--- a/src/event/ngx_event_quic.c
+++ b/src/event/ngx_event_quic.c
@@ -53,6 +53,14 @@ typedef struct {
 } ngx_quic_streams_t;
 
 
+typedef struct {
+    size_t                            in_flight;
+    size_t                            window;
+    size_t                            ssthresh;
+    ngx_msec_t                        recovery_start;
+} ngx_quic_congestion_t;
+
+
 /*
  * 12.3.  Packet Numbers
  *
@@ -103,6 +111,7 @@ struct ngx_quic_connection_s {
 #endif
 
     ngx_quic_streams_t                streams;
+    ngx_quic_congestion_t             congestion;
     ngx_uint_t                        max_data;
 
     uint64_t                          cur_streams;
@@ -171,6 +180,8 @@ static ngx_int_t ngx_quic_handle_ack_fra
     ngx_quic_header_t *pkt, ngx_quic_ack_frame_t *f);
 static ngx_int_t ngx_quic_handle_ack_frame_range(ngx_connection_t *c,
     ngx_quic_send_ctx_t *ctx, uint64_t min, uint64_t max);
+static void ngx_quic_handle_stream_ack(ngx_connection_t *c,
+    ngx_quic_frame_t *f);
 
 static ngx_int_t ngx_quic_handle_ordered_frame(ngx_connection_t *c,
     ngx_quic_frames_stream_t *fs, ngx_quic_frame_t *frame,
@@ -227,6 +238,10 @@ static ngx_chain_t *ngx_quic_stream_send
 static ngx_quic_frame_t *ngx_quic_alloc_frame(ngx_connection_t *c, size_t size);
 static void ngx_quic_free_frame(ngx_connection_t *c, ngx_quic_frame_t *frame);
 
+static void ngx_quic_congestion_ack(ngx_connection_t *c,
+    ngx_quic_frame_t *frame);
+static void ngx_quic_congestion_lost(ngx_connection_t *c, ngx_msec_t sent);
+
 
 static SSL_QUIC_METHOD quic_method = {
 #if BORINGSSL_API_VERSION >= 10
@@ -586,6 +601,11 @@ ngx_quic_new_connection(ngx_connection_t
 
     qc->streams.max_data = qc->tp.initial_max_data;
 
+    qc->congestion.window = ngx_min(10 * qc->tp.max_packet_size,
+                                    ngx_max(2 * qc->tp.max_packet_size, 14720));
+    qc->congestion.ssthresh = NGX_MAX_SIZE_T_VALUE;
+    qc->congestion.recovery_start = ngx_current_msec;
+
     qc->dcid.len = pkt->dcid.len;
     qc->dcid.data = ngx_pnalloc(c->pool, pkt->dcid.len);
     if (qc->dcid.data == NULL) {
@@ -1610,9 +1630,12 @@ static ngx_int_t
 ngx_quic_handle_ack_frame_range(ngx_connection_t *c, ngx_quic_send_ctx_t *ctx,
     uint64_t min, uint64_t max)
 {
-    ngx_uint_t         found;
-    ngx_queue_t       *q;
-    ngx_quic_frame_t  *f;
+    ngx_uint_t              found;
+    ngx_queue_t            *q;
+    ngx_quic_frame_t       *f;
+    ngx_quic_connection_t  *qc;
+
+    qc = c->quic;
 
     found = 0;
 
@@ -1623,6 +1646,10 @@ ngx_quic_handle_ack_frame_range(ngx_conn
         f = ngx_queue_data(q, ngx_quic_frame_t, queue);
 
         if (f->pnum >= min && f->pnum <= max) {
+            ngx_quic_congestion_ack(c, f);
+
+            ngx_quic_handle_stream_ack(c, f);
+
             q = ngx_queue_next(q);
             ngx_queue_remove(&f->queue);
             ngx_quic_free_frame(c, f);
@@ -1646,10 +1673,50 @@ ngx_quic_handle_ack_frame_range(ngx_conn
         return NGX_ERROR;
     }
 
+    if (!qc->push.timer_set) {
+        ngx_post_event(&qc->push, &ngx_posted_events);
+    }
+
     return NGX_OK;
 }
 
