Adds new protocol troubleshooting page (#20816)

Author: angelampcosta

src/content/docs/speed/optimization/protocol/troubleshooting/protocol-troubleshooting.mdx

@@ -0,0 +1,35 @@
+---
+pcx_content_type: troubleshooting
+title: Troubleshoot protocol issues
+
+---
+
+This guide covers common HTTP/2 and HTTP/3 issues, including origin incompatibility, multiplexing errors, and browser errors, with steps to diagnose and resolve them.
+
+## H2 to Origin - Origin incompatibility
+
+- The origin's `max_concurrent_streams` is negotiated during the handshake process. 
+- If a `GOAWAY(0)` is received, it is likely due to a server restart or another reason causing the server to refuse new streams.
+- For more information, refer to [RFC 9113 - SETTINGS_MAX_CONCURRENT_STREAMS](https://datatracker.ietf.org/doc/html/rfc9113).
+
+## H2 Multiplexing - Origin incompatibility/issues
+
+- Multiplexing issues can arise due to incorrect server configurations. 
+- Use [netlogs](https://www.chromium.org/developers/design-documents/network-stack/netlog/) to identify `SETTINGS_MAX_CONCURRENT_STREAMS` violations or unexpected `GOAWAY` frames.
+- For more information, refer to [Stream Concurrency Issues](https://datatracker.ietf.org/doc/html/rfc9113#name-stream-concurrency).
+
+## Generic browser errors 
+
+Common browser errors include:
+
+- `ERR_HTTP2_PROTOCOL_ERROR`
+- `ERR_HTTP3_PROTOCOL_ERROR`
+- `ERR_QUIC_PROTOCOL_ERROR`
+
+These errors do not necessarily indicate a protocol-level issue. Follow these steps:
+
+1. Attempt reproduction using HTTP/1.1.
+2. If the issue persists in HTTP/1.1, address the underlying error before testing HTTP/2 or HTTP/3.
+3. If the issue does not persist, analyze netlogs for HTTP/2 or HTTP/3-specific issues.
+
+For more information, refer to [Chromium URL Request Header](https://chromium.googlesource.com/chromium/src/+/HEAD/net/url_request/url_request.h).