tor/src/or/relay.c

/* Copyright (c) 2001 Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2009, The Tor Project, Inc. */
/* See LICENSE for licensing information */

/**
 * \file relay.c
 * \brief Handle relay cell encryption/decryption, plus packaging and
 *    receiving from circuits, plus queuing on circuits.
 **/

#include "or.h"
#include "mempool.h"

static int relay_crypt(circuit_t *circ, cell_t *cell,
                       cell_direction_t cell_direction,
                       crypt_path_t **layer_hint, char *recognized);
static edge_connection_t *relay_lookup_conn(circuit_t *circ, cell_t *cell,
                                            cell_direction_t cell_direction,
                                            crypt_path_t *layer_hint);

static int
connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
                                   edge_connection_t *conn,
                                   crypt_path_t *layer_hint);
static void
circuit_consider_sending_sendme(circuit_t *circ, crypt_path_t *layer_hint);
static void
circuit_resume_edge_reading(circuit_t *circ, crypt_path_t *layer_hint);
static int
circuit_resume_edge_reading_helper(edge_connection_t *conn,
                                   circuit_t *circ,
                                   crypt_path_t *layer_hint);
static int
circuit_consider_stop_edge_reading(circuit_t *circ, crypt_path_t *layer_hint);

/** Stats: how many relay cells have originated at this hop, or have
 * been relayed onward (not recognized at this hop)?
 */
uint64_t stats_n_relay_cells_relayed = 0;
/** Stats: how many relay cells have been delivered to streams at this
 * hop?
 */
uint64_t stats_n_relay_cells_delivered = 0;

/** Update digest from the payload of cell. Assign integrity part to
 * cell.
 */
static void
relay_set_digest(crypto_digest_env_t *digest, cell_t *cell)
{
  char integrity[4];
  relay_header_t rh;

  crypto_digest_add_bytes(digest, cell->payload, CELL_PAYLOAD_SIZE);
  crypto_digest_get_digest(digest, integrity, 4);
//  log_fn(LOG_DEBUG,"Putting digest of %u %u %u %u into relay cell.",
//    integrity[0], integrity[1], integrity[2], integrity[3]);
  relay_header_unpack(&rh, cell->payload);
  memcpy(rh.integrity, integrity, 4);
  relay_header_pack(cell->payload, &rh);
}

/** Does the digest for this circuit indicate that this cell is for us?
 *
 * Update digest from the payload of cell (with the integrity part set
 * to 0). If the integrity part is valid, return 1, else restore digest
 * and cell to their original state and return 0.
 */
static int
relay_digest_matches(crypto_digest_env_t *digest, cell_t *cell)
{
  char received_integrity[4], calculated_integrity[4];
  relay_header_t rh;
  crypto_digest_env_t *backup_digest=NULL;

  backup_digest = crypto_digest_dup(digest);

  relay_header_unpack(&rh, cell->payload);
  memcpy(received_integrity, rh.integrity, 4);
  memset(rh.integrity, 0, 4);
  relay_header_pack(cell->payload, &rh);

//  log_fn(LOG_DEBUG,"Reading digest of %u %u %u %u from relay cell.",
//    received_integrity[0], received_integrity[1],
//    received_integrity[2], received_integrity[3]);

  crypto_digest_add_bytes(digest, cell->payload, CELL_PAYLOAD_SIZE);
  crypto_digest_get_digest(digest, calculated_integrity, 4);

  if (memcmp(received_integrity, calculated_integrity, 4)) {
//    log_fn(LOG_INFO,"Recognized=0 but bad digest. Not recognizing.");
// (%d vs %d).", received_integrity, calculated_integrity);
    /* restore digest to its old form */
    crypto_digest_assign(digest, backup_digest);
    /* restore the relay header */
    memcpy(rh.integrity, received_integrity, 4);
    relay_header_pack(cell->payload, &rh);
    crypto_free_digest_env(backup_digest);
    return 0;
  }
  crypto_free_digest_env(backup_digest);
  return 1;
}

/** Apply <b>cipher</b> to CELL_PAYLOAD_SIZE bytes of <b>in</b>
 * (in place).
 *
 * If <b>encrypt_mode</b> is 1 then encrypt, else decrypt.
 *
 * Return -1 if the crypto fails, else return 0.
 */
static int
relay_crypt_one_payload(crypto_cipher_env_t *cipher, char *in,
                        int encrypt_mode)
{
  int r;
  (void)encrypt_mode;
  r = crypto_cipher_crypt_inplace(cipher, in, CELL_PAYLOAD_SIZE);

  if (r) {
    log_warn(LD_BUG,"Error during relay encryption");
    return -1;
  }
  return 0;
}

/** Receive a relay cell:
 *  - Crypt it (encrypt if headed toward the origin or if we <b>are</b> the
 *    origin; decrypt if we're headed toward the exit).
 *  - Check if recognized (if exitward).
 *  - If recognized and the digest checks out, then find if there's a stream
 *    that the cell is intended for, and deliver it to the right
 *    connection_edge.
 *  - If not recognized, then we need to relay it: append it to the appropriate
 *    cell_queue on <b>circ</b>.
 *
 * Return -<b>reason</b> on failure.
 */
int
circuit_receive_relay_cell(cell_t *cell, circuit_t *circ,
                           cell_direction_t cell_direction)
{
  or_connection_t *or_conn=NULL;
  crypt_path_t *layer_hint=NULL;
  char recognized=0;
  int reason;

  tor_assert(cell);
  tor_assert(circ);
  tor_assert(cell_direction == CELL_DIRECTION_OUT ||
             cell_direction == CELL_DIRECTION_IN);
  if (circ->marked_for_close)
    return 0;

  if (relay_crypt(circ, cell, cell_direction, &layer_hint, &recognized) < 0) {
    log_warn(LD_BUG,"relay crypt failed. Dropping connection.");
    return -END_CIRC_REASON_INTERNAL;
  }

  if (recognized) {
    edge_connection_t *conn = relay_lookup_conn(circ, cell, cell_direction,
                                                layer_hint);
    if (cell_direction == CELL_DIRECTION_OUT) {
      ++stats_n_relay_cells_delivered;
      log_debug(LD_OR,"Sending away from origin.");
      if ((reason=connection_edge_process_relay_cell(cell, circ, conn, NULL))
          < 0) {
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "connection_edge_process_relay_cell (away from origin) "
               "failed.");
        return reason;
      }
    }
    if (cell_direction == CELL_DIRECTION_IN) {
      ++stats_n_relay_cells_delivered;
      log_debug(LD_OR,"Sending to origin.");
      if ((reason = connection_edge_process_relay_cell(cell, circ, conn,
                                                       layer_hint)) < 0) {
        log_warn(LD_OR,
                 "connection_edge_process_relay_cell (at origin) failed.");
        return reason;
      }
    }
    return 0;
  }

  /* not recognized. pass it on. */
  if (cell_direction == CELL_DIRECTION_OUT) {
    cell->circ_id = circ->n_circ_id; /* switch it */
    or_conn = circ->n_conn;
  } else if (! CIRCUIT_IS_ORIGIN(circ)) {
    cell->circ_id = TO_OR_CIRCUIT(circ)->p_circ_id; /* switch it */
    or_conn = TO_OR_CIRCUIT(circ)->p_conn;
  } else {
    log_fn(LOG_PROTOCOL_WARN, LD_OR,
           "Dropping unrecognized inbound cell on origin circuit.");
    return 0;
  }

  if (!or_conn) {
    // XXXX Can this splice stuff be done more cleanly?
    if (! CIRCUIT_IS_ORIGIN(circ) &&
        TO_OR_CIRCUIT(circ)->rend_splice &&
        cell_direction == CELL_DIRECTION_OUT) {
      or_circuit_t *splice = TO_OR_CIRCUIT(circ)->rend_splice;
      tor_assert(circ->purpose == CIRCUIT_PURPOSE_REND_ESTABLISHED);
      tor_assert(splice->_base.purpose == CIRCUIT_PURPOSE_REND_ESTABLISHED);
      cell->circ_id = splice->p_circ_id;
      cell->command = CELL_RELAY; /* can't be relay_early anyway */
      if ((reason = circuit_receive_relay_cell(cell, TO_CIRCUIT(splice),
                                               CELL_DIRECTION_IN)) < 0) {
        log_warn(LD_REND, "Error relaying cell across rendezvous; closing "
                 "circuits");
        /* XXXX Do this here, or just return -1? */
        circuit_mark_for_close(circ, -reason);
        return reason;
      }
      return 0;
    }
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Didn't recognize cell, but circ stops here! Closing circ.");
    return -END_CIRC_REASON_TORPROTOCOL;
  }

  log_debug(LD_OR,"Passing on unrecognized cell.");

