[tor-commits] [tor/main] Prop#324: Stream flow control functions
dgoulet at torproject.org
dgoulet at torproject.org
Mon Oct 4 14:49:58 UTC 2021
commit a89a71cd7b4658eba8465f31b5e1bc21e3325a53
Author: Mike Perry <mikeperry-git at torproject.org>
Date: Tue Sep 28 22:28:26 2021 +0000
Prop#324: Stream flow control functions
---
src/core/or/congestion_control_flow.c | 697 ++++++++++++++++++++++++++++++++++
src/core/or/congestion_control_flow.h | 48 +++
2 files changed, 745 insertions(+)
diff --git a/src/core/or/congestion_control_flow.c b/src/core/or/congestion_control_flow.c
new file mode 100644
index 0000000000..6742bb38bb
--- /dev/null
+++ b/src/core/or/congestion_control_flow.c
@@ -0,0 +1,697 @@
+/* Copyright (c) 2019-2021, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file congestion_control_flow.c
+ * \brief Code that implements flow control for congestion controlled
+ * circuits.
+ */
+
+#define TOR_CONGESTION_CONTROL_FLOW_PRIVATE
+
+#include "core/or/or.h"
+
+#include "core/or/relay.h"
+#include "core/mainloop/connection.h"
+#include "core/or/connection_edge.h"
+#include "core/mainloop/mainloop.h"
+#include "core/or/congestion_control_common.h"
+#include "core/or/congestion_control_flow.h"
+#include "core/or/congestion_control_st.h"
+#include "core/or/circuitlist.h"
+#include "core/or/trace_probes_cc.h"
+#include "feature/nodelist/networkstatus.h"
+#include "trunnel/flow_control_cells.h"
+
+#include "core/or/connection_st.h"
+#include "core/or/cell_st.h"
+#include "app/config/config.h"
+
+/** Cache consensus parameters */
+static uint32_t xoff_client;
+static uint32_t xoff_exit;
+
+static uint32_t xon_change_pct;
+static uint32_t xon_rate_bytes;
+static uint32_t xon_ewma_cnt;
+
+/** In normal operation, we can get a burst of up to 32 cells before
+ * returning to libevent to flush the outbuf. This is a heuristic from
+ * hardcoded values and strange logic in connection_bucket_get_share(). */
+#define MAX_EXPECTED_CELL_BURST 32
+
+/**
+ * The following three are for dropmark rate limiting. They define when
+ * we scale down our XON, XOFF, and xmit byte counts. Early scaling
+ * is beneficial because it limits the ability of spurious XON/XOFF
+ * to be sent after large amounts of data without XON/XOFF. At these
+ * limits, after 10MB of data (or more), an adversary can only inject
+ * (log2(10MB)-log2(200*500))*100 ~= 1000 cells of fake XOFF/XON before
+ * the xmit byte * count will be halved enough to triggering a limit. */
+#define XON_COUNT_SCALE_AT 200
+#define XOFF_COUNT_SCALE_AT 200
+#define ONE_MEGABYTE (UINT64_C(1) << 20)
+#define TOTAL_XMIT_SCALE_AT 10*ONE_MEGABYTE
+
+static const congestion_control_t *
+edge_get_ccontrol(const edge_connection_t *edge)
+{
+ if (edge->cpath_layer)
+ return edge->cpath_layer->ccontrol;
+ else if (edge->on_circuit)
+ return edge->on_circuit->ccontrol;
+ else
+ return NULL;
+}
+
+void
+flow_control_new_consensus_params(const networkstatus_t *ns)
+{
