Segmentation and reassembly (SAR)
Segmentation and reassembly (SAR) provides a way of handling larger upper transport layer messages in a mesh network, with a purpose of enhancing the Bluetooth Mesh throughput. The segmentation and reassembly mechanism is used by the lower transport layer.
The lower transport layer defines how the upper transport layer PDUs are segmented and reassembled into multiple Lower Transport PDUs, and sends them to the lower transport layer on a peer device. If the Upper Transport PDU fits, it is sent in a single Lower Transport PDU. For longer packets, which do not fit into a single Lower Transport PDU, the lower transport layer performs segmentation, splitting the Upper Transport PDU into multiple segments.
The lower transport layer on the receiving device reassembles the segments into a single Upper
Transport PDU before passing it up the stack. Delivery of a segmented message is acknowledged by the
lower transport layer of the receiving node, while an unsegmented message delivery is not
acknowledged. However, an Upper Transport PDU that fits into one Lower Transport PDU can also be
sent as a single-segment segmented message when acknowledgment by the lower transport layer is
required. Set the send rel flag (see bt_mesh_msg_ctx) to use the reliable message
transmission and acknowledge single-segment segmented messages.
The transport layer is able to transport up to 32 segments with its SAR mechanism, with a maximum message (PDU) size of 384 octets. To configure message size for the Bluetooth Mesh stack, use the following Kconfig options:
CONFIG_BT_MESH_RX_SEG_MAXto set the maximum number of segments in an incoming message.CONFIG_BT_MESH_TX_SEG_MAXto set the maximum number of segments in an outgoing message.
The Kconfig options CONFIG_BT_MESH_TX_SEG_MSG_COUNT and
CONFIG_BT_MESH_RX_SEG_MSG_COUNT define how many outgoing and incoming segmented
messages can be processed simultaneously. When more than one segmented message is sent to the same
destination, the messages are queued and sent one at a time.
Incoming and outgoing segmented messages share the same pool for allocation of their segments. This
pool size is configured through the CONFIG_BT_MESH_SEG_BUFS Kconfig option.
Both incoming and outgoing messages allocate segments at the start of the transaction. The outgoing
segmented message releases its segments one by one as soon as they are acknowledged by the receiver,
while the incoming message releases the segments first after the message is fully received.
Keep this in mind when defining the size of the buffers.
SAR does not impose extra overhead on the access layer payload per segment.
Segmentation and reassembly (SAR) Configuration models
With Bluetooth Mesh Protocol Specification version 1.1, it became possible to configure SAR behavior, such as intervals, timers and retransmission counters, over a mesh network using SAR Configuration models:
The following SAR behavior applies regardless of the presence of a SAR Configuration Server on a node.
Transmission of segments is separated by a segment transmission interval (see the SAR Segment Interval Step state). Other configurable time intervals and delays available for the segmentation and reassembly are:
Interval between unicast retransmissions (see the states SAR Unicast Retransmissions Interval Step and SAR Unicast Retransmissions Interval Increment).
Interval between multicast retransmissions (see the SAR Multicast Retransmissions Interval Step state).
Segment reception interval (see the SAR Receiver Segment Interval Step state).
Acknowledgment delay increment (see the SAR Acknowledgment Delay Increment state).
When the last segment marked as unacknowledged is transmitted, the lower transport layer starts a
retransmissions timer. The initial value of the SAR Unicast Retransmissions timer depends on the
value of the TTL field of the message. If the TTL field value is greater than 0, the initial
value for the timer is set according to the following formula:
If the TTL field value is 0, the initial value of the timer is set to the unicast
retransmissions interval step.
The initial value of the SAR Multicast Retransmissions timer is set to the multicast retransmissions interval.
When the lower transport layer receives a message segment, it starts a SAR Discard timer. The discard timer tells how long the lower transport layer waits before discarding the segmented message the segment belongs to. The initial value of the SAR Discard timer is the discard timeout value indicated by the SAR Discard Timeout state.
SAR Acknowledgment timer holds the time before a Segment Acknowledgment message is sent for a received segment. The initial value of the SAR Acknowledgment timer is calculated using the following formula:
The SegN field value identifies the total number of segments the Upper Transport PDU is
segmented into.
Four counters are related to SAR behavior:
Two unicast retransmissions counts (see SAR Unicast Retransmissions Count state and SAR Unicast Retransmissions Without Progress Count state)
Multicast retransmissions count (see SAR Multicast Retransmissions Count state)
Acknowledgment retransmissions count (see SAR Acknowledgment Retransmissions Count state)
If the number of segments in the transmission is higher than the value of the SAR Segments Threshold state, Segment Acknowledgment messages are retransmitted using the value of the SAR Acknowledgment Retransmissions Count state.
