cat << 'EOF' > /etc/sysctl.d/99-latency.conf
# --- Queueing & TCP Congestion ---
# Pair CAKE/FQ with BBR for minimal bufferbloat and fast recovery
net.core.default_qdisc = cake
net.ipv4.tcp_congestion_control = bbr

# --- Reduce Bufferbloat & Latency Spikes ---
# Shrink max backlog queues to drop/process stale packets faster
net.core.netdev_max_backlog = 1000
net.ipv4.tcp_max_syn_backlog = 1024
net.ipv4.tcp_max_tw_buckets = 10000

# --- Fast Packet Processing & TCP Timers ---
# Enable Fast Open to eliminate SYN-ACK round trips
net.ipv4.tcp_fastopen = 3
# Disable slow start after idle so connection bursts don't ramp up latency
net.ipv4.tcp_slow_start_after_idle = 0
# Enable selective ACKs and explicit congestion notification
net.ipv4.tcp_sack = 1
net.ipv4.tcp_dsack = 1
net.ipv4.tcp_ecn = 1
# Reduce orphan retries for faster socket cleanup
net.ipv4.tcp_orphan_retries = 1

# --- Memory & Socket Buffers (Optimized for Small Packets / Low Delay) ---
# Keep socket buffers modest to prevent deep queuing at the socket layer
net.core.rmem_default = 262144
net.core.wmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_max = 4194304
net.ipv4.tcp_rmem = 4096 87380 4194304
net.ipv4.tcp_wmem = 4096 65536 4194304

# --- Neighbor / ARP Table Latency Fixes ---
# Keep ARP entries warm and prevent stale neighbor lookup delays
net.ipv4.neigh.default.gc_stale_time = 120
net.ipv4.neigh.default.base_reachable_time_ms = 30000
EOF