Packet Structure

All UMSH packets begin with a one-byte Frame Control Field (FCF). Optional common fields then follow in a fixed order, followed by packet-type-specific fields.

Top-Level Packet Layout

+--------+-----------+--------+--------------+------------+-----------+---------+------+
|  FCF   |  OPTIONS  | FHOPS  | DST/CHANNEL  |    SRC     |  SECINFO  | PAYLOAD | MIC  |
+--------+-----------+--------+--------------+------------+-----------+---------+------+
   1 B      variable    0/1 B      0/2/3 B     0/3/32 B     0/5/7 B      var.   0-16 B

Where:

• OPTIONS are present if the FCF options flag is set
• FHOPS is present if the FCF flood hop count flag is set
• DST is a 3-byte destination hint (2 bytes in MAC Ack packets)
• CHANNEL is a 2-byte channel identifier
• SRC is a compact 3-byte source hint (when S flag is clear) or 32-byte source public key (when S flag is set); in multicast and blind unicast packets with encryption enabled, SRC is encrypted inside the ciphertext rather than appearing as a separate field
• SECINFO is present on authenticated/encrypted packet types
• MIC is present on authenticated/encrypted packet types; MAC acks carry an ack tag instead

Frame Control Field

The Frame Control Field is one byte:

  7   6   5   4   3   2   1   0
+-------+-----------+---+---+---+
| VER   | PKT TYPE  | S | O | H |
+-------+-----------+---+---+---+
 2 bits    3 bits     1   1   1

Where:

• VER = protocol version (this specification defines version 3, i.e., both bits set)
• PKT TYPE = packet type
• S = full 32-byte source address included (when clear, a compact source hint is used instead; see Source Address for hint size by packet type)
• O = options present
• H = flood hop count present

Packet Type Values

Common Optional Fields

Options Field

Options use the same delta-length encoding as CoAP (RFC 7252 §3.1). Each option is encoded as a delta from the previous option’s number, a length, and a value. The sequence is terminated by a 0xFF byte.

Option Encoding

Each option begins with a single byte containing two 4-bit fields:

  7   6   5   4   3   2   1   0
+---------------+---------------+
| Option Delta  | Option Length |
+---------------+---------------+
     4 bits          4 bits

Followed by optional extended delta bytes, optional extended length bytes, and then the option value:

+---------------+---------------+
| Option Delta  | Option Length |  (1 byte)
+---------------+---------------+
| Extended Delta (0-2 bytes)    |
+-------------------------------+
| Extended Length (0-2 bytes)   |
+-------------------------------+
| Option Value (0 or more bytes)|
+-------------------------------+

Delta and length interpretation:

The option delta is the difference between this option’s number and the previous option’s number (or zero for the first option). Options must appear in order of increasing option number. Multiple options with the same number are permitted (delta = 0).

End-of-Options Marker

The byte 0xFF (delta nibble = 15, length nibble = 15) terminates the options field. It is always present when the O flag is set in the FCF.

Example

Two options — option 3 (1-byte value) followed by option 9 (2-byte value):

+------+-------+  +------+-------+-------+  +------+
| 0x31 |  val  |  | 0x62 |  val  |  val  |  | 0xFF |
+------+-------+  +------+-------+-------+  +------+
 delta=3 opt 3     delta=6 opt 9             marker
 len=1   val (1B)  len=2   val (2B)

Flood Hop Count

If present, FHOPS is a single byte containing two 4-bit fields:

  7   6   5   4   3   2   1   0
+---------------+---------------+
| FHOPS_REM     | FHOPS_ACC     |
+---------------+---------------+
     4 bits          4 bits

Where:

• FHOPS_REM (high nibble) = hops remaining — the number of additional flood hops permitted. Decremented by each forwarding repeater. When zero, no further flood forwarding is allowed.
• FHOPS_ACC (low nibble) = hops accumulated — the number of flood hops already traversed. Incremented by each forwarding repeater.

The sum FHOPS_REM + FHOPS_ACC is constant across forwarding hops and equals the original flood hop limit set by the sender. The maximum flood radius is 15 hops; longer paths can be achieved by combining source routing with flooding (see Routing Implications).

FHOPS_ACC enables the destination to determine how many flood hops the packet traversed, which is used for MAC ack routing when no trace route is available.