"A ""Protocol Independent Multicast - Sparse Mode"" Join message sent by a given node identifies one or more multicast distribution trees that that node wishes to join. Each tree is identified by the combination of a multicast group address and a source address (where the source address is possibly a ""wild card""). Under certain conditions it can be useful@ when joining a tree@ to specify additional information related to the construction of the tree. However@ there has been no way to do so until now. This document describes a modification of the Join message that allows a node to associate attributes with a particular tree. The attributes are encoded in Type-Length-Value format."