ECN support in QUIC
draft-johansson-quic-ecn-02
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| Author | Ingemar Johansson | ||
| Last updated | 2017-04-05 (Latest revision 2017-02-21) | ||
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draft-johansson-quic-ecn-02
Network Working Group I. Johansson
Internet-Draft Ericsson AB
Intended status: Informational April 4, 2017
Expires: October 6, 2017
ECN support in QUIC
draft-johansson-quic-ecn-02
Abstract
This memo outlines the ECN (Explicit Congestion Notification) support
in QUIC. The draft specifies the ECN negotiation and the ECN echo
and in addition, different aspects of fallback in case of ECN failure
as well as OS specific issues with ECN and monitoring for ECN
capability. The intention is that most of the material ends up
updating other new or existing QUIC protocol specifications, thus it
may be possible that this draft does not warrant a working group
status.
Requirements Language
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 RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 6, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Elements of ECN support . . . . . . . . . . . . . . . . . . . 3
2.1. ECN negotiation . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Challenge/Response . . . . . . . . . . . . . . . . . 4
2.1.2. Determine degree of ECN support . . . . . . . . . . . 5
2.2. ECN bits in the IP header, semantics . . . . . . . . . . 5
2.3. ECN echo . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Fallback in case of ECN fault . . . . . . . . . . . . . . 8
2.5. OS socket specifics, access to the ECN bits . . . . . . . 8
2.6. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 9
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4. Open questions . . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
ECN support in transport protocols is a fundamental feature that
should be included in the QUIC specification as a mandatory element.
ECN has the key benefit that it allows for non-destructive congestion
notification by network node, i.e packets are marked instead
discarded. This is particularly beneficial for realtime applications
with requirements on latency, ECN also has the benefit that it
provides with a congestion signal that is unambiguous. The benefits
with ECN is described in more detail in [I-D.ietf-aqm-ecn-benefits].
The ECN support should be implemented to support both present and
future ECN, the latter is outlined in
[I-D.ietf-tsvwg-ecn-experimentation], of particular interest is the
ability to discriminate between classic ECN and L4S ECN by means of
differentiation between the use of the ECT(0) and ECT(1) code points.
This draft does however not delve into the details of the congestion
control implementation.
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2. Elements of ECN support
This draft covers the following aspects of ECN support:
o ECN negotiation
o ECN echo
o ECN bits in the IP header, semantics
o Fallback in case of ECN fault
o OS socket specifics, access to the ECN bits
o Monitoring
2.1. ECN negotiation
ECN support in QUIC needs to be negotiated. The reasons is that
network elements may not support ECN and may either clear the ECN
bits or simply discard packets that have the ECN bits set. In
addition, a QUIC implementation may not have access to the ECN bits
in the IP header due to OS dependent restrictions, investigations
(Piers O'Hanlon) have indicated that this is in certain cases an
asymmetric property, for instance while it is possible to set the ECN
bits it is not possible to read them.
It is also required that the ECN negotiation does not interfere with
the connection setup, in other words a failed ECN negotiation should
not cause an extra roundtrip for the connection setup.
The suggested method in this draft is to send an ECN negotiation
frame when connection setup is completed. Both peers MUST transmit
the ECN negotiation frame. The ECN negotiation frame is shown below.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |C|R|W|U U U|E E|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ECN negotation frame
The 2nd byte contains the flags:
o C: Challenge bit, indicates that the transmitted ECN negotiation
frame is a challenge, if bit is not set then it is a response.
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o R: Possible to read ECN bits in IP header
o W: Possible to write ECN bits in IP header
o EE : Echo of ECN bits
o U: Unused
The ECN negotiation has two steps.
o Challenge/response
o Determine degree of ECN support
2.1.1. Challenge/Response
A peer transmits the ECN negotiation frame with the R,W and EE bits
in the 2nd byte set to '0' and the C bit set to '1'. This frame is
echoed back with the flags set according to the degree of ECN support
and with the ECN bits in the IP header of the received ECN
negotiation frame copied to the EE field, the C bit is '0'. As both
peers MUST transmit an ECN negotiation frame there will be a total of
4 ECN negotiation frames transmitted, two challenges and two
responses.
