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Reliability Mechanism

Sequence Numbers​

SRSPP uses the 14-bit sequence count field from the SPP primary header for reliability. Sequence numbers wrap around at 16383 (0x3FFF).

Acknowledgments​

The receiver acknowledges received packets using a combined cumulative and selective ACK scheme:

  • Cumulative ACK — the highest sequence number received in-order. All packets up to and including this number are acknowledged.
  • Selective ACK Bitmap — a 16-bit bitmap indicating which out-of-order packets have been received. Bit N indicates receipt of the packet N positions beyond the cumulative ACK.

Example: the receiver has received packets 0, 1, 2, 5, and 7. Packets 3, 4, and 6 are missing.

Receiver's state
0
✓
1
✓
2
✓
3
gap
4
gap
5
✓
6
gap
7
✓
↑
↑
↑
↑
↑
ACK: Cumulative = 2, Selective bitmap
0
0
1
0
1
0
0
0
0
0
0
0
0
0
0
0
Covered by cumulative ACK (seq 0–2)
Received out-of-order (bitmap = 1)
Missing — needs retransmission (bitmap = 0)

The bitmap starts at the first packet after the cumulative ACK. Each bit corresponds to one packet: 1 = received, 0 = missing. The sender retransmits only the missing packets (3, 4, and 6).

Retransmission​

The sender maintains a retransmission timer for each unacknowledged packet. When a timer expires, the packet is retransmitted. After a configurable number of failed retransmissions, the packet is considered lost.

ACK Timing​

The receiver can be configured for:

  • Immediate ACK: Send ACK immediately upon receiving any data packet
  • Delayed ACK: Wait for a timeout before sending ACK (allows batching)

Segmentation​

Large messages that exceed the MTU are segmented using the SPP sequence flags:

  • Unsegmented (0b11) — complete message in one packet
  • First (0b01) — first segment of a multi-packet message
  • Continuation (0b00) — middle segment
  • Last (0b10) — final segment

The receiver reassembles segments in order. All segments of a message share consecutive sequence numbers.

Flow Control​

SRSPP uses a sliding window for flow control:

  • Window size (WIN): Maximum number of unacknowledged packets in flight
  • Send buffer (BUF): Total bytes that can be queued for transmission

The sender blocks when either the window is full or the buffer is exhausted.