# $OpenBSD: ackpri,v 1.3 2006/10/07 04:48:01 mcbride Exp $

# Use a simple priority queue to prioritize empty (no payload) TCP ACKs,
# which dramatically improves throughput on (asymmetric) links when the
# reverse direction is saturated. The empty ACKs use an insignificant
# part of the bandwidth, but if they get delayed, downloads suffer
# badly, so prioritize them.

# Example: 512/128 kbps ADSL. Download is 50 kB/s. When a concurrent
# upload saturates the uplink, download drops to 7 kB/s. With the
# priority queue below, download drops only to 48 kB/s.

# Replace lo0 with your real external interface

ext_if="lo0"

# For a 512/128 kbps ADSL with PPPoE link, using "bandwidth 100Kb"
# is optimal. Some experimentation might be needed to find the best
# value. If it's set too high, the priority queue is not effective, and
# if it's set too low, the available bandwidth is not fully used.
# A good starting point would be real_uplink_bandwidth * 90 / 100.

altq on $ext_if priq bandwidth 100Kb queue { q_pri, q_def }
queue q_pri priority 7
queue q_def priority 1 priq(default)

pass out on $ext_if proto tcp from $ext_if to any queue (q_def, q_pri)

pass in  on $ext_if proto tcp from any to $ext_if queue (q_def, q_pri)