ECN and DSCP support for HTTPS's Connect-UDP

Introduction Connect-UDP, as currently defined, limits the Explicit Congestion Notification (ECN) exchange between the HTTP server and the target. There is no support for carrying the ECN bits between the HTTP Connect-UDP client and the HTTP server proxying the UDP flow. Thus, it is not possible to establish the end-to-end ECN information flow necessary to support either classic ECN or L4S , . Diffserv enables differential network treatment of packets. Connect-UDP, as currently defined, lacks support for carrying the DSCP field through the tunnel. This document specifies Connect-UDP extensions that enable end-to-end ECN and DSCP for proxied connections: the zero-bytes extension adds no per-packet overhead by encoding the ECN and DSCP values directly into Context IDs. This document specifies negotiation to provide an initial set of Context IDs and capsules for dynamic updates. An alternative to this extension is Connect-IP ; however, it carries a full IP header between the HTTP client and server, resulting in significantly more overhead than this extension. This extension is defined such that it allows clients to optimistically start sending UDP packets in HTTP Datagrams, i.e. before receiving the response to its UDP proxying request, as described in .

Conventions 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.

Zero-byte Combined ECN and DSCP Extension For a zero-overhead encoding, the ECN and DSCP bits are indicated by using different Context IDs. An example use of three additional Context IDs to only encode the ECN bit used together with a DSCP of 0 is shown in . ECN-only Encoding Table

Context ID Value	ECN bit	ECN Value
0	0b00	Not-ECT
2	0b01	ECT(1)
4	0b10	ECT(0)
6	0b11	CE

No new Context ID value is defined to represent Not-ECT, since using a Context ID without this extension would, by default, imply Not-ECT. Additionally, Context IDs are defined to represent the combination of an ECN value other than Not-ECT and the DSCP values. If an application uses more DSCP values than just zero, additional Context IDs must be defined. This extension results in four times as many Context IDs within a single Connect-UDP stream for each DSCP value used. We expect that this is acceptable in most cases, as a total of 31 client initiated Context IDs can be encoded in a single byte, thus resulting in no packet expansion for applications that use up to 8 DSCP values. An endpoint enabling this extension MUST define all three ECN values, even if the ECN-enabled application expects that only one ECT value (and CE) is used. This is because of transmission errors or erroneous remarking in the network, where the other ECT codepoint, as well as Not-ECT, may be observed. Negotiation of the context ID values is defined using both HTTP headers and capsules in .

Negotiating Extensions Usage This section defines capability negotiation and Context ID configuration for the zero-bytes combined ECN and DSCP extensions. Note that Context Identifiers are defined as QUIC varints (see Section 16 of ) and support values up to 4,611,686,018,427,387,903 (2^62-1), which is larger than what a Structure Header Integer supports (limited to 999,999,999,999,999). We foresee no issues with this limitation, as Context Identifiers should primarily use the single-byte representation for efficiency, i.e., they should rarely exceed 63.

ECN DSCP Context Assignment Format In this extension four Context IDs need to be configured for each DSCP value. A configuration of ECN and DSCP signaling is represented by a five-tuple with the following format:

ECN DSCP CONTEXT ASSIGNMENT Format

DSCP_VALUE:: The DSCP value in the IP valid for the following Context IDs.
Reserved:: Reserving two bits to achieve byte alignment for the DSCP_VALUE.
NOT_ECN_CONTEXT:: The Context ID used to indicate the payload was marked as Not-ECN-Capable.
ECT_1_CONTEXT:: The Context ID used to indicate the payload was marked with ECN value ECT(1).
ECT_0_CONTEXT:: The Context ID used to indicate the payload was marked with ECN value ECT(0).
CE_CONTEXT:: The Context ID used to indicate the payload was marked with ECN value CE.

HTTP Structured field ECN-DSCP-Context-ID is a Structured Header Field . Its value is a List consisting of zero or more Inner Lists, where the Inner List contains five Integer Items. The Integer Items MUST be non-negative, as they are DSCP values and Context ID defined in . The DSCP value and four ECN Context IDs are those defined in , in that order. When the header is included in an Extended Connect request, it indicates, first of all, support for this ECN extension. Secondly, it may define one or more 5-item inner lists of DSCP values and corresponding ECN Context IDs for the requestor-to-responder direction. If no 5-item inner lists of Context IDs are included, then this header only indicates support for the extension, and the Context IDs MAY be signaled using capsules. When the request includes the ECN-DSCP-Context-ID header, the responder MAY include this header in the response. If included with one or more 5-item inner lists, it defines Context IDs defined by the server, usable in either direction. The following example indicates support for this extension and defines two sets of client initiated Context IDs: ID= 10, 12, 14, 16 (Not-ECN-Capable, ECT(1), ECT(0), CE) combined with DSCP 46 for Expedited Forwarding (EF): 46 ; and ID=0, 4, 6, 8 combined with the default DSCP value of 0. Note that the default context ID of 0 is (re-)used to indicate the default ECN value of Not-ECN Capable.

