Fix: Can't Load NREL ROUTEe Vehicle Models? [SSL Error]

Hey guys,

Are you encountering frustrating issues while trying to load external vehicle models in NREL's ROUTEe powertrain library? You're not alone! Many users have faced similar problems, particularly with the "external" pretrained models. This article dives deep into the common causes behind these errors and provides step-by-step solutions to get you back on track. We'll break down the technical jargon and offer practical advice, ensuring you can seamlessly integrate these powerful models into your projects.

Understanding the Issue: SSL Certificate Verification Failure

If you're seeing errors like URLError: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self-signed certificate in certificate chain (_ssl.c:1016)>, you've likely stumbled upon an SSL certificate verification problem. This error typically arises when your system can't verify the SSL certificate of the server hosting the external vehicle models. Let's break down what this means and how to tackle it.

What are SSL Certificates?

SSL (Secure Sockets Layer) certificates are digital certificates that authenticate a website's identity and enable an encrypted connection. They ensure that the data transmitted between your computer and the server remains private and secure. When your system encounters a self-signed certificate, it means the certificate wasn't issued by a trusted Certificate Authority (CA). While self-signed certificates aren't inherently insecure, your system needs to be explicitly told to trust them.

Why is this happening with ROUTEe?

The NREL ROUTEe powertrain library relies on fetching vehicle models from external URLs. If the server hosting these models uses a self-signed certificate (or one that your system doesn't recognize), the SSL verification process will fail, leading to the CERTIFICATE_VERIFY_FAILED error. This is a common issue in development environments or when dealing with internal servers that use custom certificates.

Step-by-Step Solutions to Fix Model Loading Errors

Now that we understand the root cause, let's explore several solutions to resolve the model loading issues in NREL ROUTEe. We'll start with the simplest approaches and move towards more advanced techniques.

Solution 1: Temporarily Disabling SSL Verification (Use with Caution!)

The quickest workaround is to temporarily disable SSL verification within your Python code. However, this is strongly discouraged for production environments as it compromises security. This method should only be used for testing or development purposes when you fully understand the risks involved.

import ssl
import nrel.routee.powertrain as pt

# This is insecure and only for testing!
ssl._create_default_https_context = ssl._create_unverified_context

model_name = 'Transit_Bus_Diesel'
model = pt.load_model(model_name)
print(f"Model '{model_name}' loaded successfully.")

Explanation:

• We import the ssl module.
• We override the default HTTPS context with an unverified context using ssl._create_default_https_context = ssl._create_unverified_context. This tells Python to skip SSL certificate verification.
• We then proceed to load the model as usual.

Warning: Remember to remove this code when you're done testing and move to a secure solution.

Solution 2: Adding the Certificate to Your System's Trust Store

A more secure approach is to add the server's certificate to your system's trust store. This tells your operating system that you trust the certificate, allowing Python to verify it. The exact steps vary depending on your operating system.

For macOS:

1. Obtain the Certificate: You'll need to get the certificate file from the server administrator or download it using your web browser (e.g., by inspecting the security details of the website). It usually comes in a .crt or .pem format.
2. Add to Keychain: Open Keychain Access (search for it in Spotlight). Drag and drop the certificate file into the System keychain. You might be prompted for your administrator password.
3. Trust the Certificate: Double-click the certificate in Keychain Access. Expand the Trust section. In the When using this certificate dropdown, select Always Trust. Close the certificate window and enter your administrator password again if prompted.

For Windows:

1. Obtain the Certificate: Similar to macOS, get the certificate file.
2. Open Certificate Manager: Press Win + R, type certmgr.msc, and press Enter.
3. Import the Certificate: In the left pane, navigate to Trusted Root Certification Authorities > Certificates. Right-click in the right pane, select All Tasks > Import.... The Certificate Import Wizard will appear.
4. Follow the Wizard: Click Next, browse to your certificate file, and click Next. Ensure Trusted Root Certification Authorities is selected as the certificate store, then click Next and Finish.

For Linux:

1. Obtain the Certificate: Get the certificate file.
2. Copy to Trust Store: Copy the certificate file to the system's trusted certificate directory. The location varies depending on your distribution. Common locations include /usr/local/share/ca-certificates/ or /etc/ca-certificates/trust/anchors/.

   sudo cp your_certificate.crt /usr/local/share/ca-certificates/
   

3. Update Certificate Store: Update the system's certificate store. The command varies depending on your distribution.
   • Debian/Ubuntu:

     sudo update-ca-certificates
     

   • CentOS/RHEL:

     sudo update-ca-trust
     

Solution 3: Using certifi Package

The certifi package provides a curated bundle of trusted root certificates from the Mozilla project. It's often used by Python libraries to verify SSL certificates. You can instruct urllib (which is used by nrel.routee.powertrain) to use certifi's certificate bundle.

