interledger-spsp-0.4.0 doesn't have any documentation.
interledger-spsp
Client and server implementations of the Simple Payment Setup Protocol (SPSP), an application-layer protocol within the Interledger Protocol Suite.
This uses a simple HTTPS request to establish a shared key between the sender and receiver that is used to authenticate ILP packets sent between them. SPSP uses the STREAM transport protocol for sending money and data over ILP.