Configuring UDP Sockets for ECN for Common Platforms
draft-duke-tsvwg-udp-ecn-00
This document is an Internet-Draft (I-D).
Anyone may submit an I-D to the IETF.
This I-D is not endorsed by the IETF and has no formal standing in the
IETF standards process.
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | Martin Duke | ||
| Last updated | 2024-08-23 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-duke-tsvwg-udp-ecn-00
tsvwg M. Duke
Internet-Draft Google
Intended status: Informational 23 August 2024
Expires: 24 February 2025
Configuring UDP Sockets for ECN for Common Platforms
draft-duke-tsvwg-udp-ecn-00
Abstract
Explicit Congestion Notification (ECN) applies to all transport
protocols in principle. However, it had limited applications for UDP
until QUIC became widely deployed. As a result, documentation of UDP
socket APIs for ECN on various platforms is sparse. This document
records the results of experimenting with these APIs in order to get
ECN working on UDP for Chromium on Apple, Linux, and Windows
platforms.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://example.com/LATEST. Status information for this document may
be found at https://datatracker.ietf.org/doc/draft-duke-tsvwg-udp/.
Discussion of this document takes place on the TSVWG Working Group
mailing list (mailto:tsvwg@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/tsvwg/. Subscribe at
https://www.ietf.org/mailman/listinfo/tsvwg/.
Source for this draft and an issue tracker can be found at
https://github.com/martinduke/draft-tsvwg-udp-ecn.
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 https://datatracker.ietf.org/drafts/current/.
Duke Expires 24 February 2025 [Page 1]
Internet-Draft udp-ecn August 2024
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 24 February 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Receiving ECN marks . . . . . . . . . . . . . . . . . . . . . 3
3.1. Setting the socket to report incoming ECN marks . . . . . 4
3.1.1. Linux and Apple . . . . . . . . . . . . . . . . . . . 4
3.1.2. Windows . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Retrieving ECN marks on incoming packets . . . . . . . . 5
3.2.1. Linux . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. Apple . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.3. Windows . . . . . . . . . . . . . . . . . . . . . . . 5
4. Sending ECN marks . . . . . . . . . . . . . . . . . . . . . . 6
4.1. On a per-socket basis . . . . . . . . . . . . . . . . . . 6
4.1.1. Linux and Apple . . . . . . . . . . . . . . . . . . . 6
4.1.2. Windows . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. On a per-packet basis . . . . . . . . . . . . . . . . . . 6
4.2.1. Linux . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Apple . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2.3. Microsoft . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
Duke Expires 24 February 2025 [Page 2]
Internet-Draft udp-ecn August 2024
1. Introduction
[RFC3168] reserves two bits in the IP header for Explicit Congestion
Notification (ECN), which provides network feedback to endpoint
congestion controllers. This has historically mostly been relevant
to TCP ([RFC9293]), where any incoming ECN marks are internally
consumed by the kernel, and therefore imply no application interface
except enabling and disabling the capability.
The Stream Control Transport Protocol (SCTP) ([RFC9260]) has long
supported ECN in its design. SCTP is sometimes carried over DTLS and
UDP ([RFC8261]). In principle, user-space implementers might have
leveraged UDP ECN APIs to deliver ECN markings between SCTP and the
UDP socket. The author is not aware of any such efforts.
However, QUIC [RFC9000] runs over UDP and has seen wider deployment
than SCTP. The Low Latency, Low Loss, Scalable Throughput (L4S)
experiment ([RFC9330]) and QUIC have combined to increase interest in
ECN over UDP.
The Chromium Projects ([CHROMIUM]) provide a widely-deployed protocol
library that includes QUIC. An effort to provide ECN support for
QUIC on the many platforms on which Chromium is deployed revealed
that many ECN-related UDP socket interfaces are poorly documented.
This document provides a record of that experience, to encourage
further support for ECN in other QUIC implementations, and indeed any
consumer of ECN markings that operates over UDP.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document is not a general tutorial on UDP socket programming,
and assumes familiarity with basic socket concepts like binding,
socket options, and common system error codes.
