TN5250 Clients May Fail with a STATE_CHECK Error (220942)

CAUSE

The TN5250 data stream that is exchanged is composed of a header followed by the 5250 client data stream. The header is variable in length and composed of two parts:

• A fixed part
  
  This is six octets long and contains the logical record length, record type, and a reserved field.
  
• A variable part
  
  This includes a one-octet length field, flags, and an opcode field. The opcode field indicates the operation requested (for example, get/put/read immediate).

The 5250 client data stream follows, and is terminated by two octets, (IAC) with value 0xFF, and (EOR) with value 0xFF. The (IAC) (EOR) combination marks the end of the record. To uniquely identify these two octets as the termination, each 0xFF in the data stream is doubled.

The problem occurs when the doubled IAC character 0xFF sent by the emulator is exactly between the two SEND buffers (first one ends with FF and second one begins with the remaining FF). The TN5250 translator fails to undouble it (0xFFFF should become 0xFF), so an extra character is sent to APPC. APPC is waiting for a SEND buffer with n characters, and receives a send buffer with n+1 characters. This behavior generates a STATE_CHECK AP_SEND_DATA_NOT_LL_BDY.