+#define CC_XOFF_CLIENT_DFLT 500
+#define CC_XOFF_CLIENT_MIN 1
+#define CC_XOFF_CLIENT_MAX 10000
+ xoff_client = networkstatus_get_param(ns, "cc_xoff_client",
+ CC_XOFF_CLIENT_DFLT,
+ CC_XOFF_CLIENT_MIN,
+ CC_XOFF_CLIENT_MAX)*RELAY_PAYLOAD_SIZE;
+
+#define CC_XOFF_EXIT_DFLT 500
+#define CC_XOFF_EXIT_MIN 1
+#define CC_XOFF_EXIT_MAX 10000
+ xoff_exit = networkstatus_get_param(ns, "cc_xoff_exit",
+ CC_XOFF_EXIT_DFLT,
+ CC_XOFF_EXIT_MIN,
+ CC_XOFF_EXIT_MAX)*RELAY_PAYLOAD_SIZE;
+
+#define CC_XON_CHANGE_PCT_DFLT 25
+#define CC_XON_CHANGE_PCT_MIN 1
+#define CC_XON_CHANGE_PCT_MAX 99
+ xon_change_pct = networkstatus_get_param(ns, "cc_xon_change_pct",
+ CC_XON_CHANGE_PCT_DFLT,
+ CC_XON_CHANGE_PCT_MIN,
+ CC_XON_CHANGE_PCT_MAX);
+
+#define CC_XON_RATE_BYTES_DFLT (500)
+#define CC_XON_RATE_BYTES_MIN (1)
+#define CC_XON_RATE_BYTES_MAX (5000)
+ xon_rate_bytes = networkstatus_get_param(ns, "cc_xon_rate",
+ CC_XON_RATE_BYTES_DFLT,
+ CC_XON_RATE_BYTES_MIN,
+ CC_XON_RATE_BYTES_MAX)*RELAY_PAYLOAD_SIZE;
+
+#define CC_XON_EWMA_CNT_DFLT (2)
+#define CC_XON_EWMA_CNT_MIN (1)
+#define CC_XON_EWMA_CNT_MAX (100)
+ xon_ewma_cnt = networkstatus_get_param(ns, "cc_xon_ewma_cnt",
+ CC_XON_EWMA_CNT_DFLT,
+ CC_XON_EWMA_CNT_MIN,
+ CC_XON_EWMA_CNT_MAX);
+}
+
+/**
+ * Send an XOFF for this stream, and note that we sent one
+ */
+static void
+circuit_send_stream_xoff(edge_connection_t *stream)
+{
+ xoff_cell_t xoff;
+ uint8_t payload[CELL_PAYLOAD_SIZE];
+ ssize_t xoff_size;
+
+ memset(&xoff, 0, sizeof(xoff));
+ memset(payload, 0, sizeof(payload));
+
+ xoff_cell_set_version(&xoff, 0);
+
+ if ((xoff_size = xoff_cell_encode(payload, CELL_PAYLOAD_SIZE, &xoff)) < 0) {
+ log_warn(LD_BUG, "Failed to encode xon cell");
+ return;
+ }
+
+ if (connection_edge_send_command(stream, RELAY_COMMAND_XOFF,
+ (char*)payload, (size_t)xoff_size) == 0) {
+ stream->xoff_sent = true;
+ }
+}
+
+/**
+ * Compute the recent drain rate (write rate) for this edge
+ * connection and return it, in KB/sec (1000 bytes/sec).
+ *
+ * Returns 0 if the monotime clock is busted.
+ */
+static inline uint32_t
+compute_drain_rate(const edge_connection_t *stream)
+{
+ if (BUG(!is_monotime_clock_reliable())) {
+ log_warn(LD_BUG, "Computing drain rate with stalled monotime clock");
+ return 0;
+ }
+
+ uint64_t delta = monotime_absolute_usec() - stream->drain_start_usec;
+
+ if (delta == 0) {
+ log_warn(LD_BUG, "Computing stream drain rate with zero time delta");
+ return 0;
+ }
+
+ /* Overflow checks */
+ if (stream->prev_drained_bytes > INT32_MAX/1000 || /* Intermediate */
+ stream->prev_drained_bytes/delta > INT32_MAX/1000) { /* full value */
+ return INT32_MAX;
+ }
+
+ /* kb/sec = bytes/usec * 1000 usec/msec * 1000 msec/sec * kb/1000bytes */
+ return MAX(1, (uint32_t)(stream->prev_drained_bytes * 1000)/delta);
+}
+
+/**
+ * Send an XON for this stream, with appropriate advisory rate information.
+ *
+ * Reverts the xoff sent status, and stores the rate information we sent,
+ * in case it changes.