3. Receiving ECN marks
Network devices can change the ECN bits in the IP header. Since this
feedback is required at the packet sender, the packet receiver needs
to extract this codepoint from the UDP socket in order to report to
the sender.
Duke Expires 24 February 2025 [Page 3]
Internet-Draft udp-ecn August 2024
There are two components to this: setting the socket to report
incoming ECN marks, and retrieving the value for each incoming
packet.
3.1. Setting the socket to report incoming ECN marks
3.1.1. Linux and Apple
To report ECN, applications set a socket option to true using a
setsockopt() call.
IPv6 sockets require a socket option of level IPPROTO_IPV6 and type
IPV6_RECVTCLASS.
IPv4 sockets require a socket option of level IPPROTO_IP and type
IP_RECVTOS.
For dual-stack sockets, on Linux hosts the application sets both the
IPV6_RECVTCLASS and IP_RECVTOS options to receive ECN markings on all
incoming packets. On Apple hosts, the application only sets
IPV6_RECVTCLASS; setting IP_RECVTOS will return an error.
At the time of writing, an example implementation can be found at
[CHROMIUM-POSIX].
3.1.2. Windows
Windows documentation recommends using the function WSASetRecvIPEcn()
to enable ECN reporting regardless of the IP version.
However, this can also be accomplished by calling setsockopt() and
using options of level IPPROTO_IP and type IP_RECVECN for IPv4, and
IPPROTO_IPV6 and IPV6_RECVECN for IPv6. The author was unable to
identify any online documentation of these options at the time of
writing.
For dual-stack socekts, WSASetRecvIPEcn() will not enable ECN
reporting for IPv4. This requires a separate setsockopt() call using
the IP_RECVECN option.
Experiments revealed that if a socket is bound to a IPv6-mapped IPv4
address (i.e. it is of the format ::ffff:<IPv4 address>), calling to
WSASetRecvIpEcn() returned error EINVAL. The author was not able to
formulate a generalized rule about this behavior.
At the time of writing, an example implementation can be found at
[CHROMIUM-WINDOWS].
Duke Expires 24 February 2025 [Page 4]
Internet-Draft udp-ecn August 2024
3.2. Retrieving ECN marks on incoming packets
All platforms described in this document require the use of a
recvmsg() call to read data from the socket to retrieve ECN
information, because that information is encoded in the control data
that is returned from that function. Those platforms all return zero
or more "cmsg" that contain requested information about the arriving
packet.
Examples of the technique described below can be found at
[CHROMIUM-POSIX] and [CHROMIUM-WINDOWS].
3.2.1. Linux
If the incoming packet is IPv4, Linux will include a cmsg of level
IPPROTO_IP and type IP_TOS.
If the incoming packet is IPv6, Linux will include a cmsg of level
IPPROTO_IPV6 and type IP_TCLASS.
The resulting byte of data is the entire Type-of-Service byte from
the IPv4 header. The ECN mark constitutes the two least-significant
bits of this byte.
3.2.2. Apple
If the incoming packet is IPv4, the socket will include a cmsg of
level IPPROTO_IP and type IP_RECVTOS.
If the incoming packet is IPv6, the socket will include a cmsg of
level IPPROTO_IPV6 and type IP_RECVTCLASS.
The resulting byte of data is the entire Type-of-Service (TOS) byte
from the IPv4 header. The ECN mark constitutes the two least-
significant bits of this byte.
3.2.3. Windows
If the incoming packet is IPv4, the socket will include a cmsg of
level IPPROTO_IP and type IP_ECN.
If the incoming packet is IPv6, the socket will include a cmsg of
level IPPROTO_IPV6 and type IPV6_ECN.
The resulting integer solely consists of the ECN mark, and requires
no further bitwise operations.
