/*
* Simple 802.11 rate-control algorithm for iPXE.
*
* Copyright (c) 2009 Joshua Oreman <oremanj@rwcr.net>.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
* 02110-1301, USA.
*/
FILE_LICENCE ( GPL2_OR_LATER );
#include <stdlib.h>
#include <ipxe/net80211.h>
/**
* @file
*
* Simple 802.11 rate-control algorithm
*/
/** @page rc80211 Rate control philosophy
*
* We want to maximize our transmission speed, to the extent that we
* can do that without dropping undue numbers of packets. We also
* don't want to take up very much code space, so our algorithm has to
* be pretty simple
*
* When we receive a packet, we know what rate it was transmitted at,
* and whether it had to be retransmitted to get to us.
*
* When we send a packet, we hear back how many times it had to be
* retried to get through, and whether it got through at all.
*
* Indications of TX success are more reliable than RX success, but RX
* information helps us know where to start.
*
* To handle all of this, we keep for each rate and each direction (TX
* and RX separately) some state information for the most recent
* packets on that rate and the number of packets for which we have
* information. The state is a 32-bit unsigned integer in which two
* bits represent a packet: 11 if it went through well, 10 if it went
* through with one retry, 01 if it went through with more than one
* retry, or 00 if it didn't go through at all. We define the
* "goodness" for a particular (rate, direction) combination as the
* sum of all the 2-bit fields, times 33, divided by the number of
* 2-bit fields containing valid information (16 except when we're
* starting out). The number produced is between 0 and 99; we use -1
* for rates with less than 4 RX packets or 1 TX, as an indicator that
* we do not have enough information to rely on them.
*
* In deciding which rates are best, we find the weighted average of
* TX and RX goodness, where the weighting is by number of packets
* with data and TX packets are worth 4 times as much as RX packets.
* The weighted average is called "net goodness" and is also a number
* between 0 and 99. If 3 consecutive packets fail transmission
* outright, we automatically ratchet down the rate; otherwise, we
* switch to the best rate whenever the current rate's goodness falls
* below some threshold, and try increasing our rate when the goodness
* is very high.
*
* This system is optimized for iPXE's style of usage. Because normal
* operation always involves receiving something, we'll make our way
* to the best rate pretty quickly. We tend to follow the lead of the
* sending AP in choosing rates, but we won't use rates for long that
* don't work well for us in transmission. We assume iPXE won't be
* running for long enough that rate patterns will change much, so we
* don't have to keep time counters or the like. And if this doesn't
* work well in practice there are many ways it could be tweaked.
*
* To avoid staying at 1Mbps for a long time, we don't track any
* transmitted packets until we've set our rate based on received
* packets.
*/
/** Two-bit packet status indicator for a packet with no retries */
#define RC_PKT_OK 0x3
/** Two-bit packet status indicator for a packet with one retry */
#define RC_PKT_RETRIED_ONCE 0x2
/** Two-bit packet status indicator for a TX packet with multiple retries
*
* It is not possible to tell whether an RX packet had one or multiple
* retries; we rely instead on the fact that failed RX packets won't
* get to us at all, so if we receive a lot of RX packets on a certain
* rate it must be pretty good.
*/
#define RC_PKT_RETRIED_MULTI 0x1
/** Two-bit packet status indicator for a TX packet that was never ACKed
*
* It is not possible to tell whether an RX packet was setn if it
* didn't get through to us, but if we don't see one we won't increase
* the goodness for its rate. This asymmetry is part of why TX packets
* are weighted much more heavily than RX.
*/
#define RC_PKT_FAILED 0x0
/** Number of times to weight TX packets more heavily than RX packets */
#define RC_TX_FACTOR 4
/** Number of consecutive failed TX packets that cause an automatic rate drop */
#define RC_TX_EMERG_FAIL 3
/** Minimum net goodness below which we will search for a better rate */
#define RC_GOODNESS_MIN 85
/** Maximum net goodness above which we will try to increase our rate */
#define RC_GOODNESS_MAX 95
/** Minimum (num RX + @c RC_TX_FACTOR * num TX) to use a certain rate */
#define RC_UNCERTAINTY_THRESH 4
/** TX direction */
#define TX 0
/** RX direction */
#define RX 1
/** A rate control context */
struct rc80211_ctx
{
/** Goodness state for each rate, TX and RX */
u32 goodness[2][NET80211_MAX_RATES];
/** Number of packets recorded for each rate */
u8 count[2][NET80211_MAX_RATES];
/** Indication of whether we've set the device rate yet */
int started;
/** Counter of all packets sent and received */
int packets;
};
/**
* Initialize rate-control algorithm
*
* @v dev 802.11 device
* @ret ctx Rate-control context, to be stored in @c dev->rctl
*/
struct rc80211_ctx * rc80211_init ( struct net80211_device *dev __unused )
{
struct rc80211_ctx *ret = zalloc ( sizeof ( *ret ) );
return ret;
}
/**
* Calculate net goodness for a certain rate
*
* @v ctx Rate-control context
* @v rate_idx Index of rate to calculate net goodness for
*/
static int rc80211_calc_net_goodness ( struct rc80211_ctx *ctx,
int rate_idx )
{
int sum[2], num[2], dir, pkt;
for ( dir = 0; dir < 2; dir++ ) {
u32 good = ctx->goodness[dir][rate_idx];
num[dir] = ctx->count[dir][rate_idx];
sum[dir] = 0;
for ( pkt = 0; pkt < num[dir]; pkt++ )
sum[dir] += ( good >> ( 2 * pkt ) ) & 0x3;
}
if ( ( num[TX] * RC_TX_FACTOR + num[RX] ) < RC_UNCERTAINTY_THRESH )
return -1;
return ( 33 * ( sum[TX] * RC_TX_FACTOR + sum[RX] ) /
( num[TX] * RC_TX_FACTOR + num[RX] ) );
}
/**
* Determine the best rate to switch to and return it
*
* @v dev 802.11 device
* @ret rate_idx Index of the best rate to switch to
*/
static int rc80211_pick_best ( struct net80211_device *dev )
{
struct rc80211_ctx *ctx = dev->rctl;
int best_net_good = 0, best_rate = -1, i;
for ( i = 0; i < dev->nr_rates; i++ ) {
int net_good = rc80211_calc_net_goodness ( ctx, i );
if ( net_good > best_net_good ||
( best_net_good > RC_GOODNESS_MIN &&
net_good > RC_GOODNESS_MIN ) ) {
best_net_good = net_good;
best_rate = i;
}
}
if ( best_rate >= 0 ) {
int old_good = rc80211_calc_net_goodness ( ctx, dev->rate );
if ( old_good != best_net_good )
DBGC ( ctx, "802.11 RC %p switching from goodness "
"%d to %d\n", ctx, old_good, best_net_good );
ctx->started = 1;
return best_rate;
}
return dev->rate;
}
/**
* Set 802.11 device rate
*
* @v dev 802.11 device
* @v rate_idx Index of rate to switch to
*
* This is a thin wrapper around net80211_set_rate_idx to insert a
* debugging message where appropriate.
