This document specifies mechanisms to establish shared symmetric keys between groups of entities. It defines: — symmetric key-based key establishment mechanisms for multiple entities with a key distribution centre (KDC); and — symmetric key establishment mechanisms based on a general tree-based logical key structure with both individual rekeying and batch rekeying. It also defines key establishment mechanisms based on a key chain with group forward secrecy, group backward secrecy or both group forward and backward secrecy. This document also describes the required content of messages which carry keying material or are necessary to set up the conditions under which the keying material can be established. This document does not specify information that has no relation with key establishment mechanisms, nor does it specify other messages such as error messages. The explicit format of messages is not within the scope of this document. This document does not specify the means to be used to establish the initial secret keys required to be shared between each entity and the KDC, nor key lifecycle management. This document also does not explicitly address the issue of interdomain key management.