diff --git a/src/trunnel/flow_control_cells.c b/src/trunnel/flow_control_cells.c
new file mode 100644
index 0000000000..df44756d6b
--- /dev/null
+++ b/src/trunnel/flow_control_cells.c
@@ -0,0 +1,382 @@
+/* flow_control_cells.c -- generated by Trunnel v1.5.3.
+ * https://gitweb.torproject.org/trunnel.git
+ * You probably shouldn't edit this file.
+ */
+#include <stdlib.h>
+#include "trunnel-impl.h"
+
+#include "flow_control_cells.h"
+
+#define TRUNNEL_SET_ERROR_CODE(obj) \
+  do {                              \
+    (obj)->trunnel_error_code_ = 1; \
+  } while (0)
+
+#if defined(__COVERITY__) || defined(__clang_analyzer__)
+/* If we're running a static analysis tool, we don't want it to complain
+ * that some of our remaining-bytes checks are dead-code. */
+int flowcontrolcells_deadcode_dummy__ = 0;
+#define OR_DEADCODE_DUMMY || flowcontrolcells_deadcode_dummy__
+#else
+#define OR_DEADCODE_DUMMY
+#endif
+
+#define CHECK_REMAINING(nbytes, label)                           \
+  do {                                                           \
+    if (remaining < (nbytes) OR_DEADCODE_DUMMY) {                \
+      goto label;                                                \
+    }                                                            \
+  } while (0)
+
+xoff_cell_t *
+xoff_cell_new(void)
+{
+  xoff_cell_t *val = trunnel_calloc(1, sizeof(xoff_cell_t));
+  if (NULL == val)
+    return NULL;
+  return val;
+}
+
+/** Release all storage held inside 'obj', but do not free 'obj'.
+ */
+static void
+xoff_cell_clear(xoff_cell_t *obj)
+{
+  (void) obj;
+}
+
+void
+xoff_cell_free(xoff_cell_t *obj)
+{
+  if (obj == NULL)
+    return;
+  xoff_cell_clear(obj);
+  trunnel_memwipe(obj, sizeof(xoff_cell_t));
+  trunnel_free_(obj);
+}
+
+uint8_t
+xoff_cell_get_version(const xoff_cell_t *inp)
+{
+  return inp->version;
+}
+int
+xoff_cell_set_version(xoff_cell_t *inp, uint8_t val)
+{
+  if (! ((val == 0))) {
+     TRUNNEL_SET_ERROR_CODE(inp);
+     return -1;
+  }
+  inp->version = val;
+  return 0;
+}
+const char *
+xoff_cell_check(const xoff_cell_t *obj)
+{
+  if (obj == NULL)
+    return "Object was NULL";
+  if (obj->trunnel_error_code_)
+    return "A set function failed on this object";
+  if (! (obj->version == 0))
+    return "Integer out of bounds";
+  return NULL;
+}
+
+ssize_t
+xoff_cell_encoded_len(const xoff_cell_t *obj)
+{
+  ssize_t result = 0;
+
+  if (NULL != xoff_cell_check(obj))
+     return -1;
+
+
+  /* Length of u8 version IN [0] */
+  result += 1;
+  return result;
+}
+int
+xoff_cell_clear_errors(xoff_cell_t *obj)
+{
+  int r = obj->trunnel_error_code_;
+  obj->trunnel_error_code_ = 0;
+  return r;
+}
+ssize_t
+xoff_cell_encode(uint8_t *output, const size_t avail, const xoff_cell_t *obj)
+{
+  ssize_t result = 0;
+  size_t written = 0;
+  uint8_t *ptr = output;
+  const char *msg;
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  const ssize_t encoded_len = xoff_cell_encoded_len(obj);
+#endif
+
+  if (NULL != (msg = xoff_cell_check(obj)))
+    goto check_failed;
+
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  trunnel_assert(encoded_len >= 0);
+#endif
+
+  /* Encode u8 version IN [0] */
+  trunnel_assert(written <= avail);
+  if (avail - written < 1)
+    goto truncated;
+  trunnel_set_uint8(ptr, (obj->version));
+  written += 1; ptr += 1;
+
+
+  trunnel_assert(ptr == output + written);
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  {
+    trunnel_assert(encoded_len >= 0);
+    trunnel_assert((size_t)encoded_len == written);
+  }
+
+#endif
+
+  return written;
+
+ truncated:
+  result = -2;
+  goto fail;
+ check_failed:
+  (void)msg;
+  result = -1;
+  goto fail;
+ fail:
+  trunnel_assert(result < 0);
+  return result;
+}
+
+/** As xoff_cell_parse(), but do not allocate the output object.