  ++stats_n_relay_cells_relayed; /* XXXX no longer quite accurate {cells}
                                  * we might kill the circ before we relay
                                  * the cells. */

  append_cell_to_circuit_queue(circ, or_conn, cell, cell_direction);
  return 0;
}

/** Do the appropriate en/decryptions for <b>cell</b> arriving on
 * <b>circ</b> in direction <b>cell_direction</b>.
 *
 * If cell_direction == CELL_DIRECTION_IN:
 *   - If we're at the origin (we're the OP), for hops 1..N,
 *     decrypt cell. If recognized, stop.
 *   - Else (we're not the OP), encrypt one hop. Cell is not recognized.
 *
 * If cell_direction == CELL_DIRECTION_OUT:
 *   - decrypt one hop. Check if recognized.
 *
 * If cell is recognized, set *recognized to 1, and set
 * *layer_hint to the hop that recognized it.
 *
 * Return -1 to indicate that we should mark the circuit for close,
 * else return 0.
 */
static int
relay_crypt(circuit_t *circ, cell_t *cell, cell_direction_t cell_direction,
            crypt_path_t **layer_hint, char *recognized)
{
  relay_header_t rh;

  tor_assert(circ);
  tor_assert(cell);
  tor_assert(recognized);
  tor_assert(cell_direction == CELL_DIRECTION_IN ||
             cell_direction == CELL_DIRECTION_OUT);

  if (cell_direction == CELL_DIRECTION_IN) {
    if (CIRCUIT_IS_ORIGIN(circ)) { /* We're at the beginning of the circuit.
                                    * We'll want to do layered decrypts. */
      crypt_path_t *thishop, *cpath = TO_ORIGIN_CIRCUIT(circ)->cpath;
      thishop = cpath;
      if (thishop->state != CPATH_STATE_OPEN) {
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "Relay cell before first created cell? Closing.");
        return -1;
      }
      do { /* Remember: cpath is in forward order, that is, first hop first. */
        tor_assert(thishop);

        if (relay_crypt_one_payload(thishop->b_crypto, cell->payload, 0) < 0)
          return -1;

        relay_header_unpack(&rh, cell->payload);
        if (rh.recognized == 0) {
          /* it's possibly recognized. have to check digest to be sure. */
          if (relay_digest_matches(thishop->b_digest, cell)) {
            *recognized = 1;
            *layer_hint = thishop;
            return 0;
          }
        }

        thishop = thishop->next;
      } while (thishop != cpath && thishop->state == CPATH_STATE_OPEN);
      log_fn(LOG_PROTOCOL_WARN, LD_OR,
             "Incoming cell at client not recognized. Closing.");
      return -1;
    } else { /* we're in the middle. Just one crypt. */
      if (relay_crypt_one_payload(TO_OR_CIRCUIT(circ)->p_crypto,
                                  cell->payload, 1) < 0)
        return -1;
//      log_fn(LOG_DEBUG,"Skipping recognized check, because we're not "
//             "the client.");
    }
  } else /* cell_direction == CELL_DIRECTION_OUT */ {
    /* we're in the middle. Just one crypt. */

    if (relay_crypt_one_payload(TO_OR_CIRCUIT(circ)->n_crypto,
                                cell->payload, 0) < 0)
      return -1;

    relay_header_unpack(&rh, cell->payload);
    if (rh.recognized == 0) {
      /* it's possibly recognized. have to check digest to be sure. */
      if (relay_digest_matches(TO_OR_CIRCUIT(circ)->n_digest, cell)) {
        *recognized = 1;
        return 0;
      }
    }
  }
  return 0;
}

/** Package a relay cell from an edge:
 *  - Encrypt it to the right layer
 *  - Append it to the appropriate cell_queue on <b>circ</b>.
 */
static int
circuit_package_relay_cell(cell_t *cell, circuit_t *circ,
                           cell_direction_t cell_direction,
                           crypt_path_t *layer_hint)
{
  or_connection_t *conn; /* where to send the cell */

  if (cell_direction == CELL_DIRECTION_OUT) {
    crypt_path_t *thishop; /* counter for repeated crypts */
    conn = circ->n_conn;
    if (!CIRCUIT_IS_ORIGIN(circ) || !conn) {
      log_warn(LD_BUG,"outgoing relay cell has n_conn==NULL. Dropping.");
      return 0; /* just drop it */
    }

    relay_set_digest(layer_hint->f_digest, cell);

    thishop = layer_hint;
    /* moving from farthest to nearest hop */
    do {
      tor_assert(thishop);
      /* XXXX RD This is a bug, right? */
      log_debug(LD_OR,"crypting a layer of the relay cell.");
      if (relay_crypt_one_payload(thishop->f_crypto, cell->payload, 1) < 0) {
        return -1;
      }

      thishop = thishop->prev;
    } while (thishop != TO_ORIGIN_CIRCUIT(circ)->cpath->prev);

  } else { /* incoming cell */
    or_circuit_t *or_circ;
    if (CIRCUIT_IS_ORIGIN(circ)) {
      /* We should never package an _incoming_ cell from the circuit
       * origin; that means we messed up somewhere. */
      log_warn(LD_BUG,"incoming relay cell at origin circuit. Dropping.");
      assert_circuit_ok(circ);
      return 0; /* just drop it */
    }
    or_circ = TO_OR_CIRCUIT(circ);
    conn = or_circ->p_conn;
    relay_set_digest(or_circ->p_digest, cell);
    if (relay_crypt_one_payload(or_circ->p_crypto, cell->payload, 1) < 0)
      return -1;
  }
  ++stats_n_relay_cells_relayed;

  append_cell_to_circuit_queue(circ, conn, cell, cell_direction);
  return 0;
}

/** If cell's stream_id matches the stream_id of any conn that's
 * attached to circ, return that conn, else return NULL.
 */
static edge_connection_t *
relay_lookup_conn(circuit_t *circ, cell_t *cell,
                  cell_direction_t cell_direction, crypt_path_t *layer_hint)
{
  edge_connection_t *tmpconn;
  relay_header_t rh;

  relay_header_unpack(&rh, cell->payload);

  if (!rh.stream_id)
    return NULL;

  /* IN or OUT cells could have come from either direction, now
   * that we allow rendezvous *to* an OP.
   */

  if (CIRCUIT_IS_ORIGIN(circ)) {
    for (tmpconn = TO_ORIGIN_CIRCUIT(circ)->p_streams; tmpconn;
         tmpconn=tmpconn->next_stream) {
      if (rh.stream_id == tmpconn->stream_id &&
          !tmpconn->_base.marked_for_close &&
          tmpconn->cpath_layer == layer_hint) {
        log_debug(LD_APP,"found conn for stream %d.", rh.stream_id);
        return tmpconn;
      }
    }
  } else {
    for (tmpconn = TO_OR_CIRCUIT(circ)->n_streams; tmpconn;
         tmpconn=tmpconn->next_stream) {
      if (rh.stream_id == tmpconn->stream_id &&
          !tmpconn->_base.marked_for_close) {
        log_debug(LD_EXIT,"found conn for stream %d.", rh.stream_id);
        if (cell_direction == CELL_DIRECTION_OUT ||
            connection_edge_is_rendezvous_stream(tmpconn))
          return tmpconn;
      }
    }
    for (tmpconn = TO_OR_CIRCUIT(circ)->resolving_streams; tmpconn;
         tmpconn=tmpconn->next_stream) {
      if (rh.stream_id == tmpconn->stream_id &&
          !tmpconn->_base.marked_for_close) {
        log_debug(LD_EXIT,"found conn for stream %d.", rh.stream_id);
        return tmpconn;
      }
    }
  }
  return NULL; /* probably a begin relay cell */
}

/** Pack the relay_header_t host-order structure <b>src</b> into
 * network-order in the buffer <b>dest</b>. See tor-spec.txt for details
 * about the wire format.
 */
void
relay_header_pack(char *dest, const relay_header_t *src)
{
  *(uint8_t*)(dest) = src->command;

  set_uint16(dest+1, htons(src->recognized));
  set_uint16(dest+3, htons(src->stream_id));
  memcpy(dest+5, src->integrity, 4);
  set_uint16(dest+9, htons(src->length));
}

/** Unpack the network-order buffer <b>src</b> into a host-order
 * relay_header_t structure <b>dest</b>.
 */
void
relay_header_unpack(relay_header_t *dest, const char *src)
{
  dest->command = *(uint8_t*)(src);

  dest->recognized = ntohs(get_uint16(src+1));
  dest->stream_id = ntohs(get_uint16(src+3));
  memcpy(dest->integrity, src+5, 4);
  dest->length = ntohs(get_uint16(src+9));
}

/** Convert the relay <b>command</b> into a human-readable string. */
static const char *
relay_command_to_string(uint8_t command)
{
  switch (command) {
    case RELAY_COMMAND_BEGIN: return "BEGIN";
    case RELAY_COMMAND_DATA: return "DATA";
    case RELAY_COMMAND_END: return "END";
    case RELAY_COMMAND_CONNECTED: return "CONNECTED";
    case RELAY_COMMAND_SENDME: return "SENDME";
    case RELAY_COMMAND_EXTEND: return "EXTEND";
    case RELAY_COMMAND_EXTENDED: return "EXTENDED";
    case RELAY_COMMAND_TRUNCATE: return "TRUNCATE";
    case RELAY_COMMAND_TRUNCATED: return "TRUNCATED";
    case RELAY_COMMAND_DROP: return "DROP";
    case RELAY_COMMAND_RESOLVE: return "RESOLVE";
    case RELAY_COMMAND_RESOLVED: return "RESOLVED";
    case RELAY_COMMAND_BEGIN_DIR: return "BEGIN_DIR";
    case RELAY_COMMAND_ESTABLISH_INTRO: return "ESTABLISH_INTRO";
    case RELAY_COMMAND_ESTABLISH_RENDEZVOUS: return "ESTABLISH_RENDEZVOUS";
    case RELAY_COMMAND_INTRODUCE1: return "INTRODUCE1";
    case RELAY_COMMAND_INTRODUCE2: return "INTRODUCE2";
    case RELAY_COMMAND_RENDEZVOUS1: return "RENDEZVOUS1";
    case RELAY_COMMAND_RENDEZVOUS2: return "RENDEZVOUS2";
    case RELAY_COMMAND_INTRO_ESTABLISHED: return "INTRO_ESTABLISHED";
    case RELAY_COMMAND_RENDEZVOUS_ESTABLISHED:
      return "RENDEZVOUS_ESTABLISHED";
    case RELAY_COMMAND_INTRODUCE_ACK: return "INTRODUCE_ACK";
    default: return "(unrecognized)";
  }
}

