## HBONE

Along with pass-through traffic, Ztunnel supports the "HBONE" (HTTP-Based Overlay Network) protocol.
This is not really so much a new protocol, but rather a name we came up with to refer to the expectations of clients and servers communicating in the mesh.

HBONE is just a standard HTTP `CONNECT` tunnel, over mutual TLS with mesh (SPIFFE) certificates, on a well known port (15008).

// -----------------------------------------------------------------------------
// Dynamic cluster API.

// Update an existing context with a new set of servers defined in a ServerDef
// proto. Servers can be added to and removed from the list of remote workers
// in the context. A New set of servers identified by the ServerDef must be up
// when the context is updated.
//
// This API is for experimental usage and may be subject to change.

This is useful when uploading to cache servers that routinely redirect or reject upload requests,
as it avoids uploading the cache entry just to have it rejected (e.g. the cache entry is larger than the cache will allow) or redirected.
This additional check incurs extra latency when the server accepts the request, but reduces latency when the request is rejected or redirected.

Not all HTTP servers and proxies reliably implement Expect-Continue.

        return buildRedirectRequest(userResponse, method)
      }

      HTTP_CLIENT_TIMEOUT -> {
        // 408's are rare in practice, but some servers like HAProxy use this response code. The
        // spec says that we may repeat the request without modifications. Modern browsers also
        // repeat the request (even non-idempotent ones.)

  @Throws(IOException::class)
  fun connectionPreface() {
    this.withLock {
      if (closed) throw IOException("closed")
      if (!client) return // Nothing to write; servers don't send connection headers!
      if (logger.isLoggable(FINE)) {
        logger.fine(format(">> CONNECTION ${CONNECTION_PREFACE.hex()}"))
      }
      sink.write(CONNECTION_PREFACE)
      sink.flush()
    }
  }

// server with a chunked body when the body has zero bytes, since GETs with
// bodies (while acceptable according to specs), even zero-byte chunked
// bodies, are approximately never seen in the wild and confuse most
// servers. See Issue 18257, as one example.
//
// The only reason we'd send such a request is if the user set the Body to a
// non-nil value (say, io.NopCloser(bytes.NewReader(nil))) and didn't

	// volumeBindingMode indicates how PersistentVolumeClaims should be
	// provisioned and bound.  When unset, VolumeBindingImmediate is used.
	// This field is only honored by servers that enable the VolumeScheduling feature.
	// +optional
	VolumeBindingMode *VolumeBindingMode `json:"volumeBindingMode,omitempty" protobuf:"bytes,7,opt,name=volumeBindingMode"`

	// +optional
	AllowVolumeExpansion *bool

	// VolumeBindingMode indicates how PersistentVolumeClaims should be
	// provisioned and bound.  When unset, VolumeBindingImmediate is used.
	// This field is only honored by servers that enable the VolumeScheduling feature.
	// +optional
	VolumeBindingMode *VolumeBindingMode

	// Restrict the node topologies where volumes can be dynamically provisioned.

	// volumeBindingMode indicates how PersistentVolumeClaims should be
	// provisioned and bound.  When unset, VolumeBindingImmediate is used.
	// This field is only honored by servers that enable the VolumeScheduling feature.
	// +optional
	VolumeBindingMode *VolumeBindingMode `json:"volumeBindingMode,omitempty" protobuf:"bytes,7,opt,name=volumeBindingMode"`

		// but Envoy does not respect this (https://github.com/envoyproxy/envoy/issues/20885).
		// To counter this, we bump up the default to 60s to avoid overloading DNS servers.
		DnsRefreshRate:  durationpb.New(60 * time.Second),
		ServiceSettings: make([]*meshconfig.MeshConfig_ServiceSettings, 0),

		EnablePrometheusMerge: wrappers.Bool(true),