Example of ECN-DSCP-Context-ID header A well formed ECDN-DSCP-Context-ID header field SHALL NOT contain the same context ID in multiple inner lists or multiple positions within the same list.

ECN DSCP Context ID Assignment and ACK Capsules The ECN_DSCP_CONTEXT_ASSIGN capsule is used to assign additional Context ID values after negotiation and initial assignment in the HTTP header.

ECN_DSCP_CONTEXT_ASSIGN Capsule Format Type and Length as defined by Section 3.2 of the HTTP Capsule specification . The capsule value is the ECN_DSCP_CONTEXT_ASSIGNMENT defined above in . Thus, the capsule value consists of zero or more ECN_DSCP_CONTEXT_ASSIGNMENT five-tuples. The ECN_DSCP_CONTEXT_ACK capsule confirms the registration of Context IDs that were received via an ECN_DSCP_CONTEXT_ASSIGN capsule.

ECN_DSCP_CONTEXT_ACK Capsule Format An endpoint MUST only send an ECN_DSCP_CONTEXT_ACK capsule if it received an ECN_DSCP_CONTEXT_ASSIGN capsule with the same ECN_DSCP_CONTEXT_ASSIGNMENT. If an endpoint receives an ECN_DSCP_CONTEXT_ACK capsule for an ECN_DSCP_CONTEXT_ASSIGNMENT it did not attempt to register, that capsule is considered malformed.

Tunnels and DSCP and ECN marking interactions

Tunnel Endpoint Marking When a Tunnel Endpoint receives an IP/UDP packet belonging to a Connect-UDP request with the ECN DSCP extension enabled, the DSCP value and two ECN bits in the incoming IP/UDP packet are used to select the appropriate Context ID. If a not-yet-known DSCP value is received, the endpoint can register new Context ID assignments using the ECN_DSCP_CONTEXT_ASSIGN capsule and optimistically start using them. The Tunnel Endpoint on egress sets the DSCP that belongs to the received Context ID and corresponding ECN values in the IP packet it creates for this UDP Proxying payload. If the tunnel endpoint receives an unknown Context ID the endpoint MAY drop the packet or alternative buffer it for a limited time to enable the ECN_DSCP_CONTEXT_ASSIGN capsule to be received and processed. A Tunnel endpoint which is unable to read or set the ECN Field SHALL NOT enable the ECN extension.

DSCP Remarking Considerations The tunnel may interconnect two different administrative domains that use DSCP values differently. Thus, the endpoints likely need to perform remarking of DSCP field values, similar to what an inter-domain router would. Depending on use cases and deployment, the HTTP client can be in different network domains with different DSCP usages. An HTTP server that, based on user identification, connects the HTTP client to different network domains behind it may also need to support multiple external domains. The above complications in handling DSCP make it impossible to provide a standardized remarking instruction. Instead, the deployment will have to define whether remarking is handled by the HTTP server, the HTTP client, or both, considering the tunnel a specific network domain in itself.

Tunnel Transport Connection ECN Interactions and Congestion Control The primary goal of the ECN DSCP extension is to enable ECN usage between the proxy and the target and to have the end-to-end transport react to that ECN. However, different potential models exist for providing ECN interactions for the tunnel, i.e., between the HTTP client and server. The choice depends on how the tunnel is configured and what additional support has been implemented for the Connect-UDP protocol. The default deployment would be to use congestion controlled transport protocols between the HTTP endpoints or proxies for the tunneled ECN enabled packets. This includes all HTTP versions before HTTP/3 , as well as HTTP/3 sending packets over reliable streams as well as over congestion controlled datagrams. In this deployment on the ingress to each congestion controlled transport an Active Queue Management (AQM) is recommended that can mark the tunneled packet's ECN field (or drop them) in case there is sufficient queue build up. For tunnels using HTTP/3 with datagrams, where the QUIC connection disables congestion control on packets containing HTTP datagrams as discussed in Section 6 of , the ECN marking on tunneled packets can be propagated between the IP packet of the transport connection and the end-to-end packet. This represents a specific implementation of IP-in-IP tunnels with tightly coupled shim headers as discussed in . It is implemented as Feed-Forward-and-Up as discussed in , and MUST use the normal mode on tunnel ingress and follow the specified default behavior on egress as defined in .