+ */
+static void
+circuit_send_stream_xon(edge_connection_t *stream)
+{
+ xon_cell_t xon;
+ uint8_t payload[CELL_PAYLOAD_SIZE];
+ ssize_t xon_size;
+
+ memset(&xon, 0, sizeof(xon));
+ memset(payload, 0, sizeof(payload));
+
+ xon_cell_set_version(&xon, 0);
+ xon_cell_set_kbps_ewma(&xon, stream->ewma_drain_rate);
+
+ if ((xon_size = xon_cell_encode(payload, CELL_PAYLOAD_SIZE, &xon)) < 0) {
+ log_warn(LD_BUG, "Failed to encode xon cell");
+ return;
+ }
+
+ /* Store the advisory rate information, to send advisory updates if
+ * it changes */
+ stream->ewma_rate_last_sent = stream->ewma_drain_rate;
+
+ if (connection_edge_send_command(stream, RELAY_COMMAND_XON, (char*)payload,
+ (size_t)xon_size) == 0) {
+ /* Revert the xoff sent status, so we can send another one if need be */
+ stream->xoff_sent = false;
+ }
+}
+
+/**
+ * Process a stream XOFF, parsing it, and then stopping reading on
+ * the edge connection.
+ *
+ * Record that we have recieved an xoff, so we know not to resume
+ * reading on this edge conn until we get an XON.
+ *
+ * Returns false if the XOFF did not validate; true if it does.
+ */
+bool
+circuit_process_stream_xoff(edge_connection_t *conn,
+ const crypt_path_t *layer_hint,
+ const cell_t *cell)
+{
+ (void)cell;
+ bool retval = true;
+
+ if (BUG(!conn)) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XOFF on invalid stream?");
+ return false;
+ }
+
+ /* Make sure this XOFF came from the right hop */
+ if (layer_hint && layer_hint != conn->cpath_layer) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XOFF from wrong hop.");
+ return false;
+ }
+
+ if (edge_get_ccontrol(conn) == NULL) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XOFF for non-congestion control circuit");
+ return false;
+ }
+
+ if (conn->xoff_received) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got multiple XOFF on connection");
+ return false;
+ }
+
+ /* If we are near the max, scale everything down */
+ if (conn->num_xoff_recv == XOFF_COUNT_SCALE_AT) {
+ log_info(LD_EDGE, "Scaling down for XOFF count: %d %d %d",
+ conn->total_bytes_xmit,
+ conn->num_xoff_recv,
+ conn->num_xon_recv);
+ conn->total_bytes_xmit /= 2;
+ conn->num_xoff_recv /= 2;
+ conn->num_xon_recv /= 2;
+ }
+
+ conn->num_xoff_recv++;
+
+ /* Client-side check to make sure that XOFF is not sent too early,
+ * for dropmark attacks. The main sidechannel risk is early cells,
+ * but we also check to make sure that we have not received more XOFFs
+ * than could have been generated by the bytes we sent.
+ */
+ if (TO_CONN(conn)->type == CONN_TYPE_AP || conn->hs_ident != NULL) {
+ uint32_t limit = 0;
+
+ /* TODO: This limit technically needs to come from negotiation,
+ * and be bounds checked for sanity, because the other endpoint
+ * may have a different consensus */
+ if (conn->hs_ident)
+ limit = xoff_client;
+ else
+ limit = xoff_exit;
+
+ if (conn->total_bytes_xmit < limit*conn->num_xoff_recv) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got extra XOFF for bytes sent. Got %d, expected max %d",
+ conn->num_xoff_recv, conn->total_bytes_xmit/limit);
+ /* We still process this, because the only dropmark defenses
+ * in C tor are via the vanguards addon's use of the read valid
+ * cells. So just signal that we think this is not valid protocol
+ * data and proceed. */
+ retval = false;
+ }
+ }
+
+ // TODO: Count how many xoffs we have; log if "too many", for shadow
+ // analysis of chatter. Possibly add to extra-info?
+
+ log_info(LD_EDGE, "Got XOFF!");
+ connection_stop_reading(TO_CONN(conn));
+ conn->xoff_received = true;
+
+ return retval;
+}
+
+/**
+ * Process a stream XON, and if it validates, clear the xoff
+ * flag and resume reading on this edge connection.