SAR states
There are two states defined related to segmentation and reassembly:
SAR Transmitter state
SAR Receiver state
The SAR Transmitter state is a composite state that controls the number and timing of transmissions of segmented messages. It includes the following states:
SAR Segment Interval Step
SAR Unicast Retransmissions Count
SAR Unicast Retransmissions Without Progress Count
SAR Unicast Retransmissions Interval Step
SAR Unicast Retransmissions Interval Increment
SAR Multicast Retransmissions Count
SAR Multicast Retransmissions Interval Step
The SAR Receiver state is a composite state that controls the number and timing of Segment Acknowledgment transmissions and the discarding of reassembly of a segmented message. It includes the following states:
SAR Segments Threshold
SAR Discard Timeout
SAR Acknowledgment Delay Increment
SAR Acknowledgment Retransmissions Count
SAR Receiver Segment Interval Step
SAR Segment Interval Step
SAR Segment Interval Step state holds a value that controls the interval between transmissions of segments of a segmented message. The interval is measured in milliseconds.
Use the CONFIG_BT_MESH_SAR_TX_SEG_INT_STEP Kconfig option to set the default
value. Segment transmission interval is then calculated using the following formula:
SAR Unicast Retransmissions Count
SAR Unicast Retransmissions Count holds a value that defines the maximum number of retransmissions
of a segmented message to a unicast destination. Use the
CONFIG_BT_MESH_SAR_TX_UNICAST_RETRANS_COUNT Kconfig option to set the default
value for this state.
SAR Unicast Retransmissions Without Progress Count
This state holds a value that defines the maximum number of retransmissions of a segmented message
to a unicast address that will be sent if no acknowledgment was received during the timeout, or if
an acknowledgment with already confirmed segments was received. Use the Kconfig option
CONFIG_BT_MESH_SAR_TX_UNICAST_RETRANS_WITHOUT_PROG_COUNT to set the maximum number
of retransmissions.
SAR Unicast Retransmissions Interval Step
The value of this state controls the interval step used for delaying the retransmissions of unacknowledged segments of a segmented message to a unicast address. The interval step is measured in milliseconds.
Use the CONFIG_BT_MESH_SAR_TX_UNICAST_RETRANS_INT_STEP Kconfig option to set the
default value. This value is then used to calculate the interval step using the following formula:
SAR Unicast Retransmissions Interval Increment
SAR Unicast Retransmissions Interval Increment holds a value that controls the interval increment used for delaying the retransmissions of unacknowledged segments of a segmented message to a unicast address. The increment is measured in milliseconds.
Use the Kconfig option CONFIG_BT_MESH_SAR_TX_UNICAST_RETRANS_INT_INC to set the
default value. The Kconfig option value is used to calculate the increment using the following
formula:
SAR Multicast Retransmissions Count
The state holds a value that controls the total number of retransmissions of a segmented message to
a multicast address. Use the Kconfig option
CONFIG_BT_MESH_SAR_TX_MULTICAST_RETRANS_COUNT to set the total number of
retransmissions.
SAR Multicast Retransmissions Interval Step
This state holds a value that controls the interval between retransmissions of all segments in a segmented message to a multicast address. The interval is measured in milliseconds.
Use the Kconfig option CONFIG_BT_MESH_SAR_TX_MULTICAST_RETRANS_INT to set the
default value that is used to calculate the interval using the following formula:
SAR Discard Timeout
The value of this state defines the time in seconds that the lower transport layer waits after
receiving segments of a segmented message before discarding that segmented message. Use the Kconfig
option CONFIG_BT_MESH_SAR_RX_DISCARD_TIMEOUT to set the default value. The discard
timeout will be calculated using the following formula:
SAR Acknowledgment Delay Increment
This state holds a value that controls the delay increment of an interval used for delaying the transmission of an acknowledgment message after receiving a new segment. The increment is measured in segments.
Use the Kconfig option CONFIG_BT_MESH_SAR_RX_ACK_DELAY_INC to set the default
value. The increment value is calculated to be
\(\verb|CONFIG_BT_MESH_SAR_RX_ACK_DELAY_INC| + 1.5\).
SAR Segments Threshold
SAR Segments Threshold state holds a value that defines a threshold in number of segments of a
segmented message for acknowledgment retransmissions. Use the Kconfig option
CONFIG_BT_MESH_SAR_RX_SEG_THRESHOLD to set the threshold.
When the number of segments of a segmented message is above this threshold, the stack will
additionally retransmit every acknowledgment message the number of times given by the value of
CONFIG_BT_MESH_SAR_RX_ACK_RETRANS_COUNT.
SAR Acknowledgment Retransmissions Count
The SAR Acknowledgment Retransmissions Count state controls the number of retransmissions of Segment
Acknowledgment messages sent by the lower transport layer. It gives the total number of
retranmissions of an acknowledgment message that the stack will additionally send when the size of
segments in a segmented message is above the CONFIG_BT_MESH_SAR_RX_SEG_THRESHOLD
value.
Use the Kconfig option CONFIG_BT_MESH_SAR_RX_ACK_RETRANS_COUNT to set the default
value for this state. The maximum number of transmissions of a Segment Acknowledgment message is
\(\verb|CONFIG_BT_MESH_SAR_RX_ACK_RETRANS_COUNT| + 1\).
SAR Receiver Segment Interval Step
The SAR Receiver Segment Interval Step defines the segments reception interval step used for delaying the transmission of an acknowledgment message after receiving a new segment. The interval is measured in milliseconds.
Use the Kconfig option CONFIG_BT_MESH_SAR_RX_SEG_INT_STEP to set the default value
and calculate the interval using the following formula: