Configuring UDP Sockets for Explicit Congestion Notification (ECN) for Common Platforms

Internet Engineering Task Force (IETF)                           M. Duke
Request for Comments: 10055                                       Google
Category: Informational                                     October 2026
ISSN: 2070-1721


 Configuring UDP Sockets for Explicit Congestion Notification (ECN) for
                            Common Platforms

Abstract

   Explicit Congestion Notification (ECN) applies to all transport
   protocols in principle.  However, it had limited deployment for UDP
   until QUIC became widely adopted.  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.

   This document provides a snapshot of the ECN state of affairs as
   supported by these platforms at the time of writing.  Readers should
   refer to the documentation of the various platforms for more up-to-
   date information.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc10055.

Copyright Notice

   Copyright (c) 2026 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Receiving ECN Codepoints
     3.1.  Setting the Socket to Report Incoming ECN Codepoints
       3.1.1.  Linux, Apple, and FreeBSD
       3.1.2.  Windows
     3.2.  Retrieving ECN Codepoints on Incoming Packets
       3.2.1.  Linux
       3.2.2.  Apple and FreeBSD
       3.2.3.  Windows
   4.  Sending ECN Codepoints
     4.1.  On a Per-Socket Basis
       4.1.1.  Apple, FreeBSD, and Linux
       4.1.2.  Windows
     4.2.  On a Per-Packet Basis
       4.2.1.  Apple, FreeBSD, and Linux
       4.2.2.  Windows
   5.  Security Considerations
   6.  IANA Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Author's Address

1.  Introduction

   [RFC3168] defines a 2-bit field in the IP header for Explicit
   Congestion Notification (ECN), which provides network feedback to
   endpoint congestion controllers.  Historically, this has mostly been
   relevant to TCP [RFC9293], where any incoming ECN codepoints are
   internally consumed by the kernel and, therefore, imply no
   application interface except for enabling and disabling the
   capability.

   The Stream Control Transmission 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 codepoints between SCTP and the
   UDP socket.  At the time of publication, the IETF is not aware of any
   such efforts.

   [RFC6679] defines ECN over RTP over UDP.  The IETF is aware of a
   research implementation but cannot confirm any commercial
   deployments.  [RFC8888] proposes a different feedback format to work
   with [RFC6679], and there are a few implementations of this.

   QUIC [RFC9000] runs over UDP and has seen wider deployment than SCTP.
   The Low Latency, Low Loss, Scalable Throughput (L4S) architecture
   [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 Informational document provides a record of that experience to
   encourage further support for ECN in other QUIC implementations and
   any consumer of ECN codepoints that operates over UDP.  It is not a
   Standards Track document and does not bind platforms to any API nor
   suggest any such API.

   Many socket APIs continue to reference the "ToS (Type of Service)
   byte", including the IP_TOS label, even though [RFC2474] obsoleted
   that in 1998.  That 8-bit field now contains a 6-bit Differentiated
   Services Code Point (DSCP) and the 2-bit ECN field.

   This document focuses on the APIs for the C and C++ languages.  Other
   languages are likely to have different syntax and capabilities.

   The "ecn-examples" code repository [EXAMPLES] is extremely compact
   code that can verify the information in this document.

   This document addresses access to the ECN field in the IP header via
   socket APIs.  It does not address UDP transport-layer options
   [RFC9868], which are a separate extension mechanism operating at a
   different layer.

2.  Conventions and Definitions

   This document is not a general tutorial on UDP socket programming and
   does assume familiarity with basic socket concepts like binding,
   socket options, and common system error codes.

   Throughout this document, "Apple" refers to both macOS and iOS.
   "Linux" guidance also applies to Android.

3.  Receiving ECN Codepoints

   Network devices can change the ECN codepoint 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.

   There are two components to this: 1) setting the socket to report
   incoming ECN marks and 2) retrieving the ECN codepoint for each
   incoming packet.

   Note that Apple platforms additionally provide a framework for
   network connections that allows receiving ECN flags when using UDP
   without traditional socket option semantics.  When sending or
   receiving UDP datagrams, IP protocol metadata carries ECN information
   in both directions.  See [APPLE-NETWORK-FRAMEWORK].

3.1.  Setting the Socket to Report Incoming ECN Codepoints

3.1.1.  Linux, Apple, and FreeBSD

   To receive ECN codepoints, applications set a socket option to "true"
   using a setsockopt() call.

   On all platforms, IPv4 sockets require the IPPROTO_IP-level socket
   option with the name IP_RECVTOS to be set.

   On all platforms, IPv6 sockets require the IPPROTO_IPV6-level socket
   option with the name IPV6_RECVTCLASS to be set.

