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It's unclear if zero circ_id with non zero MSB allowed by specification. Why 0x80..00 should be allowed and 0x00..00 shouldn't? With current implementation initiator never choose zero circ_id with or without set of MSB. But command_process_create_cell() refuse only 0x00..00 and passes 0x80..00

  if (cell->circ_id == 0) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
           "Received a create cell (type %d) from %s with zero circID; "
           " ignoring.", (int)cell->command,
           channel_get_actual_remote_descr(chan));
    return;
  }