+ */
+static ssize_t
+xoff_cell_parse_into(xoff_cell_t *obj, const uint8_t *input, const size_t len_in)
+{
+  const uint8_t *ptr = input;
+  size_t remaining = len_in;
+  ssize_t result = 0;
+  (void)result;
+
+  /* Parse u8 version IN [0] */
+  CHECK_REMAINING(1, truncated);
+  obj->version = (trunnel_get_uint8(ptr));
+  remaining -= 1; ptr += 1;
+  if (! (obj->version == 0))
+    goto fail;
+  trunnel_assert(ptr + remaining == input + len_in);
+  return len_in - remaining;
+
+ truncated:
+  return -2;
+ fail:
+  result = -1;
+  return result;
+}
+
+ssize_t
+xoff_cell_parse(xoff_cell_t **output, const uint8_t *input, const size_t len_in)
+{
+  ssize_t result;
+  *output = xoff_cell_new();
+  if (NULL == *output)
+    return -1;
+  result = xoff_cell_parse_into(*output, input, len_in);
+  if (result < 0) {
+    xoff_cell_free(*output);
+    *output = NULL;
+  }
+  return result;
+}
+xon_cell_t *
+xon_cell_new(void)
+{
+  xon_cell_t *val = trunnel_calloc(1, sizeof(xon_cell_t));
+  if (NULL == val)
+    return NULL;
+  return val;
+}
+
+/** Release all storage held inside 'obj', but do not free 'obj'.
+ */
+static void
+xon_cell_clear(xon_cell_t *obj)
+{
+  (void) obj;
+}
+
+void
+xon_cell_free(xon_cell_t *obj)
+{
+  if (obj == NULL)
+    return;
+  xon_cell_clear(obj);
+  trunnel_memwipe(obj, sizeof(xon_cell_t));
+  trunnel_free_(obj);
+}
+
+uint8_t
+xon_cell_get_version(const xon_cell_t *inp)
+{
+  return inp->version;
+}
+int
+xon_cell_set_version(xon_cell_t *inp, uint8_t val)
+{
+  if (! ((val == 0))) {
+     TRUNNEL_SET_ERROR_CODE(inp);
+     return -1;
+  }
+  inp->version = val;
+  return 0;
+}
+uint32_t
+xon_cell_get_kbps_ewma(const xon_cell_t *inp)
+{
+  return inp->kbps_ewma;
+}
+int
+xon_cell_set_kbps_ewma(xon_cell_t *inp, uint32_t val)
+{
+  inp->kbps_ewma = val;
+  return 0;
+}
+const char *
+xon_cell_check(const xon_cell_t *obj)
+{
+  if (obj == NULL)
+    return "Object was NULL";
+  if (obj->trunnel_error_code_)
+    return "A set function failed on this object";
+  if (! (obj->version == 0))
+    return "Integer out of bounds";
+  return NULL;
+}
+
+ssize_t
+xon_cell_encoded_len(const xon_cell_t *obj)
+{
+  ssize_t result = 0;
+
+  if (NULL != xon_cell_check(obj))
+     return -1;
+
+
+  /* Length of u8 version IN [0] */
+  result += 1;
+
+  /* Length of u32 kbps_ewma */
+  result += 4;
+  return result;
+}
+int
+xon_cell_clear_errors(xon_cell_t *obj)
+{
+  int r = obj->trunnel_error_code_;
+  obj->trunnel_error_code_ = 0;
+  return r;
+}
+ssize_t
+xon_cell_encode(uint8_t *output, const size_t avail, const xon_cell_t *obj)
+{
+  ssize_t result = 0;