Duke Expires 24 February 2025 [Page 5]
Internet-Draft udp-ecn August 2024
4. Sending ECN marks
Existing ECN specifications envision a particular connection
consistently sending the same ECN marking. It might transition that
marking after successfully completing a handshake, recognizing the
path or the peer do not support ECN, or transitioning to a new path.
Therefore, using a socket option to configure a consistent marking is
generally more resource-efficient.
However, some server designs receive all incoming packets on a single
socket. As the many connections that constitute this packet stream
may have different support for ECN, it is suitable to configure
outgoing ECN on a per-packet basis.
4.1. On a per-socket basis
4.1.1. Linux and Apple
Both Linux and Apple platforms set the outgoing ECN for IPv4 packets
with a socket option of level IPPROTO_IP and type IP_TOS.
For IPv6 packets, they use level IPPROTO_IPV6 and type IPV6_TCLASS.
This setsockopt() call also sets the Differentiated Services Code
Point (DSCP) bits that make up the rest of the TOS byte.
Applications making this call will generally want to preserve any
existing DSCP setting, which might require a getsockopt() call.
For dual-stack sockets, we hypothesize that Linux sockets will
require an additional setsockopt() call with IP_TOS. Apple sockets
will not and will return an error if this call is made. Our
experiments did not test this hypothesis.
An example of the technique described above can be found at
[CHROMIUM-POSIX].
4.1.2. Windows
The author did not experiment with setting a windows socket to send
an ECN mark.
4.2. On a per-packet basis
Packets can be individually marked with ECN codepoints using the
control information that accompanies a sendmsg() call.
Duke Expires 24 February 2025 [Page 6]
Internet-Draft udp-ecn August 2024
4.2.1. Linux
Linux uses a cmsg with level IPPROTO_IP and type IP_TOS for IPv4
packets.
Linux uses a cmsg with level IPPROTO_IPV6 and type IPV6_TCLASS for
IPv4 packets.
In either case, note that the six most significant bits of the data
are the DSCP code point, and in general should be preserved while
changing the ECN bits.
4.2.2. Apple
The author has not conducted experiments with setting ECN marks on a
per-packet basis on an Apple socket.
4.2.3. Microsoft
Windows uses a cmsg with level IPPROTO_IP and type IP_ECN for IPv4
packets.
Windows uses a cmsg with level IPPROTO_IPV6 and type IPV6_ECN for
IPv6 packets.
An example of the technique described above can be found at
[CHROMIUM-WINDOWS].
5. Security Considerations
The security implications of ECN are documented in [RFC3168] and
[RFC9330]. This document is a guide to enabling these capabilities,
which incurs no additional security considerations.
Note that implementing ECN capabilities on some platforms, but not
others, can help to fingerprint the operating system in use by a
host, which can have privacy implications. This document aims to
mitigate that possibility.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
Duke Expires 24 February 2025 [Page 7]
Internet-Draft udp-ecn August 2024
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[CHROMIUM] "The Chromium Projects", n.d.,
<https://www.chromium.org/chromium-projects/>.
[CHROMIUM-POSIX]
"udp_socket_posix.cc", n.d.,
<https://source.chromium.org/chromium/chromium/
src/+/main:net/socket/udp_socket_posix.cc>.
[CHROMIUM-WINDOWS]
"udp_socket_win.cc", n.d.,
<https://source.chromium.org/chromium/chromium/
src/+/main:net/socket/udp_socket_posix.cc>.
[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,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/info/rfc9260>.
[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
Duke Expires 24 February 2025 [Page 8]
Internet-Draft udp-ecn August 2024
[RFC9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC9330, January 2023,
<https://www.rfc-editor.org/info/rfc9330>.
Acknowledgments
The author would like to thank Ryan Hamilton, who provided constant
advice through this effort. Randall Meyer from Apple and Nick Grifka
from Microsoft provided useful hints about the behavior of their
respective operating systems.
Author's Address
Martin Duke
Google
Email: martin.h.duke@gmail.com
Duke Expires 24 February 2025 [Page 9]