/** Make a relay cell out of <b>relay_command</b> and <b>payload</b>, and send
 * it onto the open circuit <b>circ</b>. <b>stream_id</b> is the ID on
 * <b>circ</b> for the stream that's sending the relay cell, or 0 if it's a
 * control cell.  <b>cpath_layer</b> is NULL for OR->OP cells, or the
 * destination hop for OP->OR cells.
 *
 * If you can't send the cell, mark the circuit for close and return -1. Else
 * return 0.
 */
int
relay_send_command_from_edge(uint16_t stream_id, circuit_t *circ,
                             uint8_t relay_command, const char *payload,
                             size_t payload_len, crypt_path_t *cpath_layer)
{
  cell_t cell;
  relay_header_t rh;
  cell_direction_t cell_direction;
  /* XXXX NM Split this function into a separate versions per circuit type? */

  tor_assert(circ);
  tor_assert(payload_len <= RELAY_PAYLOAD_SIZE);

  memset(&cell, 0, sizeof(cell_t));
  cell.command = CELL_RELAY;
  if (cpath_layer) {
    cell.circ_id = circ->n_circ_id;
    cell_direction = CELL_DIRECTION_OUT;
  } else if (! CIRCUIT_IS_ORIGIN(circ)) {
    cell.circ_id = TO_OR_CIRCUIT(circ)->p_circ_id;
    cell_direction = CELL_DIRECTION_IN;
  } else {
    return -1;
  }

  memset(&rh, 0, sizeof(rh));
  rh.command = relay_command;
  rh.stream_id = stream_id;
  rh.length = payload_len;
  relay_header_pack(cell.payload, &rh);
  if (payload_len)
    memcpy(cell.payload+RELAY_HEADER_SIZE, payload, payload_len);

  log_debug(LD_OR,"delivering %d cell %s.", relay_command,
            cell_direction == CELL_DIRECTION_OUT ? "forward" : "backward");

  /* If we are sending an END cell and this circuit is used for a tunneled
   * directory request, advance its state. */
  if (relay_command == RELAY_COMMAND_END && circ->dirreq_id)
    geoip_change_dirreq_state(circ->dirreq_id, DIRREQ_TUNNELED,
                              DIRREQ_END_CELL_SENT);

  if (cell_direction == CELL_DIRECTION_OUT && circ->n_conn) {
    /* if we're using relaybandwidthrate, this conn wants priority */
    circ->n_conn->client_used = approx_time();
  }

  if (cell_direction == CELL_DIRECTION_OUT) {
    origin_circuit_t *origin_circ = TO_ORIGIN_CIRCUIT(circ);
    if (origin_circ->remaining_relay_early_cells > 0 &&
        (relay_command == RELAY_COMMAND_EXTEND ||
         (cpath_layer != origin_circ->cpath &&
          !CIRCUIT_PURPOSE_IS_ESTABLISHED_REND(circ->purpose)))) {
      /* If we've got any relay_early cells left, and we're sending
       * an extend cell or (we're not talking to the first hop and we're
       * not talking to a rendezvous circuit), use one of them.
       * Don't worry about the conn protocol version:
       * append_cell_to_circuit_queue will fix it up. */
      /* XXX For now, clients don't use RELAY_EARLY cells when sending
       * relay cells on rendezvous circuits. See bug 1038. Eventually,
       * we can take this behavior away in favor of having clients avoid
       * rendezvous points running 0.2.1.3-alpha through 0.2.1.18. -RD */
      cell.command = CELL_RELAY_EARLY;
      --origin_circ->remaining_relay_early_cells;
      log_debug(LD_OR, "Sending a RELAY_EARLY cell; %d remaining.",
                (int)origin_circ->remaining_relay_early_cells);
      /* Memorize the command that is sent as RELAY_EARLY cell; helps debug
       * task 878. */
      origin_circ->relay_early_commands[
          origin_circ->relay_early_cells_sent++] = relay_command;
    } else if (relay_command == RELAY_COMMAND_EXTEND) {
      /* If no RELAY_EARLY cells can be sent over this circuit, log which
       * commands have been sent as RELAY_EARLY cells before; helps debug
       * task 878. */
      smartlist_t *commands_list = smartlist_create();
      int i = 0;
      char *commands = NULL;
      for (; i < origin_circ->relay_early_cells_sent; i++)
        smartlist_add(commands_list, (char *)
            relay_command_to_string(origin_circ->relay_early_commands[i]));
      commands = smartlist_join_strings(commands_list, ",", 0, NULL);
      log_warn(LD_BUG, "Uh-oh.  We're sending a RELAY_COMMAND_EXTEND cell, "
               "but we have run out of RELAY_EARLY cells on that circuit. "
               "Commands sent before: %s", commands);
      tor_free(commands);
      smartlist_free(commands_list);
    }
  }

  if (circuit_package_relay_cell(&cell, circ, cell_direction, cpath_layer)
      < 0) {
    log_warn(LD_BUG,"circuit_package_relay_cell failed. Closing.");
    circuit_mark_for_close(circ, END_CIRC_REASON_INTERNAL);
    return -1;
  }
  return 0;
}

/** Make a relay cell out of <b>relay_command</b> and <b>payload</b>, and
 * send it onto the open circuit <b>circ</b>. <b>fromconn</b> is the stream
 * that's sending the relay cell, or NULL if it's a control cell.
 * <b>cpath_layer</b> is NULL for OR->OP cells, or the destination hop
 * for OP->OR cells.
 *
 * If you can't send the cell, mark the circuit for close and
 * return -1. Else return 0.
 */
int
connection_edge_send_command(edge_connection_t *fromconn,
                             uint8_t relay_command, const char *payload,
                             size_t payload_len)
{
  /* XXXX NM Split this function into a separate versions per circuit type? */
  circuit_t *circ;
  tor_assert(fromconn);
  circ = fromconn->on_circuit;

  if (fromconn->_base.marked_for_close) {
    log_warn(LD_BUG,
             "called on conn that's already marked for close at %s:%d.",
             fromconn->_base.marked_for_close_file,
             fromconn->_base.marked_for_close);
    return 0;
  }

  if (!circ) {
    if (fromconn->_base.type == CONN_TYPE_AP) {
      log_info(LD_APP,"no circ. Closing conn.");
      connection_mark_unattached_ap(fromconn, END_STREAM_REASON_INTERNAL);
    } else {
      log_info(LD_EXIT,"no circ. Closing conn.");
      fromconn->edge_has_sent_end = 1; /* no circ to send to */
      fromconn->end_reason = END_STREAM_REASON_INTERNAL;
      connection_mark_for_close(TO_CONN(fromconn));
    }
    return -1;
  }

  return relay_send_command_from_edge(fromconn->stream_id, circ,
                                      relay_command, payload,
                                      payload_len, fromconn->cpath_layer);
}

/** How many times will I retry a stream that fails due to DNS
 * resolve failure or misc error?
 */
#define MAX_RESOLVE_FAILURES 3

/** Return 1 if reason is something that you should retry if you
 * get the end cell before you've connected; else return 0. */
static int
edge_reason_is_retriable(int reason)
{
  return reason == END_STREAM_REASON_HIBERNATING ||
         reason == END_STREAM_REASON_RESOURCELIMIT ||
         reason == END_STREAM_REASON_EXITPOLICY ||
         reason == END_STREAM_REASON_RESOLVEFAILED ||
         reason == END_STREAM_REASON_MISC;
}

/** Called when we receive an END cell on a stream that isn't open yet,
 * from the client side.
 * Arguments are as for connection_edge_process_relay_cell().
 */
static int
connection_ap_process_end_not_open(
    relay_header_t *rh, cell_t *cell, origin_circuit_t *circ,
    edge_connection_t *conn, crypt_path_t *layer_hint)
{
  struct in_addr in;
  routerinfo_t *exitrouter;
  int reason = *(cell->payload+RELAY_HEADER_SIZE);
  int control_reason = reason | END_STREAM_REASON_FLAG_REMOTE;
  (void) layer_hint; /* unused */

  if (rh->length > 0 && edge_reason_is_retriable(reason) &&
      !connection_edge_is_rendezvous_stream(conn)  /* avoid retry if rend */
      ) {
    log_info(LD_APP,"Address '%s' refused due to '%s'. Considering retrying.",
             safe_str(conn->socks_request->address),
             stream_end_reason_to_string(reason));
    exitrouter =
      router_get_by_digest(circ->build_state->chosen_exit->identity_digest);
    switch (reason) {
      case END_STREAM_REASON_EXITPOLICY:
        if (rh->length >= 5) {
          uint32_t addr = ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+1));
          int ttl;
          if (!addr) {
            log_info(LD_APP,"Address '%s' resolved to 0.0.0.0. Closing,",
                     safe_str(conn->socks_request->address));
            connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
            return 0;
          }
          if (rh->length >= 9)
            ttl = (int)ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+5));
          else
            ttl = -1;

          if (get_options()->ClientDNSRejectInternalAddresses &&
              is_internal_IP(addr, 0)) {
            log_info(LD_APP,"Address '%s' resolved to internal. Closing,",
                     safe_str(conn->socks_request->address));
            connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
            return 0;
          }
          client_dns_set_addressmap(conn->socks_request->address, addr,
                                    conn->chosen_exit_name, ttl);
        }
        /* check if he *ought* to have allowed it */
        if (exitrouter &&
            (rh->length < 5 ||
             (tor_inet_aton(conn->socks_request->address, &in) &&
              !conn->chosen_exit_name))) {
          log_info(LD_APP,
                 "Exitrouter '%s' seems to be more restrictive than its exit "
                 "policy. Not using this router as exit for now.",
                 exitrouter->nickname);
          policies_set_router_exitpolicy_to_reject_all(exitrouter);
        }
        /* rewrite it to an IP if we learned one. */
        if (addressmap_rewrite(conn->socks_request->address,
                               sizeof(conn->socks_request->address),
                               NULL)) {
          control_event_stream_status(conn, STREAM_EVENT_REMAP, 0);
        }
        if (conn->chosen_exit_optional ||
            conn->chosen_exit_retries) {
          /* stop wanting a specific exit */
          conn->chosen_exit_optional = 0;
          /* A non-zero chosen_exit_retries can happen if we set a
           * TrackHostExits for this address under a port that the exit
           * relay allows, but then try the same address with a different
           * port that it doesn't allow to exit. We shouldn't unregister
           * the mapping, since it is probably still wanted on the
           * original port. But now we give away to the exit relay that
           * we probably have a TrackHostExits on it. So be it. */
          conn->chosen_exit_retries = 0;
          tor_free(conn->chosen_exit_name); /* clears it */
        }
        if (connection_ap_detach_retriable(conn, circ, control_reason) >= 0)
          return 0;
        /* else, conn will get closed below */
        break;
      case END_STREAM_REASON_CONNECTREFUSED:
        if (!conn->chosen_exit_optional)
          break; /* break means it'll close, below */
        /* Else fall through: expire this circuit, clear the
         * chosen_exit_name field, and try again. */
      case END_STREAM_REASON_RESOLVEFAILED:
      case END_STREAM_REASON_TIMEOUT:
      case END_STREAM_REASON_MISC:
        if (client_dns_incr_failures(conn->socks_request->address)
            < MAX_RESOLVE_FAILURES) {
          /* We haven't retried too many times; reattach the connection. */
          circuit_log_path(LOG_INFO,LD_APP,circ);
          tor_assert(circ->_base.timestamp_dirty);
          circ->_base.timestamp_dirty -= get_options()->MaxCircuitDirtiness;

