This document defines the 6top Protocol (6P), which enables distributed scheduling in 6TiSCH networks. 6P allows neighbor nodes in a 6TiSCH network to add/delete TSCH cells to one another. 6P is part of the 6TiSCH Operation Sublayer (6top), the next higher layer of the IEEE802.15.4 TSCH medium access control layer. The 6top Scheduling Function (SF) decides when to add/delete cells, and triggers 6P Transactions. Several SFs can be defined, each identified by a different 6top Scheduling Function Identifier (SFID). This document lists the requirements for an SF, but leaves the definition of the SF out of scope. Different SFs are expected to be defined in future companion specifications.