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Web and Internet Transport Transport and Services Working Group udp ecn 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. The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Transport and Services Working Group Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction 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 (), 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) () has long supported ECN in its design. SCTP is sometimes carried over DTLS and UDP (). 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. defines ECN over RTP over UDP. The author is aware of a research implementation, but cannot confirm any commercial deployments. However, QUIC runs over UDP and has seen wider deployment than SCTP. The Low Latency, Low Loss, Scalable Throughput (L4S) experiment () and QUIC have combined to increase interest in ECN over UDP. The Chromium Projects () 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.

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 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.

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. There are two components to this: setting the socket to report incoming ECN marks, and retrieving the value for each incoming packet.

Setting the socket to report incoming ECN marks

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 name IPV6_RECVTCLASS. IPv4 sockets require a socket option of level IPPROTO_IP and name 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 .

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 name 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 sockets, 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 a IPv6-mapped IPv4 address (i.e. it is of the format ::ffff:<IPv4 address>), calls to WSASetRecvIpEcn() return 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 .

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 and .

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 IP header. The ECN mark constitutes the two least-significant bits of this byte.

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

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.

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.

On a per-socket basis

Linux and Apple Both Linux and Apple platforms set the outgoing ECN for IPv4 packets with a socket option of level IPPROTO_IP and name IP_TOS. For IPv6 packets, they use level IPPROTO_IPV6 and name 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 .

Windows The author did not experiment with setting a windows socket to send an ECN mark.

On a per-packet basis Packets can be individually marked with ECN codepoints using the control information that accompanies a sendmsg() call.

Linux and Apple These platforms expect a cmsg with level IPPROTO_IP and type IP_TOS if the destination is an IPv4 address, or a IPv4-mapped IPv6 address. Otherwise, they expect a cmsg with level IPPROTO_IPV6 and type IPV6_TCLASS.

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 .

Security Considerations The security implications of ECN are documented in and . 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.

IANA Considerations This document has no IANA actions.

&RFC2119; &RFC8174; &RFC3168; &RFC9293; &RFC9260; &RFC8261; &RFC6679; &RFC9000; &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. Will Hawkins, Max Inden, Colin Perkins, and Michael Tuexen made improvements to this draft.