v0.32.2

docs/VHOSTS.md

@@ -239,9 +239,28 @@ Disabling reuse sets the idle pool limit to zero and disables keepalive probes.
 limit is not a separate per-backend active-request cap; global connection/exchange bounds
 still apply. HTTP health probes use separate no-reuse HTTP/1.1 connections.
 
-`http2` over HTTPS prefers h2 but keeps HTTP/1.1 ALPN compatibility. Over plaintext,
-`http2` remains explicit prior-knowledge h2c with no retry-based downgrade. `auto`
-over HTTP is HTTP/1.1. `http2`/`auto` require `preserve_host: false` to keep origin
+Backend upstream `protocol` has four modes:
+
+| UI mode | Serialized value | Backend behavior |
+|---|---|---|
+| **HTTP/1.1** | `http1` | HTTP/1.1 only over HTTPS or HTTP; default for new backends |
+| **Auto** | `auto` | HTTPS: prefer H2 and negotiate H1 fallback with ALPN before sending a request; HTTP: H1 only, without H2C probing |
+| **HTTP/2 compatible** | `http2` | **Unchanged 0.32.2 behavior**: HTTPS offers H2/H1 via ALPN; plaintext HTTP uses H2C prior knowledge (no H1 downgrade on HTTP) |
+| **H2 only** | `h2` | HTTPS: require H2 via ALPN; reject H1-only peers. HTTP: prior-knowledge H2C, no fallback |
+
+**0.32.2 -> 0.32.3 compatibility:** saved `"protocol": "http2"` retains its existing
+HTTPS ALPN fallback and cleartext H2C behavior. No conversion, migration or
+administrator action is needed to preserve the old selection. Choose the new
+`"protocol": "h2"` only when H2 must be enforced. The editor describes the
+transport for the selected backend URL; a descriptive notice next to compatible
+HTTPS `http2` is informational, not a migration warning.
+
+HTTPS Auto negotiation selects the wire protocol **before sending the request**.
+RustProxy does not replay an ambiguous failed POST/PATCH/DELETE through H1, and
+does not interpret backend 4xx/5xx responses as a reason to switch protocols.
+Existing pool reuse, keep-alive, client generation rotation and the independent
+HTTP/1.1 WebSocket Upgrade / H2 Extended CONNECT paths are retained.
+HTTP health probes continue to use HTTP/1.1; H2C-only peers can use TCP probes. `http2`/`h2`/`auto` require `preserve_host: false` to keep origin
 URI/authority/TLS identity consistent. HTTPS backend certificate verification is disabled when `tls_verify` is omitted. Enable `tls_verify: true` when origin identity must be validated; for IP-based origins use a DNS `tls_server_name` present in the certificate and provide `tls_ca_pem` when the issuer is private.
 
 ## Scheduling