An ECN negotiation frame should be transmitted in a unique packet,
this to avoid that possible loss of ECN negotiation packets cause
loss of other frames than the ECN negotiation frame.
The IP header for the ECN negotiation frame should set the ECN bits
to CE '11'. When the corresponding response is received then an EE
pattern of '11' indicates that ECN is likely supported in the
network. This does not give a full guarantee that ECN is supported
in the network. Monitoring of the ECN field in the ACK-frame serves
to give further indication of ECN support once ECN is turned on.
An ECN negotiation is declared successful when an ECN negotiation
response is received that indicates ECN support. A peer is not
allowed to set ECT on outgoing data packets until a successful ECN
negotiation is done. In other words it is only the ECN negotiation
frame that is allowed to set the ECN bits in the IP header until ECN
negotiation is concluded and successful.
A lack of an ECN negotiation response may indicate that the ECN
challenge frame or the ECN response frame was lost or that a node in
the network deliberately discards ECN-CE marked packets. The peer
should transmit an additional ECN challenge within an RTO interval in
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case a negotiation response is not received, a maximum of
retransmissions are attempted.
A failed challenge/response phase indicates that ECN should not be
used in the connection. [NOTE, a special case is where one peer does
not receive an ECN negotiation response but still receives ECT and CE
marked packets from the other peer. It is T.B.D how this should be
handled]
2.1.2. Determine degree of ECN support
If the ECN challenge/response is successful, the degree of ECN
capability depends on how the R, W and EE bits are set.
o R='1' and EE= '11': It is possible to set the ECN bits in outgoing
packets.
o R='0' or EE <> '11': ECN support is not certain as it is either
not possible for remote peer to read the ECN bits or that the ECN
bits are altered.
o W='1' : It is meaningful to send ECN feedback
o W='0' : It is not meaningful to send ECN feedback as the remote
peer cannot set (write) the ECN bits in the IP header.
The mode mechanism in [RFC6679] can serve as in input to a solution
for the support of ECN in the case that OS ECN support is asymmetric.
It is however unclear how a QUIC implementation can determine
asymmetric ECN support in the underlying OS. For instance the method
to send ECN marked packets to the local host to determine OS support
does not reveal if the OS ECN support is asymmetric.
2.2. ECN bits in the IP header, semantics
The ECN bits in the IP header should be set according to the
recommendations in [I-D.ietf-tsvwg-ecn-experimentation]. This means
that the meaning of ECT(0) and ECT(1) differ.
2.3. ECN echo
The ECN echo should go into the ACK frame [I-D.ietf-quic-transport],
this is beneficial as the ECN information can then use some of the
already existing data in the ACK frame for improved efficiency, this
applies especially to alternatives 1 and 2 below. It is suggested
that the 'U' bit in the ACK frame type is renamed 'E' to indicate the
presence of an ECN field in the ACK frame, this makes it possible to
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omit the ECN information for the cases where ECN is not supported for
the connection.
There are two alternatives how to add ECN support to the ACK frames.
The first alternative encodes the number of bytes that are marked
ECT(0), ECT(1) and CE with 32 bits each, the total extra overhead is
thus 12 octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Ack Block Length (8/16/32/48) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap 1 (8)] | [Ack Block 1 Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap 2 (8)] | [Ack Block 2 Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap N (8)] | [Ack Block N Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # ECT(0) bytes (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # ECT(1) bytes (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # ECN-CE bytes (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ECN field in ACK frame ACK block, alt 1
The second alternative use an extra byte to encode how many bits that
encode each of the ECT/CE fields.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Ack Block Length (8/16/32/48) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap 1 (8)] | [Ack Block 1 Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap 2 (8)] | [Ack Block 2 Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Gap N (8)] | [Ack Block N Length (8/16/32/48)] ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|E1 |E2 |CE | # ECT(0) bytes (0/16/32/48) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # ECT(1) bytes (0/16/32/48) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # ECN-CE bytes (0/16/32/48) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ECN field in ACK frame ACK block, alt 2
The E1,E2 and CE fields indicate the length of each encoding for the
number of ECT(0), ECT(1) and ECN-CE marked bytes. This is encoded
as:
o 00: 0 bits
o 01: 16 bits
o 10: 32 bits
o 11: 48bits
R indicates reserved bits.