          if (conn->chosen_exit_optional) {
            /* stop wanting a specific exit */
            conn->chosen_exit_optional = 0;
            tor_free(conn->chosen_exit_name); /* clears it */
          }
          if (connection_ap_detach_retriable(conn, circ, control_reason) >= 0)
            return 0;
          /* else, conn will get closed below */
        } else {
          log_notice(LD_APP,
                     "Have tried resolving or connecting to address '%s' "
                     "at %d different places. Giving up.",
                     safe_str(conn->socks_request->address),
                     MAX_RESOLVE_FAILURES);
          /* clear the failures, so it will have a full try next time */
          client_dns_clear_failures(conn->socks_request->address);
        }
        break;
      case END_STREAM_REASON_HIBERNATING:
      case END_STREAM_REASON_RESOURCELIMIT:
        if (exitrouter) {
          policies_set_router_exitpolicy_to_reject_all(exitrouter);
        }
        if (conn->chosen_exit_optional) {
          /* stop wanting a specific exit */
          conn->chosen_exit_optional = 0;
          tor_free(conn->chosen_exit_name); /* clears it */
        }
        if (connection_ap_detach_retriable(conn, circ, control_reason) >= 0)
          return 0;
        /* else, will close below */
        break;
    } /* end switch */
    log_info(LD_APP,"Giving up on retrying; conn can't be handled.");
  }

  log_info(LD_APP,
           "Edge got end (%s) before we're connected. Marking for close.",
       stream_end_reason_to_string(rh->length > 0 ? reason : -1));
  circuit_log_path(LOG_INFO,LD_APP,circ);
  /* need to test because of detach_retriable */
  if (!conn->_base.marked_for_close)
    connection_mark_unattached_ap(conn, control_reason);
  return 0;
}

/** Helper: change the socks_request-&gt;address field on conn to the
 * dotted-quad representation of <b>new_addr</b> (given in host order),
 * and send an appropriate REMAP event. */
static void
remap_event_helper(edge_connection_t *conn, uint32_t new_addr)
{
  struct in_addr in;

  in.s_addr = htonl(new_addr);
  tor_inet_ntoa(&in, conn->socks_request->address,
                sizeof(conn->socks_request->address));
  control_event_stream_status(conn, STREAM_EVENT_REMAP,
                              REMAP_STREAM_SOURCE_EXIT);
}

/** An incoming relay cell has arrived from circuit <b>circ</b> to
 * stream <b>conn</b>.
 *
 * The arguments here are the same as in
 * connection_edge_process_relay_cell() below; this function is called
 * from there when <b>conn</b> is defined and not in an open state.
 */
static int
connection_edge_process_relay_cell_not_open(
    relay_header_t *rh, cell_t *cell, circuit_t *circ,
    edge_connection_t *conn, crypt_path_t *layer_hint)
{
  if (rh->command == RELAY_COMMAND_END) {
    if (CIRCUIT_IS_ORIGIN(circ) && conn->_base.type == CONN_TYPE_AP) {
      return connection_ap_process_end_not_open(rh, cell,
                                                TO_ORIGIN_CIRCUIT(circ), conn,
                                                layer_hint);
    } else {
      /* we just got an 'end', don't need to send one */
      conn->edge_has_sent_end = 1;
      conn->end_reason = *(cell->payload+RELAY_HEADER_SIZE) |
                         END_STREAM_REASON_FLAG_REMOTE;
      connection_mark_for_close(TO_CONN(conn));
      return 0;
    }
  }

  if (conn->_base.type == CONN_TYPE_AP &&
      rh->command == RELAY_COMMAND_CONNECTED) {
    tor_assert(CIRCUIT_IS_ORIGIN(circ));
    if (conn->_base.state != AP_CONN_STATE_CONNECT_WAIT) {
      log_fn(LOG_PROTOCOL_WARN, LD_APP,
             "Got 'connected' while not in state connect_wait. Dropping.");
      return 0;
    }
    conn->_base.state = AP_CONN_STATE_OPEN;
    log_info(LD_APP,"'connected' received after %d seconds.",
             (int)(time(NULL) - conn->_base.timestamp_lastread));
    if (rh->length >= 4) {
      uint32_t addr = ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE));
      int ttl;
      if (!addr || (get_options()->ClientDNSRejectInternalAddresses &&
                    is_internal_IP(addr, 0))) {
        char buf[INET_NTOA_BUF_LEN];
        struct in_addr a;
        a.s_addr = htonl(addr);
        tor_inet_ntoa(&a, buf, sizeof(buf));
        log_info(LD_APP,
                 "...but it claims the IP address was %s. Closing.", buf);
        connection_edge_end(conn, END_STREAM_REASON_TORPROTOCOL);
        connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
        return 0;
      }
      if (rh->length >= 8)
        ttl = (int)ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+4));
      else
        ttl = -1;
      client_dns_set_addressmap(conn->socks_request->address, addr,
                                conn->chosen_exit_name, ttl);

      remap_event_helper(conn, addr);
    }
    circuit_log_path(LOG_INFO,LD_APP,TO_ORIGIN_CIRCUIT(circ));
    /* don't send a socks reply to transparent conns */
    if (!conn->socks_request->has_finished)
      connection_ap_handshake_socks_reply(conn, NULL, 0, 0);

    /* Was it a linked dir conn? If so, a dir request just started to
     * fetch something; this could be a bootstrap status milestone. */
    log_debug(LD_APP, "considering");
    if (TO_CONN(conn)->linked_conn &&
        TO_CONN(conn)->linked_conn->type == CONN_TYPE_DIR) {
      connection_t *dirconn = TO_CONN(conn)->linked_conn;
      log_debug(LD_APP, "it is! %d", dirconn->purpose);
      switch (dirconn->purpose) {
        case DIR_PURPOSE_FETCH_CERTIFICATE:
          if (consensus_is_waiting_for_certs())
            control_event_bootstrap(BOOTSTRAP_STATUS_LOADING_KEYS, 0);
          break;
        case DIR_PURPOSE_FETCH_CONSENSUS:
          control_event_bootstrap(BOOTSTRAP_STATUS_LOADING_STATUS, 0);
          break;
        case DIR_PURPOSE_FETCH_SERVERDESC:
          control_event_bootstrap(BOOTSTRAP_STATUS_LOADING_DESCRIPTORS,
                                  count_loading_descriptors_progress());
          break;
      }
    }

    /* handle anything that might have queued */
    if (connection_edge_package_raw_inbuf(conn, 1) < 0) {
      /* (We already sent an end cell if possible) */
      connection_mark_for_close(TO_CONN(conn));
      return 0;
    }
    return 0;
  }
  if (conn->_base.type == CONN_TYPE_AP &&
      rh->command == RELAY_COMMAND_RESOLVED) {
    int ttl;
    int answer_len;
    uint8_t answer_type;
    if (conn->_base.state != AP_CONN_STATE_RESOLVE_WAIT) {
      log_fn(LOG_PROTOCOL_WARN, LD_APP, "Got a 'resolved' cell while "
             "not in state resolve_wait. Dropping.");
      return 0;
    }
    tor_assert(SOCKS_COMMAND_IS_RESOLVE(conn->socks_request->command));
    answer_len = cell->payload[RELAY_HEADER_SIZE+1];
    if (rh->length < 2 || answer_len+2>rh->length) {
      log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
             "Dropping malformed 'resolved' cell");
      connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
      return 0;
    }
    answer_type = cell->payload[RELAY_HEADER_SIZE];
    if (rh->length >= answer_len+6)
      ttl = (int)ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+
                                  2+answer_len));
    else
      ttl = -1;
    if (answer_type == RESOLVED_TYPE_IPV4 && answer_len == 4) {
      uint32_t addr = ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+2));
      if (get_options()->ClientDNSRejectInternalAddresses &&
          is_internal_IP(addr, 0)) {
        char buf[INET_NTOA_BUF_LEN];
        struct in_addr a;
        a.s_addr = htonl(addr);
        tor_inet_ntoa(&a, buf, sizeof(buf));
        log_info(LD_APP,"Got a resolve with answer %s.  Rejecting.", buf);
        connection_ap_handshake_socks_resolved(conn,
                                               RESOLVED_TYPE_ERROR_TRANSIENT,
                                               0, NULL, 0, TIME_MAX);
        connection_mark_unattached_ap(conn, END_STREAM_REASON_TORPROTOCOL);
        return 0;
      }
    }
    connection_ap_handshake_socks_resolved(conn,
                   answer_type,
                   cell->payload[RELAY_HEADER_SIZE+1], /*answer_len*/
                   cell->payload+RELAY_HEADER_SIZE+2, /*answer*/
                   ttl,
                   -1);
    if (answer_type == RESOLVED_TYPE_IPV4 && answer_len == 4) {
      uint32_t addr = ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+2));
      remap_event_helper(conn, addr);
    }
    connection_mark_unattached_ap(conn,
                              END_STREAM_REASON_DONE |
                              END_STREAM_REASON_FLAG_ALREADY_SOCKS_REPLIED);
    return 0;
  }

  log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
         "Got an unexpected relay command %d, in state %d (%s). Dropping.",
         rh->command, conn->_base.state,
         conn_state_to_string(conn->_base.type, conn->_base.state));
  return 0; /* for forward compatibility, don't kill the circuit */
//  connection_edge_end(conn, END_STREAM_REASON_TORPROTOCOL);
//  connection_mark_for_close(conn);
//  return -1;
}