+ *
+ * Also, use provided rate information to rate limit
+ * reading on this edge (or packagaing from it onto
+ * the circuit), to avoid XON/XOFF chatter.
+ *
+ * Returns true if the XON validates, false otherwise.
+ */
+bool
+circuit_process_stream_xon(edge_connection_t *conn,
+ const crypt_path_t *layer_hint,
+ const cell_t *cell)
+{
+ xon_cell_t *xon;
+ bool retval = true;
+
+ if (BUG(!conn)) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XON on invalid stream?");
+ return false;
+ }
+
+ /* Make sure this XON came from the right hop */
+ if (layer_hint && layer_hint != conn->cpath_layer) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XON from wrong hop.");
+ return false;
+ }
+
+ if (edge_get_ccontrol(conn) == NULL) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got XON for non-congestion control circuit");
+ return false;
+ }
+
+ if (xon_cell_parse(&xon, cell->payload+RELAY_HEADER_SIZE,
+ CELL_PAYLOAD_SIZE-RELAY_HEADER_SIZE) < 0) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Received malformed XON cell.");
+ return false;
+ }
+
+ /* If we are near the max, scale everything down */
+ if (conn->num_xon_recv == XON_COUNT_SCALE_AT) {
+ log_info(LD_EDGE, "Scaling down for XON count: %d %d %d",
+ conn->total_bytes_xmit,
+ conn->num_xoff_recv,
+ conn->num_xon_recv);
+ conn->total_bytes_xmit /= 2;
+ conn->num_xoff_recv /= 2;
+ conn->num_xon_recv /= 2;
+ }
+
+ conn->num_xon_recv++;
+
+ /* Client-side check to make sure that XON is not sent too early,
+ * for dropmark attacks. The main sidechannel risk is early cells,
+ * but we also check to see that we did not get more XONs than make
+ * sense for the number of bytes we sent.
+ */
+ if (TO_CONN(conn)->type == CONN_TYPE_AP || conn->hs_ident != NULL) {
+ uint32_t limit = 0;
+
+ /* TODO: This limit technically needs to come from negotiation,
+ * and be bounds checked for sanity, because the other endpoint
+ * may have a different consensus */
+ if (conn->hs_ident)
+ limit = MIN(xoff_client, xon_rate_bytes);
+ else
+ limit = MIN(xoff_exit, xon_rate_bytes);
+
+ if (conn->total_bytes_xmit < limit*conn->num_xon_recv) {
+ log_fn(LOG_PROTOCOL_WARN, LD_EDGE,
+ "Got extra XON for bytes sent. Got %d, expected max %d",
+ conn->num_xon_recv, conn->total_bytes_xmit/limit);
+
+ /* We still process this, because the only dropmark defenses
+ * in C tor are via the vanguards addon's use of the read valid
+ * cells. So just signal that we think this is not valid protocol
+ * data and proceed. */
+ retval = false;
+ }
+ }
+
+ log_info(LD_EDGE, "Got XON: %d", xon->kbps_ewma);
+
+ /* Adjust the token bucket of this edge connection with the drain rate in
+ * the XON. Rate is in bytes from kilobit (kpbs). */
+ uint64_t rate = xon_cell_get_kbps_ewma(xon) * 1000;
+ if (rate == 0 || INT32_MAX < rate) {
+ /* No rate. */
+ rate = INT32_MAX;
+ }
+ token_bucket_rw_adjust(&conn->bucket, (uint32_t) rate, (uint32_t) rate);
+
+ if (conn->xoff_received) {
+ /* Clear the fact that we got an XOFF, so that this edge can
+ * start and stop reading normally */
+ conn->xoff_received = false;
+ connection_start_reading(TO_CONN(conn));
+ }
+
+ xon_cell_free(xon);
+
+ return retval;
+}
+
+/**
+ * Called from sendme_stream_data_received(), when data arrives
+ * from a circuit to our edge's outbuf, to decide if we need to send
+ * an XOFF.
+ *
+ * Returns the amount of cells remaining until the buffer is full, at
+ * which point it sends an XOFF, and returns 0.
+ *
+ * Returns less than 0 if we have queued more than a congestion window
+ * worth of data and need to close the circuit.
