy raised a question about whether discussion of the ECN
Pitfalls could be improved or restructured - this is expected to be
addressed in the next revision.
Revision 02 updates the title, and also the description of mechanisms
that help with partial ECN support.
We think this draft is ready for wider review. Comments are welcome
to the authors or via the IETF AQM or TSVWG mailing lists.
Revision 03 includes updates from the mailing list and WG discussions
at the Dallas IETF meeting.
The section "Avoiding Capacity Overshoot" was removed, since this
refers primarily to an AQM benefit, and the additional benefits of
ECN are already stated. Separated normative and informative
references
Revision 04 (WG Review during WGLC)
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Updated the abstract.
Added a table of contents.
Addressed various (some conflicting) comments during WGLC with new
text.
The section on Network Support for ECN was moved, and some
suggestions for rewording sections were implemented.
Decided not to remove section headers for 2.1 and 2.2 - to ensure the
document clearly calls-out the benefits.
Updated references. Updated text to improve consistency of terms and
added definitions for key terms.
Note: The group suggested this document should not define
recommendations for end hosts or routers, but simply state the things
needs to enable deployment to be successful.
Revision 05 (after WGLC comments)
Updated abstract to avoid suggesting that this describes new methods
for deployment.
Added ECN-field definition, and sorted terms in order.
Added an opening para to each "benefit" to say what this is. Sought
to remove redundancy between sections.
Added new section on Codepoints to avoid saying the same thing twice.
Reworked sections 3 and 4 to clarify discussion and to remove
unnecessary text.
Reformatted Summary to refer to sections describing things, rather
than appear as a list of new recommendations. Reordered to match the
new document order.
Note: This version expects an update to RFC5405.bis that will
indicate UDP ECN requirements (normative).
Revision 06
Corrections from Miria.
Revision 07
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Update to include IESG feedback from: Spencer, Dan, Benoit, Joel.
Corrected Non-ECN to Not-ECN where appropriate, added table of
codepoints, clarified sentences describing "conservative" behaviour,
added requirement to not do ToS-based routing (Junos enhanced hash),
etc. Ammended Acknowledgments section.
Revision 08
Typo and definition correction from Bob Briscoe.
10. References
10.1. Normative References
[ID.RFC5405.bis]
Eggert, Lars., Fairhurst, Gorry., and Greg. Shepherd,
"Unicast UDP Usage Guidelines", 2015.
[RFC2474] "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers".
[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>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <http://www.rfc-editor.org/info/rfc6040>.
[RFC7567] Baker, F. and G. Fairhurst, "IETF Recommendations
Regarding Active Queue Management", Internet-draft draft-
ietf-aqm-recommendation-06, October 2014.
10.2. 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.
[BA11] Bauer, Steven., Beverly, Robert., and Arthur. Berger,
"Measuring the State of ECN Readiness in Servers, Clients,
and Routers, ACM IMC", 2011.
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[Fla13] Flach, Tobias., Dukkipati, Nandita., Terzis, Andreas.,
Raghavan, Barath., Cardwell, Neal., Cheng, Yuchung., Jain,
Ankur., Hao, Shuai., Katz-Bassett, Ethan., and Ramesh.
Govindan, "Reducing web latency: the virtue of gentle
aggression.", SIGCOMM 2013, October 2013.
[ID.Acc.ECN]
Briscoe, Bob., Scheffeneger, Richard., and Mirja.
Kuehlewind, "More Accurate ECN Feedback in TCP, Work-in-
Progress".
[ID.AQM.eval]
Kuhn, Nicolas., Natarajan, Preethi., Ros, David., and
Naeem. Khademi, "AQM Characterization Guidelines (Work-in-
progress, draft-ietf-aqm-eval-guidelines)", 2015.
[ID.DCTCP]
Bensley, S., Eggert, Lars., and D. Thaler, "Microsoft's
Datacenter TCP (DCTCP): TCP Congestion Control for
Datacenters (Work-in-progress, draft-bensley-tcpm-dctcp)",
2015.
[ID.ECN-Encap]
Briscoe, B., Kaippallimalil, J., and P. Thaler,
"Guidelines for Adding Congestion Notification to
Protocols that Encapsulate IP", Internet-draft, IETF work-
in-progress draft-ietf-tsvwg-ecn-encap-guidelines.
[ID.Fallback]
Kuehlewind, Mirja. and Brian. Trammell, "A Mechanism for
ECN Path Probing and Fallback, draft-kuehlewind-tcpm-ecn-
fallback, Work-in-Progress".
[RFC0768] Postel, J., "User Datagram Protocol", 1980.
[RFC1349] "Type of Service in the Internet Protocol Suite".
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<http://www.rfc-editor.org/info/rfc3649>.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<http://www.rfc-editor.org/info/rfc3758>.
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[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC4774, November 2006,
<http://www.rfc-editor.org/info/rfc4774>.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
DOI 10.17487/RFC5562, June 2009,
<http://www.rfc-editor.org/info/rfc5562>.
[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>.
[RFC6789] Briscoe, B., Ed., Woundy, R., Ed., and A. Cooper, Ed.,
"Congestion Exposure (ConEx) Concepts and Use Cases",
RFC 6789, DOI 10.17487/RFC6789, December 2012,
<http://www.rfc-editor.org/info/rfc6789>.
[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.
[TR15] Tranmmel, Brian., Kuehlewind, Mirja., Boppart, Damiano,
Learmonth, Iain., and Gorry. Fairhurst, "Enabling
internet-wide deployment of Explicit Congestion
Notification Tramwell, B., Kuehlewind, M., Boppart, D.,
Learmonth, I., Fairhurst, G. & Scheffnegger, Passive and
Active Measurement Conference (PAM)", March 2015.
Authors' Addresses
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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
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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