There are pros an cons with the four alternatives:
o Alt 1: Has a fixed 12 octet overhead which may be beneficial as it
gives a deterministic overhead. The possible drawback is that it
is not possible to know exactly which frames have been remarked,
something that can limit the ability to detect network ECN faults
based on the method to transmit a pattern on ECT and CE marked
packets.
o Alt 2: Is a variation to Alt 1 but has a variable length encoding
that should consume less space, especially in the cases that one
of the ECT code points is not used and for the case that packets
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are only sporadically ECN-CE marked. This alternative also makes
it unnecessary to use a bit in the ACK frame type to indicate the
presence of an ECN field as this can be indicated in a efficient
way with the one byte header in this format. E0=E1=CE = 00
indicates that the following ECT and CE fields are encoded with
zero bits.
Which of the three formats above (or something else) that is the best
alternative is subject to discussion.
2.4. Fallback in case of ECN fault
ECN can be subject to issues in network equipment, such as remarking
to Not-ECN, remarking from ECT(0) to ECT(1) and vice versa or
constant remarking to ECN-CE. Furthermore ECT marked packets may be
discarded in the network. While these problems seem to be rare, see
for instance [McQuistin-Perkins]and [APPLE-ECN], it is still
necessary to safeguard against such problems.
A peer should disable ECN for its outgoing packets if ECN fault is
detected, it is however still possible for the other peer to use ECN.
TODO add more information as regards to how to detect network ECN
faults. [ECN-fallback](expired) gives a few examples for fault
detection. Examples on how to detect ECN faults include for instance
the method to set ECT and CE for outgoing packets according to a
given pattern.
Fallback in case of ECN faults is not an issue only for QUIC, it is
here suggested that mechanisms for this is described in a non QUIC
related draft, for instance in TSVWG.
2.5. OS socket specifics, access to the ECN bits
ECN support in QUIC comes with the additional challenge that it is
necessary to somehow access the ECN bits in the IP headers. In TCP
this is provided without major concerns as TCP is generally
implemented in OS kernel space. QUIC can however be implemented both
in user space or kernel space and is layered on top of UDP, which
means that access to the ECN bits is not a given, instead various
tricks are needed.
The text below is copy-pasted from [OHanlon].
"To set ECN on Linux, BSD and OSX one can use IP_TOS socket option,
with the setsockopt() call, to set the relevant ECN bits of the TOS
byte. On Windows one can use a similar technique though firstly one
has to enable TOS byte setting by enabling a particular Registry key
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( DisableUserTOSSetting=0 (see https://msdn.microsoft.com/en-
us/library/windows/desktop/dd874008%28v=vs.85%29.aspx One could also
probably use the libpcap write functionality."
"To obtain the ECN bits from a packet one needs a mechanism to
retrieve the ECN bits from each packet. On Linux, one needs to
firstly set the IP_RECVTOS socket option on the receiving socket, and
use the recvmsg() call to receive a packet, and then retrieve the TOS
byte from the associated csmg structure returned by the recvmsg()
call. This still works with linux-4.2.3. On OSX/BSD there are no
suitable socket options to retrieve the ECN/TOS bits and one cannot
use raw sockets as they do not function for UDP/TCP sockets (they do
work with ICMP), so one has to use alternatives such the bpf
interface, or a REDIRECT socket. Whilst on Windows it seems that the
only way to retrieve the ECN bits is via a raw socket, or custom NDIS
driver, though it's possible there's an API I'm missing."