Tunnel Transport Connection DSCP Interactions For HTTP tunnels not using HTTP/3 , HTTP/3 using reliable streams, or HTTP/3 with datagrams but without disabling congestion control, the tunnel will consist of one or possibly several chained congestion-controlled transport connections. These transport connections can use only a single DSCP code point to avoid inconsistent network treatment that might confuse the congestion controller and retransmission mechanism. Thus, even if the tunneled packets use different DSCP values, the transport connection must settle on a single, suitable DSCP value. However, if the QUIC multipath extension is used, each path can have a different DSCP value. In this latter case, packets with different DSCP values can be mapped to different paths with the appropriate network treatment as indicated by their DSCP values. For tunnels using HTTP/3 with datagrams and where the QUIC connection disables congestion control on packets containing HTTP datagrams, as discussed in Section 6 of , the QUIC packets can be marked using the most suitable DSCP value based on the encapsulated packet. In cases where the tunnel connection is sent into a different network domain than the one on which the tunneled packet was received, a suitable remapping must occur for the domain to which the tunnel packet will be sent. The HTTP tunnel MUST NOT coalesce different tunneled payloads that are not mapped to the same DSCP in a single QUIC packet.

QUIC Aware Forwarding An HTTP endpoint that supports this extension and QUIC Aware Forwarding MUST preserve ECN markings on forwarded packets in both directions to ensure end-to-end ECN functionality. Using this extension in combination with QUIC Aware Forwarding, rather than relying solely on the latter, also ensures that ECN black holes do not occur, for example, on long-header packets or packets sent before the QUIC Aware Forwarding path is established for short-header packets. Thus, supporting both provides a consistent ECN experience.

Security Considerations Connect-UDP requires HTTPS , which ensures that Context ID assignments and capsule exchanges are confidential and integrity-protected, preventing on-path third parties from manipulating ECN and DSCP mappings.

DSCP and ECN Fields This specification main functionality is to remap the IP header's fields DSCP and ECN to context-IDs between the ingress and egress of the MASQUE tunnel for IP/UDP flows mapped to CONNECT-UDP request. It also possible to apply on path marking of outer ECN fields to the end-to-end packet on egress. Thus, this specification does not change the risks with on-path manipulation of the ECN and DSCP fields.

Resource Consumption Each ECN_DSCP_CONTEXT_ASSIGN capsule allocates state at the receiving endpoint. However, this state is very limited as per CONNECT-UDP each inner assignment maps four IP TOS (DSCP+ECN) byte values to four context value. Normally only a small subset out of the potential 256 different DSCP + ECN combination will be used. Malicous endpoints that tries to register more than 256 mappings can be dealt by closing the CONNECT-UDP request stream.

ECN Tunnel Safety The ECN propagation rules in and the safety requirements in apply to this extension. In particular, if the tunnel egress does not support ECN propagation, the ingress MUST clear the outer ECN field to Not-ECT to prevent CE marks from being silently discarded, which would break congestion control feedback loops.

IANA Considerations

HTTP Field Names IANA is requested to register one new permanent Field name in the Hypertext Transfer Protocol (HTTP) Field Name Registry (At time of writing residing at: https://www.iana.org/assignments/http-fields/http-fields.xhtml).

ECN-DSCP-Context-ID

Field Name:: ECN-DSCP-Context-ID
Status:: Permanent
Structured Type:: List
Reference:: RFC-TO-BE

HTTP Capsule Type IANA is requested to register two new HTTP Capsule Types in the permanent range (0x00-0x3f).

ECN_DSCP_CONTEXT_ASSIGN

Value:: TBA_1
Capsule Type:: ECN_DSCP_CONTEXT_ASSIGN
Status:: permanent
Reference:: RFC-TO-BE
Change Controller:: IETF
Contact:: MASQUE Working Group masque@ietf.org
Notes:: None

ECN_DSCP_CONTEXT_ACK

Value:: TBA_2
Capsule Type:: ECN_DSCP_CONTEXT_ACK
Status:: permanent
Reference:: RFC-TO-BE
Change Controller:: IETF
Contact:: MASQUE Working Group masque@ietf.org
Notes:: None

Acknowledgements The authors like to acknowledge that David Schinazi early had another proposal for adding ECN to Connect-UDP .