C2MS12 — C2MS: Optional Transfer Frame/Packet attributes should be described in schema

Non-self-describing CCSDS attributes for either transfer frames or space packets should be provided as dependent attributes of the frame messages. This would enable full integration between a sender and receiver. Right now, whether these optional fields are provided is unknown, causing the proper rendering of the frame to not be possible without outside negotiation (beyond the specification).

Examples:

Frame Header Error Control of the AOS transfer frame (AOS Space Data Link Protocol (ccsds.org)), where nothing prior to it in the definition specifies whether it is there or not:

4.1.2.6 Frame Header Error Control
4.1.2.6.1 If implemented, Bits 48-63 of the Transfer Frame Primary Header shall contain the Frame Header Error Control. NOTE – The 10-bit Master Channel Identifier, the 6-bit Virtual Channel Identifier, and the 8-bit Signaling Field may all be protected by an optional error detecting and correcting code, whose check symbols are contained within this 16-bit field.
4.1.2.6.2 The presence or absence of the optional Frame Header Error Control shall be established by management.
4.1.2.6.3 If present, the Frame Header Error Control shall exist in every Transfer Frame transmitted within the same Physical Channel.

Space Packets and the secondary header (Space Packet Protocol (ccsds.org)): A space packet might have a secondary header with a variable length time code, a variable length ancillary data field, or both. A space packet might have a secondary header, a user data field, or both. The length is not specified for any of them. When present, the format of the time code can be one of several options, but which one is not specified:

4.1.3.3.3 Secondary Header Flag 4.1.3.3.3.1 Bit 4 of the Packet Primary Header shall contain the Secondary Header Flag. 4.1.3.3.3.2 The Secondary Header Flag shall indicate the presence or absence of the Packet Secondary Header within this Space Packet. It shall be ‘1’ if a Packet Secondary Header is present; it shall be ‘0’ if a Packet Secondary Header is not present. 4.1.3.3.3.3 The Secondary Header Flag shall be static with respect to the APID and managed data path throughout a Mission Phase. 4.1.3.3.3.4 The Secondary Header Flag shall be set to ‘0’ for Idle Packets

4.1.4.2.1.5 If present, the Packet Secondary Header shall consist of either: a) a Time Code Field (variable length) only; b) an Ancillary Data Field (variable length) only; or c) a Time Code Field followed by an Ancillary Data Field. 4.1.4.2.1.6 The chosen option shall remain static for a specific managed data path throughout all Mission Phases. NOTE – The format of the Packet Secondary Header is shown in figure 4-3. ANCILLARY DATA FIELD (optional) variable PACKET SECONDARY HEADER TIME CODE FIELD (optional) variable Figure 4-3: Packet Secondary Header

4.1.4.2.2 Time Code Field 4.1.4.2.2.1 If present, the Time Code Field shall consist of an integral number of octets. 4.1.4.2.2.2 The Time Code Field shall consist of one of the CCSDS segmented binary or unsegmented binary time codes specified in reference [3].CCSDS RECOMMENDED STANDARD FOR SPACE PACKET PROTOCOL CCSDS 133.0-B-2 Page 4-8 June 2020 NOTE – The time codes defined in reference [3] consist of an optional P-Field (Preamble Field), which identifies the time code and its characteristics and a mandatory T[1]Field (Time Field). Examples of time codes are CCSDS Unsegmented Time Code and CCSDS Day Segmented Time Code. Examples of characteristics are ambiguity period, epoch, length, and resolution. 4.1.4.2.2.3 The time code selected shall be fixed for a given managed data path throughout all Mission Phases. 4.1.4.2.2.4 If the characteristics of the chosen time code are fixed, the corresponding P[1]field (as described in reference [3]) need not be present. If the characteristics are allowed to change, the P-field shall be present so as to identify the changes. 4.1.4.2.2.5 The presence or absence of the P-field in the Time Code Field shall be fixed for a given managed data path throughout all Mission Phases. If present, it shall immediately precede the T-field that is defined in reference [3]

4.1.4.3 User Data Field 4.1.4.3.1 If present, the User Data Field shall follow, without gap, either the Packet Secondary Header (if a Packet Secondary Header is present) or the Packet Primary Header (if a Packet Secondary Header is not present). 4.1.4.3.2 The User Data Field shall be mandatory if a Packet Secondary Header is not present; otherwise, it is optional. 4.1.4.3.3 If present, the User Data Field shall consist of an integral number of octets. 4.1.4.3.4 If the Packet is not an Idle Packet, then the User Data Field shall contain application data supplied by the sending user. If the Packet is an Idle Packet, the User Data Field shall contain Idle Data. NOTE – The bit pattern of Idle Data is set by the mission and is not specified in this Recommended Standard

4.1.4.3 User Data Field 4.1.4.3.1 If present, the User Data Field shall follow, without gap, either the Packet Secondary Header (if a Packet Secondary Header is present) or the Packet Primary Header (if a Packet Secondary Header is not present). 4.1.4.3.2 The User Data Field shall be mandatory if a Packet Secondary Header is not present; otherwise, it is optional. 4.1.4.3.3 If present, the User Data Field shall consist of an integral number of octets. 4.1.4.3.4 If the Packet is not an Idle Packet, then the User Data Field shall contain application data supplied by the sending user. If the Packet is an Idle Packet, the User Data Field shall contain Idle Data. NOTE – The bit pattern of Idle Data is set by the mission and is not specified in this Recommended Standard