TCP Message Framing for AV Control: Parse Complete Replies

TCP message framing belongs to the application protocol. Keep received bytes in a buffer and extract complete replies using the device's documented delimiter, length field or other framing rule. Never assume that one socket read equals one command response, even when that assumption appears to work on a bench.

This problem often appears only after installation, when network scheduling or device load changes. A parser that succeeded with short replies can fail when a reply arrives in two reads or when several replies arrive together.

Separate transport from the reply format

RFC 9293 defines TCP as a reliable, ordered byte stream. Its segment boundaries are not application message boundaries. Your device protocol supplies the additional rule that turns that stream into individual replies.

ASCII also does not define an application's end-of-message convention. RFC 20's character table distinguishes carriage return and line feed; the relevant device manual must tell you whether either character terminates a reply. A terminal showing a new visual line is not sufficient evidence.

Build the parser around retained bytes

Append each received chunk to a session-local buffer. While the buffer contains a complete frame, remove that frame and pass it to the response interpreter. Preserve any remaining partial frame for the next read. Bound the maximum buffer size using the protocol's limits and an explicit error policy.

For a delimiter parser, account for the delimiter itself being split between reads. For a length parser, validate the length before allocating or waiting for a large body. Reject impossible lengths rather than allowing malformed traffic to consume memory indefinitely.

The following is conceptual parser logic, not a device command:

receive chunk
append chunk to this connection's buffer
while a complete documented frame exists:
    extract one frame
    dispatch it to the appropriate response handler
retain the incomplete remainder

Keep framing and transaction matching independent

A correctly framed reply can still belong to the wrong request. Decide how responses are correlated: transaction identifier, command name or a single-outstanding-request policy. Unsolicited status messages need their own path; otherwise an event can accidentally complete the current command.

Clear the receive buffer when retiring a connection. A partial reply from an old session must not be joined to the greeting on a new one. Keep raw bytes in a protected diagnostic log alongside decoded text so hidden control characters remain visible during support work.

Test the failure patterns deliberately

Feed a saved, valid reply to the parser one byte at a time, then all at once. Next feed two replies in one chunk and divide a reply immediately before its terminator. All variations should produce the same application messages.

Add tests for an oversized frame, an unexpected greeting and a connection closing halfway through a reply. Specify whether each creates a parse fault, incomplete transaction or reconnect. Do not silently display a truncated value as a successful observation.

Keep sanitized captures of the actual device's shortest reply, longest expected reply and documented error shapes. Use them to test parser changes before updating a deployed driver. Preserve bytes rather than only screenshots of decoded text: hidden endings and binary length fields cannot be reconstructed reliably from terminal appearance. Include capture context and device firmware so a future response-format change can be distinguished from a regression in the buffering logic.

These tests verify your integration logic; they do not imply any particular controller product exposes programmable parsing. See the UDP versus TCP troubleshooting guide for transport diagnosis, and bring the device's documented frame format to a partner review when selecting a compatible driver.