   If the IPv6 socket is not IPv6 only, on Linux hosts it is required to
   also set the IPPROTO_IP-level socket option IP_RECVTOS to receive ECN
   codepoints for UDP/IPv4 packets.  This step is not necessary for
   Apple or FreeBSD.

   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 codepoint reporting regardless of the IP version.  This
   function dates to Windows 10 Build 20348, according to [WINDOWS-DOC].

   However, this can also be accomplished by calling setsockopt() and
   using options of level IPPROTO_IP and name IP_RECVECN for IPv4 and
   level IPPROTO_IPV6 and name IPV6_RECVECN for IPv6.  These options are
   documented at [WINDOWS-SOCKOPT].

   For IPv6 sockets that are not IPv6 only, WSASetRecvIPEcn() will not
   enable ECN reporting for IPv4.  This requires a separate setsockopt()
   call using the IP_RECVECN option.

   If a socket is bound to an IPv4-mapped IPv6 address (i.e., it is of
   the format ::ffff:<IPv4 address>; see Section 2.5.5.2 of [RFC4291]),
   then calls to WSASetRecvIpEcn() return the error EINVAL.  These
   sockets should instead use an explicit setsockopt() call to set
   IP_RECVECN.

   At the time of writing, an example implementation can be found at
   [CHROMIUM-WINDOWS].

3.2.  Retrieving ECN Codepoints on Incoming Packets

   All platforms described in this document require the use of a
   recvmsg() call to read data from the socket and to retrieve the ECN
   codepoint, because that information is provided as ancillary data.
   Those platforms all return zero or more "cmsg"s 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 a UDP/IPv4 message is received, Linux will include a cmsg of level
   IPPROTO_IP and type IP_TOS.  The cmsg data contains an unsigned char.
   This applies to IPv4 sockets and to IPv6 sockets that are not IPv6
   only.

   If a UDP/IPv6 message is received, Linux will include a cmsg of level
   IPPROTO_IPV6 and type IPV6_TCLASS.  The cmsg data contains an int.
   This applies to IPv6 sockets.

   The cmsg data contains the entire IP header byte, which includes the
   DSCP and the ECN codepoint.  The ECN codepoint constitutes the two
   least significant bits of this byte.

   The same applies to the Linux-specific recvmmsg() call.

3.2.2.  Apple and FreeBSD

   If a UDP/IPv4 message is received on an IPv4 socket, the ancillary
   data will contain a cmsg of level IPPROTO_IP and type IP_RECVTOS.
   The cmsg data contains an unsigned char.

   If a UDP/IPv6 or UDP/IPv4 message is received on an IPv6 socket, the
   ancillary data will contain a cmsg of level IPPROTO_IPV6 and type
   IPV6_TCLASS.  The cmsg data contains an int.

   The cmsg data contains the entire IP header byte, which includes the
   DSCP and the ECN codepoint.  The ECN codepoint constitutes the two
   least significant bits of this byte.

3.2.3.  Windows

   If the incoming packet is UDP/IPv4, the socket will include a cmsg of
   level IPPROTO_IP and type IP_ECN.  The cmsg data contains an int.

   If the incoming packet is UDP/IPv6, the socket will include a cmsg of
   level IPPROTO_IPV6 and type IPV6_ECN.  The cmsg data contains an int.

   The cmsg data solely consists of the ECN codepoint and requires no
   further bitwise operations.

4.  Sending ECN Codepoints

   Existing ECN specifications ([RFC3168] and [RFC9330]) envision a
   particular connection that is consistently sending the same ECN
   codepoint.  It might transition that marking after successfully
   completing a handshake, recognizing that the path or the peer does
   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 provide the ECN
   codepoint on a per-packet basis.

   Note that Apple platforms additionally provide a framework for
   network connections that allows sending ECN flags when using UDP
   without traditional socket option semantics.  When sending or
   receiving UDP datagrams, IP protocol metadata carries ECN information
   in both directions.  See [APPLE-NETWORK-FRAMEWORK].

4.1.  On a Per-Socket Basis

4.1.1.  Apple, FreeBSD, and Linux

   For sending UDP/IPv4 packets on an IPv4 socket, Apple, FreeBSD, and
   Linux platforms allow the outgoing ECN codepoint to be configured by
   using the IPPROTO_IP-level socket option with the name IP_TOS.  The
   value has the type int.

   For sending UDP/IPv6 packets on an IPv6 socket, Apple, FreeBSD, and
   Linux platforms allow the outgoing ECN codepoint to be configured by
   using the IPPROTO_IPV6-level socket option with the name IPV6_TCLASS.
   The value has the type int.

   For sending UDP/IPv4 packets on an IPv6 socket, Linux platforms allow
   the outgoing ECN codepoint to be configured by using the IPPROTO_IP-
   level socket option with the name IP_TOS.

