Browse All TLS Extensions

max_fragment_length

TLS Extension Code: 1

Extension Details

Extension Name
max_fragment_length
Extension Code (Decimal)
1
Extension Code (Hex)
0x0001
RFC Reference
RFC 6066

What is max_fragment_length?

The Maximum Fragment Length extension allows clients with limited buffer sizes to negotiate a smaller maximum fragment length for TLS records. By default, TLS allows records up to 2^14 bytes (16KB). This extension can negotiate smaller sizes: 2^9 (512 bytes), 2^10 (1KB), 2^11 (2KB), or 2^12 (4KB). This is particularly useful for IoT devices and embedded systems with memory constraints. Defined in RFC 6066, this extension helps optimize memory usage on resource-constrained devices.

Role in JA3 Fingerprinting

How This Extension Affects Fingerprinting

The max_fragment_length extension is part of the TLS Client Hello packet that JA3 analyzes to create a unique fingerprint of your browser or HTTP client.

JA3 Construction: JA3 concatenates TLS parameters including extensions in a specific format:
TLS_VERSION,CIPHERS,EXTENSIONS,CURVES,POINT_FORMATS

Different browsers and HTTP clients support different sets of TLS extensions, making this a key differentiator in fingerprinting. The presence, absence, or order of extensions like max_fragment_length can reveal:

  • Browser type and version (Chrome, Firefox, Safari, Edge)
  • Operating system (Windows, macOS, Linux, Android, iOS)
  • HTTP library (curl, Python requests, Go http.Client, Node.js)
  • Bot detection - automated tools often have distinctive extension sets

Test Your TLS Configuration

See if your browser or HTTP client includes the max_fragment_length extension in its TLS handshake:

Test Your JA3 Fingerprint

The JA3 tool will show all TLS extensions your client advertises, including max_fragment_length.

Browser & Client Support

Modern Browser Support:

  • Chrome/Edge: Full support for max_fragment_length
  • Firefox: Full support for max_fragment_length
  • Safari: Full support for max_fragment_length

HTTP Client Libraries:

  • curl: Support varies by OpenSSL/LibreSSL version
  • Python requests: Limited TLS extension control
  • curl_cffi: Can mimic browser TLS configurations
  • Scrapfly API: Automatically matches real browser TLS fingerprints

Related TLS Resources

TLS Extensions
Testing Tools

Technical References