 
+static void
+ngx_quic_handle_stream_ack(ngx_connection_t *c, ngx_quic_frame_t *f)
+{
+    uint64_t                sent, unacked;
+    ngx_event_t            *wev;
+    ngx_quic_stream_t      *sn;
+    ngx_quic_connection_t  *qc;
+
+    if (f->type < NGX_QUIC_FT_STREAM0 || f->type > NGX_QUIC_FT_STREAM7) {
+        return;
+    }
+
+    qc = c->quic;
+
+    sn = ngx_quic_find_stream(&qc->streams.tree, f->u.stream.stream_id);
+    if (sn == NULL) {
+        return;
+    }
+
+    wev = sn->c->write;
+    sent = sn->c->sent;
+    unacked = sent - sn->acked;
+
+    if (unacked >= NGX_QUIC_STREAM_BUFSIZE && wev->active) {
+        wev->ready = 1;
+        ngx_post_event(wev, &ngx_posted_events);
+    }
+
+    sn->acked += f->u.stream.length;
+
+    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, sn->c->log, 0,
+                   "quic stream ack %uL acked:%uL, unacked:%uL",
+                   f->u.stream.length, sn->acked, sent - sn->acked);
+}
+
+
 static ngx_int_t
 ngx_quic_handle_ordered_frame(ngx_connection_t *c, ngx_quic_frames_stream_t *fs,
     ngx_quic_frame_t *frame, ngx_quic_frame_handler_pt handler)
@@ -2263,11 +2330,14 @@ ngx_quic_output_frames(ngx_connection_t 
 {
     size_t                  len, hlen, n;
     ngx_int_t               rc;
+    ngx_uint_t              need_ack;
     ngx_queue_t            *q, range;
     ngx_quic_frame_t       *f;
+    ngx_quic_congestion_t  *cg;
     ngx_quic_connection_t  *qc;
 
     qc = c->quic;
+    cg = &qc->congestion;
 
     if (ngx_queue_empty(&ctx->frames)) {
         return NGX_OK;
@@ -2283,6 +2353,7 @@ ngx_quic_output_frames(ngx_connection_t 
 
     do {
         len = 0;
+        need_ack = 0;
         ngx_queue_init(&range);
 
         do {
@@ -2295,6 +2366,14 @@ ngx_quic_output_frames(ngx_connection_t 
                 break;
             }
 
+            if (f->need_ack) {
+                need_ack = 1;
+            }
+
+            if (need_ack && cg->in_flight + len + n > cg->window) {
+                break;
+            }
+
             q = ngx_queue_next(q);
 
             f->first = ngx_current_msec;
@@ -2306,6 +2385,10 @@ ngx_quic_output_frames(ngx_connection_t 
 
         } while (q != ngx_queue_sentinel(&ctx->frames));
 
+        if (ngx_queue_empty(&range)) {
+            break;
+        }
+
         rc = ngx_quic_send_frames(c, &range);
 
         if (rc == NGX_OK) {
@@ -2321,6 +2404,11 @@ ngx_quic_output_frames(ngx_connection_t 
                 ngx_queue_add(&ctx->sent, &range);
             }
 
+            cg->in_flight += len;
+
+            ngx_log_debug1(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                           "quic congestion send if:%uz", cg->in_flight);
+
         } else if (rc == NGX_DONE) {
 
             /* no ack is expected for this frames, can free them */
@@ -2386,6 +2474,8 @@ ngx_quic_send_frames(ngx_connection_t *c
 
     ngx_memzero(&pkt, sizeof(ngx_quic_header_t));
 
+    now = ngx_current_msec;
+
     p = src;
     out.data = src;
 
@@ -2409,6 +2499,7 @@ ngx_quic_send_frames(ngx_connection_t *c
 
         p += len;
         f->pnum = ctx->pnum;
+        f->last = now;
     }
 
     if (start->level == ssl_encryption_initial) {
@@ -2476,9 +2567,6 @@ ngx_quic_send_frames(ngx_connection_t *c
     /* len == NGX_OK || NGX_AGAIN */
     ctx->pnum++;
 
-    now = ngx_current_msec;
-    start->last = now;
-
     return pkt.need_ack ? NGX_OK : NGX_DONE;
 }
 
@@ -2621,6 +2709,8 @@ ngx_quic_retransmit(ngx_connection_t *c,
 
         } while (q != ngx_queue_sentinel(&ctx->sent));
 
+        ngx_quic_congestion_lost(c, start->last);
+
         /* NGX_DONE is impossible here, such frames don't get into this queue */
         if (ngx_quic_send_frames(c, &range) != NGX_OK) {
             return NGX_ERROR;
@@ -2781,6 +2871,12 @@ ngx_quic_create_stream(ngx_connection_t 
 
     log->connection = sn->c->number;
 
+    if ((id & NGX_QUIC_STREAM_UNIDIRECTIONAL) == 0
+        || (id & NGX_QUIC_STREAM_SERVER_INITIATED))
+    {
+        sn->c->write->ready = 1;
+    }
+
     cln = ngx_pool_cleanup_add(pool, 0);
     if (cln == NULL) {
         ngx_close_connection(sn->c);
@@ -2899,7 +2995,8 @@ static ssize_t
 ngx_quic_stream_send(ngx_connection_t *c, u_char *buf, size_t size)
 {
     u_char                 *p, *end;
-    size_t                  fsize, limit;
+    size_t                  fsize, limit, n, len;
+    uint64_t                sent, unacked;
     ngx_connection_t       *pc;
     ngx_quic_frame_t       *frame;
     ngx_quic_stream_t      *qs;
@@ -2923,8 +3020,22 @@ ngx_quic_stream_send(ngx_connection_t *c
     limit = qc->ctp.max_packet_size - NGX_QUIC_MAX_SHORT_HEADER - 25
             - EVP_GCM_TLS_TAG_LEN;
 