+ */
+int
+flow_control_decide_xoff(edge_connection_t *stream)
+{
+ size_t total_buffered = connection_get_outbuf_len(TO_CONN(stream));
+ uint32_t buffer_limit_xoff = 0;
+
+ if (BUG(edge_get_ccontrol(stream) == NULL)) {
+ log_err(LD_BUG, "Flow control called for non-congestion control circuit");
+ return -1;
+ }
+
+ /* Onion services and clients are typically localhost edges, so they
+ * need different buffering limits than exits do */
+ if (TO_CONN(stream)->type == CONN_TYPE_AP || stream->hs_ident != NULL) {
+ buffer_limit_xoff = xoff_client;
+ } else {
+ buffer_limit_xoff = xoff_exit;
+ }
+
+ if (total_buffered > buffer_limit_xoff) {
+ if (!stream->xoff_sent) {
+ log_info(LD_EDGE, "Sending XOFF: %ld %d",
+ total_buffered, buffer_limit_xoff);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xoff_sending), stream);
+
+ circuit_send_stream_xoff(stream);
+
+ /* Clear the drain rate. It is considered wrong if we
+ * got all the way to XOFF */
+ stream->ewma_drain_rate = 0;
+ }
+ }
+
+ /* If the outbuf has accumulated more than the expected burst limit of
+ * cells, then assume it is not draining, and call decide_xon. We must
+ * do this because writes only happen when the socket unblocks, so
+ * may not otherwise notice accumulation of data in the outbuf for
+ * advisory XONs. */
+ if (total_buffered > MAX_EXPECTED_CELL_BURST*RELAY_PAYLOAD_SIZE) {
+ flow_control_decide_xon(stream, 0);
+ }
+
+ /* Flow control always takes more data; we rely on the oomkiller to
+ * handle misbehavior. */
+ return 0;
+}
+
+/**
+ * Returns true if the stream's drain rate has changed significantly.
+ *
+ * Returns false if the monotime clock is stalled, or if we have
+ * no previous drain rate information.
+ */
+static bool
+stream_drain_rate_changed(const edge_connection_t *stream)
+{
+ if (!is_monotime_clock_reliable()) {
+ return false;
+ }
+
+ if (!stream->ewma_rate_last_sent) {
+ return false;
+ }
+
+ if (stream->ewma_drain_rate >
+ (100+(uint64_t)xon_change_pct)*stream->ewma_rate_last_sent/100) {
+ return true;
+ }
+
+ if (stream->ewma_drain_rate <
+ (100-(uint64_t)xon_change_pct)*stream->ewma_rate_last_sent/100) {
+ return true;
+ }
+
+ return false;
+}
+
+/**
+ * Called whenever we drain an edge connection outbuf by writing on
+ * its socket, to decide if it is time to send an xon.
+ *
+ * The n_written parameter tells us how many bytes we have written
+ * this time, which is used to compute the advisory drain rate fields.
+ */
+void
+flow_control_decide_xon(edge_connection_t *stream, size_t n_written)
+{
+ size_t total_buffered = connection_get_outbuf_len(TO_CONN(stream));
+
+ /* Bounds check the number of drained bytes, and scale */
+ if (stream->drained_bytes >= UINT32_MAX - n_written) {
+ /* Cut the bytes in half, and move the start time up halfway to now
+ * (if we have one). */
+ stream->drained_bytes /= 2;
+
+ if (stream->drain_start_usec) {
+ uint64_t now = monotime_absolute_usec();
+
+ stream->drain_start_usec = now - (now-stream->drain_start_usec)/2;
+ }
+ }
+
+ /* Accumulate drained bytes since last rate computation */
+ stream->drained_bytes += n_written;
+
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon), stream, n_written);
+
+ /* Check for bad monotime clock and bytecount wrap */
+ if (!is_monotime_clock_reliable()) {
+ /* If the monotime clock ever goes wrong, the safest thing to do
+ * is just clear our short-term rate info and wait for the clock to
+ * become reliable again.. */
+ stream->drain_start_usec = 0;
+ stream->drained_bytes = 0;
+ } else {
+ /* If we have no drain start timestamp, and we still have
+ * remaining buffer, start the buffering counter */
+ if (!stream->drain_start_usec && total_buffered > 0) {
+ log_debug(LD_EDGE, "Began edge buffering: %d %d %ld",
+ stream->ewma_rate_last_sent,
+ stream->ewma_drain_rate,
+ total_buffered);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon_drain_start),
+ stream);
+ stream->drain_start_usec = monotime_absolute_usec();
+ stream->drained_bytes = 0;
+ }
+ }
+
+ if (stream->drain_start_usec) {
+ /* If we have spent enough time in a queued state, update our drain
+ * rate. */
+ if (stream->drained_bytes > xon_rate_bytes) {
+ /* No previous drained bytes means it is the first time we are computing
+ * it so use the value we just drained onto the socket as a baseline. It
+ * won't be accurate but it will be a start towards the right value.
