Dialing HTTP/3

This package provides a http.RoundTripper implementation that can be used on the http.Client:

roundTripper := &http3.RoundTripper{
	TLSClientConfig: &tls.Config{},  // set a TLS client config, if desired
	QUICConfig:      &quic.Config{}, // QUIC connection options
defer roundTripper.Close()
client := &http.Client{
	Transport: roundTripper,

The http.Client can then be used to perform HTTP requests over HTTP/3.

Using a quic.Transport

To use a custom quic.Transport, the function used to dial new QUIC connections can be configured:

tr := quic.Transport{}
roundTripper := &http3.RoundTripper{
	TLSClientConfig: &tls.Config{},  // set a TLS client config, if desired 
	QUICConfig:      &quic.Config{}, // QUIC connection options 
	Dial: func(ctx context.Context, addr string, tlsConf *tls.Config, quicConf *quic.Config) (quic.EarlyConnection, error) {
		a, err := net.ResolveUDPAddr("udp", addr)
		if err != nil {
			return nil, err
		return tr.DialEarly(ctx, a, tlsConf, quicConf)

This gives the application more fine-grained control over the configuration of the quic.Transport.

Using the same UDP Socket for Server and Roundtripper

Since QUIC demultiplexes packets based on their connection IDs, it is possible allows running a QUIC server and client on the same UDP socket. This also works when using HTTP/3: HTTP requests can be sent from the same socket that a server is listening on.

To achieve this using this package, first initialize a single quic.Transport, and pass a quic.EarlyListner obtained from that transport to http3.Server.ServeListener, and use the DialEarly function of the transport as the Dial function for the http3.RoundTripper.

Using 0-RTT

The use of 0-RTT was not anticipated by Go’s standard library, and Go doesn’t have 0-RTT support, neither in its crypto/tls nor in its net/http implementation (not even for TLS 1.3 on top of TCP). The http3 package therefore defines two new request methods: http3.MethodGet0RTT for GET requests and http3.MethodHead0RTT for HEAD requests.

Support for the “Early-Data” header field, as well as the “Too Early” status code (425) defined in RFC 8470 is not yet implemented. See 📝 Future Work.

It is the application’s responsibility to make sure that it is actually safe to send a request in 0-RTT, as outlined in Security Properties of 0-RTT. Requests sent in 0-RTT can be replayed on a new connection by an on-path attacker, so 0-RTT should only be used for idempotent requests. RFC 8740 defines some guidance on how to use 0-RTT in HTTP.

rt := &http3.RoundTripper{
	TLSClientConfig: &tls.Config{
		ClientSessionCache: tls.NewLRUClientSessionCache(100),
req, err := http.NewRequest(http3.MethodGet0RTT, "https://my-server/path", nil)
// ... handle error ...

The code snippet shows all the knobs that need to be turned to send a request in 0-RTT data:

  1. TLS session resumption must be enabled by configuring a tls.ClientSessionCache on the tls.Config.
  2. The request method needs to be set to http3.MethodGet0RTT.

📝 Future Work

  • Support for zstd Content Encoding: #4100
  • qlog Support: #4124
  • Happy Eyeballs Support: #3755
  • Support for Extensible Priorities (RFC 9218): #3470
  • Support for HTTP Trailers: #2266
  • Use Early-Data header field for 0-RTT requests, retry on 425 response status: #4381
  • Use url.Errors for all http3.RoundTripper errors: #4204
  • Support for 1xx Status Codes (including Early Hints, 103): #4403