Struct rubble::link::data::Header

pub struct Header(u16);

16-bit data channel header preceding the payload.

Layout (in Bluetooth 4.2):

LSB                                                                MSB
+----------+---------+---------+---------+------------+--------------+
|   LLID   |  NESN   |   SN    |   MD    |     -      |    Length    |
| (2 bits) | (1 bit) | (1 bit) | (1 bit) |  (3 bits)  |   (8 bits)   |
+----------+---------+---------+---------+------------+--------------+

Payload format depends on the value of the 2-bit LLID field:

• 0b00: Reserved value.
• 0b01: LL Data PDU Continuation fragment or empty PDU.
• 0b10: LL Data PDU Start of L2CAP message (or complete message if no fragmentation necessary).
• 0b11: LL Control PDU.

The NESN field specifies the Next Expected Sequence Number. The SN field specifies the Sequence Number of this PDU.

The MD field specifies that the device sending the packet has more data to send during this connection event. When both slave and master send a packet with the MD bit set to 0, the connection event ends.

The Length field specifies the length of payload and MIC. Prior to Bluetooth 4.2, this was a 5-bit field, resulting in payloads + MICs of up to 31 Bytes. With Bluetooth 4.2, devices can communicate their buffer sizes and optionally transmit larger packets.

Sequence Numbers

The NESN and SN fields are used for retransmission and acknowledgement. The link layer stores two 1-bit parameters for an established connection, called transmitSeqNum and nextExpectedSeqNum. When a connection is established, both start out as 0. Both parameters are repeatedly incremented by 1 when data is transmitted, using wrapping arithmetic.

When a data channel packet is sent for the first time (ie. not retransmitted), the SN field is set to transmitSeqNum. When the packet is resent, the SN field is not modified. In both cases, the NESN bit is set to nextExpectedSeqNum.

The NESN bit tells the receiver whether its last packet has arrived: When a packet is received with an NESN value equal to the receiver’s transmitSeqNum, the receiver has already sent a packet with the expected SN, but the other side hasn’t received it yet. The receiver must resend the last data channel PDU. No other data channel PDU must be sent by it.

When the received packet’s NESN bit is different from transmitSeqNum, the last PDU has been acknowledged and the receiver should increment transmitSeqNum by 1.

Similarly, the SN bit is used to distinguish retransmitted and new packets: When a packet is received with an SN value equal to the receiver’s nextExpectedSeqNum value, the packet is new (not a retransmission), and nextExpectedSeqNum should be incremented by 1. If the value is not equal to nextExpectedSeqNum, this packet is a retransmission, so nextExpectedSeqNum should not be changed.

Tuple Fields

0: u16

Implementations

impl Header

pub fn new(llid: Llid) -> Self

Creates a header with the given LLID field and all other fields set to 0 (including the payload length).

pub fn parse(raw: &[u8]) -> Self

Parses a header from raw bytes.

Panics when raw contains less than 2 Bytes.

pub fn to_u16(&self) -> u16

Returns the raw representation of the header.

The returned u16 must be transmitted LSB and LSb first as the first 2 octets of the PDU.

pub fn payload_length(&self) -> u8

Returns the length of the payload in octets as specified in the Length field.

pub fn set_payload_length(&mut self, len: u8)

Sets the payload length field to len.

Note that BLE <4.2 is restricted to 5-bit payload lengths.

pub fn llid(&self) -> Llid

Returns the LLID field (PDU type).

pub fn nesn(&self) -> SeqNum

Returns the value of the NESN field (Next Expected Sequence Number).

pub fn set_nesn(&mut self, nesn: SeqNum)

Sets the value of the NESN field.

pub fn sn(&self) -> SeqNum

Returns the value of the SN field (Sequence Number).

pub fn set_sn(&mut self, sn: SeqNum)

Sets the value of the SN field.

pub fn md(&self) -> bool

Returns whether the MD field is set (More Data).

pub fn set_md(&mut self, md: bool)

Sets the value of the MD field.

Trait Implementations

impl Clone for Header

fn clone(&self) -> Header

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Header

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'a> FromBytes<'a> for Header

fn from_bytes(bytes: &mut ByteReader<'a>) -> Result<Self, Error>

Decode a Self from a byte slice, advancing bytes to point past the data that was read.

impl ToBytes for Header

fn to_bytes(&self, writer: &mut ByteWriter<'_>) -> Result<(), Error>

Converts self to bytes and writes them into writer, advancing writer to point past the encoded value.

impl Copy for Header