HTTP/2 is the second major version of the HTTP network protocol, designed to improve upon the performance and security of its predecessor, HTTP/1.1. Despite its enhancements, you may encounter certain errors when using HTTP/2, especially when performing tasks that involve Remote Procedure Calls (RPC) over HTTP. One such error is: `RPC failed; curl 92 HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)`.

This article delves into the intricacies of this error, exploring potential causes and solutions, and providing background on the relevant technologies involved.

Understanding the Error

The error message contains several components that are essential to unpack:

• RPC (Remote Procedure Call): This is a protocol that one program can use to request service from a program located on another computer within a network. RPC protocols mirror local calls, but the service called is located on the network rather than within the same system.
• curl 92: `curl` is a command-line tool used to transfer data using various network protocols. The number `92` indicates the specific curl source code line or error code related to HTTP/2 stream issues.
• HTTP/2 stream 0: HTTP/2 uses multiplexing, which allows multiple streams to be initiated on a single TCP connection. Stream `0` is reserved for connection-level messages.
• Not closed cleanly: This indicates that the termination of the HTTP/2 stream/message did not follow the expected protocol for closure, prompting a protocol-level error.
• PROTOCOL_ERROR: This specifies that there was a violation of the expected protocol rules. Such errors are generally associated with incorrect HTTP/2 frame handling or malformed messages.

Technical Explanation

HTTP/2 Frame Structure

HTTP/2 frames are the smallest unit of communication, containing framing information and optional data. Each frame has a specific purpose, such as headers, data, priority, or settings. The error arises because:

1. Malformed Frame: The stream might be handling a malformed frame that violates HTTP/2 framing rules.
2. Unexpected Closure: The closure procedure did not follow the expected rules, leading to a disconnect in the communication channel.
3. Mismanagement of Stream 0: Stream 0 should only be used for connection management, not data transmission, so errors often occur if a protocol attempts to misdirect data through this stream.

What Causes This Error?

1. Network Instability: A fluctuating network can interrupt clean transmission, leading to abrupt stream closures.
2. Server Misconfiguration: Incorrect server settings for HTTP/2 might result in improper stream management.
3. Software Bugs: Either in client (curl, other HTTP libraries) or server software, which might mishandle the HTTP/2 protocol specifications.
4. Security Protocol Mismatch: Incompatible SSL/TLS versions between client and server can result in an improper handshake, causing protocol errors.

Troubleshooting the Error

To address the error, consider the following approaches:

1. Network Diagnosis:
   • Test your network for interruptions or latency issues.
   • Use tools like `ping` or `traceroute` to ensure stable and secure connectivity.
2. Check Server Configuration:
   • Ensure that the server is configured to support HTTP/2 properly.
   • Inspect your web server configuration (e.g., Apache, Nginx) to ensure it's compliant with HTTP/2 standards.
3. Update Software:
   • Ensure both client and server software are up-to-date to address any bugs related to HTTP/2 protocol handling.
   • Regularly update your libraries or tools (e.g., `curl`) to the latest version.
4. Adjust Security Settings:
   • Review SSL/TLS configurations and negotiate compatible ciphers and protocols.
   • Ensure both client and server support similar SSL/TLS versions.
5. Analyze Request Using Tools:
   • Use tools like `Wireshark` to analyze the network packets involved and identify at which point the protocol error occurs.
   • Inspect HTTP/2 frames for mismanagement or miscommunication.

Key Points Summary

In conclusion, the `RPC failed; curl 92 HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)` error often occurs due to a variety of networking and configuration issues. Diagnosing the root cause and following the recommended actions can help you resolve it effectively. Understanding how HTTP/2 frames work and recognizing the importance of maintaining compatible configurations are key to averting such protocol errors in the future.