1. Install certifi:

   pip install certifi
   

2. Set Environment Variable: Set the REQUESTS_CA_BUNDLE environment variable to point to certifi's certificate bundle.

   import os
   import certifi
   import nrel.routee.powertrain as pt
   
   os.environ['REQUESTS_CA_BUNDLE'] = certifi.where()
   
   model_name = 'Transit_Bus_Diesel'
   model = pt.load_model(model_name)
   print(f"Model '{model_name}' loaded successfully.")
   

Explanation:

• We import the os and certifi modules.
• We set the REQUESTS_CA_BUNDLE environment variable to the path returned by certifi.where(), which points to certifi's certificate bundle.
• We then load the model as usual. urllib will now use certifi's certificates for verification.

Solution 4: Specifying the Certificate Path Directly

You can also explicitly tell urllib to use a specific certificate file for verification. This is useful if you have the certificate file and want to avoid modifying the system's trust store or using certifi.

import nrel.routee.powertrain as pt
import urllib.request
import ssl

# Path to your certificate file
cert_path = '/path/to/your/certificate.pem'

context = ssl.create_default_context(cafile=cert_path)
opener = urllib.request.build_opener(urllib.request.HTTPSHandler(context=context))
urllib.request.install_opener(opener)

model_name = 'Transit_Bus_Diesel'
model = pt.load_model(model_name)
print(f"Model '{model_name}' loaded successfully.")

Explanation:

• We import urllib.request and ssl.
• We create an SSL context using ssl.create_default_context(cafile=cert_path), specifying the path to our certificate file.
• We build a custom opener using urllib.request.build_opener, configuring it to use the SSL context we created.
• We install this opener as the default opener for urllib using urllib.request.install_opener(opener). This ensures that all subsequent HTTPS requests use our custom context.
• We then load the model as usual.

Additional Troubleshooting Tips

If you've tried the solutions above and are still facing issues, here are some additional tips:

• Check Your Network Connection: Ensure you have a stable internet connection.
• Verify the Model Name: Double-check that you're using the correct model name from the list returned by pt.list_available_models().
• Firewall Issues: Your firewall might be blocking the connection. Check your firewall settings and ensure that Python is allowed to make outbound HTTPS connections.
• Proxy Settings: If you're behind a proxy server, you might need to configure your proxy settings in Python. You can do this using environment variables like HTTP_PROXY and HTTPS_PROXY.
• Outdated nrel.routee.powertrain Package: Ensure you have the latest version of the nrel.routee.powertrain package installed.

  pip install --upgrade nrel.routee.powertrain
  

• Python Version: While the original issue reporter used Python 3.11, it's worth testing with other Python versions to rule out any version-specific issues.

Addressing Specific Error Scenarios

Let's address some specific error scenarios reported by users:

urllib.error.URLError: <urlopen error [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self-signed certificate in certificate chain (_ssl.c:1010)> (Older Python Versions)

This error, specifically mentioning _ssl.c:1010, often indicates a similar SSL certificate verification failure but might be related to older versions of Python or OpenSSL. The solutions outlined above should still apply, but ensure you're using a recent version of certifi and that your system's certificate store is up-to-date.

Intermittent Errors

If you're experiencing intermittent errors (i.e., the model loads sometimes but fails at other times), it could indicate a temporary issue with the server hosting the models. Try again later, or contact the server administrator to inquire about the server's status.

Conclusion: Mastering Model Loading in NREL ROUTEe

Loading external vehicle models in NREL ROUTEe can sometimes be tricky, especially when SSL certificate verification comes into play. However, by understanding the underlying causes and implementing the solutions outlined in this guide, you can overcome these challenges and seamlessly integrate these powerful models into your projects. Remember to prioritize security and avoid disabling SSL verification in production environments. By following best practices and keeping your system up-to-date, you'll be well-equipped to leverage the full potential of NREL ROUTEe for your powertrain simulations and analysis.

Key Takeaways:

• SSL certificate verification failures are a common cause of model loading issues.
• Disabling SSL verification is a temporary and insecure workaround.
• Adding the certificate to your system's trust store is a secure solution.
• The certifi package provides a curated bundle of trusted root certificates.
• Explicitly specifying the certificate path offers fine-grained control.
• Troubleshooting steps include checking network connectivity, model names, firewall settings, and proxy configurations.

If you have any further questions or encounter different error scenarios, don't hesitate to consult the NREL ROUTEe documentation or seek help from the community. Happy modeling, guys!