HSMP LSP is hub & spoke multipoint Label Switched Path (LSP), which allows traffic both from root to leaf through point-to-multipoint (P2MP) LSP and also leaf to root along the reverse path.
Upstream traffic entering the HSMP LSP at any leaf node travels upstream along the tree to the root, as if it is unicast to the root.
IETF MPLS Working Group is standardizing the HSMP LSP.
The LDP based HSMP LSP is standardized in http://tools.ietf.org/html/draft-ietf-mpls-mldp-hsmp-06.
The RSVP-TE based HSMP LSP is standardized in http://tools.ietf.org/html/draft-ietf-mpls-rsvp-te-hsmp-lsp-01.