[RFC] TCP FSM Non-Compliance: SYN_SENT + SYN does not transition to SYN_RECEIVED

[lxd15938771473]

Summary:
In picoTCP v1.7.0, the TCP state machine does not properly handle simultaneous-open scenarios. A socket in SYN_SENT state receiving a pure SYN packet does not transition to SYN_RECEIVED, diverging from RFC TCP semantics.

Affected Version:

• picoTCP v1.7.0 (official Git clone)

Details:

• modules/pico_tcp.c FSM table for PICO_SOCKET_STATE_TCP_SYN_SENT:
  syn = NULL
  synack = tcp_synack
• valid_flags[SYN_SENT] does not accept standalone SYN
• Observed behavior: receiving SYN triggers reset/invalid path, not SYN_RECEIVED
• RFC: SYN_SENT receiving SYN should transition to SYN_RECEIVED

Impact:

• Interoperability issues (e.g., P2P/NAT-hole-punch)
• Fingerprinting opportunity
• Reliability degradation with certain peers

Suggested Remediation:

• Add explicit FSM transition: SYN_SENT + SYN -> SYN_RECEIVED
• Ensure handshake continues correctly

[danielinux]

This was never in the roadmap, and quite niche.
I don't have a strong opinion tho, if you want to submit a patch to picotcp-ng, which is still maintained

This repository should be archived, but I'm not sure anyone involved still has access

[lxd15938771473]

Hi, Thanks for your earlier reply regarding picoTCP. I have a quick follow-up question — do you happen to know who is currently maintaining picotcp-ng, or what would be the best way to reach the maintainers? I'm working on a related validation/report and would like to make sure I contact the right people. Thanks again! At 2026-04-06 16:11:08, "Daniele Lacamera" ***@***.***> wrote: danielinux left a comment (tass-belgium/picotcp#528) This was never in the roadmap, and quite niche. I don't have a strong opinion tho, if you want to submit a patch. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.Message ID: ***@***.***>