+  size_t written = 0;
+  uint8_t *ptr = output;
+  const char *msg;
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  const ssize_t encoded_len = xon_cell_encoded_len(obj);
+#endif
+
+  if (NULL != (msg = xon_cell_check(obj)))
+    goto check_failed;
+
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  trunnel_assert(encoded_len >= 0);
+#endif
+
+  /* Encode u8 version IN [0] */
+  trunnel_assert(written <= avail);
+  if (avail - written < 1)
+    goto truncated;
+  trunnel_set_uint8(ptr, (obj->version));
+  written += 1; ptr += 1;
+
+  /* Encode u32 kbps_ewma */
+  trunnel_assert(written <= avail);
+  if (avail - written < 4)
+    goto truncated;
+  trunnel_set_uint32(ptr, trunnel_htonl(obj->kbps_ewma));
+  written += 4; ptr += 4;
+
+
+  trunnel_assert(ptr == output + written);
+#ifdef TRUNNEL_CHECK_ENCODED_LEN
+  {
+    trunnel_assert(encoded_len >= 0);
+    trunnel_assert((size_t)encoded_len == written);
+  }
+
+#endif
+
+  return written;
+
+ truncated:
+  result = -2;
+  goto fail;
+ check_failed:
+  (void)msg;
+  result = -1;
+  goto fail;
+ fail:
+  trunnel_assert(result < 0);
+  return result;
+}
+
+/** As xon_cell_parse(), but do not allocate the output object.
+ */
+static ssize_t
+xon_cell_parse_into(xon_cell_t *obj, const uint8_t *input, const size_t len_in)
+{
+  const uint8_t *ptr = input;
+  size_t remaining = len_in;
+  ssize_t result = 0;
+  (void)result;
+
+  /* Parse u8 version IN [0] */
+  CHECK_REMAINING(1, truncated);
+  obj->version = (trunnel_get_uint8(ptr));
+  remaining -= 1; ptr += 1;
+  if (! (obj->version == 0))
+    goto fail;
+
+  /* Parse u32 kbps_ewma */
+  CHECK_REMAINING(4, truncated);
+  obj->kbps_ewma = trunnel_ntohl(trunnel_get_uint32(ptr));
+  remaining -= 4; ptr += 4;
+  trunnel_assert(ptr + remaining == input + len_in);
+  return len_in - remaining;
+
+ truncated:
+  return -2;
+ fail:
+  result = -1;
+  return result;
+}
+
+ssize_t
+xon_cell_parse(xon_cell_t **output, const uint8_t *input, const size_t len_in)
+{
+  ssize_t result;
+  *output = xon_cell_new();
+  if (NULL == *output)
+    return -1;
+  result = xon_cell_parse_into(*output, input, len_in);
+  if (result < 0) {
+    xon_cell_free(*output);
+    *output = NULL;
+  }
+  return result;
+}
diff --git a/src/trunnel/flow_control_cells.h b/src/trunnel/flow_control_cells.h
new file mode 100644
index 0000000000..b8108b9a24
--- /dev/null
+++ b/src/trunnel/flow_control_cells.h
@@ -0,0 +1,120 @@
+/* flow_control_cells.h -- generated by Trunnel v1.5.3.
+ * https://gitweb.torproject.org/trunnel.git
+ * You probably shouldn't edit this file.