/** An incoming relay cell has arrived on circuit <b>circ</b>. If
 * <b>conn</b> is NULL this is a control cell, else <b>cell</b> is
 * destined for <b>conn</b>.
 *
 * If <b>layer_hint</b> is defined, then we're the origin of the
 * circuit, and it specifies the hop that packaged <b>cell</b>.
 *
 * Return -reason if you want to warn and tear down the circuit, else 0.
 */
static int
connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
                                   edge_connection_t *conn,
                                   crypt_path_t *layer_hint)
{
  static int num_seen=0;
  relay_header_t rh;
  unsigned domain = layer_hint?LD_APP:LD_EXIT;
  int reason;

  tor_assert(cell);
  tor_assert(circ);

  relay_header_unpack(&rh, cell->payload);
//  log_fn(LOG_DEBUG,"command %d stream %d", rh.command, rh.stream_id);
  num_seen++;
  log_debug(domain, "Now seen %d relay cells here (command %d, stream %d).",
            num_seen, rh.command, rh.stream_id);

  if (rh.length > RELAY_PAYLOAD_SIZE) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Relay cell length field too long. Closing circuit.");
    return - END_CIRC_REASON_TORPROTOCOL;
  }

  /* either conn is NULL, in which case we've got a control cell, or else
   * conn points to the recognized stream. */

  if (conn && !connection_state_is_open(TO_CONN(conn)))
    return connection_edge_process_relay_cell_not_open(
             &rh, cell, circ, conn, layer_hint);

  switch (rh.command) {
    case RELAY_COMMAND_DROP:
//      log_info(domain,"Got a relay-level padding cell. Dropping.");
      return 0;
    case RELAY_COMMAND_BEGIN:
    case RELAY_COMMAND_BEGIN_DIR:
      if (layer_hint &&
          circ->purpose != CIRCUIT_PURPOSE_S_REND_JOINED) {
        log_fn(LOG_PROTOCOL_WARN, LD_APP,
               "Relay begin request unsupported at AP. Dropping.");
        return 0;
      }
      if (circ->purpose == CIRCUIT_PURPOSE_S_REND_JOINED &&
          layer_hint != TO_ORIGIN_CIRCUIT(circ)->cpath->prev) {
        log_fn(LOG_PROTOCOL_WARN, LD_APP,
               "Relay begin request to Hidden Service "
               "from intermediary node. Dropping.");
        return 0;
      }
      if (conn) {
        log_fn(LOG_PROTOCOL_WARN, domain,
               "Begin cell for known stream. Dropping.");
        return 0;
      }
      if (rh.command == RELAY_COMMAND_BEGIN_DIR) {
        /* Assign this circuit and its app-ward OR connection a unique ID,
         * so that we can measure download times. The local edge and dir
         * connection will be assigned the same ID when they are created
         * and linked. */
        static uint64_t next_id = 0;
        circ->dirreq_id = ++next_id;
        TO_CONN(TO_OR_CIRCUIT(circ)->p_conn)->dirreq_id = circ->dirreq_id;
      }

      return connection_exit_begin_conn(cell, circ);
    case RELAY_COMMAND_DATA:
      ++stats_n_data_cells_received;
      if (( layer_hint && --layer_hint->deliver_window < 0) ||
          (!layer_hint && --circ->deliver_window < 0)) {
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "(relay data) circ deliver_window below 0. Killing.");
        connection_edge_end(conn, END_STREAM_REASON_TORPROTOCOL);
        connection_mark_for_close(TO_CONN(conn));
        return -END_CIRC_REASON_TORPROTOCOL;
      }
      log_debug(domain,"circ deliver_window now %d.", layer_hint ?
                layer_hint->deliver_window : circ->deliver_window);

      circuit_consider_sending_sendme(circ, layer_hint);

      if (!conn) {
        log_info(domain,"data cell dropped, unknown stream (streamid %d).",
                 rh.stream_id);
        return 0;
      }

      if (--conn->deliver_window < 0) { /* is it below 0 after decrement? */
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "(relay data) conn deliver_window below 0. Killing.");
        return -END_CIRC_REASON_TORPROTOCOL;
      }

      stats_n_data_bytes_received += rh.length;
      connection_write_to_buf(cell->payload + RELAY_HEADER_SIZE,
                              rh.length, TO_CONN(conn));
      connection_edge_consider_sending_sendme(conn);
      return 0;
    case RELAY_COMMAND_END:
      reason = rh.length > 0 ?
        *(uint8_t *)(cell->payload+RELAY_HEADER_SIZE) : END_STREAM_REASON_MISC;
      if (!conn) {
        log_info(domain,"end cell (%s) dropped, unknown stream.",
                 stream_end_reason_to_string(reason));
        return 0;
      }
/* XXX add to this log_fn the exit node's nickname? */
      log_info(domain,"%d: end cell (%s) for stream %d. Removing stream.",
               conn->_base.s,
               stream_end_reason_to_string(reason),
               conn->stream_id);
      if (conn->socks_request && !conn->socks_request->has_finished)
        log_warn(LD_BUG,
                 "open stream hasn't sent socks answer yet? Closing.");
      /* We just *got* an end; no reason to send one. */
      conn->edge_has_sent_end = 1;
      if (!conn->end_reason)
        conn->end_reason = reason | END_STREAM_REASON_FLAG_REMOTE;
      if (!conn->_base.marked_for_close) {
        /* only mark it if not already marked. it's possible to
         * get the 'end' right around when the client hangs up on us. */
        connection_mark_for_close(TO_CONN(conn));
        conn->_base.hold_open_until_flushed = 1;
      }
      return 0;
    case RELAY_COMMAND_EXTEND:
      if (conn) {
        log_fn(LOG_PROTOCOL_WARN, domain,
               "'extend' cell received for non-zero stream. Dropping.");
        return 0;
      }
      return circuit_extend(cell, circ);
    case RELAY_COMMAND_EXTENDED:
      if (!layer_hint) {
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "'extended' unsupported at non-origin. Dropping.");
        return 0;
      }
      log_debug(domain,"Got an extended cell! Yay.");
      if ((reason = circuit_finish_handshake(TO_ORIGIN_CIRCUIT(circ),
                                       CELL_CREATED,
                                       cell->payload+RELAY_HEADER_SIZE)) < 0) {
        log_warn(domain,"circuit_finish_handshake failed.");
        return reason;
      }
      if ((reason=circuit_send_next_onion_skin(TO_ORIGIN_CIRCUIT(circ)))<0) {
        log_info(domain,"circuit_send_next_onion_skin() failed.");
        return reason;
      }
      return 0;
    case RELAY_COMMAND_TRUNCATE:
      if (layer_hint) {
        log_fn(LOG_PROTOCOL_WARN, LD_APP,
               "'truncate' unsupported at origin. Dropping.");
        return 0;
      }
      if (circ->n_conn) {
        uint8_t trunc_reason = *(uint8_t*)(cell->payload + RELAY_HEADER_SIZE);
        connection_or_send_destroy(circ->n_circ_id, circ->n_conn,
                                   trunc_reason);
        circuit_set_n_circid_orconn(circ, 0, NULL);
      }
      log_debug(LD_EXIT, "Processed 'truncate', replying.");
      {
        char payload[1];
        payload[0] = (char)END_CIRC_REASON_REQUESTED;
        relay_send_command_from_edge(0, circ, RELAY_COMMAND_TRUNCATED,
                                     payload, sizeof(payload), NULL);
      }
      return 0;
    case RELAY_COMMAND_TRUNCATED:
      if (!layer_hint) {
        log_fn(LOG_PROTOCOL_WARN, LD_EXIT,
               "'truncated' unsupported at non-origin. Dropping.");
        return 0;
      }
      circuit_truncated(TO_ORIGIN_CIRCUIT(circ), layer_hint);
      return 0;
    case RELAY_COMMAND_CONNECTED:
      if (conn) {
        log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
               "'connected' unsupported while open. Closing circ.");
        return -END_CIRC_REASON_TORPROTOCOL;
      }
      log_info(domain,
               "'connected' received, no conn attached anymore. Ignoring.");
      return 0;
    case RELAY_COMMAND_SENDME:
      if (!conn) {
        if (layer_hint) {
          layer_hint->package_window += CIRCWINDOW_INCREMENT;
          log_debug(LD_APP,"circ-level sendme at origin, packagewindow %d.",
                    layer_hint->package_window);
          circuit_resume_edge_reading(circ, layer_hint);
        } else {
          circ->package_window += CIRCWINDOW_INCREMENT;
          log_debug(LD_APP,
                    "circ-level sendme at non-origin, packagewindow %d.",
                    circ->package_window);
          circuit_resume_edge_reading(circ, layer_hint);
        }
        return 0;
      }
      conn->package_window += STREAMWINDOW_INCREMENT;
      log_debug(domain,"stream-level sendme, packagewindow now %d.",
                conn->package_window);
      connection_start_reading(TO_CONN(conn));
      /* handle whatever might still be on the inbuf */
      if (connection_edge_package_raw_inbuf(conn, 1) < 0) {
        /* (We already sent an end cell if possible) */
        connection_mark_for_close(TO_CONN(conn));
        return 0;
      }
      return 0;
    case RELAY_COMMAND_RESOLVE:
      if (layer_hint) {
        log_fn(LOG_PROTOCOL_WARN, LD_APP,
               "resolve request unsupported at AP; dropping.");
        return 0;
      } else if (conn) {
        log_fn(LOG_PROTOCOL_WARN, domain,
               "resolve request for known stream; dropping.");
        return 0;
      } else if (circ->purpose != CIRCUIT_PURPOSE_OR) {
        log_fn(LOG_PROTOCOL_WARN, domain,
               "resolve request on circ with purpose %d; dropping",
               circ->purpose);
        return 0;
      }
      connection_exit_begin_resolve(cell, TO_OR_CIRCUIT(circ));
      return 0;
    case RELAY_COMMAND_RESOLVED:
      if (conn) {
        log_fn(LOG_PROTOCOL_WARN, domain,
               "'resolved' unsupported while open. Closing circ.");
        return -END_CIRC_REASON_TORPROTOCOL;
      }
      log_info(domain,
               "'resolved' received, no conn attached anymore. Ignoring.");
      return 0;
    case RELAY_COMMAND_ESTABLISH_INTRO:
    case RELAY_COMMAND_ESTABLISH_RENDEZVOUS:
    case RELAY_COMMAND_INTRODUCE1:
    case RELAY_COMMAND_INTRODUCE2:
    case RELAY_COMMAND_INTRODUCE_ACK:
    case RELAY_COMMAND_RENDEZVOUS1:
    case RELAY_COMMAND_RENDEZVOUS2:
    case RELAY_COMMAND_INTRO_ESTABLISHED:
    case RELAY_COMMAND_RENDEZVOUS_ESTABLISHED:
      rend_process_relay_cell(circ, layer_hint,
                              rh.command, rh.length,
                              cell->payload+RELAY_HEADER_SIZE);
      return 0;
  }
  log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
         "Received unknown relay command %d. Perhaps the other side is using "
         "a newer version of Tor? Dropping.",
         rh.command);
  return 0; /* for forward compatibility, don't kill the circuit */
}