+ *
+ * We have to do this in order to have a drain rate else we could be
+ * sending a drain rate of 0 in an XON which would be undesirable and
+ * basically like sending an XOFF. */
+ if (stream->prev_drained_bytes == 0) {
+ stream->prev_drained_bytes = stream->drained_bytes;
+ }
+ uint32_t drain_rate = compute_drain_rate(stream);
+ /* Once the drain rate has been computed, note how many bytes we just
+ * drained so it can be used at the next calculation. We do this here
+ * because it gets reset once the rate is changed. */
+ stream->prev_drained_bytes = stream->drained_bytes;
+
+ if (drain_rate) {
+ stream->ewma_drain_rate =
+ (uint32_t)n_count_ewma(drain_rate,
+ stream->ewma_drain_rate,
+ xon_ewma_cnt);
+ log_debug(LD_EDGE, "Updating drain rate: %d %d %ld",
+ drain_rate,
+ stream->ewma_drain_rate,
+ total_buffered);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon_drain_update),
+ stream, drain_rate);
+ /* Reset recent byte counts. This prevents us from sending advisory
+ * XONs more frequent than every xon_rate_bytes. */
+ stream->drained_bytes = 0;
+ stream->drain_start_usec = 0;
+ }
+ }
+ }
+
+ /* If we don't have an XOFF outstanding, consider updating an
+ * old rate */
+ if (!stream->xoff_sent) {
+ if (stream_drain_rate_changed(stream)) {
+ /* If we are still buffering and the rate changed, update
+ * advisory XON */
+ log_info(LD_EDGE, "Sending rate-change XON: %d %d %ld",
+ stream->ewma_rate_last_sent,
+ stream->ewma_drain_rate,
+ total_buffered);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon_rate_change), stream);
+ circuit_send_stream_xon(stream);
+ }
+ } else if (total_buffered == 0) {
+ log_info(LD_EDGE, "Sending XON: %d %d %ld",
+ stream->ewma_rate_last_sent,
+ stream->ewma_drain_rate,
+ total_buffered);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon_partial_drain), stream);
+ circuit_send_stream_xon(stream);
+ }
+
+ /* If the buffer has fully emptied, clear the drain timestamp,
+ * so we can total only bytes drained while outbuf is 0. */
+ if (total_buffered == 0) {
+ stream->drain_start_usec = 0;
+
+ /* After we've spent 'xon_rate_bytes' with the queue fully drained,
+ * double any rate we sent. */
+ if (stream->drained_bytes >= xon_rate_bytes &&
+ stream->ewma_rate_last_sent) {
+ stream->ewma_drain_rate = MIN(INT32_MAX, 2*stream->ewma_drain_rate);
+
+ log_debug(LD_EDGE,
+ "Queue empty for xon_rate_limit bytes: %d %d",
+ stream->ewma_rate_last_sent,
+ stream->ewma_drain_rate);
+ tor_trace(TR_SUBSYS(cc), TR_EV(flow_decide_xon_drain_doubled), stream);
+ /* Resetting the drained bytes count. We need to keep its value as a
+ * previous so the drain rate calculation takes into account what was
+ * actually drain the last time. */
+ stream->prev_drained_bytes = stream->drained_bytes;
+ stream->drained_bytes = 0;
+ }
+ }
+
+ return;
+}
+
+/**
+ * Note that we packaged some data on this stream. Used to enforce
+ * client-side dropmark limits
+ */
+void
+flow_control_note_sent_data(edge_connection_t *stream, size_t len)
+{
+ /* If we are near the max, scale everything down */
