Svc::FprimeDeframer
The Svc::FprimeDeframer component receives F´ frames on its input port, takes off the header and trailer (sometimes referred to as "footer"), and passes the encapsulated payload to a downstream component (usually the Svc.FprimeRouter).
Following the F Prime Protocol frame specification, the Svc::FprimeDeframer validates the passed in Fw.Buffer to ensure it represents a valid frame (see Frame validation), extract the payload from the frame, and outputs the payload on the dataOut output port.
Internals
The Svc::FprimeDeframer component is an implementation of the DeframerInterface for the F´ communications protocol. It receives an F´ frame (in a Fw::Buffer object) on its dataIn input port, modifies the input buffer to remove the header and trailer, and sends it out through its dataOut output port.
Ownership of the buffer is transferred to the component connected to the dataOut output port. The input buffer is modified by subtracting the header and trailer size from the buffer's length, and offsetting the buffer's data pointer to point to the start of the packet data.
The Svc::FprimeDeframer does not support deframing multiple packets in a single frame (i.e. concatenated packets) as this is not supported by the F´ communications protocol.
Frame validation
The passed-in data field (of type Fw::Buffer) of the Svc.ComDataWithContext input port is validated for the following conditions:
- The buffer is large enough to contain the header and trailer
- The buffer starts with the F´ start word
- The buffer length is equal to (or larger than) the packet length field in the frame header
- The CRC field of the frame is equal to the CRC calculated over the frame header and payload
If any of these conditions are not met, the frame is dropped meaning no payload is passed to the output port and the input Fw::Buffer is deallocated.
Usage Examples
The Svc::FprimeDeframer component is used in the uplink stack of many reference F´ application such as the tutorials source code.
Diagrams
The below diagram shows a typical configuration in which the Svc::FprimeDeframer can be used. This is the configuration used in the the tutorials source code. It is receiving accumulated frames from a Svc::FrameAccumulator and passes packets to a Svc::FprimeRouter for routing to other components.
Requirements
| Requirement | Description | Rationale | Verification Method |
|---|---|---|---|
| SVC-DEFRAMER-001 | Svc::FprimeDeframer shall extract the payload field from input buffers that represent a valid F Prime frame as specified by the F Prime Protocol |
Deframe valid frames and extract payload | Unit test |
| SVC-DEFRAMER-002 | Svc::FprimeDeframer shall return ownership of input buffers that are not a valid F Prime frame as specified by the F Prime Protocol |
Drop invalid frames | Unit test |
Port Descriptions
| Kind | Name | Type | Description |
|---|---|---|---|
guarded input |
dataIn |
Svc.ComDataWithContext |
Receives a frame for deframing |
output |
dataOut |
Svc.ComDataWithContext |
Emits deframed data (F´ packets) |
sync input |
dataReturnIn |
Svc.ComDataWithContext |
Receives ownership of the emitted data back |
output |
dataReturnOut |
Svc.ComDataWithContext |
Returns ownership of the input buffer back to the sender |