Feature/tcp bbr initial

[rkarahan80]

This commit introduces the core framework for the TCP BBR congestion control algorithm within the sys/netinet/cc/ directory.

Key changes:

1. Created sys/netinet/cc/cc_bbr.c.
2. Defined struct bbr_state to hold BBR's per-connection state variables, including those for BtlBw, RTprop, pacing/cwnd gains, current BBR mode (Startup, Drain, ProbeBW, ProbeRTT), round trip counting, BtlBw windowed filter, and ProbeRTT/ProbeBW cycle tracking.
3. Implemented the required struct cc_algo bbr_algo interface with function pointers to BBR-specific handlers.
4. Implemented core BBR callback logic:
   • bbr_cb_init(): Initializes per-connection BBR state.
   • bbr_cb_destroy(): Cleans up per-connection BBR state.
   • bbr_conn_init(): Initializes BBR state for a new TCP connection, starting in BBR_STARTUP mode.
   • bbr_ack_received(): Contains the main BBR state machine logic:
     • Updates BtlBw (using a 10-sample windowed max filter) and RTprop (via bbr_rttsample).
     • Manages transitions between BBR_STARTUP, BBR_DRAIN, BBR_PROBE_BW, and BBR_PROBE_RTT modes.
     • Implements gain cycling for BBR_PROBE_BW.
     • Calculates pacing_rate and snd_cwnd based on the current BBR mode and its model of the network path.
   • bbr_cong_signal(): Handles congestion events (RTO, ECN, NDUPACK) by adjusting snd_cwnd and BBR state (e.g., RTO causes reentry into BBR_STARTUP; loss during upward probing in BBR_PROBE_BW resets pacing gain to 1.0).
   • bbr_post_recovery(): Defines behavior after TCP fast recovery.
   • bbr_after_idle(): Resets BBR to BBR_STARTUP and clears stale estimates after an idle period.
   • bbr_rttsample(): Updates the RTprop (minimum RTT) estimate.
5. Stubbed out less critical callbacks (bbr_ecnpkt_handler, bbr_newround, bbr_ctl_output).
6. Used CC_ALGO_DECLARE(bbr, &bbr_algo) for module registration.

I paused while working on integrating cc_bbr.c into the build system, as sys/netinet/cc/Makefile was not found. The next step would be to locate the correct Makefile governing this directory (possibly sys/conf/files or a higher-level Makefile in sys/netinet/) and add cc_bbr.c to it. Following that, a sysctl interface for selecting BBR would be added.

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