/** How many relay_data cells have we built, ever? */
uint64_t stats_n_data_cells_packaged = 0;
/** How many bytes of data have we put in relay_data cells have we built,
 * ever? This would be RELAY_PAYLOAD_SIZE*stats_n_data_cells_packaged if
 * every relay cell we ever sent were completely full of data. */
uint64_t stats_n_data_bytes_packaged = 0;
/** How many relay_data cells have we received, ever? */
uint64_t stats_n_data_cells_received = 0;
/** How many bytes of data have we received relay_data cells, ever? This would
 * be RELAY_PAYLOAD_SIZE*stats_n_data_cells_packaged if every relay cell we
 * ever received were completely full of data. */
uint64_t stats_n_data_bytes_received = 0;

/** While conn->inbuf has an entire relay payload of bytes on it,
 * and the appropriate package windows aren't empty, grab a cell
 * and send it down the circuit.
 *
 * Return -1 (and send a RELAY_COMMAND_END cell if necessary) if conn should
 * be marked for close, else return 0.
 */
int
connection_edge_package_raw_inbuf(edge_connection_t *conn, int package_partial)
{
  size_t amount_to_process, length;
  char payload[CELL_PAYLOAD_SIZE];
  circuit_t *circ;
  unsigned domain = conn->cpath_layer ? LD_APP : LD_EXIT;

  tor_assert(conn);

  if (conn->_base.marked_for_close) {
    log_warn(LD_BUG,
             "called on conn that's already marked for close at %s:%d.",
             conn->_base.marked_for_close_file, conn->_base.marked_for_close);
    return 0;
  }

repeat_connection_edge_package_raw_inbuf:

  circ = circuit_get_by_edge_conn(conn);
  if (!circ) {
    log_info(domain,"conn has no circuit! Closing.");
    conn->end_reason = END_STREAM_REASON_CANT_ATTACH;
    return -1;
  }

  if (circuit_consider_stop_edge_reading(circ, conn->cpath_layer))
    return 0;

  if (conn->package_window <= 0) {
    log_info(domain,"called with package_window %d. Skipping.",
             conn->package_window);
    connection_stop_reading(TO_CONN(conn));
    return 0;
  }

  amount_to_process = buf_datalen(conn->_base.inbuf);

  if (!amount_to_process)
    return 0;

  if (!package_partial && amount_to_process < RELAY_PAYLOAD_SIZE)
    return 0;

  if (amount_to_process > RELAY_PAYLOAD_SIZE) {
    length = RELAY_PAYLOAD_SIZE;
  } else {
    length = amount_to_process;
  }
  stats_n_data_bytes_packaged += length;
  stats_n_data_cells_packaged += 1;

  connection_fetch_from_buf(payload, length, TO_CONN(conn));

  log_debug(domain,"(%d) Packaging %d bytes (%d waiting).", conn->_base.s,
            (int)length, (int)buf_datalen(conn->_base.inbuf));

  if (connection_edge_send_command(conn, RELAY_COMMAND_DATA,
                                   payload, length) < 0 )
    /* circuit got marked for close, don't continue, don't need to mark conn */
    return 0;

  if (!conn->cpath_layer) { /* non-rendezvous exit */
    tor_assert(circ->package_window > 0);
    circ->package_window--;
  } else { /* we're an AP, or an exit on a rendezvous circ */
    tor_assert(conn->cpath_layer->package_window > 0);
    conn->cpath_layer->package_window--;
  }

  if (--conn->package_window <= 0) { /* is it 0 after decrement? */
    connection_stop_reading(TO_CONN(conn));
    log_debug(domain,"conn->package_window reached 0.");
    circuit_consider_stop_edge_reading(circ, conn->cpath_layer);
    return 0; /* don't process the inbuf any more */
  }
  log_debug(domain,"conn->package_window is now %d",conn->package_window);

  /* handle more if there's more, or return 0 if there isn't */
  goto repeat_connection_edge_package_raw_inbuf;
}

/** Called when we've just received a relay data cell, or when
 * we've just finished flushing all bytes to stream <b>conn</b>.
 *
 * If conn->outbuf is not too full, and our deliver window is
 * low, send back a suitable number of stream-level sendme cells.
 */
void
connection_edge_consider_sending_sendme(edge_connection_t *conn)
{
  circuit_t *circ;

  if (connection_outbuf_too_full(TO_CONN(conn)))
    return;

  circ = circuit_get_by_edge_conn(conn);
  if (!circ) {
    /* this can legitimately happen if the destroy has already
     * arrived and torn down the circuit */
    log_info(LD_APP,"No circuit associated with conn. Skipping.");
    return;
  }

  while (conn->deliver_window <= STREAMWINDOW_START - STREAMWINDOW_INCREMENT) {
    log_debug(conn->cpath_layer?LD_APP:LD_EXIT,
              "Outbuf %d, Queuing stream sendme.",
              (int)conn->_base.outbuf_flushlen);
    conn->deliver_window += STREAMWINDOW_INCREMENT;
    if (connection_edge_send_command(conn, RELAY_COMMAND_SENDME,
                                     NULL, 0) < 0) {
      log_warn(LD_APP,"connection_edge_send_command failed. Skipping.");
      return; /* the circuit's closed, don't continue */
    }
  }
}

/** The circuit <b>circ</b> has received a circuit-level sendme
 * (on hop <b>layer_hint</b>, if we're the OP). Go through all the
 * attached streams and let them resume reading and packaging, if
 * their stream windows allow it.
 */
static void
circuit_resume_edge_reading(circuit_t *circ, crypt_path_t *layer_hint)
{

  log_debug(layer_hint?LD_APP:LD_EXIT,"resuming");

  if (CIRCUIT_IS_ORIGIN(circ))
    circuit_resume_edge_reading_helper(TO_ORIGIN_CIRCUIT(circ)->p_streams,
                                       circ, layer_hint);
  else
    circuit_resume_edge_reading_helper(TO_OR_CIRCUIT(circ)->n_streams,
                                       circ, layer_hint);
}

/** A helper function for circuit_resume_edge_reading() above.
 * The arguments are the same, except that <b>conn</b> is the head
 * of a linked list of edge streams that should each be considered.
 */
static int
circuit_resume_edge_reading_helper(edge_connection_t *conn,
                                   circuit_t *circ,
                                   crypt_path_t *layer_hint)
{
  for ( ; conn; conn=conn->next_stream) {
    if (conn->_base.marked_for_close)
      continue;
    if ((!layer_hint && conn->package_window > 0) ||
        (layer_hint && conn->package_window > 0 &&
         conn->cpath_layer == layer_hint)) {
      connection_start_reading(TO_CONN(conn));
      /* handle whatever might still be on the inbuf */
      if (connection_edge_package_raw_inbuf(conn, 1)<0) {
        /* (We already sent an end cell if possible) */
        connection_mark_for_close(TO_CONN(conn));
        continue;
      }

      /* If the circuit won't accept any more data, return without looking
       * at any more of the streams. Any connections that should be stopped
       * have already been stopped by connection_edge_package_raw_inbuf. */
      if (circuit_consider_stop_edge_reading(circ, layer_hint))
        return -1;
    }
  }
  return 0;
}

