Segment ACK APDUs are used during segmented data transfer.

APDU Description (ASHRAE 135)

The BACnet-SegmentACK-PDU is used to acknowledge the receipt of one or more frames containing portions of a segmented message. It may also be used to request the next segment or segments of the segmented message. (Clause 21)

They serve two purposes:

1. Positive acknowledgment of one or more received segments (the receiver tells the sender "I got segments up to sequence_number and my current window is actual_window_size").
2. Negative acknowledgment (negative_ack: true) to request retransmission of a specific segment.

The modules BACnet.Stack.SegmentsStore (receiver) and BACnet.Stack.Segmentator (sender) automatically send or expect to receive those. Application code normally never constructs them manually.

This module implements the BACnet.Stack.EncoderProtocol.

Decoding is performed by BACnet.Protocol.APDU.decode/1 (and BACnet.Protocol.APDU.decode_segment_ack/1).

Examples

# Positive ACK after receiving segment #3 with a window of 16
iex> sack = %SegmentACK{
...>   negative_ack: false,
...>   sent_by_server: true,
...>   invoke_id: 12,
...>   sequence_number: 3,
...>   actual_window_size: 16
...> }
iex> SegmentACK.encode(sack)
{:ok, <<65, 12, 3, 16>>}

Decoding:

iex> raw = <<0x41, 0x0C, 0x03, 0x10>>
iex> BACnet.Protocol.APDU.decode(raw)
{:ok, %SegmentACK{actual_window_size: 16, invoke_id: 12, negative_ack: false, sent_by_server: true, sequence_number: 3}}