   For sending UDP/IPv4 packets on an IPv6 socket, Apple and FreeBSD
   platforms allow the outgoing ECN codepoint to be configured by using
   the IPPROTO_IPV6-level socket option with the name IPV6_TCLASS.

   Except for Apple platforms, this setsockopt() call also sets the DSCP
   that comprises the rest of the header byte.  Applications making this
   call ought to preserve any existing DSCP setting, which might require
   an additional getsockopt() call, to avoid overriding commands set by
   other code in the stack.  If there are multiple threads making
   changes to this byte, further safeguards will be necessary.

   An example of the technique described above can be found at
   [CHROMIUM-POSIX].

4.1.2.  Windows

   At the time of this writing, Windows does not provide a way to
   configure marking on a per-socket basis.

4.2.  On a Per-Packet Basis

   Packets can be individually marked with ECN codepoints using the
   ancillary data that accompanies a sendmsg() call.

4.2.1.  Apple, FreeBSD, and Linux

   For sending UDP/IPv4 packets on an IPv4 socket, Apple, FreeBSD, and
   Linux use a cmsg with level IPPROTO_IP and type IP_TOS.  On Apple and
   Linux, the type of data is int; and for FreeBSD, it is unsigned char.

   For sending UDP/IPv6 packets on an IPv6 socket, Apple, FreeBSD, and
   Linux use a cmsg with level IPPROTO_IPV6 and type IPV6_TCLASS.  The
   type of the data is int.

   For sending UDP/IPv4 packets on an IPv6 socket, Linux requires a cmsg
   with level IPPROTO_IP and type IP_TOS.  Apple and FreeBSD accept a
   cmsg with level IPPROTO_IPV6 and type IPV6_TCLASS.

   The same applies to the Linux-specific sendmmsg() call.

4.2.2.  Windows

   For IPv4 packets, Windows uses a cmsg with level IPPROTO_IP and type
   IP_ECN.

   For IPv6 packets, Windows uses a cmsg with level IPPROTO_IPV6 and
   type IPV6_ECN.

   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 peers identify 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

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [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>.

   [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>.

7.2.  Informative References

   [APPLE-NETWORK-FRAMEWORK]
              "NWProtocolIP.Metadata",
              <https://developer.apple.com/documentation/network/
              nwprotocolip/metadata>.

   [CHROMIUM] "The Chromium Projects",
              <https://www.chromium.org/chromium-projects/>.

   [CHROMIUM-POSIX]
              "udp_socket_posix.cc", commit 6bdc14d, 19 June 2026,
              <https://source.chromium.org/chromium/chromium/
              src/+/main:net/socket/udp_socket_posix.cc>.

   [CHROMIUM-WINDOWS]
              "udp_socket_win.cc", commit 85a1d1c, 10 June 2026,
              <https://source.chromium.org/chromium/chromium/
              src/+/main:net/socket/udp_socket_win.cc>.

   [EXAMPLES] "ecn-examples", commit 39e39bf, 1 December 2025,
              <https://github.com/nplab/ecn-examples>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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, <https://www.rfc-editor.org/info/rfc6679>.

   [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>.

   [RFC8888]  Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
              Control Protocol (RTCP) Feedback for Congestion Control",
              RFC 8888, DOI 10.17487/RFC8888, January 2021,
              <https://www.rfc-editor.org/info/rfc8888>.

   [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>.

   [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>.

   [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>.

   [RFC9868]  Touch, J. and C. Heard, Ed., "Transport Options for UDP",
              RFC 9868, DOI 10.17487/RFC9868, October 2025,
              <https://www.rfc-editor.org/info/rfc9868>.

   [WINDOWS-DOC]
              "WSASetRecvIPEcn function (ws2tcpip.h)", 22 February 2024,
              <https://learn.microsoft.com/en-
              us/windows/win32/api/ws2tcpip/nf-ws2tcpip-
              wsasetrecvipecn>.

   [WINDOWS-SOCKOPT]
              "MSDN - IPPROTO_IP socket options", 7 May 2026,
              <https://learn.microsoft.com/en-us/windows/win32/winsock/
              ipproto-ip-socket-options>.

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.  However, the author takes full
   responsibility for any errors above.

   Michael Tüxen wrote the "ecn-examples" code that was very helpful in
   verifying the conclusions in this document.  He also made multiple
   editorial contributions.

   Neal Cardwell, Gorry Fairhurst, Max Franke, Rodney Grimes, Will
   Hawkins, Guillaume Hétier, Max Inden, Jonathan Lennox, Colin Perkins,
   Marten Seemann, and Greg White made improvements to this document.

Author's Address

   Martin Duke
   Google
   Email: martin.h.duke@gmail.com