/** Check if the package window for <b>circ</b> is empty (at
 * hop <b>layer_hint</b> if it's defined).
 *
 * If yes, tell edge streams to stop reading and return 1.
 * Else return 0.
 */
static int
circuit_consider_stop_edge_reading(circuit_t *circ, crypt_path_t *layer_hint)
{
  edge_connection_t *conn = NULL;
  unsigned domain = layer_hint ? LD_APP : LD_EXIT;

  if (!layer_hint) {
    or_circuit_t *or_circ = TO_OR_CIRCUIT(circ);
    log_debug(domain,"considering circ->package_window %d",
              circ->package_window);
    if (circ->package_window <= 0) {
      log_debug(domain,"yes, not-at-origin. stopped.");
      for (conn = or_circ->n_streams; conn; conn=conn->next_stream)
        connection_stop_reading(TO_CONN(conn));
      return 1;
    }
    return 0;
  }
  /* else, layer hint is defined, use it */
  log_debug(domain,"considering layer_hint->package_window %d",
            layer_hint->package_window);
  if (layer_hint->package_window <= 0) {
    log_debug(domain,"yes, at-origin. stopped.");
    for (conn = TO_ORIGIN_CIRCUIT(circ)->p_streams; conn;
         conn=conn->next_stream)
      if (conn->cpath_layer == layer_hint)
        connection_stop_reading(TO_CONN(conn));
    return 1;
  }
  return 0;
}

/** Check if the deliver_window for circuit <b>circ</b> (at hop
 * <b>layer_hint</b> if it's defined) is low enough that we should
 * send a circuit-level sendme back down the circuit. If so, send
 * enough sendmes that the window would be overfull if we sent any
 * more.
 */
static void
circuit_consider_sending_sendme(circuit_t *circ, crypt_path_t *layer_hint)
{
//  log_fn(LOG_INFO,"Considering: layer_hint is %s",
//         layer_hint ? "defined" : "null");
  while ((layer_hint ? layer_hint->deliver_window : circ->deliver_window) <=
          CIRCWINDOW_START - CIRCWINDOW_INCREMENT) {
    log_debug(LD_CIRC,"Queuing circuit sendme.");
    if (layer_hint)
      layer_hint->deliver_window += CIRCWINDOW_INCREMENT;
    else
      circ->deliver_window += CIRCWINDOW_INCREMENT;
    if (relay_send_command_from_edge(0, circ, RELAY_COMMAND_SENDME,
                                     NULL, 0, layer_hint) < 0) {
      log_warn(LD_CIRC,
               "relay_send_command_from_edge failed. Circuit's closed.");
      return; /* the circuit's closed, don't continue */
    }
  }
}

/** Stop reading on edge connections when we have this many cells
 * waiting on the appropriate queue. */
#define CELL_QUEUE_HIGHWATER_SIZE 256
/** Start reading from edge connections again when we get down to this many
 * cells. */
#define CELL_QUEUE_LOWWATER_SIZE 64

#ifdef ACTIVE_CIRCUITS_PARANOIA
#define assert_active_circuits_ok_paranoid(conn) \
     assert_active_circuits_ok(conn)
#else
#define assert_active_circuits_ok_paranoid(conn)
#endif

/** The total number of cells we have allocated from the memory pool. */
static int total_cells_allocated = 0;

/** A memory pool to allocate packed_cell_t objects. */
static mp_pool_t *cell_pool = NULL;

/** Allocate structures to hold cells. */
void
init_cell_pool(void)
{
  tor_assert(!cell_pool);
  cell_pool = mp_pool_new(sizeof(packed_cell_t), 128*1024);
}

/** Free all storage used to hold cells. */
void
free_cell_pool(void)
{
  /* Maybe we haven't called init_cell_pool yet; need to check for it. */
  if (cell_pool) {
    mp_pool_destroy(cell_pool);
    cell_pool = NULL;
  }
}

/** Free excess storage in cell pool. */
void
clean_cell_pool(void)
{
  tor_assert(cell_pool);
  mp_pool_clean(cell_pool, 0, 1);
}

/** Release storage held by <b>cell</b>. */
static INLINE void
packed_cell_free(packed_cell_t *cell)
{
  --total_cells_allocated;
  mp_pool_release(cell);
}

/** Allocate and return a new packed_cell_t. */
static INLINE packed_cell_t *
packed_cell_alloc(void)
{
  ++total_cells_allocated;
  return mp_pool_get(cell_pool);
}

/** Log current statistics for cell pool allocation at log level
 * <b>severity</b>. */
void
dump_cell_pool_usage(int severity)
{
  circuit_t *c;
  int n_circs = 0;
  int n_cells = 0;
  for (c = _circuit_get_global_list(); c; c = c->next) {
    n_cells += c->n_conn_cells.n;
    if (!CIRCUIT_IS_ORIGIN(c))
      n_cells += TO_OR_CIRCUIT(c)->p_conn_cells.n;
    ++n_circs;
  }
  log(severity, LD_MM, "%d cells allocated on %d circuits. %d cells leaked.",
      n_cells, n_circs, total_cells_allocated - n_cells);
  mp_pool_log_status(cell_pool, severity);
}

/** Allocate a new copy of packed <b>cell</b>. */
static INLINE packed_cell_t *
packed_cell_copy(const cell_t *cell)
{
  packed_cell_t *c = packed_cell_alloc();
  cell_pack(c, cell);
  c->next = NULL;
  return c;
}

/** Append <b>cell</b> to the end of <b>queue</b>. */
void
cell_queue_append(cell_queue_t *queue, packed_cell_t *cell)
{
  if (queue->tail) {
    tor_assert(!queue->tail->next);
    queue->tail->next = cell;
  } else {
    queue->head = cell;
  }
  queue->tail = cell;
  cell->next = NULL;
  ++queue->n;
}

/** Append a newly allocated copy of <b>cell</b> to the end of <b>queue</b> */
void
cell_queue_append_packed_copy(cell_queue_t *queue, const cell_t *cell)
{
  packed_cell_t *copy = packed_cell_copy(cell);
#ifdef ENABLE_BUFFER_STATS
  /* Remember the exact time when this cell was put in the queue. */
  if (get_options()->CellStatistics)
    tor_gettimeofday(&copy->packed_timeval);
#endif
  cell_queue_append(queue, copy);
}

/** Remove and free every cell in <b>queue</b>. */
void
cell_queue_clear(cell_queue_t *queue)
{
  packed_cell_t *cell, *next;
  cell = queue->head;
  while (cell) {
    next = cell->next;
    packed_cell_free(cell);
    cell = next;
  }
  queue->head = queue->tail = NULL;
  queue->n = 0;
}

/** Extract and return the cell at the head of <b>queue</b>; return NULL if
 * <b>queue</b> is empty. */
static INLINE packed_cell_t *
cell_queue_pop(cell_queue_t *queue)
{
  packed_cell_t *cell = queue->head;
  if (!cell)
    return NULL;
  queue->head = cell->next;
  if (cell == queue->tail) {
    tor_assert(!queue->head);
    queue->tail = NULL;
  }
  --queue->n;
  return cell;
}

/** Return a pointer to the "next_active_on_{n,p}_conn" pointer of <b>circ</b>,
 * depending on whether <b>conn</b> matches n_conn or p_conn. */
static INLINE circuit_t **
next_circ_on_conn_p(circuit_t *circ, or_connection_t *conn)
{
  tor_assert(circ);
  tor_assert(conn);
  if (conn == circ->n_conn) {
    return &circ->next_active_on_n_conn;
  } else {
    or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
    tor_assert(conn == orcirc->p_conn);
    return &orcirc->next_active_on_p_conn;
  }
}

/** Return a pointer to the "prev_active_on_{n,p}_conn" pointer of <b>circ</b>,
 * depending on whether <b>conn</b> matches n_conn or p_conn. */
static INLINE circuit_t **
prev_circ_on_conn_p(circuit_t *circ, or_connection_t *conn)
{
  tor_assert(circ);
  tor_assert(conn);
  if (conn == circ->n_conn) {
    return &circ->prev_active_on_n_conn;
  } else {
    or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
    tor_assert(conn == orcirc->p_conn);
    return &orcirc->prev_active_on_p_conn;
  }
}

/** Add <b>circ</b> to the list of circuits with pending cells on
 * <b>conn</b>.  No effect if <b>circ</b> is already linked. */
void
make_circuit_active_on_conn(circuit_t *circ, or_connection_t *conn)
{
  circuit_t **nextp = next_circ_on_conn_p(circ, conn);
  circuit_t **prevp = prev_circ_on_conn_p(circ, conn);

  if (*nextp && *prevp) {
    /* Already active. */
    return;
  }

  if (! conn->active_circuits) {
    conn->active_circuits = circ;
    *prevp = *nextp = circ;
  } else {
    circuit_t *head = conn->active_circuits;
    circuit_t *old_tail = *prev_circ_on_conn_p(head, conn);
    *next_circ_on_conn_p(old_tail, conn) = circ;
    *nextp = head;
    *prev_circ_on_conn_p(head, conn) = circ;
    *prevp = old_tail;
  }
  assert_active_circuits_ok_paranoid(conn);
}

/** Remove <b>circ</b> from the list of circuits with pending cells on
 * <b>conn</b>.  No effect if <b>circ</b> is already unlinked. */
void
make_circuit_inactive_on_conn(circuit_t *circ, or_connection_t *conn)
{
  circuit_t **nextp = next_circ_on_conn_p(circ, conn);
  circuit_t **prevp = prev_circ_on_conn_p(circ, conn);
  circuit_t *next = *nextp, *prev = *prevp;

  if (!next && !prev) {
    /* Already inactive. */
    return;
  }

  tor_assert(next && prev);
  tor_assert(*prev_circ_on_conn_p(next, conn) == circ);
  tor_assert(*next_circ_on_conn_p(prev, conn) == circ);

  if (next == circ) {
    conn->active_circuits = NULL;
  } else {
    *prev_circ_on_conn_p(next, conn) = prev;
    *next_circ_on_conn_p(prev, conn) = next;
    if (conn->active_circuits == circ)
      conn->active_circuits = next;
  }
  *prevp = *nextp = NULL;
  assert_active_circuits_ok_paranoid(conn);
}

