Conversations

This section applies to LU 6.2 only.

Once a session is established between two LUs, the LU-LU session supports the exchange of information between two TPs, which have the exclusive use of the session to execute a transaction. This exchange of information is called a conversation. Only one conversation can use a particular session at a time, but sessions are serially reusable (many conversations can use the same session, one after another).

To initiate a conversation, a source TP sends a request to its LU, asking it to allocate a conversation with a remote TP. The invoking TP (or source TP) initiates the conversation, like the calling party in a telephone conversation. The invokable TP or target TP (the remote TP) is the partner in the conversation, like the party who receives a telephone call.

As shown in Figure 1, information is exchanged between TPs and LUs to enable one node to communicate with another. Although the TPs appear to be communicating directly, the LUs on each node are the intermediaries in every exchange.

SNA defines two types of conversations: basic and mapped. These two types of conversations use different methods to indicate the length of transmitted or received data packages to be passed between CS Linux and the TP.

• In a basic conversation, data must be formatted by the TP as logical records before being presented to the SEND function.

  A logical record consists of a two- or four-byte header starting with a two-byte length field, often represented as LL, followed by up to 32,765 bytes of data. Logical records can be grouped together and sent as a block, transmitting more than one logical record with a single call to the SEND function.

• In a mapped conversation, information is passed to the SEND function as a pointer to a single, unformatted block of data; the length of the block is passed as another parameter. The block cannot be received as one or more logical records; the receiving TP must do whatever record-level formatting is required.