+ */
+#ifndef TRUNNEL_FLOW_CONTROL_CELLS_H
+#define TRUNNEL_FLOW_CONTROL_CELLS_H
+
+#include <stdint.h>
+#include "trunnel.h"
+
+#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_XOFF_CELL)
+struct xoff_cell_st {
+  uint8_t version;
+  uint8_t trunnel_error_code_;
+};
+#endif
+typedef struct xoff_cell_st xoff_cell_t;
+#if !defined(TRUNNEL_OPAQUE) && !defined(TRUNNEL_OPAQUE_XON_CELL)
+struct xon_cell_st {
+  uint8_t version;
+  uint32_t kbps_ewma;
+  uint8_t trunnel_error_code_;
+};
+#endif
+typedef struct xon_cell_st xon_cell_t;
+/** Return a newly allocated xoff_cell with all elements set to zero.
+ */
+xoff_cell_t *xoff_cell_new(void);
+/** Release all storage held by the xoff_cell in 'victim'. (Do nothing
+ * if 'victim' is NULL.)
+ */
+void xoff_cell_free(xoff_cell_t *victim);
+/** Try to parse a xoff_cell from the buffer in 'input', using up to
+ * 'len_in' bytes from the input buffer. On success, return the number
+ * of bytes consumed and set *output to the newly allocated
+ * xoff_cell_t. On failure, return -2 if the input appears truncated,
+ * and -1 if the input is otherwise invalid.
+ */
+ssize_t xoff_cell_parse(xoff_cell_t **output, const uint8_t *input, const size_t len_in);
+/** Return the number of bytes we expect to need to encode the
+ * xoff_cell in 'obj'. On failure, return a negative value. Note that
+ * this value may be an overestimate, and can even be an underestimate
+ * for certain unencodeable objects.
+ */
+ssize_t xoff_cell_encoded_len(const xoff_cell_t *obj);
+/** Try to encode the xoff_cell from 'input' into the buffer at
+ * 'output', using up to 'avail' bytes of the output buffer. On
+ * success, return the number of bytes used. On failure, return -2 if
+ * the buffer was not long enough, and -1 if the input was invalid.
+ */
+ssize_t xoff_cell_encode(uint8_t *output, size_t avail, const xoff_cell_t *input);
+/** Check whether the internal state of the xoff_cell in 'obj' is
+ * consistent. Return NULL if it is, and a short message if it is not.
+ */
+const char *xoff_cell_check(const xoff_cell_t *obj);
+/** Clear any errors that were set on the object 'obj' by its setter
+ * functions. Return true iff errors were cleared.
+ */
+int xoff_cell_clear_errors(xoff_cell_t *obj);
+/** Return the value of the version field of the xoff_cell_t in 'inp'
+ */
+uint8_t xoff_cell_get_version(const xoff_cell_t *inp);
+/** Set the value of the version field of the xoff_cell_t in 'inp' to
+ * 'val'. Return 0 on success; return -1 and set the error code on
+ * 'inp' on failure.
+ */
+int xoff_cell_set_version(xoff_cell_t *inp, uint8_t val);
+/** Return a newly allocated xon_cell with all elements set to zero.
+ */
+xon_cell_t *xon_cell_new(void);
+/** Release all storage held by the xon_cell in 'victim'. (Do nothing
+ * if 'victim' is NULL.)
+ */
+void xon_cell_free(xon_cell_t *victim);
+/** Try to parse a xon_cell from the buffer in 'input', using up to
+ * 'len_in' bytes from the input buffer. On success, return the number
+ * of bytes consumed and set *output to the newly allocated
+ * xon_cell_t. On failure, return -2 if the input appears truncated,
+ * and -1 if the input is otherwise invalid.
+ */
+ssize_t xon_cell_parse(xon_cell_t **output, const uint8_t *input, const size_t len_in);
+/** Return the number of bytes we expect to need to encode the
+ * xon_cell in 'obj'. On failure, return a negative value. Note that
+ * this value may be an overestimate, and can even be an underestimate
+ * for certain unencodeable objects.
+ */
+ssize_t xon_cell_encoded_len(const xon_cell_t *obj);
+/** Try to encode the xon_cell from 'input' into the buffer at
+ * 'output', using up to 'avail' bytes of the output buffer. On
+ * success, return the number of bytes used. On failure, return -2 if
+ * the buffer was not long enough, and -1 if the input was invalid.