/** Remove all circuits from the list of circuits with pending cells on
 * <b>conn</b>. */
void
connection_or_unlink_all_active_circs(or_connection_t *orconn)
{
  circuit_t *head = orconn->active_circuits;
  circuit_t *cur = head;
  if (! head)
    return;
  do {
    circuit_t *next = *next_circ_on_conn_p(cur, orconn);
    *prev_circ_on_conn_p(cur, orconn) = NULL;
    *next_circ_on_conn_p(cur, orconn) = NULL;
    cur = next;
  } while (cur != head);
  orconn->active_circuits = NULL;
}

/** Block (if <b>block</b> is true) or unblock (if <b>block</b> is false)
 * every edge connection that is using <b>circ</b> to write to <b>orconn</b>,
 * and start or stop reading as appropriate. */
static void
set_streams_blocked_on_circ(circuit_t *circ, or_connection_t *orconn,
                            int block)
{
  edge_connection_t *edge = NULL;
  if (circ->n_conn == orconn) {
    circ->streams_blocked_on_n_conn = block;
    if (CIRCUIT_IS_ORIGIN(circ))
      edge = TO_ORIGIN_CIRCUIT(circ)->p_streams;
  } else {
    circ->streams_blocked_on_p_conn = block;
    tor_assert(!CIRCUIT_IS_ORIGIN(circ));
    edge = TO_OR_CIRCUIT(circ)->n_streams;
  }

  for (; edge; edge = edge->next_stream) {
    connection_t *conn = TO_CONN(edge);
    edge->edge_blocked_on_circ = block;

    if (!conn->read_event) {
      /* This connection is a placeholder for something; probably a DNS
       * request.  It can't actually stop or start reading.*/
      continue;
    }

    if (block) {
      if (connection_is_reading(conn))
        connection_stop_reading(conn);
    } else {
      /* Is this right? */
      if (!connection_is_reading(conn))
        connection_start_reading(conn);
    }
  }
}

/** Pull as many cells as possible (but no more than <b>max</b>) from the
 * queue of the first active circuit on <b>conn</b>, and write then to
 * <b>conn</b>-&gt;outbuf.  Return the number of cells written.  Advance
 * the active circuit pointer to the next active circuit in the ring. */
int
connection_or_flush_from_first_active_circuit(or_connection_t *conn, int max,
                                              time_t now)
{
  int n_flushed;
  cell_queue_t *queue;
  circuit_t *circ;
  int streams_blocked;
  circ = conn->active_circuits;
  if (!circ) return 0;
  assert_active_circuits_ok_paranoid(conn);
  if (circ->n_conn == conn) {
    queue = &circ->n_conn_cells;
    streams_blocked = circ->streams_blocked_on_n_conn;
  } else {
    queue = &TO_OR_CIRCUIT(circ)->p_conn_cells;
    streams_blocked = circ->streams_blocked_on_p_conn;
  }
  tor_assert(*next_circ_on_conn_p(circ,conn));

  for (n_flushed = 0; n_flushed < max && queue->head; ) {
    packed_cell_t *cell = cell_queue_pop(queue);
    tor_assert(*next_circ_on_conn_p(circ,conn));

#ifdef ENABLE_BUFFER_STATS
    /* Calculate the exact time that this cell has spent in the queue. */
    if (get_options()->CellStatistics && !CIRCUIT_IS_ORIGIN(circ)) {
      struct timeval flushed_from_queue;
      uint32_t cell_waiting_time;
      or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
      tor_gettimeofday(&flushed_from_queue);
      cell_waiting_time = (uint32_t)
            tv_mdiff(&cell->packed_timeval, &flushed_from_queue);

      orcirc->total_cell_waiting_time += cell_waiting_time;
      orcirc->processed_cells++;
    }
#endif

    /* If we just flushed our queue and this circuit is used for a
     * tunneled directory request, possibly advance its state. */
    if (queue->n == 0 && TO_CONN(conn)->dirreq_id)
      geoip_change_dirreq_state(TO_CONN(conn)->dirreq_id,
                                DIRREQ_TUNNELED,
                                DIRREQ_CIRC_QUEUE_FLUSHED);

    connection_write_to_buf(cell->body, CELL_NETWORK_SIZE, TO_CONN(conn));

    packed_cell_free(cell);
    ++n_flushed;
    if (circ != conn->active_circuits) {
      /* If this happens, the current circuit just got made inactive by
       * a call in connection_write_to_buf().  That's nothing to worry about:
       * circuit_make_inactive_on_conn() already advanced conn->active_circuits
       * for us.
       */
      assert_active_circuits_ok_paranoid(conn);
      goto done;
    }
  }
  tor_assert(*next_circ_on_conn_p(circ,conn));
  assert_active_circuits_ok_paranoid(conn);
  conn->active_circuits = *next_circ_on_conn_p(circ, conn);

  /* Is the cell queue low enough to unblock all the streams that are waiting
   * to write to this circuit? */
  if (streams_blocked && queue->n <= CELL_QUEUE_LOWWATER_SIZE)
    set_streams_blocked_on_circ(circ, conn, 0); /* unblock streams */

  /* Did we just ran out of cells on this queue? */
  if (queue->n == 0) {
    log_debug(LD_GENERAL, "Made a circuit inactive.");
    make_circuit_inactive_on_conn(circ, conn);
  }
 done:
  if (n_flushed)
    conn->timestamp_last_added_nonpadding = now;
  return n_flushed;
}

/** Add <b>cell</b> to the queue of <b>circ</b> writing to <b>orconn</b>
 * transmitting in <b>direction</b>. */
void
append_cell_to_circuit_queue(circuit_t *circ, or_connection_t *orconn,
                             cell_t *cell, cell_direction_t direction)
{
  cell_queue_t *queue;
  int streams_blocked;
  if (direction == CELL_DIRECTION_OUT) {
    queue = &circ->n_conn_cells;
    streams_blocked = circ->streams_blocked_on_n_conn;
  } else {
    or_circuit_t *orcirc = TO_OR_CIRCUIT(circ);
    queue = &orcirc->p_conn_cells;
    streams_blocked = circ->streams_blocked_on_p_conn;
  }
  if (cell->command == CELL_RELAY_EARLY && orconn->link_proto < 2) {
    /* V1 connections don't understand RELAY_EARLY. */
    cell->command = CELL_RELAY;
  }

  cell_queue_append_packed_copy(queue, cell);

  /* If we have too many cells on the circuit, we should stop reading from
   * the edge streams for a while. */
  if (!streams_blocked && queue->n >= CELL_QUEUE_HIGHWATER_SIZE)
    set_streams_blocked_on_circ(circ, orconn, 1); /* block streams */

  if (queue->n == 1) {
    /* This was the first cell added to the queue.  We need to make this
     * circuit active. */
    log_debug(LD_GENERAL, "Made a circuit active.");
    make_circuit_active_on_conn(circ, orconn);
  }

  if (! buf_datalen(orconn->_base.outbuf)) {
    /* There is no data at all waiting to be sent on the outbuf.  Add a
     * cell, so that we can notice when it gets flushed, flushed_some can
     * get called, and we can start putting more data onto the buffer then.
     */
    log_debug(LD_GENERAL, "Primed a buffer.");
    connection_or_flush_from_first_active_circuit(orconn, 1, approx_time());
  }
}

/** Append an encoded value of <b>addr</b> to <b>payload_out</b>, which must
 * have at least 18 bytes of free space.  The encoding is, as specified in
 * tor-spec.txt:
 *   RESOLVED_TYPE_IPV4 or RESOLVED_TYPE_IPV6  [1 byte]
 *   LENGTH                                    [1 byte]
 *   ADDRESS                                   [length bytes]
 * Return the number of bytes added, or -1 on error */
int
append_address_to_payload(char *payload_out, const tor_addr_t *addr)
{
  uint32_t a;
  switch (tor_addr_family(addr)) {
  case AF_INET:
    payload_out[0] = RESOLVED_TYPE_IPV4;
    payload_out[1] = 4;
    a = tor_addr_to_ipv4n(addr);
    memcpy(payload_out+2, &a, 4);
    return 6;
  case AF_INET6:
    payload_out[0] = RESOLVED_TYPE_IPV6;
    payload_out[1] = 16;
    memcpy(payload_out+2, tor_addr_to_in6_addr8(addr), 16);
    return 18;
  case AF_UNSPEC:
  default:
    return -1;
  }
}

/** Given <b>payload_len</b> bytes at <b>payload</b>, starting with an address
 * encoded as by append_address_to_payload(), try to decode the address into
 * *<b>addr_out</b>.  Return the next byte in the payload after the address on
 * success, or NULL on failure. */
const char *
decode_address_from_payload(tor_addr_t *addr_out, const char *payload,
                            int payload_len)
{
  if (payload_len < 2)
    return NULL;
  if (payload_len < 2+(uint8_t)payload[1])
    return NULL;

  switch (payload[0]) {
  case RESOLVED_TYPE_IPV4:
    if (payload[1] != 4)
      return NULL;
    tor_addr_from_ipv4n(addr_out, get_uint32(payload+2));
    break;
  case RESOLVED_TYPE_IPV6:
    if (payload[1] != 16)
      return NULL;
    tor_addr_from_ipv6_bytes(addr_out, payload+2);
    break;
  default:
    tor_addr_make_unspec(addr_out);
    break;
  }
  return payload + 2 + (uint8_t)payload[1];
}

/** Fail with an assert if the active circuits ring on <b>orconn</b> is
 * corrupt.  */
void
assert_active_circuits_ok(or_connection_t *orconn)
{
  circuit_t *head = orconn->active_circuits;
  circuit_t *cur = head;
  if (! head)
    return;
  do {
    circuit_t *next = *next_circ_on_conn_p(cur, orconn);
    circuit_t *prev = *prev_circ_on_conn_p(cur, orconn);
    tor_assert(next);
    tor_assert(prev);
    tor_assert(*next_circ_on_conn_p(prev, orconn) == cur);
    tor_assert(*prev_circ_on_conn_p(next, orconn) == cur);
    cur = next;
  } while (cur != head);
}