The Message Control Header is located in front of each DICOM message fragment (see Figure E.2-1). Its presence is mandatory for all DICOM Abstract Syntaxes (see Annex B for further discussion on Abstract Syntaxes).
The Message Control Header shall be made of one byte with the least significant bit (bit 0) taking one of the following values:
a) If bit 0 is set to 1, the following fragment shall contain Message Command information.
b) If bit 0 is set to 0, the following fragment shall contain Message Data Set information.
The next least significant bit (bit 1) shall be defined by the following rules:
a) If bit 1 is set to 1, the following fragment shall contain the last fragment of a Message Data Set or of a Message Command.
b) If bit 1 is set to 0, the following fragment does not contain the last fragment of a Message Data Set or of a Message Command.
Bits 2 through 7 are always set to 0 by the sender and never checked by the receiver.
Note: The Message Control Header, in the Transport data flow, is the 1st byte in each PDV. The Transfer Syntax, negotiated at association establishment, defines the encoding for the Command/Data fragment.
Figure E.2-1 PRESENTATION DATA VALUE AND THE MESSAGE CONTROL HEADER