+    len = size;
+    sent = c->sent;
+    unacked = sent - qs->acked;
+
+    if (unacked >= NGX_QUIC_STREAM_BUFSIZE) {
+        ngx_log_debug0(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic send hit buffer size");
+        len = 0;
+
+    } else if (unacked + len > NGX_QUIC_STREAM_BUFSIZE) {
+        len = NGX_QUIC_STREAM_BUFSIZE - unacked;
+    }
+
     p = (u_char *) buf;
-    end = (u_char *) buf + size;
+    end = (u_char *) buf + len;
+    n = 0;
 
     while (p < end) {
 
@@ -2951,6 +3062,7 @@ ngx_quic_stream_send(ngx_connection_t *c
 
         c->sent += fsize;
         p += fsize;
+        n += fsize;
 
         ngx_sprintf(frame->info, "stream 0x%xi len=%ui level=%d",
                     qs->id, fsize, frame->level);
@@ -2958,7 +3070,19 @@ ngx_quic_stream_send(ngx_connection_t *c
         ngx_quic_queue_frame(qc, frame);
     }
 
-    return size;
+    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                   "quic stream send %uz sent:%O, unacked:%uL",
+                   n, c->sent, (uint64_t) c->sent - qs->acked);
+
+    if (n != size) {
+        c->write->ready = 0;
+    }
+
+    if (n == 0) {
+        return NGX_AGAIN;
+    }
+
+    return n;
 }
 
 
@@ -3122,6 +3246,83 @@ ngx_quic_alloc_frame(ngx_connection_t *c
 
 
 static void
+ngx_quic_congestion_ack(ngx_connection_t *c, ngx_quic_frame_t *f)
+{
+    ssize_t                 n;
+    ngx_msec_t              timer;
+    ngx_quic_congestion_t  *cg;
+    ngx_quic_connection_t  *qc;
+
+    qc = c->quic;
+    cg = &qc->congestion;
+
+    n = ngx_quic_create_frame(NULL, f);
+
+    cg->in_flight -= n;
+
+    timer = f->last - cg->recovery_start;
+
+    if ((ngx_msec_int_t) timer <= 0) {
+        return;
+    }
+
+    if (cg->window < cg->ssthresh) {
+        cg->window += n;
+
+        ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic congestion slow start win:%uz, ss:%uz, if:%uz",
+                       cg->window, cg->ssthresh, cg->in_flight);
+
+    } else {
+        cg->window += qc->tp.max_packet_size * n / cg->window;
+
+        ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                       "quic congestion avoidance win:%uz, ss:%uz, if:%uz",
+                       cg->window, cg->ssthresh, cg->in_flight);
+    }
+
+    /* prevent recovery_start from wrapping */
+
+    timer = cg->recovery_start - ngx_current_msec + qc->tp.max_idle_timeout * 2;
+
+    if ((ngx_msec_int_t) timer < 0) {
+        cg->recovery_start = ngx_current_msec - qc->tp.max_idle_timeout * 2;
+    }
+}
+
+
+static void
+ngx_quic_congestion_lost(ngx_connection_t *c, ngx_msec_t sent)
+{
+    ngx_msec_t              timer;
+    ngx_quic_congestion_t  *cg;
+    ngx_quic_connection_t  *qc;
+
+    qc = c->quic;
+    cg = &qc->congestion;
+
+    timer = sent - cg->recovery_start;
+
+    if ((ngx_msec_int_t) timer <= 0) {
+        return;
+    }
+
+    cg->recovery_start = ngx_current_msec;
+    cg->window /= 2;
+
+    if (cg->window < qc->tp.max_packet_size * 2) {
+        cg->window = qc->tp.max_packet_size * 2;
+    }
+
+    cg->ssthresh = cg->window;
+
+    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, c->log, 0,
+                   "quic congestion lost win:%uz, ss:%uz, if:%uz",
+                   cg->window, cg->ssthresh, cg->in_flight);
+}
+
+
+static void
 ngx_quic_free_frame(ngx_connection_t *c, ngx_quic_frame_t *frame)
 {
     ngx_quic_connection_t  *qc;

--- a/src/event/ngx_event_quic.h
+++ b/src/event/ngx_event_quic.h
@@ -31,7 +31,7 @@
 #define NGX_QUIC_STREAM_SERVER_INITIATED     0x01
 #define NGX_QUIC_STREAM_UNIDIRECTIONAL       0x02
 
-#define NGX_QUIC_STREAM_BUFSIZE              16384
+#define NGX_QUIC_STREAM_BUFSIZE              65536
 
 
 typedef struct {
@@ -70,6 +70,7 @@ struct ngx_quic_stream_s {
     ngx_connection_t          *parent;
     ngx_connection_t          *c;
     uint64_t                   id;
+    uint64_t                   acked;
     ngx_buf_t                 *b;
     ngx_quic_frames_stream_t   fs;
 };