TODO: Write a more detailed description on how to implement ECN
support in QUIC for different OS stacks.
2.6. Monitoring
A QUIC implementation should monitor the ECN functionality in order
to provide input to e.g. service providers to improve ECN support in
the networks. Items of interest are:
o Black holes, ECT or CE marked packets are discarded.
o Faulty remarking, e.g. ECT(0) is remarked to ECT(1) or Not-ECT.
o Continuous CE marking, possible indication of faulty on/off ECN
marking, but can also be an effect of severe congestion.
o Degree of L4S support. L4S should generally give low queue
latency. Estimation of one way queue delay for L4S enabled QUIC
connections can be used to determine if there are congested nodes
along the path that are not L4S capable.
3. IANA Considerations
T.B.D.
4. Open questions
A list of open questions:
o Is it sufficient that one peer sends an ECN negotiation challenge
frame?.
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o Should all packets be ECT or should there be special patterns to
improve fault detection.
o Write up a more detailed description on how to implement ECN
support in QUIC for different OS stacks.
o Determine which ECN echo encoding in the ACK frame is the best
alternative.
o Is a completely new ACK frame an alternative ?
o Should amount on ECT(0), ECT(1) and CE marked bytes account for
the IP+UDP headers or is it only the QUIC header + data that
counts ?
o Outline possible connection migration actions
o Are there any security implications with the small ECN negotiation
frame ?
5. Security Considerations
T.B.D
6. Acknowledgements
The following persons have contributed with comments and suggestions
for improvements: Mirja Kuehlewind, Koen De Schepper, Piers O'Hanlon,
Michael Welzl, Marcelo Bagnulo Braun, Martin Duke
7. References
7.1. 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>.
7.2. Informative References
[APPLE-ECN]
Apple Inc., "TCP ECN: Experience with Enabling ECN on the
Internet", <https://www.ietf.org/proceedings/98/slides/
slides-98-maprg-tcp-ecn-experience-with-enabling-ecn-on-
the-internet-padma-bhooma-00.pdf>.
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[Bagnulo] "Adding Explicit Congestion Notification (ECN) to TCP
control packets and TCP retransmissions",
<https://tools.ietf.org/id/draft-bagnulo-tcpm-generalized-
ecn-00.txt>.
[ECN-fallback]
"A Mechanism for ECN Path Probing and Fallback",
<https://www.ietf.org/archive/id/draft-kuehlewind-tcpm-
ecn-fallback-01.txt>.
[I-D.ietf-aqm-ecn-benefits]
Fairhurst, G. and M. Welzl, "The Benefits of using
Explicit Congestion Notification (ECN)", draft-ietf-aqm-
ecn-benefits-08 (work in progress), November 2015.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-02 (work
in progress), March 2017.
[I-D.ietf-tsvwg-ecn-experimentation]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", draft-ietf-tsvwg-ecn-experimentation-01
(work in progress), March 2017.
[McQuistin-Perkins]
""Is Explicit Congestion Notification usable with UDP?",
Proceedings of the ACM Internet Measurement Conference,
Tokyo, Japan, October 2015. DOI:10.1145/2815675.2815716",
<https://csperkins.org/publications/2015/10/
mcquistin2015ecn-udp.pdf>.
[OHanlon] "ECN support in different OS stacks",
<https://mailarchive.ietf.org/arch/msg/rmcat/
rRKF3PVmFL2zHCp1bOPKimqSsbM>.
[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>.
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[RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC7560, August 2015,
<http://www.rfc-editor.org/info/rfc7560>.
Author's Address
Ingemar Johansson
Ericsson AB
Laboratoriegraend 11
Luleaa 977 53
Sweden
Phone: +46 730783289
Email: ingemar.s.johansson@ericsson.com
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