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a305bb92b9
This is a simple algorithm to replace the classic slow start phase of the congestion control algorithms. It should reduce the high packet loss during this step. Implemented only for Cubic.
130 lines
3.8 KiB
C
130 lines
3.8 KiB
C
/* RFC 9406: HyStart++: Modified Slow Start for TCP. */
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/* HyStart++ constants */
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#define HYSTART_MIN_RTT_THRESH 4U /* ms */
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#define HYSTART_MAX_RTT_THRESH 16U /* ms */
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#define HYSTART_MIN_RTT_DIVISOR 8
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#define HYSTART_N_RTT_SAMPLE 8
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#define HYSTART_CSS_GROWTH_DIVISOR 4
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#define HYSTART_CSS_ROUNDS 5
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#define HYSTART_LIMIT 8 /* Must be infinite if paced */
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#define QUIC_CLAMP(a, b, c) ({ \
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typeof(a) _a = (a); \
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typeof(b) _b = (b); \
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typeof(c) _c = (c); \
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(void) (&_a == &_b); \
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(void) (&_b == &_c); \
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_b < _a ? _a : _b > _c ? _c : _b; })
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struct quic_hystart {
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/* Current round minimum RTT. */
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uint32_t curr_rnd_min_rtt;
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/* Last round minimum RTT. */
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uint32_t last_rnd_min_rtt;
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/* Conservative Slow State baseline minimum RTT */
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uint32_t css_baseline_min_rtt;
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uint32_t rtt_sample_count;
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uint32_t css_rnd_count;
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uint64_t wnd_end;
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};
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/* Reset <h> Hystart++ algorithm state.
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* Never fail.
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*/
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static inline void quic_cc_hystart_reset(struct quic_hystart *h)
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{
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h->curr_rnd_min_rtt = UINT32_MAX;
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h->last_rnd_min_rtt = UINT32_MAX;
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h->css_baseline_min_rtt = UINT32_MAX;
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h->rtt_sample_count = 0;
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h->css_rnd_count = 0;
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h->wnd_end = UINT64_MAX;
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}
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/* Track the minimum RTT. */
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static inline void quic_cc_hystart_track_min_rtt(struct quic_cc *cc,
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struct quic_hystart *h,
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unsigned int latest_rtt)
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{
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if (h->wnd_end == UINT64_MAX)
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return;
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h->curr_rnd_min_rtt = QUIC_MIN(h->curr_rnd_min_rtt, latest_rtt);
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h->rtt_sample_count++;
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}
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/* RFC 9406 4.2. Algorithm Details
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* At the start of each round during standard slow start [RFC5681] and CSS,
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* initialize the variables used to compute the last round's and current round's
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* minimum RTT.
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*
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* Never fail.
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*/
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static inline void quic_cc_hystart_start_round(struct quic_hystart *h, uint64_t pn)
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{
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if (h->wnd_end != UINT64_MAX) {
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/* Round already started */
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return;
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}
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h->wnd_end = pn;
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h->last_rnd_min_rtt = h->curr_rnd_min_rtt;
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h->rtt_sample_count = 0;
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}
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/* RFC 9406 4.2. Algorithm Details
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* For rounds where at least N_RTT_SAMPLE RTT samples have been obtained and
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* currentRoundMinRTT and lastRoundMinRTT are valid, check to see if delay
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*increase triggers slow start exit.
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*
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* Depending on <h> HyStart++ algorithm state, returns 1 if the underlying
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* congestion control algorithm may enter the Conservative Slow Start (CSS)
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* state, 0 if not.
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*/
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static inline int quic_cc_hystart_may_enter_cs(struct quic_hystart *h)
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{
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uint32_t rtt_thresh;
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if (h->rtt_sample_count < HYSTART_N_RTT_SAMPLE ||
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h->curr_rnd_min_rtt == UINT32_MAX || h->last_rnd_min_rtt == UINT32_MAX)
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return 0;
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rtt_thresh = QUIC_CLAMP(HYSTART_MIN_RTT_THRESH,
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h->last_rnd_min_rtt / HYSTART_MIN_RTT_DIVISOR,
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HYSTART_MAX_RTT_THRESH);
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if (h->curr_rnd_min_rtt + rtt_thresh >= h->last_rnd_min_rtt) {
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h->css_baseline_min_rtt = h->curr_rnd_min_rtt;
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h->rtt_sample_count = 0;
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return 1;
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}
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return 0;
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}
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/* RFC 9406 4.2. Algorithm Details
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* For CSS rounds where at least N_RTT_SAMPLE RTT samples have been obtained,
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* check to see if the current round's minRTT drops below baseline (cssBaselineMinRtt)
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* indicating that slow start exit was spurious.
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*
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* Return 1 if slow start exit was spurious, 0 if not. If the slow start
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* exist was spurious, the caller must update the underlying congestion control
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* algorithm to make it re-enter slow start state.
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*/
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static inline int quic_cc_hystart_may_reenter_ss(struct quic_hystart *h)
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{
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if (h->rtt_sample_count < HYSTART_N_RTT_SAMPLE)
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return 0;
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h->css_rnd_count++;
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h->rtt_sample_count = 0;
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if (h->curr_rnd_min_rtt >= h->css_baseline_min_rtt) {
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return 0;
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}
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h->css_baseline_min_rtt = UINT32_MAX;
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return 1;
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}
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