*/
static inline void rc80211_set_rate ( struct net80211_device *dev,
int rate_idx )
{
DBGC ( dev->rctl, "802.11 RC %p changing rate %d->%d Mbps\n", dev->rctl,
dev->rates[dev->rate] / 10, dev->rates[rate_idx] / 10 );
net80211_set_rate_idx ( dev, rate_idx );
}
/**
* Check rate-control state and change rate if necessary
*
* @v dev 802.11 device
*/
static void rc80211_maybe_set_new ( struct net80211_device *dev )
{
struct rc80211_ctx *ctx = dev->rctl;
int net_good;
net_good = rc80211_calc_net_goodness ( ctx, dev->rate );
if ( ! ctx->started ) {
rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
return;
}
if ( net_good < 0 ) /* insufficient data */
return;
if ( net_good > RC_GOODNESS_MAX && dev->rate + 1 < dev->nr_rates ) {
int higher = rc80211_calc_net_goodness ( ctx, dev->rate + 1 );
if ( higher > net_good || higher < 0 )
rc80211_set_rate ( dev, dev->rate + 1 );
else
rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
}
if ( net_good < RC_GOODNESS_MIN ) {
rc80211_set_rate ( dev, rc80211_pick_best ( dev ) );
}
}
/**
* Update rate-control state
*
* @v dev 802.11 device
* @v direction One of the direction constants TX or RX
* @v rate_idx Index of rate at which packet was sent or received
* @v retries Number of times packet was retried before success
* @v failed If nonzero, the packet failed to get through
*/
static void rc80211_update ( struct net80211_device *dev, int direction,
int rate_idx, int retries, int failed )
{
struct rc80211_ctx *ctx = dev->rctl;
u32 goodness = ctx->goodness[direction][rate_idx];
if ( ctx->count[direction][rate_idx] < 16 )
ctx->count[direction][rate_idx]++;
goodness <<= 2;
if ( failed )
goodness |= RC_PKT_FAILED;
else if ( retries > 1 )
goodness |= RC_PKT_RETRIED_MULTI;
else if ( retries )
goodness |= RC_PKT_RETRIED_ONCE;
else
goodness |= RC_PKT_OK;
ctx->goodness[direction][rate_idx] = goodness;
ctx->packets++;
rc80211_maybe_set_new ( dev );
}
/**
* Update rate-control state for transmitted packet
*
* @v dev 802.11 device
* @v retries Number of times packet was transmitted before success
* @v rc Return status code for transmission
*/
void rc80211_update_tx ( struct net80211_device *dev, int retries, int rc )
{
struct rc80211_ctx *ctx = dev->rctl;
if ( ! ctx->started )
return;
rc80211_update ( dev, TX, dev->rate, retries, rc );
/* Check if the last RC_TX_EMERG_FAIL packets have all failed */
if ( ! ( ctx->goodness[TX][dev->rate] &
( ( 1 << ( 2 * RC_TX_EMERG_FAIL ) ) - 1 ) ) ) {
if ( dev->rate == 0 )
DBGC ( dev->rctl, "802.11 RC %p saw %d consecutive "
"failed TX, but cannot lower rate any further\n",
dev->rctl, RC_TX_EMERG_FAIL );
else {
DBGC ( dev->rctl, "802.11 RC %p lowering rate (%d->%d "
"Mbps) due to %d consecutive TX failures\n",
dev->rctl, dev->rates[dev->rate] / 10,
dev->rates[dev->rate - 1] / 10,
RC_TX_EMERG_FAIL );
rc80211_set_rate ( dev, dev->rate - 1 );
}
}
}
/**
* Update rate-control state for received packet
*
* @v dev 802.11 device
* @v retry Whether the received packet had been retransmitted
* @v rate Rate at which packet was received, in 100 kbps units
*/
void rc80211_update_rx ( struct net80211_device *dev, int retry, u16 rate )
{
int ridx;
for ( ridx = 0; ridx < dev->nr_rates && dev->rates[ridx] != rate;
ridx++ )
;
if ( ridx >= dev->nr_rates )
return; /* couldn't find the rate */
rc80211_update ( dev, RX, ridx, retry, 0 );
}
/**
* Free rate-control context
*
* @v ctx Rate-control context
*/
void rc80211_free ( struct rc80211_ctx *ctx )
{
free ( ctx );
}
