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5. MMS Context Management (Introduction to MMS Nr.5)

    5.1 The basics

    MMS provides services for managing the context of communications between two MMS nodes on a network. These services are used to establish and terminate application associations and for handling protocol errors between two MMS nodes. The node that initiates the association with another node is referred to as the calling node. The responding node is referred to as the called node.

    In a MMS environment, two MMS applications establish an application association between themselves using the MMS Initiate service. This process of establishing an application association consists of an exchange of some parameters and a negotiation of other parameters. The exchanged parameters include information about restrictions that pertain to each node that are determined solely by that node (e.g. which MMS services are supported). The negotiated parameters are items where the called node either accepts the parameter proposed by the calling node or adjusts it downward as it requires (e.g. the maximum message size). The calling application issues an Initiate service request that contains information about the calling node's restrictions and a proposed set of the negotiated parameters. The called node examines the negotiated parameters and adjusts them as necessary to meets its requirements and then returns the results of this negotiation and the information about it's restrictions in the Initiate response. Once the calling node receives the Initiate confirmation the application association is established and other MMS service requests can then be exchanged between the applications.

    Once an application association is established either node can assume the role of client or server independent of which node was the calling or called node. For any given set of MMS services one application assumes the client role while the other assumes the role of server or VMD. Whether or not a particular MMS application is a client, server (VMD), or both is determined solely by the developer of the application.

    5.2 Associations versus Connections

    Although many people may refer to network connections and application associations interchangeably there is a distinct difference. A connection is an attribute of the underlying network layers that represents a virtual circuit between two nodes. For instance, telephone networks require that two parties establish a connection between themselves (by dialing and answering) before they can communicate at all. An application association is an agreement between two networked applications governing their communications. It is analogous to the two parties agreeing to use a particular language and to not speak about religion or politics over the telephone. Application associations exist independent of any underlying network connections (or lack thereof).

    In a connection oriented environment the MMS Initiate service is used to signal to the lower layers that a connection must be established. The Initiate service request is carried by the network through the layers as each layer goes through its connection establishment procedure until the Initiate indication is received by the called node. The connection does not exist until after all the layers in both nodes have completed their connection establishment procedures and the calling node has received the Initiate confirmation. Because of this, the association and the connection occur simultaneously in a connection oriented environment.

    In a connectionless environment, it is not strictly necessary to send the Initiate request before two nodes can actually communicate. In an environment where the Initiate service request is not used before other service requests are issued by a MMS client to a VMD, each application must have all the knowledge regarding the other application's exchanged and negotiated parameters via some local means (e.g. a configuration file). This foreknowledge of the other MMS application's restrictions is the application association from a MMS perspective. Whether an Initiate service request is used or not, application associations between two MMS applications must exist before communications can take place. In some connectionless environment such as MiniMAP, MMS nodes still use the Initiate service to establish the application association before communicating.

    5.3 Context Management Services

    The following services are specified for context management:

    Services Description
    This service is used to exchange and negotiate the parameters required for two MMS applications to have an application association.
    This service is used by a client to request that a previously existing application association be terminated in a graceful manner. The conclude service allows the server to decline to terminate the association due to ongoing activities such as a download sequence or file transfer.
    This service is used to terminate an application association in an ungraceful way. Because the server does not have the opportunity to decline an abort, an abort may result in the loss of data. Although the MMS standard does not provide any protocol for an Abort service, most MMS implementations use other network services for providing this service.
    This service is used by a client to cancel an outstanding MMS service request (e.g. TakeControl) that has not yet been responded to by the server.
    This service is used by either the client or server to notify the other MMS application that it had received an unsupported service request or a message that was not properly encoded.

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