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TOR_WESTWOOD: Implement Prop#324 TOR_WESTWOOD
This commit is contained in:
Mike Perry
David Goulet
@@ -0,0 +1,231 @@
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/* Copyright (c) 2019-2021, The Tor Project, Inc. */
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/* See LICENSE for licensing information */
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/**
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* \file congestion_control_westwood.c
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* \brief Code that implements the TOR_WESTWOOD congestion control algorithm
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* from Proposal #324.
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*/
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#define TOR_CONGESTION_CONTROL_WESTWOOD_PRIVATE
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#include "core/or/or.h"
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#include "core/or/crypt_path.h"
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#include "core/or/or_circuit_st.h"
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#include "core/or/sendme.h"
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#include "core/or/congestion_control_st.h"
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#include "core/or/congestion_control_common.h"
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#include "core/or/congestion_control_westwood.h"
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#include "core/or/circuitlist.h"
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#include "core/or/circuituse.h"
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#include "core/or/origin_circuit_st.h"
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#include "core/or/channel.h"
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#include "feature/nodelist/networkstatus.h"
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#define USEC_ONE_MS (1000)
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#define WESTWOOD_CWND_BACKOFF_M 75
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#define WESTWOOD_RTT_BACKOFF_M 100
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#define WESTWOOD_RTT_THRESH 33
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#define WESTWOOD_MIN_BACKOFF 0
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/**
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* Cache westwood consensus parameters.
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*/
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void
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congestion_control_westwood_set_params(congestion_control_t *cc)
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{
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tor_assert(cc->cc_alg == CC_ALG_WESTWOOD);
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cc->westwood_params.cwnd_backoff_m =
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networkstatus_get_param(NULL, "cc_westwood_cwnd_m",
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WESTWOOD_CWND_BACKOFF_M,
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0,
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100);
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cc->westwood_params.rtt_backoff_m =
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networkstatus_get_param(NULL, "cc_westwood_rtt_m",
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WESTWOOD_RTT_BACKOFF_M,
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50,
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100);
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cc->westwood_params.rtt_thresh =
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networkstatus_get_param(NULL, "cc_westwood_rtt_thresh",
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WESTWOOD_RTT_THRESH,
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0,
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100);
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cc->westwood_params.min_backoff =
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networkstatus_get_param(NULL, "cc_westwood_min_backoff",
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WESTWOOD_MIN_BACKOFF,
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0,
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1);
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}
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/**
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* Return the RTT threshhold that signals congestion.
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*
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* Computed from the threshold parameter that specifies a
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* percent between the min and max RTT obseved so far.
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*/
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static inline uint64_t
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westwood_rtt_signal(const congestion_control_t *cc)
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{
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return ((100 - cc->westwood_params.rtt_thresh)*cc->min_rtt_usec +
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cc->westwood_params.rtt_thresh*(cc)->max_rtt_usec)/100;
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}
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/**
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* Compute a backoff to reduce the max RTT.
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*
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* This may be necessary to ensure that westwood does not have
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* a runaway condition where congestion inflates the max RTT, which
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* inflates the congestion threshold. That cannot happen with one
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* Westwood instance, but it may happen in aggregate. Hence, this is
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* a safety parameter, in case we need it.
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*/
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static inline uint64_t
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westwood_rtt_max_backoff(const congestion_control_t *cc)
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{
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return cc->min_rtt_usec +
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(cc->westwood_params.rtt_backoff_m *
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(cc->max_rtt_usec - cc->min_rtt_usec))/100;
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}
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/**
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* Returns true if the circuit is experiencing congestion, as per
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* TOR_WESTWOOD rules.
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*/
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static inline bool
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westwood_is_congested(const congestion_control_t *cc)
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{
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/* If the local channel is blocked, that is always congestion */
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if (cc->blocked_chan)
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return true;
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/* If the min RTT is within 1ms of the signal, then there is not enough
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* range in RTTs to signify congestion. Treat that as not congested. */
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if (westwood_rtt_signal(cc) < cc->min_rtt_usec ||
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westwood_rtt_signal(cc) - cc->min_rtt_usec < USEC_ONE_MS)
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return false;
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/* If the EWMA-smoothed RTT exceeds the westwood RTT threshhold,
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* then it is congestion. */
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if (cc->ewma_rtt_usec > westwood_rtt_signal(cc))
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return true;
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return false;
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}
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/**
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* Process a SENDME and update the congestion window according to the
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* rules specified in TOR_WESTWOOD of Proposal #324.
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*
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* Essentially, this algorithm uses a threshhold of 'rtt_thresh', which
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* is a midpoint between the min and max RTT. If the RTT exceeds this
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* threshhold, then queue delay due to congestion is assumed to be present,
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* and the algirithm reduces the congestion window. If the RTT is below the
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* threshhold, the circuit is not congested (ie: queue delay is low), and we
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* increase the congestion window.
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*
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* The congestion window is updated only once every congestion window worth of
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* packets, even if the signal persists. It is also updated whenever the
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* upstream orcon blocks, or unblocks. This minimizes local client queues.