+ if (stream->total_bytes_xmit >= TOTAL_XMIT_SCALE_AT-len) {
+ log_info(LD_EDGE, "Scaling down for flow control xmit bytes:: %d %d %d",
+ stream->total_bytes_xmit,
+ stream->num_xoff_recv,
+ stream->num_xon_recv);
+
+ stream->total_bytes_xmit /= 2;
+ stream->num_xoff_recv /= 2;
+ stream->num_xon_recv /= 2;
+ }
+
+ stream->total_bytes_xmit += len;
+}
+
+/** Returns true if an edge connection uses flow control */
+bool
+edge_uses_flow_control(const edge_connection_t *stream)
+{
+ bool ret = (stream->on_circuit && stream->on_circuit->ccontrol) ||
+ (stream->cpath_layer && stream->cpath_layer->ccontrol);
+
+ /* All circuits with congestion control use flow control */
+ return ret;
+}
+
+/**
+ * Returns the max RTT for the circuit that carries this stream,
+ * as observed by congestion control.
+ */
+uint64_t
+edge_get_max_rtt(const edge_connection_t *stream)
+{
+ if (stream->on_circuit && stream->on_circuit->ccontrol)
+ return stream->on_circuit->ccontrol->max_rtt_usec;
+ else if (stream->cpath_layer && stream->cpath_layer->ccontrol)
+ return stream->cpath_layer->ccontrol->max_rtt_usec;
+
+ return 0;
+}
+
+/** Returns true if a connection is an edge conn that uses flow control */
+bool
+conn_uses_flow_control(connection_t *conn)
+{
+ bool ret = false;
+
+ if (CONN_IS_EDGE(conn)) {
+ edge_connection_t *edge = TO_EDGE_CONN(conn);
+
+ if (edge_uses_flow_control(edge)) {
+ ret = true;
+ }
+ }
+
+ return ret;
+}
+
diff --git a/src/core/or/congestion_control_flow.h b/src/core/or/congestion_control_flow.h
new file mode 100644
index 0000000000..6c318027ea
--- /dev/null
+++ b/src/core/or/congestion_control_flow.h
@@ -0,0 +1,48 @@
+/* Copyright (c) 2019-2021, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file congestion_control_flow.h
+ * \brief APIs for stream flow control on congestion controlled circuits.
+ **/
+
+#ifndef TOR_CONGESTION_CONTROL_FLOW_H
+#define TOR_CONGESTION_CONTROL_FLOW_H
+
+#include "core/or/crypt_path_st.h"
+#include "core/or/circuit_st.h"
+#include "core/or/edge_connection_st.h"
+
+void flow_control_new_consensus_params(const struct networkstatus_t *);
+
+bool circuit_process_stream_xoff(edge_connection_t *conn,
+ const crypt_path_t *layer_hint,
+ const cell_t *cell);
+bool circuit_process_stream_xon(edge_connection_t *conn,
+ const crypt_path_t *layer_hint,
+ const cell_t *cell);
+
+int flow_control_decide_xoff(edge_connection_t *stream);
+void flow_control_decide_xon(edge_connection_t *stream, size_t n_written);
+
+void flow_control_note_sent_data(edge_connection_t *stream, size_t len);
+
+bool edge_uses_flow_control(const edge_connection_t *stream);
+
+bool conn_uses_flow_control(connection_t *stream);
+
+uint64_t edge_get_max_rtt(const edge_connection_t *);
+
+/* Private section starts. */
+#ifdef TOR_CONGESTION_CONTROL_FLOW_PRIVATE
+
+/*
+ * Unit tests declaractions.
+ */
+#ifdef TOR_UNIT_TESTS
+
+#endif /* defined(TOR_UNIT_TESTS) */
+
+#endif /* defined(TOR_CONGESTION_CONTROL_FLOW_PRIVATE) */
+
+#endif /* !defined(TOR_CONGESTION_CONTROL_FLOW_H) */
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