+ */
+ssize_t xon_cell_encode(uint8_t *output, size_t avail, const xon_cell_t *input);
+/** Check whether the internal state of the xon_cell in 'obj' is
+ * consistent. Return NULL if it is, and a short message if it is not.
+ */
+const char *xon_cell_check(const xon_cell_t *obj);
+/** Clear any errors that were set on the object 'obj' by its setter
+ * functions. Return true iff errors were cleared.
+ */
+int xon_cell_clear_errors(xon_cell_t *obj);
+/** Return the value of the version field of the xon_cell_t in 'inp'
+ */
+uint8_t xon_cell_get_version(const xon_cell_t *inp);
+/** Set the value of the version field of the xon_cell_t in 'inp' to
+ * 'val'. Return 0 on success; return -1 and set the error code on
+ * 'inp' on failure.
+ */
+int xon_cell_set_version(xon_cell_t *inp, uint8_t val);
+/** Return the value of the kbps_ewma field of the xon_cell_t in 'inp'
+ */
+uint32_t xon_cell_get_kbps_ewma(const xon_cell_t *inp);
+/** Set the value of the kbps_ewma field of the xon_cell_t in 'inp' to
+ * 'val'. Return 0 on success; return -1 and set the error code on
+ * 'inp' on failure.
+ */
+int xon_cell_set_kbps_ewma(xon_cell_t *inp, uint32_t val);
+
+
+#endif
diff --git a/src/trunnel/flow_control_cells.trunnel b/src/trunnel/flow_control_cells.trunnel
new file mode 100644
index 0000000000..9d07b568a9
--- /dev/null
+++ b/src/trunnel/flow_control_cells.trunnel
@@ -0,0 +1,20 @@
+/* This file contains the xon and xoff cell definitions, for flow control. */
+
+/* xoff cell definition. Tells the other endpoint to stop sending, because
+ * we have too much data queued for this stream. */
+struct xoff_cell {
+  /* Version field. */
+  u8 version IN [0x00];
+}
+
+/* xon cell declaration. Tells the other endpoint to resume sending and/or
+ * update its sending rate on this stream based on advisory information. */
+struct xon_cell {
+  /* Version field. */
+  u8 version IN [0x00];
+
+  /* Advisory field: The ewma rate of socket drain we have seen so far
+   * on this stream, in kilobytes/sec (1000 bytes/sec). May be zero,
+   * which means no rate advice. */
+  u32 kbps_ewma;
+}
diff --git a/src/trunnel/include.am b/src/trunnel/include.am
index 6c3a5ff06b..00a96536f1 100644
--- a/src/trunnel/include.am
+++ b/src/trunnel/include.am
@@ -12,6 +12,7 @@ TRUNNELINPUTS = \
 	src/trunnel/pwbox.trunnel \
 	src/trunnel/channelpadding_negotiation.trunnel \
 	src/trunnel/sendme_cell.trunnel \
+	src/trunnel/flow_control_cells.trunnel \
 	src/trunnel/socks5.trunnel \
 	src/trunnel/circpad_negotiation.trunnel
 
@@ -26,6 +27,7 @@ TRUNNELSOURCES = \
 	src/trunnel/hs/cell_rendezvous.c \
 	src/trunnel/channelpadding_negotiation.c \
 	src/trunnel/sendme_cell.c                    \
+	src/trunnel/flow_control_cells.c                    \
 	src/trunnel/socks5.c \
 	src/trunnel/netinfo.c \
 	src/trunnel/circpad_negotiation.c
@@ -43,6 +45,7 @@ TRUNNELHEADERS = \
 	src/trunnel/hs/cell_rendezvous.h \
 	src/trunnel/channelpadding_negotiation.h \
 	src/trunnel/sendme_cell.h                    \
+	src/trunnel/flow_control_cells.h                    \
 	src/trunnel/socks5.h                    \
 	src/trunnel/netinfo.h \
 	src/trunnel/circpad_negotiation.h