Multicast User Authentication

The procedure for joining a network-level IP multicast group an open one---a request is made by the receiving host, using MLD (IPv6) or IGMP (IPv4) , and the multicast routing protocol (typically PIM-SM ) is responsibile for building a Data Distribution Tree (DDT) to ensure that the data are delivered to the receiving host(s). The procedure for joining an application-level group clearly depends on the application. When IP multicast is used as the data distribution technology, then it is desirable to be able to limit delivery of the network-level multicast data packets to those hosts that have receiving users who are valid members of the application-level group. The anyone-can-send, anyone-can-receive nature of IP multicast has resulted in restricted deployment of multicast distribution technology, since it is impossible to generate any revenue from services based on standard multicast. However, several pieces of the problem have received significant attention in recent years. The problem of security and key management for application-level groups has been explored by the Multicast Security (MSEC) working group, and a framework devised . The use of AAA protocols (RADIUS, Diameter) to manage network-level access has been standardized. These protocols (especially Diameter) can be extended to permit controlling access to application-level groups. The requirements for "well-managed" multicast have been stated in , and a framework for satisfying these requirements with the help of AAA functionality has been described in . Finally, work is under way on securing the network infrastructure and the exchanges between adjacent multicast routers . However, one key piece is missing. To minimize the resource wastage that would result from delivering multicast traffic to hosts that have no entitlement to receive them, it is necessary to authenticate and authorize receiving users and to correlate their right to access a group with the action of putting the data on that part of the network that is directly connected to the receiving host.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. They indicate requirement levels for compliant PIM-SM implementations.

The design of IP multicast ensures that there is no relationship between the receiving hosts and the sending host(s) in a network-level multicast group. Multicast sending hosts do not even know whether there are receiving hosts or not, much less who they are or whether they are entitled to receive the data. The receiving host issues a network-level "join" on behalf of a receiving user, using IGMP (IPv4) or MLD (IPv6) , and a designated access router is responsible for grafting itself onto the data distribution tree. The network exercises no control over this process---it is required to provide the data flow. Although specifications exist for encrypting the user data, thus ensuring that only legitimate users can decrypt these data, these specifications provide no way to ensure that the data distribution tree is not extended when a non-authorized receiving user makes a request to join the tree. Thus, key management and receiving user access control have to be considered as separate problems. Given the lack of a relationship between the sending user(s) and the receiving user(s), it is difficult to create and enforce an appropriate business model.

Several scenarios can cause a need for re-authentication and re-authorization: When a user changes the group that he/she wishes to attach to; When a user changes the access router used for connection (e.g., wireless roaming); When a user changes the medium used for physical connectivity (e.g., cellular to wireless, etc.).

The fact of delivery of group data needs to be recorded, to enable revenue to be earned. This is only one of a range of accounting issues that may need to be addressed, which points to the need for a general solution.

There is a wide range of authentication and authorization procedures that may be desired by an Internet Service Provider, including some that may not yet be standardized. This implies the adoption of a very general framework for such procedures.

It is conceivable that a solution could be found for the above issues that would be based on standard network protocols and separate (proprietary or standard) group management protocols. For example, the key management and distribution protocol associated with the application-level group could have authentication as one of its features. However, the separation of the network-level controls from the application-level controls enables a significant class of security attacks. It is therefore important that control of access to the network resources and control of access to the application-level resources be strongly coupled.

Access to the network is different from access to a group. As an example, the authorization to watch a particular video presentation may be associated with a specific family member, while the authorization to use the network connection may be associated with an entire family (or to anyone present in the house). While existing AAA procedures are designed to control network level access, they must be extended (or alternatives found) if group access must be controlled.

Two levels of action are apparent: the action of joining the network-level data distribution tree, and the action of joining the group, with its accompanying security properties. Joining the data distribution tree should not occur unless and until the receiving user has been authenticated and authorized. One way to ensure that this relationship is enforced is to carry the receiving user authentication material in the network-level join packet. To support multiple types of authentication methods, the Extensible Authentication Protocol (EAP) provides a standardized solution. To support a method-independent and efficient re-authentication, the EAP Re-Authentication Protocol (ERP) provides a possible solution. ERP is applicable for mobile receivers . To permit correlating the join actions (at the group level and the network level) with the accounting procedures, the EAP/ERP packets that are delivered to the access router by the extended network-level join can be forwarded to the local AAA server for a decision, using existing AAA protocols, such as RADIUS or Diameter. In keeping with the statement in that "A CP may delegate AAA responsibility to an NSP.", we observe that the NSP can distribute the responsibility among a collection of local AAA servers, and that there is sufficient generality in the AAA architectural model that a wide range of policies could be implemented, in support of a wide range of business models.

Pending incorporation of the material into this document, readers are invited to access Islam, et al..

TBD.

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