Advertising the WebSockets support in the HTTPS resource record

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Table of Contents

1. Introduction

The mechanisms for running the WebSocket Protocol over a single stream of an HTTP/2 and HTTP/3 connection are defined in [RFC8441] and [RFC9220]. For bootstrapping WebSockets from HTTP/2 and HTTP/3 the extended CONNECT is used. The support for the extended CONNECT is advertised using HTTP/2 and HTTP/3 settings (see [RFC9113] and [RFC9114]). A client needs to establish an HTTP/2 or HTTP/3 connection and wait for the setting frames to be exchanged to discover whether it can try to use WebSockets over HTTP/2 or HTTP/3. The request still may be rejected because the settings advertise the support for the extended CONNECT but not explicitly the support for the WebSockets Protocol. The clients may choose to attempt HTTP/2 or HTTP/3 first and fall back to HTTP/1.1 or HTTP/2 if the WebSocket Protocol is not supported. This may add a delay. The other option is to try to use WebSockets over HTTP/2 or HTTP/3 only on connections that are already established and where it is known the extended CONNECT is supported. This approach leads to WebSockets over HTTP/2 or HTTP/3 being used less frequently.¶

This specification adds a way to advertise the support for WebSockets over HTTP versions using HTTPS resource record [HTTPSRR]. The client may choose to try using an HTTP/2 or HTTP/3 connection only if the support for the protocol is advertised. This will eliminate the delay in most cases and increase usage of WebSockets over HTTP/2 and HTTP/3.¶

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.¶

3. Extending HTTPS DNS resource record

This specification introduces the "wss" SvcParamKey (see [HTTPSRR]) that indicates a set of HTTP versions that support the WebSocket Protocol on the particular service endpoint. The HTTP versions are identified using alpn-id specified in [HTTPSRR].¶

The presentation value SHALL be a comma-separated list of one or more alpn-ids. The wire format values for the "wss" SvcParamKey consists of at least one alpn-id prefixed by its length as a single octet, and these length-value pairs are concatenated to form the SvcParamValue. These pairs MUST exactly fill the SvcParamValue; otherwise, the SvcParamValue is malformed.¶

All alpn-ids listed in the "wss" MUST also be present in the "alpn" key.¶

example.net              IN HTTPS 1 . alpn=h2,h3 wss=h2,h3

4. The Client Behavior

Upon receiving an HTTPS RR, a client should use the "wss" SvcParamKey as an indication of whether a particular service endpoint supports the WebSocket Protocol over HTTP /2 or HTTP/3.¶

If the key is present, that is a strong indication that the service endpoint supports WebSockets over HTTP/2 or HTTP/3 protocol and the client can attempt using WebSockets over HTTP/2 or HTTP/3 protocol. Due to difficulties of deployments, the client may discover that the feature, although advertised, is not supported and in this case, the client should fall back to using HTTP/1.1.¶

If the "no-default-alpn" key is present, the WebSocket Protocol over HTTP/1.1 is not supported by the endpoint. Otherwise, it might be supported whether the "wss" key is present or not.¶

If the "wss" key is not present, the client should not try using WebSockets over HTTP/2 and HTTP/3, and should directly use HTTP/1.1.¶

5. Security Considerations

This specification only adds a new SvcParamKey that is a hint of whether the WebSockets over HTTP/2 and HTTP/3 are supported. Therefore, it does not introduce additional security considerations beyond one described in [HTTPSRR], [RFC8441] and [RFC9220].¶

6. IANA Considerations

This specification adds the following entry to the Service Parameter Keys (SvcParamKeys) registry:¶

Acknowledgments

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