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*/
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int
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congestion_control_westwood_process_sendme(congestion_control_t *cc,
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const circuit_t *circ,
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const crypt_path_t *layer_hint)
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{
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tor_assert(cc && cc->cc_alg == CC_ALG_WESTWOOD);
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tor_assert(circ);
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/* Update ack counter until next congestion signal event is allowed */
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if (cc->next_cc_event)
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cc->next_cc_event--;
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/* If we were unable to update our circuit estimates, Westwood must
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* *not* update its cwnd, otherwise it could run to infinity, or to 0.
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* Just update inflight from the sendme and return. */
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if (!congestion_control_update_circuit_estimates(cc, circ, layer_hint)) {
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cc->inflight = cc->inflight - cc->sendme_inc;
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return 0;
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}
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/* We only update anything once per window */
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if (cc->next_cc_event == 0) {
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if (!westwood_is_congested(cc)) {
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if (cc->in_slow_start) {
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cc->cwnd = MAX(cc->cwnd + CWND_INC_SS(cc),
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cc->bdp[cc->bdp_alg]);
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} else {
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cc->cwnd = cc->cwnd + CWND_INC(cc);
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}
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} else {
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if (cc->westwood_params.min_backoff)
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cc->cwnd = MIN(cc->cwnd*cc->westwood_params.cwnd_backoff_m/100,
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cc->bdp[cc->bdp_alg]);
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else
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cc->cwnd = MAX(cc->cwnd*cc->westwood_params.cwnd_backoff_m/100,
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cc->bdp[cc->bdp_alg]);
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cc->in_slow_start = 0;
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// Because Westwood's congestion can runaway and boost max rtt,
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// which increases its congestion signal, we backoff the max rtt
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// too.
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cc->max_rtt_usec = westwood_rtt_max_backoff(cc);
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log_info(LD_CIRC, "CC: TOR_WESTWOOD congestion. New max RTT: %"PRIu64,
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cc->max_rtt_usec/1000);
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}
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/* cwnd can never fall below 1 increment */
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cc->cwnd = MAX(cc->cwnd, cc->cwnd_min);
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/* Schedule next update */
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cc->next_cc_event = CWND_UPDATE_RATE(cc);
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if (CIRCUIT_IS_ORIGIN(circ)) {
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log_info(LD_CIRC,
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"CC: TOR_WESTWOOD Circuit %d "
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"CWND: %"PRIu64", "
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"INFL: %"PRIu64", "
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"NCCE: %"PRIu64", "
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"WRTT: %"PRIu64", "
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"WSIG: %"PRIu64", "
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"SS: %d",
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CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
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cc->cwnd,
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cc->inflight,
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cc->next_cc_event,
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cc->ewma_rtt_usec/1000,
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westwood_rtt_signal(cc)/1000,
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cc->in_slow_start
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);
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} else {
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log_info(LD_CIRC,
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"CC: TOR_WESTWOOD Circuit %"PRIu64":%d "
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"CWND: %"PRIu64", "
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"INFL: %"PRIu64", "
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"NCCE: %"PRIu64", "
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"WRTT: %"PRIu64", "
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"WSIG: %"PRIu64", "
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"SS: %d",
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circ->n_chan->global_identifier, circ->n_circ_id,
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cc->cwnd,
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cc->inflight,
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cc->next_cc_event,
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cc->ewma_rtt_usec/1000,
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westwood_rtt_signal(cc)/1000,
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cc->in_slow_start
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);
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}
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}
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/* Update inflight with ack */
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cc->inflight = cc->inflight - cc->sendme_inc;
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return 0;
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}
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@@ -0,0 +1,33 @@
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/* Copyright (c) 2019-2021, The Tor Project, Inc. */
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/* See LICENSE for licensing information */
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/**
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* \file congestion_control_westwood.h
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* \brief Private-ish APIs for the TOR_WESTWOOD congestion control algorithm
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**/
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#ifndef TOR_CONGESTION_CONTROL_WESTWOOD_H
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#define TOR_CONGESTION_CONTROL_WESTWOOD_H
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#include "core/or/crypt_path_st.h"
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#include "core/or/circuit_st.h"
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/* Processing SENDME cell. */
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int congestion_control_westwood_process_sendme(struct congestion_control_t *cc,
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const circuit_t *circ,
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const crypt_path_t *layer_hint);
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void congestion_control_westwood_set_params(struct congestion_control_t *cc);
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/* Private section starts. */
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#ifdef TOR_CONGESTION_CONTROL_WESTWOOD_PRIVATE
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/*
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* Unit tests declaractions.
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*/
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#ifdef TOR_UNIT_TESTS
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#endif /* defined(TOR_UNIT_TESTS) */
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#endif /* defined(TOR_CONGESTION_CONTROL_WESTWOOD_PRIVATE) */
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#endif /* !defined(TOR_CONGESTION_CONTROL_WESTWOOD_H) */
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