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Overview

CCSDS SPP (133.0-B-2) defines the packet format used by all higher layers. Each Space Packet has a 6-byte primary header containing:

  • APID (Application Process Identifier): an 11-bit value that identifies which application or service the packet belongs to. The receiver uses the APID to dispatch incoming packets to the correct handler.
  • Sequence Count: a 14-bit counter that increments per packet per APID. Used by the transport layer to detect gaps and reorder.
  • Sequence Flags: indicate whether the packet is unsegmented, or the first/continuation/last segment of a larger payload.
  • Data Length: the number of bytes in the data field.

When a payload is too large for a single Space Packet (constrained by the maximum frame size), the segmenter splits it into multiple packets with appropriate sequence flags. The reassembler on the receiving side collects the segments and reconstructs the original payload.

The Encapsulation Packet Protocol (CCSDS 133.1-B-3) extends SPP with encapsulation packets that can wrap non-CCSDS data or serve as idle fill when the link has no data to send but must maintain frame synchronization.

Space Packets are extracted from transfer frames by the link reader, which parses the data field using the First Header Pointer and packet length fields to find packet boundaries even when packets span multiple frames.

  • SPP — Space Packet header format
  • Encapsulation — Encapsulation Packet for non-CCSDS data