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Configuration Parameters

SRSPP does not negotiate parameters at runtime. Both endpoints must be configured with compatible values.

  • APID — application process ID used for routing. Must match on sender and receiver.
  • WIN (default 8) — window size: maximum number of unacknowledged packets in flight. Receiver's WIN should be >= sender's WIN.
  • BUF (default 4096) — send buffer size in bytes. Limits total queued data.
  • MTU (default 512) — maximum transmission unit: largest payload per packet. Messages larger than MTU are segmented.
  • REASM (default 8192) — maximum reassembled message size on the receiver. Must be >= the sender's largest message.
  • RTO Policy (default Fixed) — retransmission timeout strategy (see below).
  • Max Retransmits (default 3) — attempts before declaring a packet lost and signaling an error to the application.
  • ACK Delay (default 100 ticks) — time the receiver waits before sending a delayed ACK, allowing multiple packets to be acknowledged in one ACK.

RTO Policy​

The retransmission timeout is governed by a pluggable RTO policy. The sender queries the policy each time it starts a retransmission timer, passing the current time so the policy can adapt dynamically.

Two built-in policies are provided:

  • Fixed — returns a constant timeout. Suitable for ISL links with stable, predictable latency.
  • Orbit-aware — adapts the timeout based on a contact schedule. During a line-of-sight window, uses a short RTO (the link is active, real loss should be detected quickly). Outside a window, sets the RTO to the time until the next window plus a configurable margin — this prevents the sender from declaring packets lost during normal orbital gaps. If no future windows exist in the schedule, falls back to the short RTO. The contact schedule is stored in a fixed-size buffer suitable for embedded systems.

Custom policies can be implemented by providing a different RTO computation.

Compatibility Constraints​

SRSPP does not negotiate parameters at runtime. Both endpoints must be configured with compatible values. The simplest approach is to share the same constants between sender and receiver applications. With matching defaults, the sender will never exceed what the receiver can handle.

  • APID must match for routing to work
  • Receiver's WIN should be >= sender's WIN (to buffer all in-flight packets)
  • Receiver's REASM must be >= sender's largest message size