A survey of key management for secure group communication <s> Intra-Domain Group Key Management <s> This document defines the "administratively scoped IPv4 multicast space" to be the range 239.0.0.0 to 239.255.255.255. In addition, it describes a simple set of semantics for the implementation of Administratively Scoped IP Multicast. Finally, it provides a mapping between the IPv6 multicast address classes [RFC1884] and IPv4 multicast address classes. <s> BIB001 </s> A survey of key management for secure group communication <s> Intra-Domain Group Key Management <s> Hydra is a scaleable decentralised architecture to create and distribute symmetric cryptographic keys to large multicast-based groups. The group is divided into a number of TTL-scoped regions in order to achieve flexible and efficient key management, particularly in face of group membership changes. Hydra does not employ a manager for subgroup managers, and hence, it is not vulnerable to failures of single entities. <s> BIB002
DeCleene et al. [2001] present an intraregion group key management protocol (IGKMP). IGKMP is divided in administratively scoped areas BIB001 ]. There is a Domain Key Distributor (DKD) and many Area Key Distributors (AKD). Each AKD is responsible for one area. The group key is generated by the DKD and is propagated to the members through the AKDs. The key managers (DKD and AKD) are placed in a multicast group, named All-KD-group (see Figure 11) . The All-KD-group is used by the DKD to transmit the rekey messages to the AKDs. All areas in the domain use the same group key. Therefore, data packets do not need to be translated when passing from one area to another. In addition, the DKD does not need to keep track of all group members, it only has to keep track of the AKDs. Nevertheless, since Intra-domain protocol employs a central controller (namely, DKD) for controlling the subgroup controllers (namely, AKDs), it presents the undesirable feature of allowing the whole group to be disrupted if the DKD is compromised. Moreover, if an AKD is unavailable no members in that area are able to access the group communication, since they will not be able to access AKDs from other areas. BIB002 proposed Hydra. In Hydra, the large group is divided into smaller subgroups, and a server called the Hydra Server (HS) controls each subgroup. Hydra is a decentralized group key management scheme without a central subgroup controller. If a membership change takes place at HS i , and a new key must be generated, it can generate the new group key and send this key to the other HS j involved in that session. The case when one or more HSs become unavailable will not cause a problem for the remaining HSs. In order to have the group key distributed to all HSs a synchronized group key distribution protocol (SGKDP) is employed. The SGKDP protocol ensures that only a single valid HS is generating the new group key at every given time.