Encapsulation Packet
Implementation of CCSDS 133.1-B-3 Encapsulation Packet Protocol. Encapsulation
packets carry non-CCSDS protocol data (e.g. IP datagrams) over CCSDS space
links. They share the same packet layer as Space Packets but use a different
header format, distinguished by a Packet Version Number of 7 (binary 111)
instead of 0.
Header
The mandatory header is 2 bytes. A variable-length packet length field follows, sized by the Length of Length field.
| Field | Bits | Description |
|---|---|---|
| Packet Version | 3 | Always 111 (7) |
| Protocol ID | 4 | Identifies the encapsulated protocol |
| Length of Length | 2 | Number of bytes for the packet length field |
| User Defined | 4 | Mission-specific |
| Protocol ID Ext | 4 | Extends Protocol ID when Protocol ID = 0010 |
| CCSDS Defined | 1 | Reserved for CCSDS use |
Protocol ID
The 4-bit Protocol ID identifies what is encapsulated:
| Value | Name | Description |
|---|---|---|
0000 | Idle | Idle packet (all-zeros payload) |
0001 | IPE | Internet Protocol Extension |
0010 | CCSDS Defined | Type specified by extension field |
0111 | User Defined | Mission-specific protocol |
1000 | IPv4 | Internet Protocol version 4 |
1001 | IPv6 | Internet Protocol version 6 |
When Protocol ID is 0010 (CCSDS Defined), the 4-bit Protocol ID
Extension field selects the specific CCSDS protocol.
Packet Length
The Length of Length field determines how many bytes encode the packet length:
| Length of Length | Bytes | Maximum packet size |
|---|---|---|
00 | 0 | Implicit/undefined |
01 | 1 | 255 |
10 | 2 | 65 535 |
11 | 4 | 4 294 967 295 |
The total header size is 2 bytes (mandatory) plus the packet length bytes (0, 1, 2, or 4).
Comparison with SPP
Both SPP and Encapsulation Packets live in the same packet layer and can be multiplexed on the same virtual channel. They are distinguished by the Packet Version Number in the first 3 bits: 0 for SPP, 7 for Encapsulation. This lets a receiver demultiplex them without any additional framing.