Spark uses at least 4 logical communication channels:
1. Control messages - Akka based
2. JARs and other files - Jetty based (HttpServer)
3. Computation results - Java NIO based
4. Web UI - Jetty based
The aim of this feature is to enable SSL for (1) and (2).
Spark configuration is sent through (1). Spark configuration may contain sensitive information like credentials for accessing external data sources or streams. Application JAR files (2) may include the application logic and therefore they may include information about the structure of the external data sources, and credentials as well.