VTAM® performs relatively little error checking on the FMH-5. More checks are performed when VTAM receives an FMH-5 for an application program than when an application program gives an FMH-5 to VTAM for processing. In both cases, VTAM checks the length of the FMH-5 by summing up the totals of the length fields of the various subfields and comparing the result to the overall length field for the FMH-5. For FMH-5s supplied as part of conversation allocation, VTAM checks that:

• The application program is not requesting a conversation with a control operator transaction program (specified by a transaction program name of X'06F1').
• The application program is not requesting a full-duplex conversation when full-duplex is not supported.
• The sync point capability specified is supported by the session.
• Security subfields, if present, are accepted by the partner LU.

For FMH-5s received from another LU, VTAM also checks that:

• RECONNECT support is not specified in the synchronization field.
• Valid characters are used in the network name in the logical unit of work field.
• The application program is not requesting a full-duplex conversation when full-duplex is not supported.
• Security subfields, if present, are accepted by the application program.
• Only one of each type of access security subfield is present.
• An unrecognizable security subfield is not present.

The application program must check for many of the potential errors. The application program ensures that:

• If a user identifier field is present, either a password field is present or the already-verified indicator or the persistent verification signed-on indicator is set.
• If a profile subfield is present, a user identifier subfield is also present.
• Any other security information in the FMH-5 that is not checked by VTAM is accurate.
• If specified, security indicators are in architecturally valid combinations.
• The transaction name is not null (length of 0).
• The transaction name does not specify an SNA service transaction program unless the conversation is authorized.
• The conversation type (basic or mapped) is correct.
• No attempt is made to allocate a conversation with a mode name of SNASVCMG unless the transaction programs have control operator status.