When an application program issues SEND OPTCD=LMPEO, it passes FM data to VTAM® and also passes information for an initial RH. VTAM reformats the FM data into one or more request units which form a chain or partial chain of RUs, and then sends each generated request to the other LU in the session as seen in Figure 1. None of these generated requests is larger than the maximum RU size specified in the BIND for sending in that direction on the session (that is, from the application program to the other LU in the session). If there is sufficient data, the first RU generated by VTAM has the maximum RU size allowed. If the amount of data specified by SEND is less than the maximum RU size, or if the maximum RU size in the BIND is 0, only a single request unit is generated. A maximum RU size of 0 specified in the BIND is inconsistent with the purpose of LMPEO, because VTAM does not break the data into multiple RUs if 0 is specified, no matter how large the message is.

The field in the BIND that specifies the maximum RU size is described in Request unit size. There are two such fields, one for each direction of data flow in the session (PLU to SLU and SLU to PLU). If the application program is the SLU, LMPEO uses the first field (byte 9) as the maximum RU size; if the application program is the PLU, LMPEO uses the second field (byte 10) as the maximum RU size.

The location of the FM data to be sent by LMPEO varies depending upon whether the buffer-list option (OPTCD=BUFFLST) is used with LMPEO. BUFFLST also allows the application program to influence how LMPEO splits the FM data to be sent. Without the BUFFLST option (OPTCD=NBUFFLST), LMPEO sends the FM data from the area pointed to directly by the RPL AREA field, and splits the FM data (if necessary) into multiple RUs without regard for the data content. See The buffer-list (BUFFLST) option for information on using BUFFLST with LMPEO.

If SEND OPTCD=NLMPEO is issued, then VTAM does not split the data into separate RUs and does not enforce the maximum RU size restriction. In that case, the application program must ensure that each RU is within the size limits agreed upon at BIND time.

If OPTCD=LMPEO and the RU being sent is not an FM data request (for example, CONTROL is not DATA), then SEND is rejected with (RTNCD,FDB2)=(X'14',X'77').

If the VTAM encrypt/decrypt facility is used, each of the requests generated by VTAM for a SEND is enciphered, if appropriate. For each SEND, all or none of the FM data is enciphered. This is under the control of the RPL CRYPT operand and the NIB ENCR operand. See Sending and receiving enciphered data requests for information on cryptographic requirements.