Fix Retransmission Timeout calculation according to RFC6298

[danielinux]

Fixed RTO calculation according to RFC6298

- Fixed bug in smooth average RTT calculation
- Actually assign calculated RTO value (previously fixed to 1s)
- Enforce min/max RTO according to RFC
- Enforce Karn to only account for new ACK in RTO calculation

Depends on merging #33 first.

[Copilot]

Pull request overview

This PR implements RFC 6298-compliant retransmission timeout (RTO) calculation for TCP, fixing several bugs in the existing implementation.

Changes:

• Corrected the smooth RTT (SRTT) calculation formula to properly implement RFC 6298's exponential weighted moving average
• Enabled actual assignment of calculated RTO values (previously hardcoded to 1 second) with min/max clamping
• Implemented Karn's algorithm to exclude retransmitted segments from RTT measurements

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/wolfip.c	Implements RFC 6298 RTO calculation, adds control RTO timer for connection management states, fixes SRTT formula, enforces RTO bounds, and applies Karn's algorithm
src/test/unit/unit.c	Adds comprehensive test coverage for new RTO calculation logic and control state RTO behavior
README.md	Documents RFC 6298 and RFC 5681 compliance for RTO computation

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[Copilot]

The SRTT update formula does not match RFC 6298. According to RFC 6298 section 2.3, SRTT should be updated as SRTT = (7/8) * SRTT + (1/8) * R'. The current implementation shifts the sample by 3 (<< 3), but SRTT is already scaled by 8. This double-scales the new sample. The correct formula should be t->sock.tcp.srtt = (7U * t->sock.tcp.srtt + sample_ms) >> 3; to properly compute the weighted average.

Suggested change

	t->sock.tcp.srtt = (7U * t->sock.tcp.srtt + (sample_ms << 3)) >> 3;
	t->sock.tcp.srtt = (7U * t->sock.tcp.srtt + sample_ms) >> 3;

[Copilot]

The RTTVAR update does not follow RFC 6298. According to RFC 6298 section 2.3, RTTVAR should be updated as RTTVAR = (3/4) * RTTVAR + (1/4) * |SRTT - R'|. Here RTTVAR is scaled by 4, and err_ms is the absolute difference. However, the formula shifts err_ms by 2 (<< 2), which scales it by 4 instead of just adding it. The correct formula should be t->sock.tcp.rttvar = (3U * t->sock.tcp.rttvar + err_ms) >> 2;.

Suggested change

	t->sock.tcp.rttvar = (3U * t->sock.tcp.rttvar + (err_ms << 2)) >> 2;
	t->sock.tcp.rttvar = (3U * t->sock.tcp.rttvar + err_ms) >> 2;